ML113050588
| ML113050588 | |
| Person / Time | |
|---|---|
| Issue date: | 12/29/2011 |
| From: | Office of Nuclear Reactor Regulation, Office of New Reactors |
| To: | |
| Mensah, T M, NRR/DPR, 415-3610 | |
| Shared Package | |
| ML113050591 | List: |
| References | |
| NRC-2011-0013 RIS-11-012, Rev 1 | |
| Download: ML113050588 (56) | |
See also: RIS 2011-12
Text
RESPONSE TO PUBLIC COMMENTS ON DOCKET ID NRC-2011-0013
PROPOSED GENERIC COMMUNICATIONS: DRAFT NRC REGULATORY ISSUE
SUMMARY 2011-XX: ADEQUACY OF STATION ELECTRIC DISTRIBUTION
SYSTEM VOLTAGE*
On January 18, 2011, a Notice of Opportunity for Public Comment was published in the Federal
Register to clarify the NRC staffs technical position on existing regulatory requirements and
guidance for Degraded Voltage Relay (second level undervoltage protection) protection setting
bases and Transmission Network/Offsite/Onsite station electric power system design bases. On
February 23, 2011, a Notice was published in the Federal Register extending the comment
period to March 19, 2011, based on the request from NEI (ADAMS Accession No.
ML110330025). Comments were received from 14 organizations/individuals. The staff also
conducted a workshop on June 28-29, 2011, to discuss the NRCs existing regulatory
requirements and guidance for nuclear power plant electric power system degraded voltage
protection and addressed workshop participants questions
1. (1-7) Dominion 2. (8-31) Exelon 3. (32-39) Southern 4. (40-45, 59-85 & 86-96)
Resources, Inc Generation Company, LLC Nuclear Operating NEI
(ADAMS Accession No. (ADAMS Accession No. Company 1776 I Street NW
ML110540357) ML110540358) (ADAMS Accession No. Washington, DC, 20006
ML110540360) (ADAMS Accession Nos.
5. (49-51) Jerry Nicely 6. (52-58) Larry Nicholson 7. (97-108) PPL 8. (109-124) APS, Palo
Self Nextera Susquehanna, LLC Verde Nuclear Generating
(ADAMS Accession No. (ADAMS Accession No. (ADAMS Accession No. Station
ML110800530) ML110800536) ML110830675) (ADAMS Accession No.
9. (125-131) Nextera 10. (132) TVA 11. (133-137)Progress 12. (138-139) STARS
Energy (ADAMS Accession No. Energy (ADAMS Accession No.
(ADAMS Accession No. ML110840041) (ADAMS Accession No. ML110870916)
13. (140) Greg Reimers/ 14. (141) Brian Wilson
Diablo Canyon (ADAMS Accession No.
Email (ADAMS Accession ML110960076)
No. ML112010028)
The NRC staffs review and disposition of the comments are provided in the following Table.
- This response supersedes the information provided earlier in ADAMS at Accession Nos.
ML111600659 and ML112371830. These documents were incorrectly released as final
documents when in fact they were drafts. Changes between the draft and final public comment
resolution documents can be viewed in ADAMS at Accession No. ML11357A142xxxxxxxxx.
No. Section of RIS Originator Specific Comment NRC Resolution
1. SUMMARY OF Dominion Section DVR Setting Design
ISSUES - 1. DVR Resources
Setting Design Services, Inc In this manner, the DVR ensures adequate
Calculations operational (starting and running) voltage to all
safety related equipment, independent of voltage
controlling equipment external to the plant safety
related electrical distribution system.
The approach could imply that the load(s) should Disagree
start from the lowest DVR dropout setting. A
specific example for illustration is as follows: NRC Staff has the following clarification with
If voltage is at the lowest possible value above this position.
1
No. Section of RIS Originator Specific Comment NRC Resolution
dropout, starting a load will cause DVR dropout,
but, since the new steady state voltage will be If the offsite power has adequate capacity
lower than the initial value, then DVR reset cannot and capability, any voltage just above the
occur. DVR setpoint should not separate the offsite
power source from the safety bus when
Many utilities use the ABB 27N with harmonic filter starting large motors. The grid voltage is
which has a minimum 0.5% reset. Thus, with a expected to recover.
setting of 93.6% +/- 0.9%, dropout could be as low
as 92.7%. However, for motors causing more than The key point is that the voltage setting(s)
0.5% voltage dip at initial start, even if the voltage selected should ensure that adequate
at the beginning of the event was 93.2% and a load voltage is available at the component
was started, then DVR will dropout and never reset terminal(s) to operate the most limiting
causing a separation. A clarification that allows component (s) at a plant during the most
evaluation of motor starting as well-as other limiting design basis event. The offsite/onsite
conditions is: interface calculation should show that, with
the grid at the lower limit of the normal
If the DVR could possibly not cause separation operating range, voltage at the safety bus is
then the required safety functions must be always well above the degraded voltage
performed successfully. Thus, depending on the setpoint for all design basis event loading
design of the plant, a voltage value for beginning conditions (normal, abnormal and accident
the event with all required starts could be conditions including anticipated operational
determined by an iterative process. occurrence).
The safety related equipment should be
protected from two types of low voltage
issues:
1. Loss of voltage event which
implies a sudden sharp voltage
drop in grid system. Typically a
nominal delay is allowed for relay
actuation to separate onsite
busses from the grid if voltage
does not recover to normal
operating band.
2. Degraded voltage event that
postulates sustained low voltage
conditions for several seconds and
subsequent recovery to normal
operating band. If the offsite power
system does not recover to
nominal operating conditions, it is
preferable to separate from the
source.
The ABB relay with harmonic filter should be
able to reset if the grid perturbation is limited
to a short duration.
2. SUMMARY OF Dominion This section contains elements that are too Disagree
ISSUES -- 2 . Resources prescriptive. Many analyses will show that the unit
Offsite/Onsite Services, Inc loads/sequences assessed for determination of NRC Staff has the following clarification for
Design Interface DVR setpoint adequacy for equipment protection this position.
Calculations(page are the same as those for evaluating offsite power.
7) Since evaluating offsite power always involves To meet GDC 17 requirements, the licensee
higher voltages, it is clear the equipment will must demonstrate capability to safely shut
function and providing terminal voltages for this down the plant for all design basis events
equipment is bounded by DVR adequacy analysis. with the grid voltage at the lowest allowable
A clarification that helps frame adequate analysis value as afforded by the transmission system
is: operator.
If the DVR could initiate separation then offsite The voltage studies done for evaluating
power is not operable. Using the example above offsite power/onsite power interface should
when evaluating offsite power would require that use minimum expected voltage at the
the safety bus recover above 93.6%+0.9%+0.5% plant/grid interface node, demonstrating
or 95% before the earliest time delay for the DVR adequate voltage for starting and running of
expires. Since a reset also resets the time delay, plant components during normal, abnormal
multiple DVR drop outs could occur without and accident conditions. The expected plant
separation during load sequencing. Also, since loading at 100% power operation may be
92.7% was evaluated for equipment protection, higher than accident loading. Hence the
2
No. Section of RIS Originator Specific Comment NRC Resolution
evaluating 95% (used for offsite power evaluation) voltage drop in the plant auxiliary system will
would require the two sequences be substantially be higher for normal operating conditions.
different (2.3%) for the DVR adequacy evaluation The DVR setpoint should be below the
(at 92.7%) to not be bounding for equipment normal operating voltage of the plant to avoid
evaluation. Thus, providing calculation detail to multiple spurious actuations. A separate
motor terminals for offsite power evaluation is analysis may be needed for DVR setpoint.
unnecessary in many designs.
The comment implies that DVR setpoint
overlaps with system voltage during normal
operation.
Calculation details to motor terminals are
helpful in gaining margin between DVR
setpoint and normal grid operating voltages.
To avoid spurious DVR actuation during
normal plant operation and during load
sequencing, the DVR setpoint should be
lower than normal operating band for offsite
power. This can be achieved by :
1) Specifying equipment for safety related
applications to function at voltage levels
well below the nominal bus voltage.
2) Reducing the onsite system
impedance/voltage drop. This can be
achieved by reducing the cable impedance
for the limiting safety loads and tripping
non-essential loads after unit trip.
3. SUMMARY OF Dominion Part a states:
ISSUES -- 2 . Resources This transmission owner/operator supplied voltage
Offsite/Onsite Services, Inc range should address all transmission
Design Interface network and plant system operating configurations
Calculations(page and should also include voltage drop due to
7) the bounding worst case transmission system
contingency (transmission system contingencies
include trip of the nuclear power unit).
Certainly the trip of the nuclear power unit must Disagree
always be considered. However, the definition of
the worst case transmission system contingency
requires clarifying statements. Certainly some
analyses are done using "strong grid" for fault
analyses or "weak grid" for voltage analyses.
These modeled sources have a number of
contingencies built into them. Voltage drop from the The plant electrical distribution system
loss of the unit can vary considerably with system should be designed based on the grid
conditions. Arguments can be made that the worst voltage range including the bounding worst
case contingency (if different than the nuclear case grid contingency (strong or weak grid
power unit) should only be considered once it has depending on which one is bounding). In this
occurred. However, a key question should be way, the plants design ensures adequate
applied: Is the contingency of interest monitored? If voltage to plant equipment as long as grid is
the status is not monitored, then how would the operating as expected.
nuclear unit know when to apply the contingency?
Thus, if the status of a key transmission line to the Contingencies that are beyond design basis
nuclear unit switchyard is known, either by (line outages during peak grid loading
instrumentation at the plant or timely notification conditions) that occur during plant operation
by the grid operator, then the contingencies need should be evaluated uniquely to assess the
only be considered when applicable. When capability of offsite power to provide
evaluating voltage drop, most situations which shutdown capability post trip as required by
cause meaningful changes are nearby and can be GDC 17.
monitored.
GDC 17 requires that offsite power has
sufficient capacity and capability to assure
that (1) specified acceptable fuel design
limits and design conditions of the reactor
coolant pressure boundary are not exceeded
as a result of anticipated operational
occurrences, and (2) the core is cooled and
containment integrity and other vital
functions are maintained in the event of
3
No. Section of RIS Originator Specific Comment NRC Resolution
postulated accidents.
4. SUMMARY OF Dominion Part c states:
ISSUES -- 2 . Resources
Offsite/Onsite Services, Inc For multi-unit stations, a separate analysis should
Design Interface be performed for each unit assuming (1) an
Calculations(page accident in the unit being analyzed and
8) simultaneous shutdown of all other units at the
station,' or (2) an anticipated transient (anticipated
operational occurrence) in the unit being analyzed
(e.g., unit trip) and simultaneous shutdown of all
other units at that station, whichever presents the
largest load situation.
This requires clarification as an accident or
anticipated transient both require unit trip. The Disagree
word used for other units at the station is
"shutdown" which is more orderly and takes more
time. Simultaneous unit trip results in bigger This statement is consistent with GDC 17,
voltage drops from VAR support (not loading). This GL 79-36, and IEEE Standard 308-1971,
is because VARs are local. System X/R is typically Class IE Electrical Systems, Section 8,
-50 so it is 50 times more difficult to move a VAR a Multi-Unit Station Considerations.
hundred miles than a watt. Thus, most analyses
show the worst voltage drop for the loss of the Multi-unit sites have been licensed in
nuclear unit is when all of the nearby units (any accordance with above documents and
type of generation) are already off. Changing that should therefore evaluate the plants
to tripping all units at the same time increases the according to their licensing basis.
voltage drop because grid compensatory actions
are not included. Even a small (minute) time
difference between losses of units can be
meaningful in the voltage result. However, if the
intent of the wording was simultaneous trip, then
this is a special case of item 3 above, which is
likely a monitored contingency.
5. SUMMARY OF Dominion Part j states:
4
No. Section of RIS Originator Specific Comment NRC Resolution
ISSUES -- 2 . Resources
Offsite/Onsite Services, Inc To provide assurance that actions taken to assure
Design Interface adequate voltage levels for safety related loads
Calculations(page do not result in excessive voltages, assuming the
8) maximum expected value of voltage at the
connection to the offsite circuit, a determination
should be made of the maximum voltage
expected at the terminals of all safety related
equipment and their starting circuits (if applicable).
If this voltage exceeds the maximum voltage rating
of any safety related equipment, immediate
remedial action should be taken.
The word "immediate" describing remedial action Disagree
should be removed from this section. Immediate
remedial action could imply control room
intervention. The control room has alarm
procedures to address high voltage should it occur.
Timeliness of remedial actions depends on how
high actual voltage is since minor incursions have
only long term implications for most equipment. The wording is consistent with GL 79-36.
Analyses of high grid voltage with light plant load
are standard and provide insights as to what grid The Offsite/Onsite design should address all
voltage upper limit should be or what compensating grid operating conditions to prevent
activities might be required for light load operations overvoltages from occurring.
(refueling). In those cases where unit trip can result
in a step increase in grid voltage (most common on The point here is that if a design problem is
higher voltage connections like 765kv), anticipated identified such as overvoltage conditions,
excursions above desired voltages should be immediate actions should be taken
addressed by compensating measures (changing (compensatory and/or permanent design
excitation for nearby units, switching in reactor changes) to address the design problem
banks, etc.). rather than taking actions after it occurs.
6. SUMMARY OF Dominion States: This interface calculation establishes
ISSUES (top of Resources operating voltage bands for all plant electrical
page 6) Services, Inc buses, which ensures that all plant components
and systems (Class 1E and Non Safety Related)
have proper voltage for starting and running in all
operational configurations (expected operational
and accident conditions).
This statement needs clarification in that not all Agree
non-safety load voltages need to be evaluated.
Typically, large motors (like reactor coolant pumps)
need to be evaluated for starting impact on the
safety bus. However, once a motor is found to be
small enough to not impact safety bus operation,
further evaluation is unnecessary. The statement in
the RIS can easily be interpreted as requiring The reference to non-safety related is being
evaluation of all non-safety loads down to the removed. However, non-safety related loads
lowest levels of distribution. should be modeled to the extent that their
operation can affect safety bus/equipment
voltage.
7. SUMMARY OF Dominion States:
ISSUES - 1. DVR Resources
Setting Design Services, Inc Licensee voltage calculations should provide the
Calculations (page basis for their DVR settings, ensuring safety related
6) equipment is supplied with adequate operating
voltage (typically a minimum of 0.9per
unit voltage at the terminals of the safety related
equipment per equipment manufacturers
requirements), based on bounding conditions for
the most limiting safety related load (in terms of
voltage) in the plant.
Clarification that voltages other than 90% voltage Agree.
are common based on detailed plant analysis
should be added. As an example, motors below 90% was mentioned as an example used for
90% voltage continue to have plenty of margin in illustration. The voltage values are plant-
5
No. Section of RIS Originator Specific Comment NRC Resolution
torque but may encroach on long time thermal specific. Evaluations like mentioned in this
limits. However, unless a motor is fully into its comment could be acceptable as long as
service factor (typically 1.15), as well as below 90% there is adequate engineering justification.
voltage, operation will be acceptable. Ensuring that voltages are within nominal
limits greatly simplifies the analysis required.
8. General Exelon General Comments:
Generation
Company, The RIS uses terms such as "LVR (loss-of-voltage
Background - LLC relay) voltage setting," "DVR (degraded
Pages 2 and 3 voltage relay) settings" and "DVR dropout setting" Disagree.
without clarifying the intent or highlighting the
differences.
IEEE 741-2006, Annex A (Reference 1), has a The terminology used in the RIS is
discussion on the tolerances to be considered and consistent with the guidance documents.
recommends following ANSI/ISA 67.04.01
(Reference 2) treating the voltage relays and
associated time delays as instruments.
For the DVR, one example might be The setpoint accuracies and methodologies
represented as follows: are beyond the scope of this RIS.
Analytical limit: Minimum voltage that assures
actuation of the relay
Allowable value, Lower: Higher than analytical limit
to allow for drift and test equipment tolerance;
abbreviated AVDO. Tech Spec value.
Dropout setpoint: Lower voltage band of nominal
setpoint. Abbreviated SPc DO (Setpoint calculated
Drop Out)
Pickup setpoint: Upper voltage band of nominal
setpoint. Abbreviated SPc PU (Setpoint calculated
Pick Up)
Allowable value, Upper: Higher than SPc PU to
allow for drift and test equipment tolerance;
abbreviated AVPU. Tech Spec value.
Maximum Dropout: Highest voltage that relay could
actuate. Only importance is for establishing reset
voltage.
Maximum Pickup: The voltage required to assure
DVR resets.
The RIS states that two sets of calculations are
required. It appears that at least three (3) The point of the RIS was to highlight that the
distinctly different calculations are required (four if DVR setting and design interface
the site has different DVR time delays for accident calculations have different requirements.
and normal conditions). These would be at different
bus voltage values. The "degraded voltage relaying The staff agrees that there are other
design calculations" would be a load flow calculations required to demonstrate the
performed at the DVR analytical limit; the "plant electrical system design basis.
voltage analysis" would be load flows and motor
starting performed at the minimum transmission
contingency voltage with an acceptance criterion of
greater than relay maximum pickup (the voltage
where DVR reset is assured) at the bus where the
degraded voltage relays are connected (generally
the medium voltage bus where the Emergency
Diesel Generator is connected); finally, the
evaluation of protective device actuation would be
performed at the analytical limit of the loss of
voltage relay setting comparing the motor running
current to the thermal damage curve and protective
6
No. Section of RIS Originator Specific Comment NRC Resolution
device characteristic curve.
Page 2, criteria b) - Some approved DVR designs
sense and trip at an emergency bus level, and take RIS is consistent with the NRC letter dated
advantage of inherent redundancy of the June 2, 1977.
emergency buses. It should be an owner's option The coincident logic is to ensure that
with respect to coincident logic. Change the "shall" spurious or inadvertent separation of a
to "may." reliable offsite power source. The
Page 2/3 - The listed 6 criteria are good for setting redundancy of the safety buses alone does
the DVR. Early correspondence of the issue not address the above concern
also included a second function for the DVR in that
the design should minimize the effects of Current wording seems adequate to address
spuriously disconnecting the offsite sources. the point that spurious trips of offsite power
Although criteria b) and c)(2) are intended to add should be precluded by the design.
robustness to the design, a few sentences should
be added to the discussion to accentuate the
point.
9. SUMMARY OF Exelon Under "Degraded Voltage Relaying Design Disagree.
ISSUES - 1. Generation Calculations," the RIS states in part "During normal
Degraded Voltage Company, plant operation, the Class 1 E safety related buses This is not a new requirement.
Relaying Design LLC should automatically separate from the power
Calculations (page supply within a short interval (typically less than 60 RIS will be revised to remove the reference
6) seconds) if sustained degraded voltage to auto separate in 60 seconds. The 60
conditions are detected." Branch Technical Position seconds time delay was identified as an
PSB-1 clause B.1.b.2 included provisions example to illustrate that the time delay
for operator manual actions to restore bus voltage chosen for the sustained degraded condition
on the Class 1 E distribution system. The (DVR settings) should be short to ensure that
sixty second time delay would not allow operator permanently connected Class 1E loads are
actions. This appears to be a new NRC not damaged.
position.
However, it should be noted that when
voltage alarms occur (alarm setpoint is set
higher than the DVR setpoint), the grid
voltage at that point may be well below the
normal operating values and is approaching
the DVR setpoint and operator actions may
be taken to improve the voltage conditions to
prevent separation from offsite power.
The time delay chosen should ensure that
until the relay automatic action is initiated, all
Class 1E equipment are protected. The
licensee must provide the bases and
justification in support of the actual delay
chosen.
10. SUMMARY OF Exelon The next to last sentence under item 1 states: 'The Disagree.
ISSUES - 1. Generation staff considers degraded voltage
Degraded Voltage Company, conditions coincident with a postulated design The point being made in the RIS is that
Relaying Design LLC basis accident to be a credible event. The event setting of the DVR should include
Calculations Page is credible in that it has occurred previously consideration of a coincident accident, in that
6 (although nonaccident). It is acknowledged that the time delay chosen for the DVR should
safety loads combined with loss of generator support the accident analysis assumptions
reactive power support will cause a decrease in consistent with the NRC1977 letter.
bus voltage. However, if the plant is operated
within the bounds of the operating procedures Operating a plant within allowable voltage
(which are reflected in the voltage regulation range should minimize the potential for
calculations as described under the subsequent degraded voltage conditions on 1E busses.
section), then the Class 1 E equipment should not However, grid perturbations cannot be
experience a degraded voltage condition. predicted. Hence the need for automatic
The sentence can be removed without diminishing protection.
the need for the DVR, or without changing
the intent of this section.
11. SUMMARY OF Exelon DVR Setting Design Calculations - Add a sentence Agree.
ISSUES - 1. Generation "The model should utilize loads on the plant
Degraded Voltage Company, distribution system consistent with the specific The suggested sentence will be added to the
Relaying Design LLC transient or accident being analyzed." RIS.
Calculations -
7
No. Section of RIS Originator Specific Comment NRC Resolution
Page 6,
12 SUMMARY OF Exelon In addition, Branch Technical Position (BTP) PSB-1 Disagree
ISSUES - 1. Generation clause B. 1 .b.2 (Reference 4) included provisions All actions required to protect the Class 1E
Degraded Voltage Company, for operator manual actions to restore bus voltage equipment from degraded voltage must be
Relaying Design LLC on the Class 1 E distribution system. The RIS automatic in accordance with 10 CFR
Calculations - specifically excludes manual load shedding under 50.55a(h)(2).
Page 6, the Offsite/Onsite Design Interface Calculations
whereas the BTP allows for manual actions to Manual actions are allowed as stated in
avoid separation from offsite power. Please clarify PSB-1, B.1.b.2 for improving the voltage in
if manual actions taken to restore voltages now response to the alarm in control room that
require prior NRC approval. alerted the operator to the degraded
condition. However, to demonstrate the
adequacy of onsite/offsite interface design
and offsite power capacity and capability, as
specified in GL 79-36, manual load shedding
should not be assumed.
13 SUMMARY OF Exelon Under "DVR Setting Design Calculations," the RIS Agree
ISSUES - 1. Generation states in part "...would allow calculation of
Degraded Voltage Company, voltages at terminals or contacts of all safety RIS will be revised to just state terminals
Relaying Design LLC related equipment with the voltage at the DVR and not Contacts.
Calculations - monitored bus at the DVR dropout setting." It is not
Page 6, clear what "contacts" are in this context. It
is assumed that the concern is motor control center
contactors and/or motor starting control circuits.
14 SUMMARY OF Exelon Under discussion of DVR setting calculations, the Disagree
ISSUES - 1. Generation RIS states that setting cannot cause any
Degraded Voltage Company, degradation of the safety related components, The DVR ensures that voltage requirements
Relaying Design LLC including actuation of their protective devices. of the Class 1E loads are always preserved
Calculations - The BTP only stated damage to normally operating for operating the equipment under accident
Page 6, safety related equipment. The RIS language and non accident conditions including all
seems broader then BTP and appears to open up abnormal operational occurrences.
the position that the DVR studies have to consider
starting of loads under non-accident conditions.
15 SUMMARY OF Exelon The DVR time delay seems to be considering Disagree.
ISSUES - 1. Generation operation down to LVR setting for evaluations. The point is that the DVR setting is based on
Degraded Voltage Company, However, there is no discussion on LVR setting the voltage requirements of the equipment,
Relaying Design LLC considerations in any original requirements or the which should equate to voltages on the grid
Calculations - RIS. Under Guidelines for Voltage Drop well below normal. It is understood that grid
Page 6, Calculations, the summary states that the plant operating procedures should prevent
voltage analysis, while supplied from the sustained voltages at such low levels but
transmission network, should be based on the regardless of what happens on the grid the
operating voltage range of the transmission DVRs will preserve the voltage limits for the
network connection. Grid operating voltage ranges equipment.
do not allow operation down to levels that would
cause sustained operation at LVR levels. Plant operation at LVR setpoint is not
Therefore, consideration for operation at the LVR expected and is not within the scope of the
setpoint would be inconsistent with this guidance. RIS.
The condition that occurred at Arkansas Nuclear
One (ANO) in 1978 would appear to be related to
inadequate operating procedures and a lack of a
rigorous analysis of the AC power distribution
system. It would not be credible for present day
operation. In addition, the operator would be
alerted by an alarm on degraded voltage conditions
(less than the analytical limit) as required by
Branch Technical Position PSB- B.l.b.1.
16 SUMMARY OF Exelon In Section "DVR Setting Design Calculation" Agree.
ISSUES - 1. Generation reference is made to 0.9 per unit voltage for
(17 not DVR Setting Company, adequate operating voltage. This would only apply 0.9 per unit voltage was mentioned as an
used)
Design LLC for the most part to rotating equipment example and was not meant to cover
Calculations - (motors). Motor Control Center (MCC) contactors, everything. RIS will be revised to delete
Page 6 battery chargers, Motor Operated Valves references to specific numbers and
(MOVs) all have less than a 90% operating voltage emphasize voltage requirements and voltage
requirement. This distinction should be requirements are plant-specific.
made and/or clarified.
18 SUMMARY OF Exelon The Degraded Voltage Relaying Design
ISSUES - 1. Generation Calculations section should include a statement to Agree.
8
No. Section of RIS Originator Specific Comment NRC Resolution
DVR Setting Company, emphasize that only steady state loading and
Design LLC steady state acceptable voltages at the class IE 0.9 per unit voltage was mentioned as an
Calculations - equipment are to be considered in determining the example and was not meant to cover
Page 6 DVR drop out settings including the allowable everything. RIS will be revised to delete
tolerances. The paragraph does mention 0.9 per references to specific numbers and
unit voltages at the terminals which is steady state emphasize voltage requirements and voltage
but a positive statement about steady state loading requirements are plant-specific.
and steady state acceptable voltages would be
helpful. Also, there are alternatives to the 90% The suggested analysis may be acceptable if
terminal voltage criterion. The concern is heating, properly developed and supported in the
causing a temperature rise, which decreases useful design. The 90% voltage criteria may not be
life of the insulation. Inspectors may read too much adequate for certain components such as
into the 90% criterion; a motor loaded to less than SOVs, motor control center contactors, etc.
nameplate will draw less than service factor current
at a lower terminal voltage. Therefore, a lower
voltage would prove adequate as long as adequate
torque is available.
19 SUMMARY OF Exelon MOVs are not steady state loads. MOVs have Disagree.
ISSUES - 1. Generation traditionally been considered transient loads and,
DVR Setting Company, therefore, not included in the steady state voltage MOVs should be addressed specifically and
Design LLC analysis. GL 89-10 (Reference 3) programs the DVR settings must support adequate
Calculations - perform these calculations. A statement in this voltages for all Class 1E equipment including
Page 6 section that MOVs loads are not to be considered MOVs.
in this calculation will be helpful if NRC agrees with
this interpretation
20 SUMMARY OF Exelon The starting voltage requirement is unclear. Some Disagree.
ISSUES - 1. Generation sites have evaluated the capability of starting each
DVR Setting Company, required safety related motor individually at the The intent of the RIS is not to prescribe DVR
Design LLC degraded voltage analytical limit. Other sites use a relay setpoint methodology for every plant.
Calculations - "block start analysis" where multiple motors are
Page 6 started simultaneously on the offsite source. There The RIS provides the conditions for which
have been violations associated with both plant specific analyses should be performed.
approaches. The RIS should describe an The specific design of the plant dictates the
acceptable methodology for determination of motor type of analyses required to demonstrate
starting voltage adequacy. adequacy of DVR setting. If the plant design
requires load sequencing on the offsite
source, then individual motor start is the
appropriate methodology. If the plant design
requires block starting accident loads, then
the DVR setpoint should be based on
multiple motor starts.
21 SUMMARY OF Exelon The RIS implies this portion of the calculations Disagree
ISSUES - 1. Generation require that the licensee demonstrate that all NRC staff disagrees with the interpretation.
DVR Setting Company, class IE motors can be started with the voltages The staff agrees that a grid voltage freeze
Design LLC just above the analytical limit of the DVR corresponding to the DVR setpoint and a
Calculations - setpoint. However, with voltage just above the DVR subsequent motor start will eventually
Page 6 drop out value, any load addition (starting or separate the plant from offsite source as the
running) will result in separating from the offsite voltage will not recover to reset the DVR.
source if no credit for external voltage controlling The 1977 NRC letter states that voltage and
equipment is taken. Therefore, the purpose of this time setpoints shall be determined from an
requirement is not clear. The intent of the starting analysis of the voltage requirements of the
voltage evaluation should be clarified. safety related loads. Safety related (Class
1E) equipment, particularly large motors,
Some stations have evaluated the performance of have starting and running voltage
protective devices during degraded grid conditions requirements.
by mechanisms other than calculations (e.g.,
technical evaluations or computations). It is When grid voltages are degraded (such as
suggested that the NRC add a statement for resulting in Class 1E bus voltages down
acceptability of the same. close to where DVRs are set based on Class
1E equipment requirements), and the grid
does not automatically recover, separation
from the grid is appropriate. The DVR is
expected to reset after a perturbation of
sustained duration when automatic actions
such as clearing the grid fault that resulted
in degraded voltage conditions.
The NRC staff will accept standard industry
practices to evaluate performance
9
No. Section of RIS Originator Specific Comment NRC Resolution
capabilities of DVR. Analyses supported by
calculation should clearly and succinctly
define plant design basis and compliance
with regulation.
22 SUMMARY OF Exelon Under Offsite/Onsite Design Interface Calculations, Agree.
ISSUES - 1. Generation Guidelines for voltage drop calculations item
DVR Setting Company, 2, i), the acceptance criteria for demonstrating RIS Section 2 (i) will be modified to state: For
Design LLC voltage adequacy would appear to be DVR each case evaluated, the calculated voltages
Calculations - Maximum Pickup (the voltage required to assure on each safety bus should demonstrate
Page 6 relay reset) and not component level voltage adequate voltage at the safety bus and down
values. to the component level. It is based on Class
1E component terminal voltage
requirements.
23 SUMMARY OF Exelon Item 2 (Offsite/Onsite Design Interface
ISSUES - 2. Generation calculations) appears to be additional requirements Disagree
Offsite/Onsite Company, for those sites licensed to the Standard Review
Design Interface LLC Plan (NUREG 0800) Chapter 8 Appendix A Branch RIS highlights the guidelines provided in GL
Calculations Technical Position PSB-1, "Adequacy of Station 79-36 and NUREG 0800, Chapter 8
Electric Distribution System Voltages." Appendix A Branch
Technical Position PSB-1, "Adequacy of
Station Electric Distribution System
Voltages.". There are no new requirements
24 SUMMARY OF Exelon Page 7- The phrase "...all operating configurations
ISSUES - 2. Generation of transmission network and plant
Offsite/Onsite Company, systems..." appears in a few sentences. The station
Design Interface LLC interface agreement with the transmission
Calculations provider integrates the considerations among the Disagree.
transmission network, the operability of the
off site sources, and the voltage regulation (drop) This is addressed in RIS Section 2 a.
calculations. The calculations identify certain
controlling parameters for the transmission As discussed in GL2006-02 Grid Reliability
network. These controlling parameters are then and Impact on Plant Risk and the Operability
incorporated into the Bases for the operability of of Offsite Power, licensees are required to
the offsite source(s). If the plant configuration provide the transmission system operator
or transmission network parameters are not (TSO), the operating voltage parameters
bounded by the calculations, then the operability of required by the plant during all modes of
the offsite sources needs to be examined. In most operation. The analyses discussed in this
cases, the plant operator has no control over comment should be the bases for the
the "configuration" of the transmission network, but information provided to the TSO.
does have agreements with the transmission
system operator that normal operating voltages The DVR protects the safety related
and post unit trip contingency voltages are equipment when a perturbation in the grid
controlled within established bounds. Add a few system results in degraded voltage
sentences detailing that the intent of the conditions and the normal operating
phrase "all operating conditions of the transmission parameters cannot be restored immediately
network" means that the controlling to protect safety related equipment. The
parameters from the transmission network that are DVR setpoint evaluation should be a
used in the calculations are consistent with separate analysis.
those utilized in the Bases for operability of the
offsite sources.
25 SUMMARY OF Exelon Page 7, item a) - Change the last sentence to read
ISSUES - 2. Generation "... include voltage drop due to all Disagree.
Offsite/Onsite Company, transmission system contingencies that are a direct
Design Interface LLC result of the transient or accident being A transient grid perturbation may be a result
Calculations analyzed (typically this will include tripping of the in a plant trip. The DVR setpoint should be
nuclear power unit)." based on bounding voltage resulting from a
transient grid condition. The actual reason
for the perturbation is not a consideration. A
plant trip may result in limiting conditions for
DVR setpoint calculation.
26 SUMMARY OF Exelon Page 7, item a) - Either add to a) or add another
ISSUES - 2. Generation section immediately after a). The transmission Agree.
Offsite/Onsite Company, system controlling parameters are assumed to
Design Interface LLC remain unchanged throughout the initial stages of RIS will be revised to state a): The unit trip
10
No. Section of RIS Originator Specific Comment NRC Resolution
Calculations the event with the exception of those effects grid contingency voltage drop value should
resulting from the event (contingency due to the be used in the accident cases in accordance
loss of the unit). For purposes of the calculation, with the plant accident analyses since a unit
the Unit trip contingency can be coincident with the trip occurs with an accident.
accident, or at a later time consistent with the
assumptions in the plant accident sequence
analyses."
27 SUMMARY OF Exelon Page 7, item b) - Delete the tabulation of sources
ISSUES - 2. Generation of power to the emergency buses and replace with Disagree.
Offsite/Onsite Company, a simple statement of "all credited sources of offsite
Design Interface LLC power to the emergency buses." The recommendation does not change the
Calculations intent of the tabulation. To maintain
consistency with GL 79-36, it is preferable to
maintain the tabulation.
28 SUMMARY OF Exelon Page 8, item c) - Change to read: "...(1) an
ISSUES - 2. Generation accident in the unit being analyzed and shutdown Disagree.
Offsite/Onsite Company, of all other units at the station consistent with the
Design Interface LLC licensing basis of the station; ... in the unit The RIS is consistent with GL 79-36.
Calculations being analyzed (e.g., unit trip) and shutdown of all The licensing basis of multi-unit sites has to
other units at that station consistent with the be uniquely considered.
licensing basis of the station, whichever represents
the largest load situation." Typically, the
licensing basis for multi-units site allow for an
orderly shutdown of the unit not being analyzed,
and do not require a "simultaneous" shutdown.
29 SUMMARY OF Exelon It is recommended that the NRC provide a positive
ISSUES - 2. Generation statement for allowing the credit for voltage Disagree.
Offsite/Onsite Company, controlling equipment external to the class IE
Design Interface LLC equipment for this calculation. Licensees perform Use of LTCs is acceptable for regulating
Calculations LOCA load sequencing under this section of the voltage during normal plant operation. LTCs
calculations and take credit for LTCs (or other do not afford protection during a transient
voltage regulating devices) to demonstrate the degraded voltage condition that can affect
adequacy of the offsite sources. In addition, operation of redundant equipment.
please clarify if MOVs are to be modeled during The following changes will be incorporated in
this scenario, even though it appears from the the RIS: Add the following in section 2
RIS that MOVs and other equipment like contactors (general) and d.
are to be evaluated with voltages obtained from the
Degraded Voltage Relaying Design Calculations All actions the electric power system is
with voltage just above the lowest set point of DGV designed to automatically initiate or control
relays. should be assumed to occur as designed
(e.g., automatic bulk or sequential loading or
automatic transfers of bulk loads from one
transformer to another, automatic starts of
components, operation of automatic voltage
controlling equipment, etc.,)
Yes. All equipment including MOVs,
contactors, solenoids, etc., should be
evaluated for adequate voltage based on the
DVR set point.
30 SUMMARY OF Exelon Under Item a), for units with LTCs, please clarify if
ISSUES - 2. Generation the analysis is to be performed with the grid Disagree
Offsite/Onsite Company, at minimum expected voltage, maximum expected
Design Interface LLC voltage, or at both. See Question 29.
Calculations Analyses for normal operation should
evaluate effect of LTC operation at the
extreme settings for impact on operating
equipment. DVR setpoint should be based
on minimum voltage required for operation of
accident mitigation loads. For units with
LTCs, it is unlikely that voltage correction
can be achieved within the short time it takes
for contactors to drop out or fuses to blow
during a sustained degraded voltage or
overvoltage condition.
31 General Exelon In general the clarifications contained in the draft
Generation RIS appear to be more restrictive and Disagree.
11
No. Section of RIS Originator Specific Comment NRC Resolution
Company, prescriptive than the cited historical regulatory
LLC documentation, and do not support plant unique Unique designs that may have been
design and current licensing bases that have been previously accepted should have
developed and accepted in previous licensing appropriate justification with NRC approval of
activities. Unique design and licensing bases that the licensing documents. Typically, detailed
have previously been accepted and calculations have not been reviewed as part
approved that may not be strictly aligned with the of Technical Specification changes. The staff
clarifications in the draft RIS may include use has relied on licensee correspondence
of an inverse time under voltage relay set between stating adequacy of DVR setpoint to approve
the DVR and LVR relays (such as .875 to license amendment requests. Onsite
0.70 PU for a maximum of 60 seconds). Some inspections are used to verify analytical
sites may not provide coincident logic to methods used to meet regulations.
preclude spurious trips; rather, the logic may
include alternate design features to conform to the Alternate methods used to demonstrate
intent of the requirements of BTP PSB-1. conformance may be acceptable provided
they meet the intent of BTP PSB-1 to protect
safety related equipment
32 Southern
General Nuclear Agree
Operating Include a definition of key terms (ex. Normal grid
Company operation, sustained degraded voltage) Additional clarifications will be provided in
the RIS wherever appropriate.
33 General Southern The RIS does not address completely the specific
Nuclear requirements in the PSB-1 (ADAMS Accession No. Disagree
Operating ML052350520), Arkansas Nuclear One (ADAMS
Company Accession No.ML0311801180), and Millstone The RIS covered the key topics intended to
(ADAMS Accession No. ML093521388) address inspection findings.
documents. In some cases specific positions in
the above documents were omitted from the RIS. The reference documents should be
reviewed for more details.
Proposed resolution: include missing positions
especially those related to determining minimum The expected offsite system voltages can
expected offsite system voltages and testing. vary between 0-110 percent. The objective of
the LVR and DVR is to afford protection and
separation from the grid when plant specific
needs cannot be satisfied.
34 General Southern The RIS lacks adequate guidance to perform the
Nuclear requested calculation(s) without additional Disagree
Operating interpretations by the licensee and auditors as to
Company the intent of the provided guidance. The RIS is consistent with GL 79-36. The
licensee is responsible for performing
Proposed resolution: Provide a guideline with calculations, in accordance with industry
examples on how to perform the calculation(s) engineering practices, with properly
including expected assumptions, other supported inputs and assumptions that
considerations, and criteria to be used for demonstrate compliance in accordance with
acceptance. 10 CFR Part 50, Appendix B, Criterion III.
The RIS addresses certain problem areas
identified mainly through NRC inspections
and to re-emphasize the existing NRC
requirements and staff positionsguidance.
35 General Southern The RIS provides some examples of plants that
Nuclear have NRC reviewed and approved analyses and Disagree
Operating goes on to point out that "backfit rule" was applied
Company because the staff believed the sites were not in The licensee must be in compliance with all
compliance with regulations even though they had regulations pertaining to onsite and offsite
approved the analysis. How is a licensee who has power systems. Unless, licensees are
an NRC approved or acceptable analysis supposed exempted from meeting certain regulatory
to know that their analysis is no longer acceptable? requirements, changes to the design and
The RIS needs more clarification with regard to licensing bases are required to meet the
individual plant licensing bases if it is to be useful regulations in accordance with 10 CFR
to licensees. 50.109. The RIS identified some of the
recent inspection findings.
Some plants have installed degraded grid alarm
systems and, at the staff request, included them in
Unit Operating Technical Specifications. Required Operator actions could be used only to
12
No. Section of RIS Originator Specific Comment NRC Resolution
operator actions related to degraded grid supplement the automatic DVR scheme by
conditions are specified in the bases and providing alarm in the control room when grid
procedures. The RIS does not discuss this voltage is below nominal operating range.
approach. Operator actions cannot be substituted for
protecting the safety related equipment from
There are a number of plants that have URIs degraded voltage. 10 CFR 50.55a(h)(2)
related to this issue. Issuance of this RIS could be requires all protective actions to be
used by inspectors to close the URIs to violations automatic.
without regard to plant specific licensing bases,
resulting in regulation by inspection. The intent of the RIS is to highlight the basis
for DVR requirements and preclude future
findings in plant designs. The RIS does not
provide new guidelines for issuing violations.
36 Summary of Southern The RIS states "The Class 1 E buses should
Issues", pg. 6, Nuclear separate from the offsite power system within a few Disagree.
. Item 1. "Degraded Operating seconds if an accident occurs coincident with
Voltage Relaying Company sustained degraded voltage conditions." The RIS correctly states that if an accident
Design signal is received during sustained
Calculations", GDC 17 describes the requirements for onsite and degraded grid conditions, it may be prudent
Line 5. offsite power systems. One of its requirements is to separate from the grid as :
that they each provide sufficient capacity and
capability to mitigate postulated events. The events 1. The duration of degraded
are described in Chapter 15 "Accident Analysis". conditions on the grid is unknown
These analyses assume Loss of Offsite Power 2. It precludes other complications
simultaneous with the event. They do not require such as double sequencing.
assuming degraded grid voltage condition prior to
an event occurring. In addition because of FERC Chapter 15 Accident Analyses assumes
and NERC requirements for voltage control, the Loss of Offsite Power as a limiting case for
likelihood of a chapter 15 accident occurring safe shutdown in view of the limited power
concurrent with a serious degraded grid voltage and resources available from the onsite
condition is not believed to be credible. power sources. The preferred power source
for all operating modes and accident related
Proposed resolution: Remove or clarify this safe shutdown is the offsite source. The
statement since proper offsite system design and DVR provides assurance that the plant
operation renders such simultaneous postulated shutdown capability is not compromised
events as incredible. when the offsite source is degraded and a
fast transfer can occur to the onsite sources
if the offsite source does not recover within
the allotted time. This preserves the Chapter
15 accident analyses.
NERC and FERC requirements for voltage
control are beneficial to nuclear plant
operators as they provide assurance that
grid parameters will be maintained within
acceptable limits for normal nuclear plant
operations. However, the transmission
system is always vulnerable to perturbations
such as line outages, overload conditions,
generation shortages etc. which are beyond
the control of the grid operator. The
magnitude and duration of these
perturbations cannot be predicted. The
safety of the nuclear plant must not be
compromised during these conditions. The
function of the DVR is to protect redundant
safety related equipment during these grid
perturbations.
The staff considers degraded voltage
condition and coincident LOCA can occur.
Until the DVR relay takes automatic action,
the offsite power is considered to have
adequate capacity and capability. Therefore,
the accident analysis assumption for a LOCA
with offsite power available applies.
37 Summary of Southern The RIS states "During normal plant operation, the
Issues", pg. 6, Nuclear Class 1 E safety related buses should Disagree.
Item 1. "Degraded Operating automatically separate from the power supply
13
No. Section of RIS Originator Specific Comment NRC Resolution
Voltage Relaying Company within a short interval (typically less than 60 See staffs response to Comment No. 9
Design seconds) if sustained degraded voltage conditions
Calculations", are detected.
Line 7 Voltages down at the DVR level should be
During normal plant operation (i.e. non LOCA), the well below the normal grid voltage levels.
degraded grid relay settings may be overly Transmission operators will be taking actions
conservative. Therefore automatic separation from when voltages fall below the normal low level
the preferred power supply may not be desired. (or post contingency low) well above the
DVR value (assuming the plant design is
Proposed resolution: Transmission Operators proper given the grid operating voltage
should be allowed time to correct the degraded range).
voltage condition while Plant Operators monitor the
The plant electrical distribution system
safety bus voltages for adequate voltage.
design should be based on the grid voltage
range including the bounding worst case grid
contingency (strong or weak grid depending
on which one is bounding). In this way, the
plants design ensures adequate voltage to
plant equipment as long as grid is operating
as expected.
GDC 17 requires that offsite power has
sufficient capacity and capability to assure
that (1) specified acceptable fuel design
limits and design conditions of the reactor
coolant pressure boundary are not exceeded
as a result of anticipated operational
occurrences, and (2) the core is cooled and
containment integrity and other vital
functions are maintained in the event of
postulated accidents.
38 Summary of Southern The discussion on time delays does not provide
Issues, Last Nuclear adequate criteria for time delay selections. Disagree
paragraph of Operating
"DVR criteria for Company Proposed Resolution: Clarify. The acceptable level of time delay is based
time delay on plant specific accident analyses and
selections. electrical systems designs.
The RIS states In this manner, the DVR ensures
Setting Design Different time delays may be selected for
adequate operational (starting and running) voltage
Calculations", pg. different plant designs. BTP PSB-1 suggests
to all safety related equipment, independent of
7 two relays with different settings to
voltage controlling equipment external to the plant
accommodate motor starts and sustained
safety related electrical distribution system. For the
degraded conditions. Typically, chapter 15
purposes of this calculation, no t credit should be
accident analyses assume 10 second time
taken for voltage controlling equipment external to
delay for onsite power sources to energize
the Class 1E distribution system such as automatic
safety busses. DVR time delay of 10seconds
load tap changers and capacitor banks.
or less may be acceptable to demonstrate
that the assumptions in the accident
Proposed Resolution: This statement needs to be
analyses remain valid.
clarified to allow reasonable assumptions for the
status of equipment external to the Class 1E
External voltage regulating equipment (tap
distribution system. For example it is unclear how
changers) does not afford timely protection
to perform motor starting calculations without
for safety related equipment during sudden
taking credit for some Non 1E voltage controlling
grid perturbations and therefore cannot be
equipment. Additionally, normal Transmission grid
credited for DVR settings including transient
switching to prepare for the next contingency to
voltages during motor starts.
maintain minimum expected transmission system
voltages should be allowed.
See staffs response to Comment Nos. 76
and 118.
39 Summary of Southern After paragraph (f) the RIS leaves out the guidance Disagree
Issues, Guidelines Nuclear in GL 79-36 concerning minimum expected values
for voltage drop Operating (item 6 of enclosure 2) was omitted from the RIS Item 6 was addressed in item(a) in the RIS
calculations, item Company guidance.
(f) pg. 8
Proposed resolution: Add item 6 of enclosure 2 in
40 General NEI 3/2/11 NEI contends that RIS represents new positions
14
No. Section of RIS Originator Specific Comment NRC Resolution
Letter Disagree
The RIS provides clarifications to avoid the
misconception of the existing requirements
and NRC staff positionsguidance.
No new requirements are identified in the
RIS.
41 NEI 3/2/11 77 Letter and PSB-1 Not consistent. Degraded
Letter Disagree
voltage event and accident coincident vs.
subsequent. Also, starting and running voltage vs. PSB just states how the scheme should
just running (term sustained in PSB-1) react to a subsequent degraded voltage.
The 77 letter indicates that the time delay
should support the accident analysis which
means coincident events.
42 NEI 3/2/11 77 Letter and 79 GL are generic communications
Letter Both communications provide guidance on
not requirements
how to meet regulatory criteria. To that
extent, they are related to compliance with
regulations, but are not themselves
regulatory requirements.
43 NEI 3/2/11 Multi-unit sites, accident and simultaneous
Letter Disagree
shutdown vs. orderly shutdown and cooldown per
GDC 5 This RIS is consistent with GDC 17, GL 79-
36, and IEEE Standard 308-1971, Class IE
Electrical Systems, Section 8, Multi-Unit
Station Considerations.
44 NEI 3/2/11 During normal plant operation, a degraded voltage
Letter Disagree
for 60 seconds resulting in separation doesnt give
the operator enough time (implied by PSB-1) to See NRC response to .Comment No. 9.
take actions
45 NEI 3/2/11 Offsite/Onsite design interface calculations doesnt
Letter Disagree
(46- 48 allow for manual actions
not
used) This means that manual actions for the
purposes of analyses of the offsite power
Also system voltages should not be assumed.
see
Nos. See GL 79-36 for more details.
86-96 PSB-1 allows manual actions after first alarm
for this at higher voltage.
NEI
Letter
49 Page 6, DVR Jerry Nicely Section DVR Setting Design Calculations states: Disagree
Setting Design At the DVR dropout setting ...... the DVR ensures
Calculations adequate operational (starting and running) voltage See Responses to Questions 1 & 2.
to all safety related equipment, independent of
voltage controlling equipment external to the plant Sustained in the context that the loss of
safety related electrical distribution system. voltage relay was designed for momentary
dip in voltage or complete loss of voltage.
BTP PSB-1 states, "sustained", implying a steady Steady state operation is expected at normal
state voltage condition and not a transient voltage operating band.
condition that exists during a motor starting event.
The 1977 NRC letter states that voltage and
The original 1977 NRC Letter, the later PSB-1, or time setpoints shall be determined from an
GL 79-36 does not require plants to demonstrate analysis of the voltage requirements of the
the ability to start motors at the DVR settings. safety related loads. Safety related
equipment, particularly large motors, have
Requiring the ability to start motors at the DVR starting and running voltage requirements.
dropout setting does not accomplish anything or This second level of protection should
make sense, since starting a motor at this voltage address these voltage requirements.
will ensure a resultant voltage below the DVR Sustained degraded voltage, as discussed in
dropout; result in not being able to be reset the the 1977 NRC letter, refers to grid voltage
relay, and as a result causing a spurious below the expected low value given normal
disconnection from offsite power and transferring to grid operation and grid post contingency
15
No. Section of RIS Originator Specific Comment NRC Resolution
the emergency diesel generator. Having a (Single, N-1). Thus, when grid voltages are
requirement to be able to start motors at the DVR degraded beyond the minimum voltage
dropout setting will result in the raising the DVR assured by the grid operator(such as
settings to a higher value and is more likely to resulting in SR bus voltages down close to
result in spurious separation from the grid which is where DVRs are set based on SR equipment
in direct conflict with PSB-1. requirements), separation from the grid is
appropriate. Proper design of the plant
electrical distribution system and setting of
the DVRs, based on the grid voltage range
(described above) should provide proper
margin such that spurious separation from
the grid should not occur.
The key point is that the voltage setting(s)
selected should ensure that adequate
voltage is available at the component
terminal(s) to operate the most limiting
component (s) at a plant during the most
limiting design basis event. The offsite/onsite
interface calculation should show that, with
the grid at the lower limit of the normal
operating range, voltage at the safety bus is
always well above the degraded voltage
setpoint for all design basis event loading
conditions (normal, abnormal and accident
conditions including anticipated operational
occurrence).
50 Page 5, Peach Jerry Nicely In the RIS section of recent inspection findings for
Bottom Peach Bottom, it was stated that since the load tap Agree
changers are not safety-related and are subject to
operational limitations and credible failures, they The finding is correct.
cannot be relied on.
The safety related equipment should be
protected by Class1E relays and not
dependent on non safety LTCs functioning.
The DVR action is independent of LTC
action. Therefore, no credit should be taken
for determining the setpoint of DVR relay.
The response time of tap changers is
relatively slow. Redundant safety related
equipment may be exposed to degraded
voltage conditions that can last for minutes
depending on tap changer response time.
51 Page 8, c) Jerry Nicely In the Offsite/Onsite Design Interface Calc section
(C) it states: an accident in the unit being analyzed Disagree
and simultaneous shutdown of all other units at the
station. For electrical system, the statement in the
RIS is consistent with GDC 17, GL 79-36,
RG 1.81 states: The Regulatory staff has and IEEE Standard 308-1971, Class IE
determined that, because of the low probability of a Electrical Systems, Section 8, Multi-Unit
major reactor accident, a suitable design basis for Station Considerations.
multi-unit nuclear power plants is the assumption
that an accident occurs in only one of the units at a
time, with all remaining units proceeding to an
orderly shutdown and a maintained cooldown
condition; 10CFR50 App A Criterion 5 states: ... in
the event of an accident in one unit, an orderly
shutdown and cooldown of the remaining units;
NUREG 0800 Section 8.2 states: ..... in the event
of an accident in one unit, with a simultaneous
orderly shutdown and cooldown of the remaining
units. Based on the above Regulatory positions,
the RIS should consider rewording the
"simultaneous shutdown" to "orderly shutdown".
52 Page 6, DVR Larry This paragraph could be interpreted to require the
Setting Design Nicholson, LOCA sequence to be modeled at the DVR dropout Disagree
Calculations Nexterra setting. LOCA sequencing modeled at the DVR
16
No. Section of RIS Originator Specific Comment NRC Resolution
dropout setting would result in separation of the See response to questions 1&2
buses from the Preferred Power Source (off-site
power) as the voltage would not recover above the The sustained degraded voltage conditions
DVR reset value. are not expected to last the total sequencing
process. The analyses should demonstrate
Proposed Resolution: Clarify the intent to show that the largest load (limiting case) for
safety related equipment will function at the accident mitigation can be successfully
selected DVR dropout setting voltage and that it is started under degraded grid conditions and
not expected to start the LOCA sequence from this the loads that are normally operating will not
voltage level. be adversely impacted. It is expected that
the grid will either recover to nominal voltage
Clarify that LOCA sequencing is evaluated using and reset the DVR for the rest of the
minimum switchyard voltage as the starting point. sequencing process or the DVR will separate
the plant from source that cannot support
53 Page 6, DVR Larry Having a sustained degraded voltage just above
Setting Design Nicholson, the LVR voltage setling (70%) is not practical Agree
Calculations Nexterra without grid collapse and does not exist in Branch
Technical Position #1 (PSB-1). Although undervoltage protection (first level)
is not within the scope of the RIS, the
Proposed Resolution: Clarify degraded voltages to licensees analysis should ensure that the
be analyzed to a credible level. LVR and DVR settings protect the Class 1E
components from voltage ranges between
the DVR and LVR settings due to sustained
degraded conditions.
54 Page 6, DVR Larry The statement that the DVR ensures adequate
Setting Design Nicholson, operational (starting and running) is the first time in
Calculations Nexterra NRC correspondence that starting equipment at Disagree
the DVR setpoint is expected. The example letter
sent to Peach Bottom in June 1977 did not require The NRC 1977 letter states that voltage and
starting of equipment at the DVR setpoint. This time setpoints shall be determined from an
requirement should be removed from the RIS since analysis of the voltage requirements of the
it is not possible to start equipment at the DVR safety related loads. Safety related (Class
setpoint and not subsequently separate from offsite 1E) equipment, particularly large motors,
power. If the equipment starts at the DVR setpoint, have starting and running voltage
the voltage will dip during the transient and must requirements. This second level of
then recover above the reset point to avoid undervoltage protection should address
separation from offsite power. Since the reset point these voltage requirements
will always be above the DVR dropout point it will
be impossible to reset the relay.
Proposed Resolution: Remove starting equipment
at the DVR setpoint as a requirement.
55 Page 7, DVR Larry It is agreed that no credit is to be taken for voltage
Setting Design Nicholson, controlling equipment external to the Class 1 E Agree
Calculations Nexterra distribution system for the establishing the
degraded voltage relay (DVR) settings; however, it Grid Interface calculations can take credit for
should be clarified that for credit may be taken for voltage correction equipment. The DVR
minimum switchyard voltage/voltage drop setpoint should be set independent of
calculations (or the Offsite/Onsite Design Interface voltage correction equipment that cannot
Calculations). operate in a timely manner to protect 1E
equipment. RIS will be clarified regarding
Proposed Resolution: Clarify that credit must be crediting voltage controlling equipment
taken for automatic load tap changers and/or external to the 1E system for Offsite/Onsite
capacitor for minimum switchyard voltage/voltage Design Interface.
drop calculations (or the Offsite/Onsite Design
Interface Calculations).
56 Page 8, c) Larry NRC Generic Letter 79-36, Enclosure 2, Item 2
Nicholson, states that For multi-unit stations a separate
Nexterra analysis should be performed for each unit
assuming (1) an accident in the unit being analyzed
and simultaneous shutdown of all other units at the
station; or (2) an anticipated transient in the unit
being analyzed (e.g., unit trip) and simultaneous
17
No. Section of RIS Originator Specific Comment NRC Resolution
shutdown of all other units at that station,
whichever presents the largest load situation."
Comment:
NRC Draft RIS re-states NRC GL 79-36 verbatim, Disagree
with an attempt to clarify "anticipated transient" by
adding in parenthesis "(anticipated operational Anticipated Operational Transient is a more
occurrence)" immediately afterwards. It is not clear general term for operational events per the
what the added parenthetical statement is meant to design except for design basis accidents
convey, other than unit trip (which already exists in This is consistent with the term used in GDC
GL 79-36). 17.
Proposed Resolution: It is recommended that this
either be removed, or stated "anticipated transient
per station licensing basis".
57. Page 8, c) Larry NRC should clarify "simultaneous shutdown" with
Nicholson, consideration to:
Nexterra
Most multi-unit station's Licensing Basis consider
an "orderly or controlled safe shutdown" of the
other unit(s) not being analyzed.
NERC Std TPL-004-0; particularly Category D
events per Table 1, where a "loss of all generating
units at a station" may result in "portions or all of
the interconnected systems may or may not
achieve a new, stable operating point'.
IEEE Std 308-1974, Clause 8, subclause 8.1.1 Disagree
"Capacity describes this as a "concurrent safe
shutdown on the remaining units".
This statement is consistent with GDC 17,
Proposed Resolution: The wording for the GL 79-36, and IEEE Standard 308-1971,
proposed RIS, sub clause 2.c should be revised to Class IE Electrical Systems, Section 8,
indicate orderly or controlled safe shutdown of the Multi-Unit Station Considerations.
remaining units, as per the stations licensing
basis instead of simultaneous shutdown.
Alternatively, the wording shutdown consistent
with the station licensing basis could be used
instead of simultaneous shutdown.
58 Page 8, e) and f) Larry These guidelines seem contradictory that you
Nicholson, cannot credit procedurally controlled operator Disagree
Nexterra actions to reduce load but you have to assume the
actions will be carried out when load is added. Adding loads manually per procedure is
conservative in terms of maximum loading,
Proposed Resolution: Delete e) Manual load but not for load reductions. Plant design
shedding should not be assumed or add should not depend on manual load shedding.
allowance to credit procedurally controlled operator This is not conservative. That was the point
actions to decrease load. of item e).
But General NEI 3/18/11 The RIS should identify that plant compliance with
not59 Letter / 1 the regulation (GDC 17) is by each plant operating Agree
within its Licensing Basis.
Second level undervoltage protection
(degraded voltage protection) applies to all
operating plants whether the plant is GDC or
pre-GDC plant.
60 General NEI 3/18/11 Definitions vary for the same words used with
Letter / 2 respect to this topic. Agree
The RIS should include definitions for key terms, Clarifications will be provided in the RIS
e.g. normal grid operation, sustained degraded wherever appropriate.
voltage, etc.
18
No. Section of RIS Originator Specific Comment NRC Resolution
61 General NEI 3/18/11 There are various documents that address the
Letter / 3 Adequacy of Station Electric Distribution System Disagree
Voltages. These documents have differences in the
methodology, terminology, and level of detail. Such The RIS provides adequate clarifications to
differences challenge the users of these guidance the existing guidance.
documents when they conflict.
The staff positions described in the NRC
Attachment 2 to the NEI supplemental comment 1977 letter is are guidance for all licensees
letter provides a table that shows the differences as to how to meet GDC 17 or applicable
between GL 79-36, (BTP) PSB-1, IEEE 741, and plants principle design criteria. This letter
the draft RIS. focuses on the need for a second level
undervoltage protection scheme to protect
The RIS should identify the guidance document the Class 1E electrical components from the
differences, establish the NRC position on each consequences of sustained degraded
conflicting topic, and provide the basis for each voltage conditions.
change in previously accepted guidance.
GL 79-36 emphasizes the electrical design
attributes to be considered for the interface
of onsite and offsite distribution systems to
ensure adequate voltages to the Class 1E
buses and safety related components for
normal, abnormal, and accident conditions to
comply with GDC 17 or applicable plants
principle design criteria requirements.
BTP PSB-1 incorporates both the above
staff positions and guidance to meet GDC 17
requirements.
IEEE 741 is only referenced in the SRP. The
NRC has not endorsed this industry
guidance.
62 Page 1, 3rd NEI 3/18/11 "The U.S. Nuclear Regulatory Commission (NRC)
paragraph Letter / 4 is issuing this Regulatory Issue Summary (RIS) to
clarify the NRC staff's technical position on existing
regulatory requirements and voltage studies
necessary for Degraded Voltage Relay (second
level undervoltage protection) setting bases and
Transmission Network/Offsite/Onsite station
electric power system design bases."
No regulatory requirements are referenced except
for a generic reference to GDC 17.
Reword the paragraph to read: Agree
'The U.S. Nuclear Regulatory Commission (NRC)
is issuing this Regulatory Issue Summary (RIS) to The RIS will be revised to incorporate this
clarify the NRC staff's technical position on comment.
Degraded Voltage Relay (second level
undervoltage protection) setting bases and
Transmission Network/Offsite/Onsite station
electric power system design bases appropriate for
meeting the regulatory requirements specified in
GDC 17.'
63 Page 2, NEI 3/18/11 "The selection of voltage and time delay setpoints
Paragraph a) Letter / 5 shall be determined from an analysis of the
operating voltage requirements of safety related
loads at all onsite system distribution levels"
Requirements for DVR settings have never used
the term "operating voltage". They instead used Disagree in part.
"sustained voltage" which by definition would be
steady state voltage, running voltage, or voltage Voltage requirements - all voltage
held at a constant value requirements of SR equipment
(BTP) PSB-1 and BTP 8-6 are silent on The term operating voltage is being
operating/running voltage in the DVR settings removed from the RIS as recommended.
section; however, running is implied by using the
19
No. Section of RIS Originator Specific Comment NRC Resolution
term "sustained" in the Time Delay settings section. The term sustained voltage used in the 1977
letter and the BTPs is referring to the voltage
To be consistent with (BTP) PSB-1, NRC letter, condition on the grid, not steady state
and BTP 8.6 remove the word "operating". voltage
64 Page 3, Arkansas NEI 3/18/11 "...assuming all onsite sources of AC power are not
Nuclear One Letter / 6 available, the offsite power system and the onsite
distribution system is of sufficient capacity and
capability to automatically start as well as operate
all required safety related loads."
This sentence implies that the NRC use of the term
"operate" does not mean the same thing as 'start', Agree
i.e., operate/start/running are not synonymous. NEI
supports this interpretation; however, it conflicts
with the words "proper voltage for starting and
running in all operational configurations" in the 1st The term operating voltage is being removed
paragraph of page 6. from RIS.
65 Page 6, 1st NEI 3/18/11 "...components are provided adequate voltage
Paragraph Letter / 7 based on the design of the Class 1E distribution
system in the plant and its most limiting operating
configuration."
There is a need to better define "most limiting Agree.
operating configuration", since experience shows
that a component is most limiting. RIS will be revised to state the following:
Reword the RIS to: "... all safety related components are
'...components are provided adequate voltage provided adequate voltage based on the
based on the design of the Class 1E distribution design of the plant power distribution system
system in the plant.' (and the offsite circuits), including the design
of the Class 1E distribution system in the
plant and its most limiting operating
configuration(s)."
Operating configurations affect limits as well
as components.
st
66 Page 6, 1 NEI 3/18/11 "The Offsite/Onsite Design Interface Calculations
paragraph Letter / 8 specify the voltage operating parameters of the
plant electrical distribution system based on the
transmission system (Offsite) operating
parameters."
This paragraph could be interpreted to require the Disagree
LOCA sequence to be modeled at the DVR dropout
setting. LOCA sequencing modeled at the DVR
dropout setting would result in separation of the This sentence is not referring to calculations
buses from the referred Power Source (off-site for setting the relays but referring to
power) as the voltage would not recover above the Offsite/Onsite Design Interface Calculations.
DVR reset value.
The RIS should state that the intent is to show
safety related equipment will function at the
selected DVR dropout setting voltage and that it is
not expected to start the LOCA sequence from this
voltage level. The RIS should state that LOCA
sequencing is typically evaluated using minimum
switchyard voltage as starting point.
67 Page 6, 1st NEI 3/18/11 "This interface calculation establishes operating
paragraph Letter / 9 voltage bands for all plant electrical buses, which
ensures that all plant components and systems
(Class IE and Non Safety Related) have proper
voltage for starting and running in all operational
configurations (expected operational and accident
conditions)."
This statement needs clarification in that not all
20
No. Section of RIS Originator Specific Comment NRC Resolution
non-safety load voltages need to be evaluated. Disagree
The statement that the DVR ensures adequate
operational (starting and running) is the first time in See response to questions 1&2 for
NRC correspondence that starting equipment at clarification.
the DVR setpoint is expected.
The 1977 NRC letter refers to SR equipment
Having a sustained degraded voltage just above voltage requirements.
the LVR voltage setting (70%) is not practical
without grid collapse and does not exist in Branch NSR items are being removed from this
Technical Position #1 ((BTP) PSB-1). sentence in the RIS. However, non safety
related loads should be modeled to the
Typically, large motors (like reactor coolant pumps) extent that their operation can affect safety
need to be evaluated for starting impact on the bus/equipment voltage.
safety bus. Once a motor is found to be small
enough to not impact safety bus operation, further
evaluation is unnecessary. The statement in the
RIS can easily be interpreted as requiring
evaluation of all non-safety loads down to the DVR setpoints are based on low voltages
lowest levels of distribution. that can occur due to sustained grid
perturbations and can potentially degrade
Technical compliance with determining the capability of onsite safety related equipment.
degraded voltage relay setpoint would not be
achievable because the RIS requires the DVR
dropout setpoint to be based on the starting voltage The NRC 1977 letter states that voltage and
required for motors. Basing the DVR setpoint time setpoints shall be determined from an
(dropout setting) on starting voltage requirements analysis of the voltage requirements of the
(rather than steady-state operating voltage) safety related loads. Safety related
appears to be a new NRC requirement/position. It equipment, particularly large motors, have
is technically flawed in that it would not actually starting and running voltage requirements.
provide the required protection for the Class1lE This second level of protection should
loads. It also appears to disagree with the intended address these voltage requirements.
purpose of the existing regulations (1977 NRC Sustained degraded voltage, as discussed in
Letters on degraded voltage protection and (BTP) the 1977 letter, refers to grid voltage below
PSB-1). the expected low value given normal grid
operation and grid post contingency (Single,
The letter sent to Peach Bottom in June 1977 did N-1). Thus, when grid voltages are degraded
not require starting of equipment at the DVR (such as resulting in SR bus voltages down
setpoint. This new requirement should be removed close to where DVRs are set based on SR
from the RIS, since it is not possible to start equipment requirements), separation from
equipment at the DVR setpoint and not the grid is appropriate. The design of the
subsequently separate from offsite power. If the plant electrical distribution system and
equipment starts at the DVR setpoint, the voltage setting of the DVRs, based on the grid
will dip during the transient and must then recover voltage range (described above) should
above the reset point to avoid separation from provide proper margin such that spurious
offsite power. Since the reset point will always be separation from the grid should not occur
above the DVR dropout point, it will be impossible due to sequencing or block loading of loads
to reset the relay. during a design basis events.
Motor starts have been discussed in several
questions above. An accident signal
concurrent with degraded grid conditions will
require motor starts. All NRC
communications discuss the requirement for
safe shutdown of the plant following
postulated events.
68 Page 6, 2nd NEI 3/18/11 "The staff considers degraded voltage conditions
paragraph Letter / 10 coincident with a postulated design basis accident
to be a credible event. DVRs should be set to
protect the safety related equipment from sustained
degraded voltage conditions."
GDC 17 should be identified as the regulatory
requirement. This RIS is creating conditions in Disagree
excess of GDC 17.
The RIS concludes that the staff considers The 1977 NRC letter staff positions guidance
degraded voltage conditions coincident with a (implementation second level undervoltage
postulated design basis accident to be a credible protection scheme) apply applies to all
21
No. Section of RIS Originator Specific Comment NRC Resolution
event; however, this is not consistent with GDC 17. operating plants and provides guidance on
how to meet regulatory criteria..
The RIS should identify that plant compliance with
the regulation (GDC 17) is by each plant operating The 1977 NRC letter indicates that the DVR
within its Licensing Basis that was developed from scheme time delays should support accident
available NRC and industry guidance. analysis assumptions which ties degraded
event with an accident.
Most licensees are committed to a version of IEEE
308, Standard Criteria for Class 1E Power Systems
for Nuclear Power Generating Stations. This
Standard defines the malfunctions, accidents,
environmental events, and operating modes (i.e.,
design basis events) that could physically damage
Class 1E power systems or lead to degradation of
system performance and for which provisions shall
be incorporated.
A degraded voltage condition coincident with a
postulated design basis accident is not among the
identified design basis events; however, the
Standard does include a requirement for the
protection from common mode failure.
69 Page 6, 2nd NEI 3/18/11 "The Class 1E buses should separate from the
paragraph Letter / 11 offsite power system within a few seconds if an
accident occurs coincident with a sustained
degraded voltage conditions."
GDC 17 describes the requirements for onsite and Disagree
offsite power systems. One of its requirements is
that they each provide sufficient capacity and
capability to mitigate postulated events. The events
are described in Chapter 15 "Accident Analysis". The NRC 1977 letter indicates that the DVR
These analyses assume Loss of offsite Power scheme time delays should support accident
simultaneous with the event. They do not require analysis assumptions which ties degraded
assuming degraded grid voltage condition prior to event with an accident.
an event occurring. In addition, because of FERC
and NERC requirements for voltage control, the
likelihood of a chapter 15 accident occurring
concurrent with a serious degraded grid voltage
condition is so low that it is believed to be not See NRC response to Comment No. 36
credible.
Remove or clarify this statement, since proper
offsite system design and operation renders such
simultaneous postulated events as incredible.
This position assumes (BTP) PSB-1 (BTP 8-6) is
part of the license and design basis for all
licensees. An equivalent position was not identified
in the NRC letters issued following the Millstone
event. Specifically, this requirement is more
stringent than the position stated on Page 2, Item
d) and may constitute a backfit to some licensees.
Care must be exercised with regards to the scope Section 2 is not about DVR schemes and
of this position. It does not apply to Section 2, separation during a degraded voltage
"Offsite/Onsite Design Interface Calculations" of the condition. It is about operation of the plant
RIS. This is confirmed in the draft RIS on Page 8, during normal, abnormal and accident
Paragraph d) which states all electric system action conditions and assuming the normal
occur "as designed". It would be beneficial to clarify operation of the grid (including the bounding
the scope limitations associated with this or any N-1 contingency and the trip of the unit for
revised position. the accident cases).
70 Page 6, 2nd NEI 3/18/11 ... Position (BTP) PSB-1 (revised later to become
paragraph Letter / 12 BTP-6), is to protect Class 1E safety related buses
22
No. Section of RIS Originator Specific Comment NRC Resolution
and components from sustained degraded voltage
conditions on the offsite power system coincident
with an accident as well as during non-accident
conditions."
A definition of the word protect is needed. It is not Disagree
clear what is being protected: the components
(MOV, motor, etc.) or the class 1E function or Protect means guard or defend safety
something else. related components against the
consequence of sustained degraded voltage
The word "coincident" should read "subsequent to" conditions.
or "followed by", per (BTP) PSB-1 and BTP 8-6.
Coincident is appropriate based on the 1977
NRC letter verbiage. The BTPs just provide a
design which would also deal with an event
when a SIAS signal would occur subsequent
to the degraded voltage condition as well
(not conflicting)
71 Page 6, 2nd NEI 3/18/11 "The Class 1E buses should separate from the
paragraph Letter / 13 offsite power system within a few seconds if an
accident occurs coincident with a sustained
degraded voltage condition."
Per (BTP) PSB-1, the text should read: Agree.
'The Class 1E buses should separate from the
offsite power system immediately if an accident RIS will be revised to incorporate the
occurs subsequent to a sustained degraded comment.
voltage condition.'
72 Page 6, 2nd NEI 3/18/11 ... Class 1E safety related buses should
paragraph Letter / 14 automatically separate from the power supply
within a short interval (typically less than 60
seconds)..."
There is no basis for "typically less than 60 Agree
seconds". In the original context of the time delay
section, it was sufficient time for an operator to RIS will be revised to delete the parenthesis
intervene" which is much greater than 60 seconds. section of the sentence as suggested.
Remove the parenthesis section of the sentence.
With this guidance during normal plant operation, Disagree
the degraded grid relay settings may be overly
conservative; automatic separation from the 10 CFR 50.55a(h(2)) requires all protective
preferred power supply may occur under conditions actions to be automatic. Operator
where this action is inappropriate. intervention is probably not possible when
voltage gets down to the DVR setpoint (grid
The RIS should allow Transmission Operators time voltage is well below normal).
to correct the degraded voltage condition while
Plant Operators monitor the safety bus voltages for Operator action takes minutes. Operation at
adequate voltage. degraded voltage conditions can degrade
equipment performance capabilities within
seconds.
73 Page 6, 3rd NEI 3/18/11 DVR Setting Design Calculations
paragraph Letter / 15
This section would be a good place to describe this
type of analysis as having a "bottom-up" approach. Disagree
Such calculations would prevent confusion of
crediting anything above the DVR voltage sensors' The approach is already described in this
values. paragraph.
In the context of DVR Setting Design Calculations,
using a steady state or sustained voltage analysis
23
No. Section of RIS Originator Specific Comment NRC Resolution
is the only way that can result in a voltage 'setting' The term sustained voltage used in the NRC
requirement. 1977 letter and the BTPs is referring to the
voltage condition on the grid, not steady
state voltage
74 Page 6, 3rd NEI 3/18/11 "... DVR ensures adequate operational (starting
paragraph Letter / 16 and running) voltage..."
The "operational voltage" cannot define both Agree
starting and running voltages.
RIS will be revised to state DVR ensures
The only place "operational voltage" is referenced adequate voltage (start and run
in the associated documents is in the tap setting conditions)
section of (BTP) PSB-1 and BTP 8-6 when a plant
is connected to the offsite power supply. The only
qualifying term used in the protection of the
equipment is 'sustained' which is synonymous with
steady state or running. Disagree
Reword the RIS to: The term sustained voltage used in the NRC
'... DVR ensures adequate sustained voltage...' 1977 letter and the BTPs is referring to the
voltage condition on the grid, not steady
state voltage
75 Page 6, 3rd NEI 3/18/11 "Licensee voltage calculations should provide the
paragraph Letter / 17 basis for their DVR settings, ensuring safety related
equipment is supplied with adequate operating
voltage (typically a minimum of 0.9 per unit voltage
at the terminals of the safety related equipment per
equipment manufacturers requirements), based on
bounding conditions for the most limiting safety
related load (in terms of voltage) in the plant."
Equipment manufacturers do not provide the same
voltage requirement to perform both running and Agreed
starting a motor. The 0.9 per unit in this context
refers to the typical running voltage requirement of This sentence is being reworded
a motor; whereas, 0.85 per unit is typical for a
starting voltage requirement.
The RIS should identify that voltages other than Starting requirements for motors have been
90% voltage are common based on detailed plant observed over a range of 0.75 to 0.85. It
analysis. depends on the particular plant and how the
motors were procured. Either way, the
For example, motors below 90% voltage continue voltage requirements must be preserved
to have plenty of margin in torque but may (starting and running). However, there could
encroach on long time thermal limits. Unless a be other components that are more sensitive
motor is fully into its service factor (typicallyl.15) to voltage for operation.
and below 90% voltage, operation will be 0.9 per unit voltage was mentioned as an
acceptable. example and was not meant to cover
everything. RIS will be revised to delete
references to specific numbers and
emphasize voltage requirements and voltage
requirements are plant-specific.
The design basis of the plant should
determine the adequacy of voltage. The RIS
clarifies the regulation.
24
No. Section of RIS Originator Specific Comment NRC Resolution
76 Page 6, 3rd NEI 3/18/11 "In this manner, the DVR ensures adequate
paragraph Letter / 18 operational (starting and running) voltage to all
safety related equipment, independent of voltage
controlling equipment external to the plant safety
related electrical distribution system."
The draft RIS suggests the DVR dropout setpoint
to be based on the starting voltage required for Disagree
motors.
Basing the DVR dropout setpoint on starting The NRC 1977 letter states that voltage and
voltage requirements rather than steady-state time setpoints shall be determined from an
operating voltage appears to be a new NRC analysis of the voltage requirements of the
requirement/position. It also appears to disagree safety related loads. Safety related (Class
with the intended purpose of the existing 1E) equipment, particularly large motors,
requirements and guidance (1977 NRC Letters on have starting and running voltage
degraded voltage protection and (BTP) PSB-1). requirements. This second level of
undervoltage protection should address
As suggested, the approach incorrectly implies that these voltage requirements. Sustained
the load(s) should start from the lowest DVR degraded voltage, as discussed in the NRC
dropout setting. A specific example for illustration is 1977 letter as well, refers to grid voltage
as follows: If the initial voltage value is at the lowest below the expected low value given normal
possible value above dropout actuation, starting a grid operation. Thus, when grid voltages are
load will cause the DVR dropout. Since the new degraded (such as resulting in Class 1E bus
steady state voltage will be lower than the initial voltages down close to where DVRs are set
value because of the added loads, the DVR reset based on Class 1E equipment
will never occur. requirements), and the grid does not
automatically recover, separation from the
Many utilities use the ABB 27N with harmonic filter grid is appropriate. Proper design of the plant
which has a minimum 0.5% reset. Thus, with a electrical distribution system and setting of
setting of 93.6% +/- 0.9%, the dropout value could the DVRs, based on the grid voltage range
be as low as 92.7%. For motors causing more than (described above) should provide proper
0.5% voltage dip at initial start, even if the voltage margin such that spurious separation from
at the beginning of the event was 93.2% and a load the grid should not occur due to sequencing
was started, the DVR will dropout and never reset. or block loading of loads during a design
This will lead to a grid separation. basis event.
Reword the RIS to remove "(starting and running)" Also, see response to questions 1 & 2.
77 Page 6, 3rd NEI 3/18/11 "For the purposes of this calculation, no credit
paragraph Letter / 19 should be taken for voltage controlling equipment
external to the Class 1E distribution system such
as automatic load tap changers and capacitor
banks."
The intent of the position appears to ensure that Disagree
the DVR setpoint(s) protect against the potential This sentence is being re-worded
loss of ESF equipment, regardless of the
component mode of operation. It does not imply
that the Class 1E bus must remain connected to
offsite power after starting a large motor with an The point being made is that calculations for
initial bus voltage corresponding to the DVR the DVR voltage settings should have cases
setpoint and no voltage regulation capability. at voltages just above the DVR voltage
settings (well below what would be based on
normal grid operations and voltage
Actually, the calculation should be performed with controlling equipment if applicable) to
the DVR monitored bus voltage at the TS limit, not demonstrate that the settings enforce the SR
the DVR setpoint. All that is required is that under equipment voltage requirements.
motor starting conditions, separation from offsite
power occurs before starting loads trip on overload. This is covered in Offsite/Onsite Design
The intent could be conveyed in more detail. Section calculations (not DVR calculations
section)
The RIS should allow reasonable assumptions for The offsite source is the preferred source of
the status of equipment external to the Class 1E power for plant shutdown. The DVR should
distribution system. For example it is unclear how not separate the plant from the grid for motor
to perform motor starting calculations without starts. In the event that grid conditions
taking credit for some Non 1E voltage controlling degrade beyond an acceptable point and an
equipment. Additionally, normal transmission grid accident signal is actuated, BTP PSB-1
switching should be allowed to prepare for the next recommends separation from the grid.
25
No. Section of RIS Originator Specific Comment NRC Resolution
grid event, so that minimum expected transmission
system voltages are maintained. Credit for voltage controlling equipment in
the Offsite/Onsite Design Section
calculations is appropriate if corrective action
can be taken in a timely manner to preclude
safety related equipment malfunctions.
78 Page 7, 1st NEI 3/18/11 "Voltage-time settings for DVR's should be selected
paragraph Letter / 20 so as to avoid spurious separation of the safety
buses from the offsite power system during unit
startup, normal operation and shutdown."
This position is new and contrary to the NRC Disagree
historical position stated on Page 2, Item (c)(3).
Either the DVR protection scheme favors ESF- This sentence is being re-worded
equipment-protection or connectivity-to-offsite-
power. Otherwise, this position would result in a
mutually exclusive requirement. The prevention of DVR setting always enforce SR equipment
spurious separation is addressed by coincident voltage requirements. Offsite/Onsite Design
logic channels (Page 2, Item (b)), not the setpoint. should ensure that there is proper margin
between where voltage is in the plant during
If no credit is to be taken for voltage controlling normal grid operation as compared to
equipment external to the Class 1E distribution voltages in the plant when the DVRs actuate.
system for the establishing the degraded voltage
relay (DVR) settings, then the RIS should state that
credit may be taken for minimum switchyard Credit for voltage controlling equipment in
voltage/voltage drop calculations (Offsite/Onsite the Offsite/Onsite Design Section
Design Interface Calculations). calculations is appropriate. However, it is not
appropriate to use it for DVR calculations
since DVR setpoint is derived from the
minimum voltage required at the component
terminal at all voltage levels. (Also see
response to question #77)
79 Page 7, 1st NEI 3/18/11 "These DVRs should disconnect the Class 1E
paragraph Letter / 21 buses from any power source other than the
emergency diesel generators (onsite sources) if the
degraded voltage condition exists for a time interval
that could prevent the Class 1E safety related
loads from achieving their safety function."
This position ensures ESF functionality, should an Disagree
undervoltage condition persist.
The BTP PSB-1 offers an option to set a
(BTP) PSB-1 was written before the application of higher voltage alarm level to support
voltage regulating devices within the nuclear power corrective action to restore voltage to normal
plant offsite power circuit boundary. The RIS operating band.
should clarify that if the calculations necessary to
support RIS positions in Section 1, "Degraded
Voltage Relaying Design Calculations" and Section Since offsite power is the preferred source of
2, "Offsite/Onsite Design Interface Calculations" power to mitigate design basis event, it is
demonstrate completion of ESF functions within important that the Offsite/Onsite Design
accident analysis assumptions, then immediate Interface calculations ensure the capacity
separation per (BTP) PSB-1, Section B(1)(b)(i), is and capability of the offsite power is
no longer the preferred NRC position. adequate to sequence or block load during
design basis events without actuating DVRs
(BTP) PSB-1 (BTP 8-6) states: with sufficient margin available at the safety
"The subsequent occurrence of a safety injection buses. Separation of the safety buses from
actuation signal (SIAS) [after an undervoltage the grid is only appropriate when the DVR
condition longer than a motor starting transient] relays actuate indicating that SR equipment
should immediately separate the Class 1E voltage requirements are not being met (not
distribution system from the offsite power system." able to protect or provide adequate voltage
to the terminals of the SR limiting
The RIS should state that this (BTP) PSB-1 components at the plant).
position is not included in the draft RIS because it
provides no added protection in terms of Analyses to determine such setpoints always
establishing the DVR setpoint(s) or in establishing should have included modeling the plant
operability of the offsite power interface. To the power distribution system such that proper
contrary, this (BTP) PSB-1 increases the voltages throughout the plant system can be
probability of separation from offsite power. calculated in all operating and accident
26
No. Section of RIS Originator Specific Comment NRC Resolution
conditions.
80 Page 7, NEI 3/18/11 "Guidelines for voltage drop calculations
paragraph 2 a) Letter / 22
a) The plant voltage analysis, while
supplied from the transmission
network, should be based on the
operating voltage range of the
transmission network connection."
It is recommended that the first sentence of Disagree
Paragraph 2(a) be deleted. It is covered by
Paragraph 2(b), as the switchyard is the "power Enclosure 2 of GL 79-36 provides guidelines
source" for the offsite power circuits. for voltage drop calculations.
Paragraph 2(a) addresses both plant and
transmission operator analyses. The purpose is to
identify that the switchyard voltage results from the
transmission operator analysis should be used as
an input to the power plant analysis. From the
nuclear power plant point of view, the
determination as to whether each offsite power Accident cases consider the unit trip grid
circuit is individually capable of performing its contingency since a trip is assumed to occur
design function is based on a postulated post-trip coincident with an accident. However, if the
switchyard voltage for the present grid unit trip is not the most limiting grid
configuration and operating level (i.e. RIS contingency (not the largest grid voltage
Paragraph 2(b)). drop), the cases which assess normal and
abnormal operation (non-accident) need to
As written, it is conceivable that a reader of this assume the bounding grid contingency
paragraph could conclude that the transmission (normal grid operating range)
"contingency analysis" is a factor in the nuclear
plant analysis regarding "when" the contingency is
postulated to occur relative to the postulated plant
event. The alteration of the present basis to include
concurrent grid/plant events is a change in position
and would be subject to backfit consideration.
81 Page 8, NEI 3/18/11 "For multi-unit stations, a separate analysis should
paragraph 2 c) Letter / 23 be performed for each unit assuming (1) an
accident in the unit being analyzed and
simultaneous shutdown of all other units at the
station; or (2) an anticipated transient (anticipated
operational occurrence) in the unit being analyzed
(e.g., unit trip) and simultaneous shutdown of all
other units at that station, whichever presents the
largest load situation.
The RIS wording should be revised to indicate Disagree
"orderly or controlled safe shutdown of the
remaining units, as per the station's licensing basis" Wording is the same as provided in GL 79-
instead of "simultaneous shutdown". Alternatively, 36
the wording 'shutdown consistent with the station
licensing basis" could be used instead of
"simultaneous shutdown'.
This statement is consistent with GDC 17,
Most multi-unit stations Licensing Basis consider GL 79-36, and IEEE Standard 308-1971,
an "orderly or controlled safe shutdown" of the Class IE Electrical Systems, Section 8,
other unit(s) not being analyzed. Multi-Unit Station Considerations.
NERC Std TPL-004-0; particularly Category D
events per Table 1, where a "loss of all generating
units at a station" may result in "portions or all of
the interconnected systems may or may not
achieve a new, stable operating point".
IEEE Std 308-1974, Clause 8, sub-clause 8.1.1
"Capacity" describes this as a "concurrent safe
shutdown on the remaining units".
27
No. Section of RIS Originator Specific Comment NRC Resolution
This RIS re-states part of GL 79-36, with an
attempt to clarify "anticipated transient" by adding
in parenthesis "(anticipated operational
occurrence)". It is not clear what the added
parenthetical statement is meant to convey, other
than unit trip (which already exists in GL 79-36).
The RIS should remove this parenthetical addition
or state '...an anticipated transient per station
licensing basis...'
82 Page 8, NEI 3/18/11 "All actions the electric power system is designed
paragraph 2 d) Letter / 24 to automatically initiate should be assumed to
occur as designed..."
This statement is consistent with GDC 17 in that Agree.
the presumption is the onsite AC sources are lost.
The postulation of concurrent malfunctions in both No change to this sentence.
the onsite and offsite sources is not required.
The RIS should retain this sentence, since it may
not have been consistently applied during recent
CDBI's.
83 Page 8, NEI 3/18/11 "e) Manual load shedding should not be assumed.
paragraph 2 e) & Letter / 25
f) f) For each event analyzed, the maximum load
necessitated by the event and the mode of
operation of the unit at the time of the event should
be assumed in addition to all loads caused by
expected automatic actions and manual actions
permitted by administrative procedures."
These guidelines seem contradictory in that e) Disagree
states that there may be no credit for procedurally
controlled operator actions to reduce load but f)
states that the manual action loads must be This guidance is consistent GL 79-36.
considered in the maximum load.
The RIS should delete "e) Manual load shedding Adding loads manually per procedure is
should not be assumed" or add allowance to credit conservative in terms of maximum loading,
procedurally controlled operator actions to but not for load reductions. Plant design for
decrease load. maximum load should not depend on manual
load shedding (not conservative). That was
the point of item e).
84 Page 8, NEI 3/18/11 Omission
paragraph 2 f) Letter / 26 Disagree
After paragraph 2 f), the RIS leaves out the
guidance in GL 79-36 concerning minimum It was not omitted. This is covered in item a)
expected values (item 6 of enclosure 2).
Add item 6 of enclosure 2 in GL 79-36 to the RIS:
"6. The voltage at the terminals of each safety load
should be calculated based on the above listed
consideration and assumptions and based on the
assumption that the grid voltage is at the "minimum
expected value". The "minimum expected value"
should be selected based on the least of the
following:
a. The minimum steady-state voltage
experience at the connection to the
b. The minimum voltage expected at the
connection to the offsite circuit due to
contingency plans which may result in
reduced voltage from this grid.
c. The minimum predicted voltage from grid
stability analysis. (e.g., load flow
28
No. Section of RIS Originator Specific Comment NRC Resolution
studies)."
85 Page 8, NEI 3/18/11 "To provide assurance that actions taken to assure
paragraph 2 j) Letter / 27 adequate voltage levels for safety related loads do
not result in excessive voltages, assuming the
maximum expected value of voltage at the
connection to the offsite circuit, a determination
should be made of the maximum voltage expected
at the terminals of all safety related actual
equipment and their starting circuits (if applicable).
If this voltage exceeds the maximum voltage rating
of any safety related equipment, immediate
remedial action should be taken."
The RIS should remove the word "immediate" Disagree
describing remedial action. Immediate remedial
action could imply control room intervention. The
control room has alarm procedures to address high
voltage should it occur. Timeliness of remedial The Offsite/Onsite design should address all
actions depends on how high actual voltage grid operating conditions to prevent
reaches and is addressed by procedures. overvoltages from occurring.
Analyses of high grid voltage with light plant load The point here is that if a design problem is
are standard and provide insights as to what the identified such as overvoltage conditions,
grid voltage upper limit should be or what immediate actions should be taken
compensating activities might be required for light (compensatory and/or permanent design
load operations (refueling). changes) to address the design problem
rather than taking actions after it occurs.
The RIS should provide examples of typical
responses to high grid voltages. For example: in It is not the intent of the RIS to highlight
those cases where unit trip can result in a step reasons for voltage perturbations.
increase in grid voltage (most common on higher
voltage connections like 765kv), anticipated
excursions above desired voltages should be
addressed by compensating measures (changing
excitation for nearby units, switching in reactor
banks, etc.).
86 General NEI 3/2/11 Page 2, Paragraph 1
Letter While NEI supports efforts to obtain greater clarity Disagree
with respect to the staff's technical position in this
important area, the draft RIS greatly oversimplifies NRC Staffs position is that the RIS is
the regulatory and licensing aspects of the intended to clarify the requirements and
degraded grid voltage protection issue. As a result associated existing staff positions guidance
of this oversimplification, the draft RIS which would apply to all plants.
inappropriately combines several generic
communications and guidance documents that Any inspection findings that questions the
affected the licensing bases of individual plants in plant-specific licensing bases will be
different ways, and fails to adequately address the reviewed by the NRR staff in accordance
significant backfitting concerns that arise when NRCs TIA process.
attempting to eliminate licensing basis variability
via a RIS (or any other guidance document).
87 General NEI 3/2/11 Page 2, Paragraph 2
Letter Unless it is revised, the draft RIS will unnecessarily Disagree
increase the potential for loss of the preferred off-
site power source and, consequently, increase Proper design of the plant electrical
reliance on emergency diesel generators. NEI distribution system, given the operating
believes that the use of emergency diesel range of the grid and the proper selection of
generators more frequently than necessary is DVR settings (based on the voltage
inconsistent with GDC 17 and results in an requirements of the 1E equipment), should
unnecessary loss of defense-in-depth. provide more than adequate operating
margin, preventing unnecessary separation
from offsite power.
88 General NEI 3/2/11 Page 2, Paragraph 3, Comment I
Letter The Draft RIS Fails to Adequately Consider Agree in part.
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection As a general matter, NRC staff positions are
The only generic obligation or legally binding guidance, as are, among other things,
29
No. Section of RIS Originator Specific Comment NRC Resolution
requirement mentioned in the Discussion section of regulatory guides and Interim Staff Guidance
the draft RIS is GDC 17. (ISG). As guidance, NRC staff positions, like
regulatory guides, are not legally binding
unless the NRC legally imposes them on a
licensee or the licensee binds itself to
complying with them in a document subject
to NRC-mandated controls. In other words,
for any particular nuclear power plant,
guidance may be part of the licensing basis
for that plant because of past NRC or
licensee actions. For instance, guidance
may be legally imposed upon a plant by
virtue of the issuance of an order or through
a license condition that imposes the
guidance on that particular plant. As another
example, a licensee may have committed to
compliance with the guidance in the plants
final safety analysis report (FSAR) or other
document subject to NRC controls (e.g., the
description of the plants quality assurance
program, an emergency plant, or a security
plan). The NRC resolutions for all the public
comments received on this RIS should be
understood in light of this explanation.
.
89 General NEI 3/2/11 Page 3, Paragraph 2, Comment I (Cont.)
Letter The Draft RIS Fails to Adequately Consider Disagree.
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection The 77 Letter is a staff position which applies
Although these letters resulted in changes to the to all operating reactors at that time and
licensing bases of the nuclear power plants that plants licensed since, on how to comply with
received them, they do not function the same way the requirements in 10 CFR Part 50, General
as generally applicable regulatory requirements. Design Criteria 17 (GDC 17).The 1977 letter,
That is, these generic communications were only as well as other staff guidance, were made
received by plants that were licensed at the time available or sent to all operating plants at
the communications were issued. Operating that time. In addition, the NRCs regulatory
licenses for the current fleet were issued during a practice-which has been understood by the
period that ranged from the late 1960s through the industry-is that staff guidance represents the
1990s. Thus, not all operating plants received and staff position until subsequently modified or
responded to the generic communications issued in withdrawn. While the staff recognizes that
1977 and 1979. there is variability among plants licensing
bases with respect to degraded voltage
protection, the NRC believes (with one
exception identified elsewhwere) that the
overall licensing basis provisions with
respect to degraded voltage protection are
consistent with the staffs position.
90 General NEI 3/2/11 Page 3, Paragraph 2, Comment I (Cont.)
Letter The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of Disagree.
Degraded Grid Voltage Protection
Further, the Branch Technical Position (BTP) The 1977 Letter is a staff positionguidance
described in the draft RIS BTP PSB-1, Rev. 0, which applies to all operating reactors at that
"Adequacy of Station Electric Distribution System time and plants licensed since, on how to
Voltages"--was issued in 1981. BTP PSB-1 and the comply with the requirements in 10 CFR
Standard Review Plan in which it is included were Part 50, General Design Criteria 17 (GDC
"prepared for the guidance of the Office of Nuclear 17).
Reactor Regulation staff responsible for the review
of applications to construct and operate nuclear
power plants.... Standard review plans are not
substitutes for regulatory guides or the
Commission's regulations and compliance with
them is not required." Thus, BTP PSB-1 would
have been directly relevant to plants licensed after
its issuance, but not before. Further, the specific
details in the information provided in the 1977
letters, Generic Letter 79-36, and BTP PSB-I are
not identical.
30
No. Section of RIS Originator Specific Comment NRC Resolution
91 General NEI 3/2/11 Page 3, Paragraph 3, Comment I (Cont.)
Letter The Draft RIS Fails to Adequately Consider Disagree.
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection NRC Staff asserts that coincident degraded
For example, the draft RIS makes several grid and accident is specified in the 77 Letter
recommendations that may be inconsistent with the and the BTP approach supports that
approved licensing bases for operating plants, position.
including:
The draft RIS proposes "Degraded See also staff response to Comment No. 36.
voltage conditions coincident with a
postulated design basis accident." BTP
PSB-1 says "subsequent occurrence."
92 General NEI 3/2/11 Page 4, First Bullet, Comment I (Cont.)
Letter The Draft RIS Fails to Adequately Consider Disagree.
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection The staff position is consistent with 1977
The draft RIS proposes "DVR Dropout setting letter and BTP PSB-1.
based on starting and running voltage." BTP PSB-1
says "sustained," implying a steady state voltage
condition and not a transient voltage condition that
exists during a motor starting event.
93 General NEI 3/2/11 Page 4, Second Bullet, Comment I (Cont.)
Letter The Draft RIS Fails to Adequately Consider Disagree
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection This statement is consistent with GDC 17,
The draft RIS proposes "separate analysis should GL 79-36, and IEEE Standard 308-1971,
be performed for each unit assuming (1) an Class IE Electrical Systems, Section 8,
accident in the unit being analyzed and Multi-Unit Station Considerations.
simultaneous shutdown of all other units at the
station."
GDC 5 says:
"... in the event of an accident in one unit, an
orderly shutdown and cooldown of the remaining
units."
94 General NEI 3/2/11 Page 4, Third Bullet, Comment I (Cont.)
Letter The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of Disagree
Degraded Grid Voltage Protection
The draft RIS proposes "During normal plant
operation, the Class 1E safety related buses
should automatically separate from the power
supply within a short interval (typically less than 60
seconds) if sustained degraded voltage conditions
are detected." BTP PSB-1 clause B.1.b.2 included
provisions for operator manual actions to restore
bus voltage on the Class 1 E distribution system.
See staffs response to Comment No. 37.
BTP PSB-1 B.1.b.2 says:
"The second time delay should be of a limited
duration such that the permanently connected
Class 1E loads will not be damaged. Following this
delay, if the operator has failed to restore adequate
voltages, the Class 1E distribution system should
be automatically separated from the offsite power
system. Bases and justification must be provided in
support of the actual delay chosen."
Manual actions for the purposes of reducing
The draft RIS specifically excludes manual load load for the design of the plant electrical
shedding under the Offsite/Onsite Design Interface distribution system should not be assumed.
Calculations whereas the BTP PSB-1 allows for This is not precluding load shedding as part
manual actions to avoid separation from offsite of normal operation when there is sufficient
power. time to do so to support adequate voltage.
See GL 79-36 for more details.
The sixty second time delay would not allow
operator actions. This appears to be a new NRC
position.
31
No. Section of RIS Originator Specific Comment NRC Resolution
95 General NEI 3/2/11 Page 5, Paragraph 2, Comment I (Cont.)
Letter The Draft RIS Fails to Adequately Consider Disagree
Licensing Basis Variability in the Area of .
Degraded Grid Voltage Protection The 77 Letter indicates that the DVR circuits
In addition to the specific examples provided should be designed assuming coincident
above, the draft RIS states "[t]he staff considers sustained degraded grid voltage and
degraded voltage conditions coincident with a accident events. Upon the onset of the
postulated design basis accident to be a credible coincident accident and degraded grid event,
event." It is unclear what exactly the staff intended the time delay for the DVR circuit should
with this statement. allow for separation of the 1E buses from the
offsite circuit(s) and connection to the 1E
onsite supplies in time to support safety
system functions to mitigate the accident in
accordance with the FSAR accident
analyses.
96 General NEI 3/2/11 Page 6-7, Comment II
Letter The Backfit Discussion Provided in the Draft
RIS is Inadequate
Given the complex regulatory and licensing history Disagree
associated with providing degraded grid voltage
protection, the backfitting discussion included in the
draft RIS is inadequate. Despite the fact that
facility-specific backfits were required as a result of
several recent inspection findings on degraded
voltage protection,1 8 the entire backfitting
discussion included in the draft RIS consists of
three sentences. Of those three sentences, only
one provides any analysis:
Specifically, NRC Staff technical
positions outlined in this draft RIS are The details of the inspection findings and
consistent with the aforementioned enforcement actions are discussed in detail
regulations [GDC 17] and generic in the applicable inspection reports and TIAs,
communications [1977 letter, Generic if applicable referenced in the RIS.
Letter 79-36, BTP PSB-1], while
providing more detailed discussion
concerning the necessary voltage
calculations supporting DVR settings
based only on voltage requirements of
Class 1E components and the Class 1E NRC staff asserts that the regulations and
distribution system design.19 staff positions articulated in the RIS are
consistent with the regulations and existing
This analysis misses the point. First, GDC 17 (like guidance documents and therefore do not
most GDC) is cast in broad, general terms; constitute new or different positions with
therefore, the fact that the specific positions respect to the backfit rule (50.109).
discussed in the draft RIS are "consistent with" the See the supplemental response attached at
design criteria does not necessarily mean that they the end of this Comment/Response Table.
escape the definition of a backfit. Specifically, there
are any number of staff positions that are
"consistent with" a broad design principle, but the
relevant inquiry when examining the backfit
definition is whether the staff position being
articulated is new or different from a previously
applicable staff position. The draft RIS does not
address this issue. Further, as discussed above,
the generic communications and guidance
discussed in the draft RIS are not completely
consistent with one another and were not equally
relevant in developing the licensing bases for all
reactor licensees. Given the variability in the
protection schemes approved by the NRC, merely
concluding that the positions provided in the draft
RIS are "consistent with" one or more of these
documents, does not address the obvious fact that
"providing more detailed discussion" on how to
demonstrate compliance with GDC 17 could
32
No. Section of RIS Originator Specific Comment NRC Resolution
constitute a staff position that is new or different
from a previously approved protection scheme.
97 General PPL Page 1-2, First Bullet Disagree
Contrary to the stated intent, PPL believes that the
RIS does transmit new requirements and staff The NRC 1977 letter states that voltage and
positions. Specific comments applicable to time setpoints shall be determined from an
Susquehanna Steam Electric Station (SSES) are analysis of the voltage requirements of the
as follows: safety related loads. Safety related
equipment, particularly large motors, have
The RIS introduces the need to consider starting and running voltage requirements.
both "starting and running" conditions This second level of protection should
during all operating configurations while address these voltage requirements.
maintaining the offsite power supply Sustained degraded voltage, as discussed in
connected to the plant electrical the 1977 letter, refers to grid voltage below
distribution system. The establishment of the expected low value given normal grid
a degraded voltage relay (DVR) to detect operation and grid post contingency. Thus,
a "sustained" degraded voltage condition when grid voltages are degraded (such as
challenges the relay's basis for resulting in SR bus voltages down close to
"protection" if its actuation (dropout) where DVRs are set based on SR equipment
setpoint must accommodate both starting requirements), separation from the grid is
and running voltage conditions. The term appropriate. The design of the plant electrical
"sustained degraded voltage" implies a distribution system and setting of the DVRs,
steady state degraded voltage condition, based on the grid voltage range (described
and excludes starting voltage above) should provide proper margin such
consideration that spurious separation from the grid should
not occur due to sequencing or block loading
of loads during a design basis events.
98 Page 6, Section 1 PPL Page 2, Paragraph 2 Disagree
RIS 201 1-XX, Page 6, Section 1. "Degraded
Voltage Relaying Design Calculations" contains the NRC Staff asserts that this statement means
statement "staff considers degraded voltage that while the events are coincident (which is
condition coincident with a postulated design basis important from the standpoint that the time
accident to be a credible event." This statement delay chosen for the DVR must support the
implies a requirement to demonstrate capability of accident analysis), it does not mean that
connected loads to start and run at the degraded connected loads must start and run at the
voltage relay dropout setting. For Susquehanna, dropout setting. The dropout setting should
sequencing of loads from the offsite power source be developed based on the voltage
cannot be demonstrated at the relay dropout requirements (starting and running) and
setpoint because operation at this voltage level therefore to develop values which are
would result in separation from the offsite bounding, the studies should be done under
transmission system. Furthermore, the statement worst starting and loading conditions, which
on page 6 of the RIS is not in agreement with other means the required voltage at the 1E bus
regulatory position documents such as GSI 171, prior to the start or run case would have to
"Engineered Safety Features Failure (ESF) from a be higher than the setpoint. The main point is
Loss of Offsite Power (LOOP) subsequent to a that the setpoint should equate to the limiting
Loss of Coolant Accident (LOCA)," which voltage at the limiting component during the
concluded a degraded voltage condition coincident bounding starting or running scenario to
with a postulated design basis accident is not a protect the 1E equipment.
credible event.
99 Page 6, Section 1 PPL Page 2, Paragraph 3
Additional clarification is necessary if starting Disagree.
transients must be included when determining the Proper design of the plant electrical
degraded voltage relay (DVR) dropout setpoint. distribution system, given the operating
This condition will increase the probability of range of the grid and the proper selection of
separating from the offsite transmission system DVR settings (based on the voltage
and increase the likelihood of a double sequencing requirements of the 1E equipment), should
event, which is a potential nuclear safety concern. provide more than adequate operating
margin, preventing unnecessary separation
from offsite power.
100 Page 8, Section c) PPL Page 2, Paragraph 3
The RIS requires performance of analyses for an Disagree.
accident in the unit being analyzed and
simultaneous shutdown of all other units at the This statement in the RIS is consistent with
station. This is not consistent with the present GDC 17, GL 79-36, and IEEE Standard
Susquehanna design and licensing basis, which is 308-1971, Class IE Electrical Systems,
an accident on one unit followed by the safe Section 8, Multi-Unit Station Considerations.
shutdown of the second (non-accident) unit. The
safe shutdown of the non-accident unit is
33
No. Section of RIS Originator Specific Comment NRC Resolution
considered a controlled shutdown, which follows
automatic operation of the safety related loads on
the accident unit. This accident response is also
consistent with the NERC requirements for the
design of the transmission system. The RIS should
be revised to be in agreement with the current
NERC requirements.
101 General PPL Page 2-3, First Bullet
The draft RIS attempts to clarify the requirements Disagree.
for setting the DVRs based on the criteria
established in the following three main documents: The 1977 Letter is a staff positionguidance
which applies to all operating reactors at that
1) NRC letters to licensees dated June 2 & 3, 1977, time and plants licensed since, on how to
2) Branch Technical Position (BTP), PSB-1 comply with the requirements in 10 CFR
Revision 0, Part 50, General Design Criteria 17 (GDC
3) Generic Letter 79-36, "Adequacy of Station 17). In addition, NRCs staff position is that
Electric Distribution Systems Voltages" while the BTPs go into some more detail,
they are consistent with the 77 Letter.
The guidance listed in the draft RIS is not
consistent with all the requirements listed in these
three documents and a new interpretation is
provided in some cases.
It should be generally recognized that a nuclear
plant operating license may not have been issued
based on the above documents. For example, the
1977 letters discussed above are not applicable to
SSES.
102 General PPL Page 3, First Bullet
The lack of regulatory clarity in the RIS could result Disagree.
in revising the degraded voltage setpoint for a
plant's DVR thus increasing the possibility of Proper design of the plant electrical
premature separation from the offsite circuit (i.e., distribution system, given the operating
undervoltage relay actuation). This relay operation range of the grid and the proper selection of
could lead to an increase in the likelihood of a DVR settings (based on the voltage
double sequencing event, which has the potential requirements of the 1E equipment), should
to create a nuclear safety concern. provide more than adequate operating
margin, preventing unnecessary separation
from offsite power.
103 General PPL Page 3, Second Bullet
The RIS introduces the need for two sets of Disagree.
calculations, one to establish the DVR relay
setpoint and one for the interface with the offsite The RIS is primarily identifying that different
transmission system. The RIS should not specify types of calculations are necessary to
the number of calculations that are necessary for a address different requirements. DVR setting
plant to meet a regulatory requirement. calculations consider the voltage of the 1E
equipment while the plant design is more
about the operating range of the grid and the
resulting voltages in the plant system (which
should be well above the DVR voltages)
104 General PPL Page 3, Third Bullet
The condition the DVR is required to "protect" Disagree.
needs to be specifically defined along with the
applicable relay setting, (i.e., relay minimum The DVRs function is specified in that it
dropout, maximum dropout, or reset). If the DVR is ensures that 1E equipment is supplied with
installed to provide a level of protection then the adequate voltage in accordance with its
analysis must demonstrate that the safety related design requirements.
equipment is capable of performing its required
safety function. An example of this would be the
case where the DVR analysis would need to
demonstrate acceptable operation at both the
starting and running equipment ratings when at the
DVR dropout setting.
105 General PPL Page 3, Fourth Bullet
A clarification of the term "sustained" is needed to Disagree.
determine if "sustained" refers to a steady state
voltage condition (i.e., no equipment starting Sustained degraded voltage, as discussed in
voltage effects) for which the DVR setting is to be the NRC 1977 letter as well, refers to grid
established. voltage below the expected low value given
34
No. Section of RIS Originator Specific Comment NRC Resolution
normal grid operation.
106 General PPL Page 3, Fifth Bullet
The guidance in the RIS is too general when Disagree.
referring to operating voltages. The specific voltage
requirements need to be specified instead of The term voltage requirements used in the
implied by a general term. The RIS needs to clarify RIS is defined in terms of equipment
that the impact of the nuclear unit trip on the manufacturer design requirements. NRC
transmission system voltage must be considered in Staff feels that this terminology is sufficiently
the plants voltage analysis. specific. Additional wording has been added
to the RIS to clarify that unit trip voltage
impact should be factored into the accident
analysis cases.
107 General PPL Page 3, Sixth Bullet
The time delays suggested are not consistent with Disagree.
PSB-l. The PSB established one time delay to
allow for operator action. The RIS does not While there may be differences, the BTPs
address this requirement. are guidance documents and represent an
approach but not necessarily the only
approach. In addition, following the guidance
documents approach will satisfy the GDC 17
requirements.
108 General PPL Page 4, First Bullet
The RIS also lacks any acknowledgement of Disagree.
preventative measures the licensees have taken to
minimize the potential for a degraded voltage Regardless of improvements made in terms
condition. Advancements in plant Ioadflow of grid operation and understanding of grid
analyses and measures to increase the reliability of operations impact on plant voltages, the
the offsite transmission system are industry plant design has always had to properly
improvements that have occurred since the address grid operating parameters and their
degraded voltage events that occurred 35 years impact on plant voltages in all modes of
ago. operation. This point was properly
emphasized in the RIS as was in the original
regulations and guidance.
109 General APS Page 2, 1.
The draft RIS asserts that there is a simple and Disagree.
singular set of design criteria that have been
applied universally to the industry. Over the years The 77 Letter provides staff positions on the
the degraded voltage performance requirements design criteria in that the voltage
have changed, as a specific issue, and on a requirements for the 1E equipment has to be
component basis (e.g., motor operated valves and ensured by the DVR circuits by automatic
contactors), for individual nuclear power plants. As separation from offsite and transfer to the
a result, each nuclear power plant has specific onsite sources.
licensing bases, and there is no singular set of
requirements that have been applied universally to
the industry.
110 General APS Page 2, 2.
The draft RIS asserts that the guidance provided to Disagree.
the industry to address the Millstone and other
degraded voltage events adequately addresses The DVR circuits will automatically separate
this potential common mode failure. The common the 1E circuits from offsite power when
mode failure potential is that multiple trains of voltage requirements are not met which will
safety equipment could be simultaneously prevent the Millstone type event
negatively impacted if off-site power is degraded. automatically.
The deterministic guidance provided does not
appear to effectively address integrated plant
response nor preclude a Millstone type event. The
use of degraded voltage relays to address this
potential failure mode is not consistent with
operating experience and lacks adequate technical
basis as described in the detailed technical
comments that follow.
111 Page 6 APS Page 2, 3.
The draft RIS (page 6 of 10) states: Disagree
"The staff considers degraded voltage conditions The point being made in the RIS is that
coincident with a postulated design basis accident setting of the DVR should include
to be a credible event." consideration of a coincident accident signal
in that the time delay chosen for the DVR
35
No. Section of RIS Originator Specific Comment NRC Resolution
It is our understanding that the established staff should support the accident analysis
interpretation is that this is not a credible event, as assumptions consistent with the NRC1977
discussed and supported by analysis in NUREG- letter.
0933, Supplement 33, dated August 2010,
Resolution of Generic Safety Issues, Issue 171,
ESF Failure from LOOP Subsequent to a LOCA,
and Brookhaven National Laboratory NUREG/CR-
6538 (BNL-NUREG-52528), Evaluation of LOCA
With Delayed Loop and Loop With Delayed LOCA
Accident Scenarios, Technical Findings Related to
GSI-1 71, 'ESF Failure from LOOP Subsequent to
LOCA' published July 1997. This appears to be a
new staff interpretation and no documented
analysis is provided to support it. Therefore, if the
scenario is credible, as the draft RIS asserts, then
GSI-171 is not adequately resolved and should be
reevaluated.
112 General APS Page 5, 4. Disagree
The recent licensing actions in the industry which
have mandated setpoint changes for the degraded Setting the DVRs in accordance with the
voltage relays (DVRs) and loss of voltage relays voltage requirements of the 1E equipment
(such as the one cited in the draft RIS for Fermi-2) coupled with a properly designed plant
only serve to increase the probability of the 'ESF electrical distribution system (and based on
Failure from LOOP Subsequent to a LOCA' event the grids allowable voltage range) must
discussed in NUREG/CR-6538 without providing provide adequate voltage margin to preclude
an advantage for any credible scenario. As such, offsite separation.
these changes may increase core damage
frequency (CDF).
Disagree
It is APS's understanding that a comprehensive
review of guidance related to degraded grid voltage NRC staff asserts that the regulations and
has not been performed using the cost-benefit and positions guidance articulated in the RIS are
risk criteria of 10 CFR 50.109 (backfit rule), nor is it consistent with the existing regulatory
apparent that risk insights have been used to requirements and NRC staff positions
inform this guidance. guidance therefore do not constitute new or
different positions with respect to the backfit
rule (50.109).
113 General APS Page 5, 5.
The draft RIS does not address the implication of Disagree.
the Branch Technical Position (BTP) PSB-1
requirement that "The Class 1 E bus load shedding The design of the plant electrical distribution
scheme should automatically prevent shedding- system and the onsite sources should
during sequencing of the emergency loads to the provide for adequate voltage to all 1E
bus." A large variety of voltage conditions could equipment in all normal, abnormal and
exist during the sequencing period while the accident conditions.
shedding is blocked, and no analytical methods are
discussed that could demonstrate that equipment Typical designs do not block the DVR or the
damage or malfunction would not occur. LOV relay when sequencing loads on the
offsite source. Hence load shedding in the
event of a loss of offsite power should be
part of the design basis. A large variation of
voltage conditions can occur during various
operating modes of a nuclear plant. The
DVR setpoint should be based on limiting
conditions. If the recommendations of BTP
PSB-1 are followed, the probability of events
such as double sequencing is minimized.
114 Page 7 APS Page 5-7, 6.
In light of the summary of the resolution of GSI- Disagree.
171, the draft RIS statement (page 7 of 10) that
"the time-delays(s) chosen for DVRs during Degraded voltage conditions can be
accident conditions should meet the accident postulated to occur at anytime. The DVR
analysis assumptions..." does not seem setpoint should accommodate the limiting
appropriate. The degraded voltage condition could case for equipment protection. If the
occur at various times during the initial energization recommendations of BTP PSB-1 are
of the accident mitigation equipment, and the relay followed, then a separation from the
time delay value only affects the additional time degraded grid coupled with accident signal is
36
No. Section of RIS Originator Specific Comment NRC Resolution
until the subsequent LOOP occurs. the preferred approach to resolve the issue
and satisfy accident analyses.
115 Page 8 APS Page 7-8, 7.
It is not feasible for multi-unit nuclear plants to Disagree
successfully demonstrate that voltage from the
offsite circuits would be adequate, as described in
the draft RIS (page 8 of 10), for:
This wording in the RIS is the same as was
"(1) an accident in the unit being analyzed and used in GL 79-36.
simultaneous shutdown of all other units at the
station; or (2) an anticipated transient (anticipated
operation occurrence) in the unit being analyzed
(e.g., unit trip) and simultaneous shutdown of all
other units at that station." TPL-004 requires transmission planning to
address simultaneous multiple transmission
North American Electric Reliability Corporation contingencies.
(NERC) Standard TPL-004 recognizes that the
design and operating constraints of the The requirements of TPL-004 are not within
transmission network are such that the loss of all the scope of RIS.
generating units at a station could result in portions
or all of the interconnected system not achieving a
new, stable operating point.
It is beyond the nuclear plant operators authority or
capability to ensure otherwise.
116 Page 6 APS Page 8, 8.
The draft RIS (page 6 of 10) contains the following
statement:
"The Class I E buses should separate
from the offsite power system within a
few seconds if an accident occurs
coincident with sustained degraded
voltage conditions."
This statement appears to reflect the position of
Revision 3 of BTP 8-6, which states, in part:
"The first time delay should be long
enough to establish the existence of a
sustained degraded voltage condition
(i.e., something longer than a motor-
starting transient). Following this delay,
an alarm in the control room should alert
the operator to the degraded condition.
The subsequent occurrence of a safety
injection actuation signal (SIAS) should
immediately separate the Class 1 E
distribution system from the offsite power
system. In addition, the degraded voltage
relay logic should appropriately function
during the occurrence of an SIAS
followed by a degraded voltage
condition." Disagree.
This is not currently a design or licensing
requirement for all existing plants. As such the RIS As a result of these Millstone events, the
process would not be the appropriate method to NRC requested that all licensees implement
communicate a new regulatory position. degraded protection as described in the
1977 Letter to ensure automatic protection of
safety buses and loads. This Letter provides
staff positionguidance, which applies to all
operating reactors at that time and plants
licensed since, on how to comply with the
requirements in 10 CFR Part 50, General
Design Criteria 17 (GDC 17).
37
No. Section of RIS Originator Specific Comment NRC Resolution
117 Page 6 APS Page 8-9, 9.
The draft RIS (page 6 of 10) contains the following
statement:
"The time delay chosen should be optimized to
ensure that permanently connected Class 1 E
loads are not damaged under sustained degraded
voltage conditions (such as sustained degraded
voltage just above the LVR voltage setting for the
duration of the DVR time delay setting)."
This deterministic approach, while appearing Disagree
conservative, has the net effect of increasing the
frequency of delayed LOOP events during The voltage studies done for evaluating
transients, even when the subsequent sustained offsite power/onsite power interface should
voltage condition is not degraded (see Comment use minimum expected voltage at the
4), with resulting adverse effects as discussed in plant/grid interface node, demonstrating
the resolution of GSI-171. It also neglects adequate voltage for starting and running of
consideration of the voltage levels that must be plant components during normal, abnormal
maintained in the event of a unit trip and coincident and accident conditions. The voltage studies
accident to prevent delayed LOOP events. Finally, for the DVR setpoints should require
the draft RIS is silent on the particulars of the plant/grid interface node voltages well below
voltage studies that would be acceptable to use to the minimum expected values (including post
determine the optimum time delay (such as the grid contingency).
plant operating conditions and voltage profile).
118 Page 6 APS Page 9, 10.
The DVR Setting Design Calculations section
(page 6 of 10) indicates that:
"models would allow calculation of voltages at
terminals or contacts of all safety related
equipment with the voltage of the DVR monitored
bus at the DVR dropout setting, providing the
necessary design basis for the DVR voltage
settings. In this manner, the DVR ensures
adequate operational (starting and running) voltage
to all safety related equipment, independent of
voltage controlling equipment external to the plant
safety related electrical distribution system."
This seems to impose a new requirement. Further, Disagree
the described model is of a nondegraded voltage
scenario that does not result in DVR actuation. The DVR dropout setting should be
Therefore, it does not demonstrate that "required developed based on the voltage
safety related components are provided adequate requirements (starting and running) and
voltage" for accidents with degraded voltage therefore to develop values which are
scenarios. That conclusion could only be bounding, the studies should be done under
demonstrated by modeling degraded voltage worst starting and loading conditions which
scenarios that involve DVR actuation. However, in means the required voltage at the 1E bus
all cases involving degraded voltage coincident prior to the start or run case would have to
with postulated accidents, such models would be higher than the setpoint. The main point is
result in delayed LOOP scenarios as discussed in that the setpoint should equate to the limiting
GSI-171. voltage at the limiting component during the
bounding starting or running scenario to
Also, it reflects a non-conservative voltage profile. protect the 1E equipment.
If the voltage at the DVR monitored bus was at the
DVR dropout setting prior to starting a motor, it The RIS does not impose any new
would be lower than that during and after starting requirements. It provides clarification on
the motor, and the voltage at the motor terminals existing requirements.
would be correspondingly lower, as well, compared
to the results using the constant bus voltage The DVR setpoint should be optimized for
methodology described in the draft RIS. motor starting transient and protection of
safety related equipment.
119 Page 5 APS Page 10, 11.
The draft RIS discussion asserts that the NRC Disagree
Office of Nuclear Reactor Regulation (NRR) Task
38
No. Section of RIS Originator Specific Comment NRC Resolution
Interface Agreement (TIA) response (TIA 2010-05) The point being made in the RIS is that
"concluded ... the time delay to transfer from a setting of the DVR should include
degraded offsite source to the standby power consideration of a coincident degraded grid
source to support the emergency core cooling and accident in that the time delay chosen
equipment operation must be consistent with for the DVR should support the accident
accident analysis time assumptions, as required by analysis assumptions consistent with the
BTP PSB-1 (NUREG 0800)." This statement is not NRC1977 letter.
included in the TIA response. The TIA response
(pages 4 and 5) quotes the Palo Verde UFSAR
description for the design requirements of the
degraded voltage relays, and this specific time
delay provision is not included in the PVNGS
This specific time delay provision was removed as
part of the PVNGS license amendment 123 Task Interface Agreement 2010-005
process and was specifically addressed in the NRC (ADAMS Accession No. ML102800340)
and APS correspondence (NRC Letter dated June provides more details regarding Palo Verde
14, 1999, and APS letter dated July 16, 1999, degraded voltage inspection finding.
Question 13). The subject matter of the TIA did not
include the time delay element of the design, with Plant specific findings are not in the scope of
regard to the accident analysis time assumptions, the RIS.
but rather was focused upon whether license
amendment 123 bounded the need to perform
design basis electrical calculations for the
degraded voltage relay low setpoint value of 3697
volts or below.
120 Page 5 APS Page 10-11, 12.
The draft RIS asserts that PVNGS erroneously
maintains that a degraded voltage condition
concurrent with a design basis accident is not
credible. PVNGS had originally implemented the
design approach included in the NRC letter
Qualification Review of the PVNGS Units 1,2 and
3, dated December 12, 1977. Based on operating
experience (LER 50-528/529/530-93-011)-and site
specific license amendment 123, PVNGS took
action to preclude such an event, by implementing
new TS LCO 3.8.1, Condition G. This approach
was consistent with the resolution of GS1-171,
alternative 3, and was approved. Disagree.
The prevention strategy was implemented to The point being made in the RIS is that
preclude a concurrent degraded voltage condition setting of the DVR should include
and design basis accident because the PSB-1 type consideration of a coincident degraded grid
design is not capable of adequately coping with and accident in that the time delay chosen
such an event. All such events would result in for the DVR should support the accident
delayed LOOP/double sequencing scenarios, as analysis assumptions consistent with the
described in GSI-171, for which there is no viable NRC1977 letter.
analytical approach.
The licensee should ensure that SI actuation
at a point just above the DVR set point
should not cause double sequencing.
See staff response to Task Interface
Agreement 2010-005 (ADAMS Accession
No. ML102800340) for more details
regarding Palo Verde degraded voltage
inspection finding.
121 Page 5 APS Page 11, 13.
PVNGS originally implemented the second level
degraded voltage protection design consistent with
NRC letter Qualification Review of the PVNGS Disagree
39
No. Section of RIS Originator Specific Comment NRC Resolution
Units 1, 2 and 3, dated December 12, 1977. As a
result, reference to PSB-1 in the draft RIS for The licensees analysis must show that
PVNGS does not reflect the historic licensing basis degraded voltage trip setpoint adequately
for PVNGS. protects the equipment powered by the 4.16
kV ESF bus from a potentially damaging
degraded voltage condition.
The NRC regulatory requirement is Criterion
17 of Appendix A to 10 CFR Part 50. The
NRC staff guidance and positions are
described in PSB-1.
See staff response to Task Interface
Agreement 2010-005 (ADAMS Accession
No. ML102800340) for more details
regarding Palo Verde degraded voltage
inspection finding.
122 Page 5 APS Page 11, 14.
Inspection Report 2009-008 is described in the
draft RIS. The specific elements of the inspection
report that require response are next described.
The inspection report states:
"the time delay of 35 seconds for transfer of safety
buses to the onsite power supplies may be too long
to prevent core damage in case of a sustained
degraded voltage condition concurrent with an
accident. This time delay could result in a delay in
supplying water to the core in case of an accident
concurrent with degraded voltage, due to the
inability of electrical equipment to respond as
required during the timeout period."
APS Response: This is a double sequencing Disagree.
scenario, which is a malfunction of an SSC with a
different result than previously evaluated pursuant The point being made in the RIS is that
to 10 CFR 50.59, for PVNGS. It could result in core setting of the DVR should include
damage regardless of the time delay value at which consideration of a coincident degraded grid
the DVR actuation (delayed LOOP) occurs. This is and accident in that the time delay chosen
the reason APS precludes such an event by for the DVR should support the accident
establishing appropriate initial conditions, with TS analysis assumptions consistent with the
LCO 3.8.1, Condition G, through license NRC1977 letter.
amendment 123.
The focus of the RIS is to clarify regulatory
requirements.
See staff response to Task Interface
Agreement 2010-005 (ADAMS Accession
No. ML102800340) for more details
regarding Palo Verde degraded voltage
inspection finding.
The double sequencing issue is a plant-
specific issue. The staff determined that the
amendment that addressed the specific
design issue (double sequencing) at PVNGS
did not change the licensing requirements for
the degraded voltage protection at PVNGS.
123 Page 5 APS Page 11-12, 15.
The inspection report states:
"A shorter time delay will not delay the time
required to provide water to the core, but will
actually improve it."
APS Response: APS is not aware of any analysis Disagree
in the GSI-171 resolution document to suggest that
40
No. Section of RIS Originator Specific Comment NRC Resolution
a shorter time delay (e.g., delayed LOOP occurring This is a plant specific issue. The issue will
sooner) would be of any benefit in preventing the be reviewed through the ROP.
failure mechanisms associated with a delayed
LOOP or assuring that water would be successfully
provided to the core. See Technical Comment 6 for
further discussion on the lack of correlation
between the DVR time delay setting, accident
analysis time, and core damage.
The PVNGS current licensing basis for the DVR
time delay is > 28.6 seconds. During the review
that led to issuance of PVNGS license amendment
123, the staff expressed a concern that a minimum
allowable time delay be specified to assure that
unnecessary separation from offsite power would
not occur. The safety evaluation for license
amendment 123 states:
"APS responded by adding a lower limit (> 28.6
seconds) to the time delay allowable value
specified for the degraded voltage function in its
revised submittal dated September 29, 1999. This
change resolves the staffs concern on this matter."
The NRC staff was aware and approved the
existing time delay values for the DVRs and the
staff considered a shorter time delay to be a
concern. The inspection report is inconsistent with
the current safety evaluation.
124 Page 5 APS Page 12, 16.
The inspection report states:
"The licensee had offered the proposition that
degraded voltage concurrent with an accident was
not credible, but the team could not find evidence
that the NRC had accepted this position, or that the
degraded voltage relays were no longer required to
perform a protective function during accidents."
APS Response: The PVNGS current licensing
basis is documented in the safety evaluation for Disagree.
PVNGS license amendment 123, which states:
See staffs response to Comment No. 123
"The licensee's proposed revision to TS 3.8.1,
Condition G is designed to preclude a degraded
voltage/double sequencing scenario from occurring
at the Palo Verde site. The staff finds this approach
acceptable...."
The safety evaluation recognizes that the
prevention strategy precludes degraded voltage
conditions from occurring. All scenarios involving
degraded voltage concurrent with an accident are
delayed LOOP/double-sequencing scenarios. The
purpose for PVNGS license amendment 123 was
to implement a method to prevent this degraded
voltage concurrent with an accident (which would
always result in a delayed LOOP and double
sequencing). APS is not aware of an accepted
method to ensure that core damage will not result,
if such an event were to occur. Design basis
calculations to justify the function of the degraded
voltage relays during accidents are not feasible,
because they would be unable to justify the
delayed LOOP/double sequencing effects
discussed in GSI-171.
125 Page 6 Nextera 1 This paragraph could be interpreted to require the
LOCA sequence to be modeled at the DVR dropout Disagree
setting. LOCA sequencing modeled at the DVR
41
No. Section of RIS Originator Specific Comment NRC Resolution
dropout setting would result in separation of the The dropout setting should be developed
buses from the Preferred Power Source (off-site based on the voltage requirements (starting
power) as the voltage would not recover above the and running) and therefore to develop values
DVR reset value. which are bounding, the studies should be
done under worst starting and loading
Clarify the intent is to show safety related conditions which means the required voltage
equipment will function at the selected DVR at the 1E bus prior to the start or run case
dropout setting voltage and that it is not expected would have to be higher than the setpoint.
to start the LOCA sequence from this voltage level. The main point is that the setpoint should
equate to the limiting voltage at the limiting
Clarify that LOCA sequencing is evaluated using component during the bounding starting or
minimum switchyard voltage as starting point. running scenario to protect the 1E
equipment.
126 Page 6 Nextera 2 Having a sustained degraded voltage just above Agree.
the LVR voltage setting (70%) is not practical
without grid collapse and does not exist in Branch The DVR setpoints are calculated based only
Technical Position #1 (PSB-1). on the voltage requirements of the 1E
equipment, not based on whether the grid
Clarify degraded voltages to be analyzed to a can sustain voltage at levels that result in
credible level. such conditions.
127 Page 6 Nextera 3 The statement that the DVR ensures adequate
operational (starting and running) is the first time in Disagree
NRC correspondence that starting equipment at
the DVR setpoint is expected. The example letter The dropout setting should be developed
sent to Peach Bottom in June 1977 did not require based on the voltage requirements (starting
starting of equipment at the DVR setpoint. This and running) and therefore to develop values
requirement should be removed from the RIS since that are bounding, the studies should be
it is not possible to start equipment at the DVR done under worst starting and loading
setpoint and not subsequently separate from offsite conditions, which means the required voltage
power. If the equipment starts at the DVR setpoint, at the 1E bus prior to the start or run case
the voltage will dip during the transient and must would have to be higher than the setpoint.
then recover above the reset point to avoid The main point is that the setpoint should
separation from offsite power. Since the reset point equate to the limiting voltage at the limiting
will always be above the DVR dropout point it will component during the bounding starting or
be impossible to reset the relay. running scenario to protect the 1E
equipment.
Remove starting of equipment at the DVR setpoint
as a requirement.
128 Page 7 Nextera 4 It is agreed that no credit is to be taken for voltage
controlling equipment external to the Class 1 E Agree
distribution system for the establishing the
degraded voltage relay (DVR) settings; however, it Additional wording has been added to the
should be clarified that for credit may be taken for Offsite/Plant distribution discussion to make
minimum switchyard voltage/voltage drop it more clear that equipment like automatic
calculations (or the Offsite/Onsite Design Interface load tap changers can be credited if the
Calculations). response time will support normal operation.
Clarify that credit may be taken for automatic load
tap changers and/or capacitor banks for minimum
switchyard voltage/voltage drop calculations (or the
Offsite/Onsite Design Interface Calculations).
129 Page 8 Nextera 5 NRC Generic Letter 79-36, Enclosure 2, Item 2
states that It is recommended that "For multi-unit
stations a separate analysis should be performed
for each unit assuming (1) an accident in the unit
being analyzed and simultaneous shutdown of all
other units "Offsite/Onsite at the station; or (2) an
anticipated transient in the unit being Design
analyzed (e.g., unit trip) and simultaneous
42
No. Section of RIS Originator Specific Comment NRC Resolution
shutdown of all Interface other units at that station,
whichever presents the largest Calculations", load
situation."
Comment:
NRC Draft RIS re-states NRC GL 79-36 verbatim, Disagree
with an attempt to clarify "anticipated transient' by
adding in parenthesis "(anticipated operational The reference added is the wording used in
occurrence)" immediately afterwards. It is not clear GDC 17 (for consistency).
what the added parenthetical statement is meant to
convey, other than unit trip (which already exists in
GL 79-36).
It is recommended that this either be removed, or
stated "anticipated transient per station licensing
basis".
130 Page 8 Nextera 6 NRC should clarify "simultaneous shutdown" with
consideration to:
Most multi-unit station's Licensing Basis consider
an "orderly or controlled safe shutdown" of the Disagree
other unit(s) not being analyzed.
NERC Std TPL-004-0; particularly Category D This wording in the RIS is the same as was
events per Table 1, where a "loss of all generating used in GL 79-36.
units at a station" may result in "portions or all of
the interconnected systems may or may not TPL-004 requires transmission planning to
achieve a new, stable operating point'. address simultaneous multiple transmission
contingencies.
IEEE Std 308-1974, Clause 8, subclause 8.1.1
"Capacity' describes this as a "concurrent safe The plant licensing basis provides basis for
shutdown on the remaining units". analyses related to multi unit sites.
The wording for the proposed RIS, subclause 2.c
should be revised to indicate "orderly or controlled
safe shutdown of the remaining units, as per the
station's licensing basis" instead of "simultaneous
shutdown". Alternatively, the wording "shutdown
consistent with the station licensing basis" could be
used instead of "simultaneous shutdown".
131 Page 8 Nextera 7 These guidelines ( e) and f) )seem contradictory
that you cannot credit procedurally controlled Disagree.
operator actions to reduce load but you have to
assume the actions will be carried out when load is These guidelines are not contradictory in that
added. one is considering load shedding (not
conservative) for design of system based on
Delete "e) Manual load shedding should not be maximum load, while the other is about load
assumed" or add allowance to credit procedurally additions that occur per procedure
controlled operator actions to decrease load. (conservative for maximum loading design).
132 General TVA Comment: The RIS suggests that demonstrating
adequate motor starting voltage is a reasonable
objective or outcome of a setpoint calculation for a
Degraded Voltage Relay whose purpose is to Disagree
protect Class 1 E equipment.
TVA's position is that such an objective or outcome
is not technically achievable for the reasons
discussed below:
1) A Voltage Relay is not a Predictive Device
The DVR dropout setting should be
Voltage sensing equipment cannot provide a developed based on the voltage
predictive function without crediting the capacity or requirements (starting and running) and
capability of the upstream system, since it cannot therefore to develop values that are
determine the capacity or capability provided bounding, the studies should be done under
during a transient condition such as a motor start. worst starting and loading conditions, which
Since the existing regulatory framework for means the required voltage at the 1E bus
43
No. Section of RIS Originator Specific Comment NRC Resolution
degraded voltage protection was based on use of prior to the start or run case would have to
voltage relaying, it is not clear how the existing be higher than the setpoint. The main point is
relaying equipment could be used to demonstrate that the setpoint should equate to the limiting
compliance with an adequate motor starting voltage at the limiting component during the
demonstration requirement. bounding starting or running scenario to
protect the 1E equipment. In addition, the
2) A Degraded Voltage Relay Protection Setpoint time delay would be determined based on
Based on Starting Voltage Does Not Provide Motor the limiting starting transient duration only
Protection (not based on allowing time for operator
action). In this manner, if the voltage drops
This method could potentially be calculated but below expected values during starting
would mean that the DVR setpoint would have (based on the 1E equipment limits) and
been determined during the starting of the most prolongs the start transient, then the DVR
limiting Class-I E motor. A degraded voltage relay will timeout and separation will occur
setpoint based on a motor starting would not (providing low starting voltage protection).
protect the motor from damage (required by
regulations) or preclude tripping of the motor's
over-current device(s) prior to transferring to the
onsite power supply (required by regulations). This
is because the DVR time delays are (by definition)
required to be longer than a motor starting transient
(1st time delay) and long enough to allow operator
intervention (2nd time delay). If starting of the
limiting (worst-case) motor was attempted in a true
degraded voltage situation (even slightly below the
DVR setpoint), the DVR scheme could not perform
either of these protective functions prior to tripping
the normal overcurrent relays. Therefore, this
would not provide any additional protection for the
Class-1 E loads.
133 General Progress Background: The draft NRC Regulatory Issue
Energy 1 Summary, 2011 -xx, Adequacy of Station Electric
Distribution System Voltages, describes a
methodology of implementation for degraded
voltage relay schemes that would impose
"Additional Conservatisms" into the settings and
time delays in an effort to further reduce the risk of Disagree.
degraded voltage operation on nuclear plant safety
related / accident mitigating electrical equipment. Additional conservatism should not be added
for the sake of adding conservatism.
"Additional Conservatisms" from this point of view Conservatism is typically added to
tends to mean that the degraded voltage relaying compensate for assumptions that cannot be
will actuate earlier in a degraded voltage event time accurately verified or proven (e.g. cable
line - meaning it would be set to actuate at a higher impedances when actual pull lengths are not
degraded voltage and/or with a shorter time delay. known).
.
In conflict with the NRC's desire to impose NERC and FERC guidelines are reviewed by
additional conservatisms on degraded voltage NRC staff for applicability to nuclear plant
protection at nuclear power plants, the North operation.
American Electric Reliability Corporation (NERC),
is developing a national standard for Frequency The DVR and loss of voltage relay settings
and Voltage Excursion Ride- Through Performance should not be in conflict with NERC or FERC
(PRC-024) for all generating stations in North recommended guidelines for grid operations.
America. The Voltage Excursion Ride-Through
Time Duration Curves currently proposed by the
NERC Standards Drafting Team shows the
competing desire for nuclear power plants to be
capable of riding through a grid induced voltage
transient without tripping.
Actuation of the degraded voltage relaying in a
nuclear power plant during a grid induced voltage
transient results in a temporary loss of power to the
safety related loads powered from the plant buses
until the loads are realigned to an emergency
power source and reenergized. This temporary loss
of power will result in a trip of the nuclear plant in
many cases and a significant challenge to
44
No. Section of RIS Originator Specific Comment NRC Resolution
continued operation of the plant in all cases.
An analysis of current settings and time delays for
several nuclear plant loss-of-voltage and degraded
voltage relay schemes against the proposed NERC
ride through criteria shows that existing settings are
already in conflict with the proposed ride-through
criteria.
Imposition of additional conservatisms into the
relay settings and time delays for nuclear
plant equipment degraded voltage protection will
further complicate efforts to coordinate
NRC required degraded voltage protection
schemes with NERC voltage transient ridethrough
capability needs.
Comment: Please coordinate NRC Staff proposed
degraded voltage relay setting
methodology changes with NERC proposed
voltage transient ride-through capability
standard (PRC-024) by engaging with NERC under
the current NRC - NERC Memorandum of
Agreement (MOA).
134 General Progress Background: The use of on-load automatic load tap
Energy 2 changing transformers for nuclear plant offsite
power supplies would aid in minimizing auxiliary
bus under voltage or degraded voltage transients
of concern to the NRC while also improving the
voltage transient ride through capability of the
nuclear plants that is of concern to NERC.
Comment: Please revise the RIS to allow the Agree
nuclear plants to use and take credit for on load
automatic load tap changing transformers for
nuclear plant offsite power supplies to prevent Load tap changers help improve voltage
degraded voltage events and improve the voltage regulation for normal plant operation. Load
transient ride through capability of the nuclear tap changers do not help protect safety
plants. related equipment during degraded grid
conditions.
Additional wording has been added to the
Offsite/Plant distribution discussion to make
it more clear that equipment like automatic
load tap changers can be credited for normal
plant operation.
135 General Progress Comment: Please also consider the italicized
Energy 3 changes below: Agree
DVR Setting Design Calculations Italicized changes suggested were
considered along with other similar
Licensee voltage calculations should provide the comments received from other stakeholders
basis for their DVR settings, ensuring safety related and clarified in the RIS revision.Italicized
equipment is supplied with adequate operating changes were not incorporated in the RIS
voltage (typically a minimum of 0.9 per unit voltage since the comments were not consistent with
at the terminals of the safety related equipment per the staffs existing guidance for DVR
equipment manufacturers requirements), based on settings.
bounding conditions for the most limiting safety
related load (in terms of voltage) in the plant.
These voltage calculations should model the plant
safety related electrical distribution system such
that the limiting voltage at the bus monitored by the
DVR can be calculated in terms of the voltage at
the terminals of the most limiting safety related
component in the plant. These models would allow
calculation of voltages at terminals (delete "or
contacts ") of all safety related equipment with the
voltage at the DVR monitored bus at the DVR
dropout setting, providing the necessary design
45
No. Section of RIS Originator Specific Comment NRC Resolution
basis for the DVR voltage settings.
In this manner, independent of voltage controlling
equipment external to the plant safety related
electrical distribution system, the DVR ensures that
all safety related equipment can continue to
operate at the degraded voltage relay drop out Contacts was deleted.
setting if previously in service, small loads will not
be damaged if successfully started at the degraded
voltage without DVR time out (the DVR either does
not drop out or resets before time out because the
started load is small), and larger loads will not be
damaged or trip on overload /protective relaying if
starting the equipment results in sustained
degraded voltage for the duration of the DVR time
delay (the DVR drops out and does not reset
because the load is large but the DVR timer times
out and sheds the load from the degraded voltage
source before the overloads and/or protective
relaying actuates).
For the purposes of this DVR Setting Design
calculation, no credit should be taken for voltage
controlling equipment external to the Class 1 E
distribution system such as automatic load tap
changers and capacitor banks because these
devices normally prevent degraded voltage from
occurring and thus, by definition, should not be
included in a bottom up analysis to determine
minimum voltage requirements for the safety
related loads. Voltage time settings for DVRs
should be selected so as to avoid spurious
separation of safety buses from the offsite power
system during unit startup, normal operation and
shutdown. These DVRs should disconnect the
Class 1 E buses from any power source other than
the emergency diesel generators (onsite sources) if
the degraded voltage condition exists for a time
interval that could prevent the Class 1 E safety
related loads from achieving their safety function.
The DVRs should also protect the Class 1 E safety
related loads from prolonged operation below
sustained degraded voltage which could result in
equipment damage.
The licensees should demonstrate that the existing
DVR settings including allowable values and time
delays are adequate so that safety related loads
can continue to operate at the degraded voltage
relay drop out setting if previously in service, small
loads will not be damaged if successfully started at
the degraded voltage without DVR time out, and
larger loads will not be damaged or trip on
overload/protective relaying if starting the
equipment results in sustained degraded voltage
for the duration of the DVR time delay resulting in
separation from offsite power and realignment to
the emergency onsite power supply. The time-
delay(s) chosen for DVRs during accident
conditions should be short enough to meet the
accident analyses assumptions and allow for
proper starting of all Class 1E safety related
equipment assuming that the DVR time delay times
out and the accident mitigating loads realign to the
onsite emergency power supply. Also, the time
delay chosen for DVRs during non-accident
condition must be short enough to not cause any
degradation of the safety related components,
including actuation of their protective devices.
46
No. Section of RIS Originator Specific Comment NRC Resolution
136 General Progress Comment: Please also consider the italicized
Energy 3 changes below: Agree
Offsite/Onsite Design Interface Calculations
The offsite power source is the preferred source of Italicized changes suggested were
power to safely shut down the plant during design considered along with other similar
basis accidents, abnormal operational occurrence, comments received from other stakeholders
and reactor trips. The licensee's voltage and clarified in the RIS revision.Italicized
calculations should provide the basis for proper changes were not incorporated in the RIS
operation of the plant safety related electrical since the comments were not consistent with
distribution system, when supplied from the offsite staffs existing guidance for offsite/onsite
circuit (from the transmission network). These design interface calculations.
calculations should demonstrate that the voltage
requirements (both starting and operational
voltages) of all plant safety related systems and
components are satisfied based on operation of the
transmission system and the plant onsite electric
power system during normal, startup, shutdown,
accident mitigation, and alternate authorized
operating configurations of transmission network
and plant systems. In this way, all safety related
systems and components will function as designed
with proper starting and running voltages during all
plant conditions and the DVRs will not actuate
(separating the transmission network supply).
Following are guidelines for voltage drop
calculations derived from Generic Letter 79-36,
which have been supplemented to add clarifying
information. They do not represent new NRC staff
positions.
137 General Progress Comment: Please also consider the italicized
Energy 3 changes below: Agree
Guidelines for voltage drop calculations
a) The plant voltage analysis, while supplied from Italicized changes suggested were
the transmission network, should be based on the considered along with other similar
operating voltage range of the transmission comments received from other stakeholders
network connection. This transmission and clarified in the RIS revision.Italicized
owner/operator supplied voltage range should changes were not incorporated in the RIS
address normal, startup, shutdown, accident since the comments were not consistent with
mitigation, and alternate authorized transmission staffs guidance provided in Generic Letter
network and plant system operating configurations 79-36.
and should also include voltage drop due to the
bounding worst case transmission system single
contingency (transmission system contingencies
include trip of the nuclear power unit). Normally in-
service and periodically tested non-safety related
equipment (such as automatic load tap changing
transformers that regulate voltage during changing
conditions) are to be included in the analysis.
b) Separate analyses should be
performed assuming the power
source to the safety buses is (1) the
unit auxiliary transformer; (2) the
startup transformer; and (3) other
available connections (e.g., from all
available connections) to the offsite
network one by one assuming the
need for electric power is initiated
by (1) an anticipated transient such
as a unit trip (e.g., anticipated
operational occurrence), or (2) an
accident, whichever presents the
bounding load demand on the
power source.
47
No. Section of RIS Originator Specific Comment NRC Resolution
138 General STARS "Statement of Staff Positions Relative to
Emergency Power Systems for Operating
Reactors"
RIS 201 1-XX states that "the NRC required
licensees to install degraded voltage protection
schemes ... as described in NRC Letters dated
June 2 & 3, 1977, 'Statement of Staff Positions
Relative to Emergency Power Systems for
Operating Reactors,' which were sent to all
licensees of all operating nuclear power plants. As
an example, see the NRC letter dated June 2,
1977, ADAMS Accession No. ML100610489, sent
to the licensee for Peach Bottom Atomic Power
Station." (Ref. 2) However, the RIS does not Disagree
recognize the latitude in response allowed to each
Licensee:
"We request that you compare the current design
of the emergency power systems at your
facility(ies) with the Staff Positions stated in the
enclosure and:
(1) propose plant modifications as necessary to
meet the Staff Positions, or
(2) provide a detailed analysis which shows your
facility design has equivalent capabilities and
protective features.
Additionally, we require that certain technical
specifications be incorporated into all facility
operating licenses."
Observations:
1. The NRC letters request some actions and
require some actions - specifically - a technical
specification change.
2. The response makes allowance for varied
responses that account for "equivalent capabilities
and protective features." These varied responses
become part of the licensees' Current Licensing
Basis.
NRC staff does not agree with this position.
3. Licensees were required to change their As a result of these Millstone events, the
operating license because the staff position. NRC requested that all licensees implement
However, this in and of itself, does not change the degraded protection as described in the
licensees' Current Licensing Basis. 1977 Letter to ensure automatic protection of
safety buses and loads. This Letter provides
4. The "1977" letters apply only to addressees, i.e., staff positionguidance, which applies to all
plants licensed before 1977. operating reactors at that time and plants
licensed since, on how to comply with the
Adequacy of Station Electric Distribution System requirements in 10 CFR Part 50, General
Voltages Design Criteria 17 (GDC 17).
The technical content, with some modifications, of
the "Statement of Staff Positions Relative to
Emergency Power Systems for Operating
Reactors" was put in the Branch Technical Position
(BTP) of the Standard Review Plan (SRP/NUREG-
0800), PSB-1, Revision 0, "Adequacy of
Station Electric Distribution System Voltages,"
dated July 1981, and in the current BTP 8-6 of
the SRP, Revision 3, "Adequacy of Station Electric
Distribution System Voltages," dated March
2007.
48
No. Section of RIS Originator Specific Comment NRC Resolution
1. Branch Technical Positions of NUREG-
0800 are not requirements but:
"represent guidelines intended to supplement the
acceptance criteria established in
Commission Regulations, guidelines presented in
Regulatory Guides, and recommendations
presented in applicable IEEE standards."
2. PSB-1 and BTP 8-6 provide subtle but
significant changes to each other and to
the original "Statement of Staff Positions
Relative to Emergency Power Systems
for Operating Reactors" (Note: these
differences will be provided in a
comment letter from the Nuclear Energy
Institute). If the original statement of staff
positions is considered a requirement,
then it is contradictory to subsequent
NRC guidance.
3. PSB-1 and BTP 8-6 represent guidance as
committed to in a licensees' Current
Licensing Basis - which, with plant specific
justification, may depart from NRC
guidelines, but are reviewed and approved by the
NRC.
139 General STARS By characterizing the new contents of RIS 2011 -
XX as clarifications to "the NRC staff's technical Disagree
position on existing regulatory requirements," the
RIS seeks to supersede the NRC reviewed and The purpose of the RIS is to clarify the NRC
approved Current Licensing Basis for many staffs technical position on existing
licensees. regulatory requirements and voltage studies
necessary for Degraded Voltage Relay
(second level undervoltage protection)
setting bases and Transmission
Network/Offsite/Onsite station electric power
system design bases. This RIS does not
transmit any new requirements or staff
positions.
A RIS is an appropriate document for NRC
staff to provide clarification on existing
Regulatory Requirements and existing NRC
Staff Positions.
140 General Greg The issue I am concerned about is the regulatory
Reimers conflict created by requiring the DVR setpoint to
(DCCP) preclude spurious actuation of the undervoltage Agree
protection function.
The NRC draft RIS 2011-XXX discusses spurious
separation at least three times.
1. The first occurrence is an accurate NRC Staff agrees with commenters position
restatement of the 1977 NRC position on use of the term spurious with respect to
that "The voltage protection shall include the design of the DVR scheme to prevent
coincidence logic to preclude spurious false actuations due to DVR component
trips of the offsite power source" (See failures or miss-operations
RIS Page 2, Item (b)).
2. The second occurrence is in the
"Degraded Voltage Relay Design The RIS will be revised to remove spurious
Calculations" section. Specifically, the from this section. The NRC Staff position is
second sentence of the first paragraph that the settings are to be selected based on
on Page 7 reads "Voltage-time settings the voltage requirements of the 1E
for DVRs should be selected so as to equipment such that when compared with
avoid spurious separation of safety the minimum expected grid voltages, there
buses from the offsite power system should be sufficient margin ensure that
during unit startup, normal operation and separation from the grid would not be
shutdown." This introduces the DVR expected during normal, abnormal or
49
No. Section of RIS Originator Specific Comment NRC Resolution
voltage and time setpoint interaction with accident conditions.
the offsite power circuits as a factor in
the setpoint determination. I believe a
conclusion of the workshop was a
common understanding that the
functional requirement of the DVR
protection is to prevent common mode
equipment failure during a sustained
degraded voltage condition. As
discussed, this can best be achieved via
a "bottom up" analysis without
consideration of offsite power capacity
and capability.
3. The third occurrence is in the
"Offsite/Onsite Design Interface
Calculation" Section. Page 8, Item (i)
reads "For each case evaluated, the
calculated voltages on each safety bus
should demonstrate adequate voltage at
the component level without separation
from the offsite circuit due to DVR
actuation."
Points #2&3 above introduce a contradiction for
those stations whose current license basis is
consistent with the Standard Technical
Specifications. Referring to NUREG-1431,
Standard Technical Specifications Westinghouse
Plants (typical TS), the degraded voltage TS bases
read "The Allowable Value is considered a limiting
value such that a [DVR] channel is OPERABLE."
Thus, at the Allowable Value lower limit, the Class
1E electrical distribution system is capable of
fulfilling its ESF supporting design function. The
offsite power LCO reads "Each offsite circuit must
be capable of maintaining rated frequency and
voltage, and accepting required loads during an
accident, while connected to the ESF buses." No
voltage values are defined for the offsite power TS
LCO. Therefore, if the offsite power circuit can
maintain the bus voltage such that the DVR lower
Allowable Limit is satisfied, then the offsite power
circuit would also be operable.
The DVR dropout and reset setpoints must be
greater than the TS lower Allowable Value due to
instrument tolerances and uncertainty. Given the
DVR favors the DGs, does not mean bus voltages
between the DVR setpoint and the TS lower
Allowable Value reduce the capability of the offsite
power circuit. Consequently, the DVR setpoint
cannot completely preclude spurious separation.
As discussed in the workshop, a voltage relay NRC Staff agrees with commenters position
cannot predict future operating conditions. on use of the term spurious with respect to
Consequently, the DVR can't distinguish between the design of the DVR scheme to prevent
voltage transients that are expected to recover and false actuations due to DVR component
those that are not. Therefore, in the context of the failures of miss-operations
original NRC position (i.e. Point #1), I believe the
term spurious was in the context of false signals NRC Staff agrees with the commenters
from within the DVR instrumentation and not any position that the intent of item i) is to specify
group of bus voltage transients. The IEEE 308 that the preferred power supply is able to
requirement that RIS Page 8, Item (i) is trying to start and run all required 1E equipment in
convey is "The preferred power supply shall be accordance with its voltage requirements
capable of starting and operating all required while not separating
loads."
Backup power Brian Wilson, Why are there not back-up power sources located
141 options CA on the roof of the fuel cell tanks with electric lines Disagree
50
No. Section of RIS Originator Specific Comment NRC Resolution
connected directly to the pumps that cool the fuel
rods back-up power sources run on both methane This comment is not related to the RIS
or propane and Ipg. A remote control panel from a regarding Adequacy of Station Electric
distant site would provide a safe environment to Distribution System Voltage. Therefore,
control a dangerous situation safely. staff did not address the comment.
51
Supplemental Response to NEI Comment No. 96 in Comment/Resolution Table (corrected) Formatted: Font: Italic, Underline
RESPONSES TO NEI 3-2-2-11 BACKFITTING COMMENTS Formatted: Centered
RIS On Adequacy of Station Electric Distribution Voltages Formatted: Centered
Comment: The RIS represents an NRC attempt to standardize varied approaches to providing protection during
degraded grid voltage conditions, as currently memorialized in the licensing bases of individual plants. However,
given the complex regulatory and licensing history associated with providing degraded grid voltage protection at each
plant, a conclusion that the guidance in the proposed RIS is consistent with prior NRC guidance is insufficient to
meet the requirements of the Backfit Rule. (NEI - pp.2-7)
NRC Response: The NRC interprets the comment as stating a general principle: if the NRC proposes to issue
generic guidance applicable to several plants - each of which has a complex regulatory and licensing history, then the
NRC complies with the Backfit Rule only if its backfitting discussion for the proposed generic guidance considers and
addresses the licensing basis for each affected plant.
The NRC disagrees with the comment, and does not believe that the NRC should, as a matter of policy, adopt such a
principle to guide the agencys implementation of the Backfit Rule. Application of such a principle would oftentimes
impose substantial resource burdens on the NRC, inasmuch as the NRC currently has no efficient way of easily
compiling and reviewingas it is difficult for the NRC to efficiently compile and review the licenses bases of selected
plants on a comparative basis. The more complex the regulatory licensing history for each licensees plant, the more
resource intensive it would be for the NRC to prepare a generic backfitting discussion that essentially constitutes a
collection of plant-specific licensing basis reviews. Upon completing the licensing basis review for each plant to which
the generic guidance is applicable, the NRC (and licensee) may well conclude that imposition of the guidance would
not represent backfitting. In that situation, the review would constitute an arguably unnecessary expenditure of NRC
1
(and licensee ) resources.
The NRC believes a more sensical approach is: if the NRC has generally maintained a consistent position (or at least Formatted: Font: 9 pt
expressed no contrary position) and has implemented that position in a consistent manner, then the NRC may issue
guidance as a statement of position, and prepare any necessary backfit discussion in connection with any NRC
action which imposes the guidance on a licensee who claims that the imposition constitutes backfitting. The NRC
action may be, among other things, a NRC determination of a license amendment application, the issuance of a
notice of violation, or issuance of an order directing the licensee to comply with the guidance. In this manner, the
specific regulatory and licensing history for that plant can be compiled and evaluated by the NRC as part of the
NRCs mandated backfitting consideration.
Consistent with this general principle, the NRC has (as part of the backfitting consideration of this RIS) reviewed its
records with respect to generic guidance on GDC017 and degraded voltage protection. Based upon that review, the
NRC believes its generic guidance on GDC-17 and degraded voltage protection has been consistent over time.
Thus, the positions presented in the degraded voltage RIS do not constitute generic backfitting. In addition, the NRC Formatted: Font: Italic
2
has reviewed its records and believes that (with one exception which is being dealt with in a plant-specific manner ),
Formatted: Font color: Auto
the NRC has taken a consistent position on a plant-specific basis. The NRC is currently in the process of applying
the guidance to a specific plant, and is preparing the appropriate documentation required by the Backfit Rule. Hence,
the NRC concludes that plant-specific evaluation of the backfitting implications of this RIS is not appropriate. Formatted: Font: 9 pt
Therefore, the NRC believes it makes more sense, that if the NRC has generally maintained a consistent position (or
at least expressed no contrary position) and has implemented that position in a consistent manner, then the NRC
may issue guidance as a statement of position, and prepare any necessary backfit discussion in connection with any
NRC action imposing the guidance on each licensee. The NRC action may be, among other things, a NRC
determination of a license amendment application, the issuance of a notice of violation, or issuance of an order Formatted: Font: (Default) Arial, 9 pt
directing the licensee to comply with the guidance. In this manner, the specific regulatory and licensing history for that Formatted: Font: (Default) Arial, 9 pt, Not
plant can be compiled and evaluated by the NRC as part of the NRCs mandated backfitting consideration. Superscript/ Subscript
For this RIS, the NRC reviewed the regulatory guidance and NRC practice in this area, and concluded that the Formatted: Font: (Default) Arial, 9 pt
guidance in the RIS is consistent with the NRCs regulations and staff guidance, as well as the NRCs approval of Formatted: Font: (Default) Arial, 9 pt, Not
Superscript/ Subscript
1
If the NRC prepared a detailed discussion of a plants licensing bases to support the issuance of guidance, then it Formatted: Font: (Default) Arial, 9 pt, Font
would be likely that the licensee would be required to verify the NRCs discussion. color: Auto, Not Superscript/ Subscript
2
This involves the Edwin C. Hatch (Hatch) Plant, for which the NRC staff has prepared a documented evaluation
supporting the invocation of the compliance exception (included in NRC Inspection Report 05000321 and Formatted: Font: (Default) Arial, 9 pt, Not
Superscript/ Subscript
366/2011009, ML1114507930). The NRCs reliance on the compliance exception is the subject of a backfit appeal by
Southern Nuclear Operating Company, the licensee of the Hatch Plant (ML111680360). The NRC has denied the Formatted: Font: (Default) Arial, 9 pt, Font
backfit appeal (ML112730194). color: Auto, Not Superscript/ Subscript
52
electrical systems in various licensing actions (with one exception which is being dealt with in a plant-specific
3
manner ). Hence, the NRC concludes that plant-specific evaluation of the backfitting implications of this RIS is not
appropriate.
The NRC emphasizes that this approach to addressing backfitting in connection with the issuance of guidance (such
as this RIS) would not be appropriate if the NRC does not have reasonable certainty that it has articulated (or at least
expressed no contrary position) and implemented a consistent position over time. However, as stated above, such is
not the case with the guidance on degraded grid voltage protection contained in this RIS. Thus, the degraded voltage
RIS does not constitute generic backfitting because it does not constitute a new or different generic NRC staff
position. If there is plant-specific backfit when applying the guidance to a specific plant, then the staff will address
backfitting in the context of that staff action (and prepare the necessary documentation to support the staffs
backfitting action). Accordingly, the NRC declines, as a matter of discretion, to prepare a plant-specific backfitting
discussions for each of the plants that may be affected by the RIS. No change was made to the RIS as a result of this
comment.
Comment: The NRC should not rely upon GDC-17 to conclude that the proposed RIS does not represent backfitting.
GDCs are cast in broad, general terms and are non-specific nature. Thus, the relevant backfitting inquiry should be
whether the proposed RISs guidance differs from any individual plants NRC-approved voltage protection scheme.
(NEI - pp.6-7) (See NRC Comment/Resolution table item No. 96)
NRC Response: In general, the NRC agrees with the comment that many (but not all) of the GDC are cast in broad,
general, performance-based language. However, the NRC disagrees with the comments implicit assertion that any
GDC which is expressed in broad, general terms or is non-specific in nature, or is expressed in performance-based
language, may not be relied upon when the NRC invokes the compliance exception under § 50.109(a)(4)(i). Nothing
in the history of the Backfit Rule suggests that the Commission intended to adopt such an interpretation of the Backfit
Rule.
Furthermore, if the NRC were to adopt the position that NRC issuance of changed or new guidance for performance-
based regulations must always be accompanied by plant-specific licensing basis analyses for all potentially-affected
plants, this would likely inhibit the adoption of performance-based rules. If the NRC must take into account the plant-
specific licensing basis considerations whenever it proposes to take generic regulatory action through the issuance of
rules and regulations, then this effectively converts the administrative process of rulemaking under the Administrative
Procedures Act (APA) to the administrative process of issuance of orders under the APA. The NRC does not see why
it must limit itself, in light of the several and broadly-worded rulemaking authorities accorded the NRC under various
provisions of the Atomic Energy Act of 1954, as amended. These rulemaking provisions include, among others,
Sections 103.a, 103.b, 161.i, 161.p, 182.a, and 183.
Moreover, the GDCs are just one NRC position for which a licensee is protected from backfitting. If there is a more
specific requirement in a plants licensing basis representing the NRC position on an acceptable way of compliance
with the performance-based requirement (e.g., a GDC), then that position is a separate and independent basis for
3
that licensee to invoke backfitting protection when the NRC proposes to impose that NRC position on that licensee.
More importantly, the NRC disagrees with the comments implicit assertion that GDC-17 is a broad, general, and
nonspecific GDC. A brief review of GDC-17 shows that its regulatory text is among the most lengthy of the GDCs,
consisting of three paragraphs. The requirements are stated in relatively specific terms using performance-based
regulatory language, but also setting forth very specific conceptual requirements, e.g., the need for electric power
from the transmission network to the onsite electrical distribution system to be supplied by two physically
independent circuits (not necessarily on separate rights of way).
For these reasons, the NRC disagrees that the relevant inquiry for GDC-17 is whether the proposed RIS differs from
any individual plants licensing and regulatory bases. No change was made to the RIS as a result of this comment.
backfit appeal by Southern Nuclear Operating Company, the licensee of the Hatch Plant (ML111680360).
3
The situation involving the imposition of a new NRC position on an acceptable way of complying with GDC-17 is
an example where the NRC acknowledges that the NRC approval of the licensees specific undervoltage protection
scheme) in a license amendment constitutes an applicable staff position for purposes of the Backfit Rule. Thus, the
NRC has acknowledged that the proposed new staff position on the Hatch Plants undervoltage protection scheme
constitutes backfitting. Thus, the issue which is the subject of the licensees backfit appeal, is whether the NRC may
rely upon the compliance exception under 10 CFR 50.109(a)(4)(i).
53
Comment: The NRCs generic communications and guidance identified in the draft RIS are not completely consistent
with one another and were not equally relevant in developing the licensing bases for all reactor licensees, citing to
various statements in the BTP PSB-1. (NEI - pp. 4-5) (See NRC Comment/Resolution table item No. 96)
NRC Response: The NRC agrees that there is some variability between the scope of depth of information presented
in the NRCs generic communications and guidance on the subject of degraded voltage protection. However, the
NRC disagrees with the comments implicit assertion that there is no consistent NRC staff position. As discussed in
response to the prior comment, there is some variability among the NRC generic communications and guidance
documents in terms of the scope of issues relating to electrical system design, as well as the detail provided. The
comment provided four bulleted examples purporting to describe inconsistent or contradictory NRC guidance.
However, none of the identified examples set forth statements which are clearly contradictory or implicitly inconsistent
with one another.
The first example identifies an apparent conflict between a draft RIS statement that degraded voltage conditions
[must be] coincident with a postulated design basis accident, and a statement for Branch Technical Position (BTP)
PSB-1 which states that The subsequent occurrence of a safety injection signalshould immediately separate the
Class 1E distribution system from the offsite power system (emphasis added). There is no conflict or inconsistency
between these two statements, when the BTP (which precedes the RIS) is read in full context. The full text of the
relevant portion of the BTP is:
Two separate time delays shall be selected for the second level of undervoltage
protection based upon the following conditions:
1) The first time delay should be of a duration that established the
existence of a sustained degraded voltage condition (i.e.,
something longer than a motor starting transient). Following this
delay, an alarm in the control room should alert the operator to the
degraded condition. The subsequent occurrence of a safety
injection actuation signal (SIAS) should immediately separate the
Class 1E distribution system from the offsite system (emphasis
added).
Coincident means, happening at the same time, and coinciding. The Random House College Dictionary,
Revised Edition (1980). Coincide means, among other things, to come to occupy the same period in time
(emphasis added). Id. Subsequent means either occurring or coming later or after, or, following in order of
succession, succeeding. Id. Coincidence simply requires that the two events or conditions happen at the same time,
or come to occupy the same period in time. It is clear that this is what the RIS was addressing - that the postulated
design basis accident must be assumed to occur at the same time as the degraded voltage (undervoltage) condition.
By contrast, the BTP addresses the order or sequence of occurrence of the two events or conditions which must,
despite the order or sequence, must also happen at the same time. The BTP states that the staff will accept a design
in which the undervoltage condition occurs first, and then (while the undervoltage condition is still present), the design
basis event occurs (and thereby draws a load on the electrical system). In either case, the result is the same in that
separation from the grid occurs and the 1E systems are powered from the onsite sources during design basis event
mitigation. The only difference is that in the case of the subsequent design basis event, the actual start of the 1E
equipment will be later than when the two events occur coincidently. Either way the design basis event assumptions
are satisfied.
Figure 1 illustrates the difference. There is no conflict or inconsistency between the RIS and the BTP.
54
Degraded Voltage Set Points
(BTP PSB-1) DVR -1 Protection - Existence of a sustained degraded
voltage condition
100% To account for ECCS motor starting, running of all 1E loads and time for
grid recovery (Long time Delay.) Causes alarm and Time delay is
bypassed on SI signal if a subsequent SI occurs. Provide adequate time
for plant or grid operator to take manual actions.
Alarm setpoint
significantly
higher than DVR
setpoint (BTP PSB-1) DVR-2 Protection with short time
delay to rideout the motor starting transients.
DVR set point based on minimum
voltage required at equipment terminal Ensures minimum voltage required for all
to operate all safety related equipment equipment to prevent control fuse blowing, relay
lockout, contactor opening etc ., concurrent DVR-
2 condition SI signal causes relay to time out if
the voltage doesnt within the time delay and
bus transfers to onsite power system.
Follow 1977 letter staff position
The selection of voltage and time
delay setpoints shall be determined from an
analysis of the operating voltage requirements of
the safety related loads at all onsite system
distribution levels;
Figure 1
The second example identifies an apparent conflict between the RIS and BTP PSB-1. The draft RIS proposes "DVR
Dropout setting based on starting and running voltage." BTP PSB-1 says "sustained degraded voltage condition,"
during the discussion concerning selection of time delays for the DVR, implying a degraded voltage condition lasting
more than a few seconds and not a transient voltage condition that exists during a motor starting event or during a
momentary grid perturbation such a lightning strike that may be cleared by automatic actions of protection schemes
and automatic breaker open/reclosure cycle. Specifically, BTP PSB-1 states: 'Two separate time delays shall be
selected for the second level of undervoltage protection based on the following conditions: 1) The first time delay
should be of a duration that established the existence of a sustained degraded voltage condition (i.e., something
longer than a motor starting transient)."
It is clear in the context of the 1977 Letter that the phrase sustained degraded voltage is referring to the grid event
and not voltage scenarios in plant electrical system operation (see fig 1). In addition, when selecting an appropriate
time delay for the DVR (which is what the RIS is referring to), one must consider the voltage drop due to large motor
starts in the plant since they can depress voltage momentarily by design and such voltage drops should therefore be
overridden by the time delay since they would mask detection of the sustained degraded grid voltage event. Also, it is
also clear that the 1977 Letter language refers to the DVR voltage setting(s) being based on the voltage requirements
of the 1E equipment. Large 1E motors, for example, have starting and running voltage requirements by design which
must be met to ensure proper operation during accident conditions. Thus, the statement that the BTP implies only
running voltage requirements are required be enforced is not correct. No change was made to the RIS as a result of
this comment. There is no conflict between the RIS and BTP PSB-1.
The third example juxtaposes: (i) the proposed RIS guidance indicating that each unit must have analysis that
assumes an accident in the unit being analyzed and simultaneous shutdown for all other units (emphasis in
original), with language in GDC 5 which states, in the event of an accident in one unit, an orderly shutdown and
cooldown of the remaining units. The language of GDC 5 concerns sharing of SSCs rather than electrical systems,
55
and the language quoted in the comment represents a criterion for allowing sharing of important to safety SSCs
among nuclear power units. This is not the same subject as the determination of how to analyze whether GDC-17s
requirements are being met. Moreover, there is no direct contradiction between simultaneous shutdown in the
proposed RIS, and orderly shutdown and cooldown in GDC 5: an orderly shutdown under GDC-5 could also be
simultaneous for purposes of GDC-17.
The fourth example identifies an apparent conflict between the RIS and BTP PSB-1. The draft RIS proposes "During
normal plant operation, the Class 1E safety related buses should automatically separate from the power supply within
a short interval (typically less than 60 seconds) if sustained degraded voltage conditions are detected." (emphasis
added). The comment suggests that a sixty second time delay would not allow operator actions, and appears to be a
new NRC position.
BTP PSB-1 clause B.1.b.2 included provisions for operator manual actions to restore bus voltage on the Class 1E
distribution system. BTP PSB-1 B.1.b.2 says: "The second time delay should be of a limited duration such that the
permanently connected Class 1E loads will not be damaged. Following this delay, if the operator has failed to restore
adequate voltages, the Class 1E distribution system should be automatically separated from the offsite power
system. Bases and justification must be provided in support of the actual delay chosen." The draft RIS specifically
excludes manual load shedding under the Offsite/Onsite Design Interface Calculations whereas the BTP PSB-1
allows for manual actions to avoid separation from offsite power. The sixty second time delay would not allow
operator actions. This appears to be a new NRC position.
The reference to 60 seconds has been removed in the revised RIS, however, the time delay chosen still has to be
justified in terms of providing automatic protection of the Class 1E equipment in addition to allowing time for operator
actions (if possible). See DVR1 and DVR2 relay and time delays depicted in Figure 1. This is in accordance with BTP
PSB-1 staff position and is an acceptable design to meet the staff position in the 1977 letter, staff position i.e., the
selection of voltage and time delay setpoints shall be determined from an analysis of the operating voltage
requirements of the safety related loads at all onsite system distribution levels. BTP PSB-1 clause B.1.b.2 included
provisions for operator manual actions to restore bus voltage on the Class 1E distribution system. BTP PSB-1 B.1.b.2
says: "The second time delay should be of a limited duration such that the permanently connected Class 1E loads will
not be damaged. Following this delay, if the operator has failed to restore adequate voltages, the Class 1E
distribution system should be automatically separated from the offsite power system. Bases and justification must be
provided in support of the actual delay chosen." The draft RIS specifically excludes manual load shedding under the
Offsite/Onsite Design Interface Calculations whereas the BTP PSB-1 allows for manual actions to avoid separation
from offsite power. Therefore, there is no conflict between the RIS and BTP PSB-1.
Comment: Given the variability in NRCs generic communications and guidance, the draft RIS statement that the
positions in the RIS are consistent with the RIS-identified NRC documents, does not address the possibility that the
RIS is new or different from a previously-approved protection scheme (and thereby constitute backfitting). (NEI - p.7)
NRC Response: As discussed above in response to NEI- pp.4-5, while the NRC agrees that there is some
variability in NRCs communications and guidance on degraded voltage protection, the NRC does not agree that
this variability is of any significance from a backfitting standpoint because there is no contradiction or material
inconsistency between the various NRC communications and guidance documents. Thus, such variability does not
provide a sufficient basis for the NRC to perform plant-specific determination on whether imposition of the RIS
constitutes backfitting. No change was made to the RIS as a result of this comment.
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