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
1
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
Resources, Inc
(ADAMS Accession No.
2. (8-31) Exelon
Generation Company, LLC
(ADAMS Accession No.
3. (32-39) Southern
Nuclear Operating
Company
(ADAMS Accession No.
4. (40-45, 59-85 & 86-96)
NEI
1776 I Street NW
Washington, DC, 20006
(ADAMS Accession Nos.
5. (49-51) Jerry Nicely
Self
(ADAMS Accession No.
6. (52-58) Larry Nicholson
Nextera
(ADAMS Accession No.
7. (97-108) PPL
Susquehanna, LLC
(ADAMS Accession No.
8. (109-124) APS, Palo
Verde Nuclear Generating
Station
(ADAMS Accession No.
9. (125-131) Nextera
Energy
(ADAMS Accession No.
10. (132) TVA
(ADAMS Accession No.
11. (133-137)Progress
Energy
(ADAMS Accession No.
12. (138-139) STARS
(ADAMS Accession No.
13. (140) Greg Reimers/
Diablo Canyon
Email (ADAMS Accession
No. ML112010028)
14. (141) Brian Wilson
(ADAMS Accession No.
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
ISSUES - 1. DVR
Setting Design
Calculations
Dominion
Resources
Services, Inc
Section DVR Setting Design
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.
The approach could imply that the load(s) should
start from the lowest DVR dropout setting. A
specific example for illustration is as follows:
If voltage is at the lowest possible value above
Disagree
NRC Staff has the following clarification with
this position.
2
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
lower than the initial value, then DVR reset cannot
occur.
Many utilities use the ABB 27N with harmonic filter
which has a minimum 0.5% reset. Thus, with a
setting of 93.6% +/- 0.9%, dropout could be as low
as 92.7%. However, for motors causing more than
0.5% voltage dip at initial start, even if the voltage
at the beginning of the event was 93.2% and a load
was started, then DVR will dropout and never reset
causing a separation. A clarification that allows
evaluation of motor starting as well-as other
conditions is:
If the DVR could possibly not cause separation
then the required safety functions must be
performed successfully. Thus, depending on the
design of the plant, a voltage value for beginning
the event with all required starts could be
determined by an iterative process.
If the offsite power has adequate capacity
and capability, any voltage just above the
DVR setpoint should not separate the offsite
power source from the safety bus when
starting large motors. The grid voltage is
expected to recover.
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).
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
ISSUES -- 2 .
Offsite/Onsite
Design Interface
Calculations(page
7)
Dominion
Resources
Services, Inc
This section contains elements that are too
prescriptive. Many analyses will show that the unit
loads/sequences assessed for determination of
DVR setpoint adequacy for equipment protection
are the same as those for evaluating offsite power.
Since evaluating offsite power always involves
higher voltages, it is clear the equipment will
function and providing terminal voltages for this
equipment is bounded by DVR adequacy analysis.
A clarification that helps frame adequate analysis
is:
If the DVR could initiate separation then offsite
power is not operable. Using the example above
when evaluating offsite power would require that
the safety bus recover above 93.6%+0.9%+0.5%
or 95% before the earliest time delay for the DVR
expires. Since a reset also resets the time delay,
multiple DVR drop outs could occur without
separation during load sequencing. Also, since
92.7% was evaluated for equipment protection,
Disagree
NRC Staff has the following clarification for
this position.
To meet GDC 17 requirements, the licensee
must demonstrate capability to safely shut
down the plant for all design basis events
with the grid voltage at the lowest allowable
value as afforded by the transmission system
operator.
The voltage studies done for evaluating
offsite power/onsite power interface should
use minimum expected voltage at the
plant/grid interface node, demonstrating
adequate voltage for starting and running of
plant components during normal, abnormal
and accident conditions. The expected plant
loading at 100% power operation may be
higher than accident loading. Hence the
3
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
evaluating 95% (used for offsite power evaluation)
would require the two sequences be substantially
different (2.3%) for the DVR adequacy evaluation
(at 92.7%) to not be bounding for equipment
evaluation. Thus, providing calculation detail to
motor terminals for offsite power evaluation is
unnecessary in many designs.
voltage drop in the plant auxiliary system will
be higher for normal operating conditions.
The DVR setpoint should be below the
normal operating voltage of the plant to avoid
multiple spurious actuations. A separate
analysis may be needed for DVR setpoint.
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
ISSUES -- 2 .
Offsite/Onsite
Design Interface
Calculations(page
7)
Dominion
Resources
Services, Inc
Part a states:
This transmission owner/operator supplied voltage
range should address all transmission
network and plant system operating configurations
and should also include voltage drop due to
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
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
loss of the unit can vary considerably with system
conditions. Arguments can be made that the worst
case contingency (if different than the nuclear
power unit) should only be considered once it has
occurred. However, a key question should be
applied: Is the contingency of interest monitored? If
the status is not monitored, then how would the
nuclear unit know when to apply the contingency?
Thus, if the status of a key transmission line to the
nuclear unit switchyard is known, either by
instrumentation at the plant or timely notification
by the grid operator, then the contingencies need
only be considered when applicable. When
evaluating voltage drop, most situations which
cause meaningful changes are nearby and can be
monitored.
Disagree
The plant electrical distribution system
should be designed 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.
Contingencies that are beyond design basis
(line outages during peak grid loading
conditions) that occur during plant operation
should be evaluated uniquely to assess the
capability of offsite power to provide
shutdown capability post trip as required by
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
4
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
postulated accidents.
4.
SUMMARY OF
ISSUES -- 2 .
Offsite/Onsite
Design Interface
Calculations(page
8)
Dominion
Resources
Services, Inc
Part c states:
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 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
word used for other units at the station is
"shutdown" which is more orderly and takes more
time. Simultaneous unit trip results in bigger
voltage drops from VAR support (not loading). This
is because VARs are local. System X/R is typically
-50 so it is 50 times more difficult to move a VAR a
hundred miles than a watt. Thus, most analyses
show the worst voltage drop for the loss of the
nuclear unit is when all of the nearby units (any
type of generation) are already off. Changing that
to tripping all units at the same time increases the
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.
Disagree
This statement is consistent with GDC 17,
GL 79-36, and IEEE Standard 308-1971,
Class IE Electrical Systems, Section 8,
Multi-Unit Station Considerations.
Multi-unit sites have been licensed in
accordance with above documents and
should therefore evaluate the plants
according to their licensing basis.
5.
SUMMARY OF
Dominion
Part j states:
5
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
ISSUES -- 2 .
Offsite/Onsite
Design Interface
Calculations(page
8)
Resources
Services, Inc
To provide assurance that actions taken to assure
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
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
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.
Analyses of high grid voltage with light plant load
are standard and provide insights as to what grid
voltage upper limit should be or what compensating
activities might be required for light load operations
(refueling). In those cases where unit trip can result
in a step 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.).
Disagree
The wording is consistent with GL 79-36.
The Offsite/Onsite design should address all
grid operating conditions to prevent
overvoltages from occurring.
The point here is that if a design problem is
identified such as overvoltage conditions,
immediate actions should be taken
(compensatory and/or permanent design
changes) to address the design problem
rather than taking actions after it occurs.
6.
SUMMARY OF
ISSUES (top of
page 6)
Dominion
Resources
Services, Inc
States: This interface calculation establishes
operating voltage bands for all plant electrical
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
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
evaluation of all non-safety loads down to the
lowest levels of distribution.
Agree
The reference to non-safety related is being
removed. However, non-safety related loads
should be modeled to the extent that their
operation can affect safety bus/equipment
voltage.
7.
SUMMARY OF
ISSUES - 1. DVR
Setting Design
Calculations (page
6)
Dominion
Resources
Services, Inc
States:
Licensee voltage calculations should provide the
basis for their DVR settings, ensuring safety related
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
are common based on detailed plant analysis
should be added. As an example, motors below
90% voltage continue to have plenty of margin in
Agree.
90% was mentioned as an example used for
illustration. The voltage values are plant-
6
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
torque but may encroach on long time thermal
limits. However, unless a motor is fully into its
service factor (typically 1.15), as well as below 90%
voltage, operation will be acceptable.
specific. Evaluations like mentioned in this
comment could be acceptable as long as
there is adequate engineering justification.
Ensuring that voltages are within nominal
limits greatly simplifies the analysis required.
8.
General
Background -
Pages 2 and 3
Exelon
Generation
Company,
General Comments:
The RIS uses terms such as "LVR (loss-of-voltage
relay) voltage setting," "DVR (degraded
voltage relay) settings" and "DVR dropout setting"
without clarifying the intent or highlighting the
differences.
IEEE 741-2006, Annex A (Reference 1), has a
discussion on the tolerances to be considered and
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
represented as follows:
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)
distinctly different calculations are required (four if
the site has different DVR time delays for accident
and normal conditions). These would be at different
bus voltage values. The "degraded voltage relaying
design calculations" would be a load flow
performed at the DVR analytical limit; the "plant
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
Disagree.
The terminology used in the RIS is
consistent with the guidance documents.
The setpoint accuracies and methodologies
are beyond the scope of this RIS.
The point of the RIS was to highlight that the
DVR setting and design interface
calculations have different requirements.
The staff agrees that there are other
calculations required to demonstrate the
electrical system design basis.
7
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
advantage of inherent redundancy of the
emergency buses. It should be an owner's option
with respect to coincident logic. Change the "shall"
to "may."
Page 2/3 - The listed 6 criteria are good for setting
the DVR. Early correspondence of the issue
also included a second function for the DVR in that
the design should minimize the effects of
spuriously disconnecting the offsite sources.
Although criteria b) and c)(2) are intended to add
robustness to the design, a few sentences should
be added to the discussion to accentuate the
point.
RIS is consistent with the NRC letter dated
June 2, 1977.
The coincident logic is to ensure that
spurious or inadvertent separation of a
reliable offsite power source. The
redundancy of the safety buses alone does
not address the above concern
Current wording seems adequate to address
the point that spurious trips of offsite power
should be precluded by the design.
9.
SUMMARY OF
ISSUES - 1.
Degraded Voltage
Relaying Design
Calculations (page
6)
Exelon
Generation
Company,
Under "Degraded Voltage Relaying Design
Calculations," the RIS states in part "During normal
plant operation, the Class 1 E 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." Branch Technical Position
PSB-1 clause B.1.b.2 included provisions
for operator manual actions to restore bus voltage
on the Class 1 E distribution system. The
sixty second time delay would not allow operator
actions. This appears to be a new NRC
position.
Disagree.
This is not a new requirement.
RIS will be revised to remove the reference
to auto separate in 60 seconds. The 60
seconds time delay was identified as an
example to illustrate that the time delay
chosen for the sustained degraded condition
(DVR settings) should be short to ensure that
permanently connected Class 1E loads are
not damaged.
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
ISSUES - 1.
Degraded Voltage
Relaying Design
Calculations Page
6
Exelon
Generation
Company,
The next to last sentence under item 1 states: 'The
staff considers degraded voltage
conditions coincident with a postulated design
basis accident to be a credible event. The event
is credible in that it has occurred previously
(although nonaccident). It is acknowledged that
safety loads combined with loss of generator
reactive power support will cause a decrease in
bus voltage. However, if the plant is operated
within the bounds of the operating procedures
(which are reflected in the voltage regulation
calculations as described under the subsequent
section), then the Class 1 E equipment should not
experience a degraded voltage condition.
The sentence can be removed without diminishing
the need for the DVR, or without changing
the intent of this section.
Disagree.
The point being made in the RIS is that
setting of the DVR should include
consideration of a coincident accident, in that
the time delay chosen for the DVR should
support the accident analysis assumptions
consistent with the NRC1977 letter.
Operating a plant within allowable voltage
range should minimize the potential for
degraded voltage conditions on 1E busses.
However, grid perturbations cannot be
predicted. Hence the need for automatic
protection.
11.
SUMMARY OF
ISSUES - 1.
Degraded Voltage
Relaying Design
Calculations -
Exelon
Generation
Company,
DVR Setting Design Calculations - Add a sentence
"The model should utilize loads on the plant
distribution system consistent with the specific
transient or accident being analyzed."
Agree.
The suggested sentence will be added to the
RIS.
8
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
Page 6,
12
SUMMARY OF
ISSUES - 1.
Degraded Voltage
Relaying Design
Calculations -
Page 6,
Exelon
Generation
Company,
In addition, Branch Technical Position (BTP) PSB-1
clause B. 1 .b.2 (Reference 4) included provisions
for operator manual actions to restore bus voltage
on the Class 1 E distribution system. The RIS
specifically excludes manual load shedding under
the Offsite/Onsite Design Interface Calculations
whereas the BTP allows for manual actions to
avoid separation from offsite power. Please clarify
if manual actions taken to restore voltages now
require prior NRC approval.
Disagree
All actions required to protect the Class 1E
equipment from degraded voltage must be
automatic in accordance with 10 CFR
50.55a(h)(2).
Manual actions are allowed as stated in
PSB-1, B.1.b.2 for improving the voltage in
response to the alarm in control room that
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
ISSUES - 1.
Degraded Voltage
Relaying Design
Calculations -
Page 6,
Exelon
Generation
Company,
Under "DVR Setting Design Calculations," the RIS
states in part "...would allow calculation of
voltages at terminals or contacts of all safety
related equipment with the voltage at the DVR
monitored bus at the DVR dropout setting." It is not
clear what "contacts" are in this context. It
is assumed that the concern is motor control center
contactors and/or motor starting control circuits.
Agree
RIS will be revised to just state terminals
and not Contacts.
14
SUMMARY OF
ISSUES - 1.
Degraded Voltage
Relaying Design
Calculations -
Page 6,
Exelon
Generation
Company,
Under discussion of DVR setting calculations, the
RIS states that setting cannot cause any
degradation of the safety related components,
including actuation of their protective devices.
The BTP only stated damage to normally operating
safety related equipment. The RIS language
seems broader then BTP and appears to open up
the position that the DVR studies have to consider
starting of loads under non-accident conditions.
Disagree
The DVR ensures that voltage requirements
of the Class 1E loads are always preserved
for operating the equipment under accident
and non accident conditions including all
abnormal operational occurrences.
15
SUMMARY OF
ISSUES - 1.
Degraded Voltage
Relaying Design
Calculations -
Page 6,
Exelon
Generation
Company,
The DVR time delay seems to be considering
operation down to LVR setting for evaluations.
However, there is no discussion on LVR setting
considerations in any original requirements or the
RIS. Under Guidelines for Voltage Drop
Calculations, the summary states that the plant
voltage analysis, while supplied from the
transmission network, should be based on the
operating voltage range of the transmission
network connection. Grid operating voltage ranges
do not allow operation down to levels that would
cause sustained operation at LVR levels.
Therefore, consideration for operation at the LVR
setpoint would be inconsistent with this guidance.
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.
Disagree.
The point is that the DVR setting is based on
the voltage requirements of the equipment,
which should equate to voltages on the grid
well below normal. It is understood that grid
operating procedures should prevent
sustained voltages at such low levels but
regardless of what happens on the grid the
DVRs will preserve the voltage limits for the
equipment.
Plant operation at LVR setpoint is not
expected and is not within the scope of the
RIS.
16
(17 not
used)
SUMMARY OF
ISSUES - 1.
DVR Setting
Design
Calculations -
Page 6
Exelon
Generation
Company,
In Section "DVR Setting Design Calculation"
reference is made to 0.9 per unit voltage for
adequate operating voltage. This would only apply
for the most part to rotating equipment
(motors). Motor Control Center (MCC) contactors,
battery chargers, Motor Operated Valves
(MOVs) all have less than a 90% operating voltage
requirement. This distinction should be
made and/or clarified.
Agree.
0.9 per unit voltage was mentioned as an
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.
18
SUMMARY OF
ISSUES - 1.
Exelon
Generation
The Degraded Voltage Relaying Design
Calculations section should include a statement to
Agree.
9
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
DVR Setting
Design
Calculations -
Page 6
Company,
emphasize that only steady state loading and
steady state acceptable voltages at the class IE
equipment are to be considered in determining the
DVR drop out settings including the allowable
tolerances. The paragraph does mention 0.9 per
unit voltages at the terminals which is steady state
but a positive statement about steady state loading
and steady state acceptable voltages would be
helpful. Also, there are alternatives to the 90%
terminal voltage criterion. The concern is heating,
causing a temperature rise, which decreases useful
life of the insulation. Inspectors may read too much
into the 90% criterion; a motor loaded to less than
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.
0.9 per unit voltage was mentioned as an
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 suggested analysis may be acceptable if
properly developed and supported in the
design. The 90% voltage criteria may not be
adequate for certain components such as
SOVs, motor control center contactors, etc.
19
SUMMARY OF
ISSUES - 1.
DVR Setting
Design
Calculations -
Page 6
Exelon
Generation
Company,
MOVs are not steady state loads. MOVs have
traditionally been considered transient loads and,
therefore, not included in the steady state voltage
analysis. GL 89-10 (Reference 3) programs
perform these calculations. A statement in this
section that MOVs loads are not to be considered
in this calculation will be helpful if NRC agrees with
this interpretation
Disagree.
MOVs should be addressed specifically and
the DVR settings must support adequate
voltages for all Class 1E equipment including
MOVs.
20
SUMMARY OF
ISSUES - 1.
DVR Setting
Design
Calculations -
Page 6
Exelon
Generation
Company,
The starting voltage requirement is unclear. Some
sites have evaluated the capability of starting each
required safety related motor individually at the
degraded voltage analytical limit. Other sites use a
"block start analysis" where multiple motors are
started simultaneously on the offsite source. There
have been violations associated with both
approaches. The RIS should describe an
acceptable methodology for determination of motor
starting voltage adequacy.
Disagree.
The intent of the RIS is not to prescribe DVR
relay setpoint methodology for every plant.
The RIS provides the conditions for which
plant specific analyses should be performed.
The specific design of the plant dictates the
type of analyses required to demonstrate
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
ISSUES - 1.
DVR Setting
Design
Calculations -
Page 6
Exelon
Generation
Company,
The RIS implies this portion of the calculations
require that the licensee demonstrate that all
class IE motors can be started with the voltages
just above the analytical limit of the DVR
setpoint. However, with voltage just above the DVR
drop out value, any load addition (starting or
running) will result in separating from the offsite
source if no credit for external voltage controlling
equipment is taken. Therefore, the purpose of this
requirement is not clear. The intent of the starting
voltage evaluation should be clarified.
Some stations have evaluated the performance of
protective devices during degraded grid conditions
by mechanisms other than calculations (e.g.,
technical evaluations or computations). It is
suggested that the NRC add a statement for
acceptability of the same.
Disagree
NRC staff disagrees with the interpretation.
The staff agrees that a grid voltage freeze
corresponding to the DVR setpoint and a
subsequent motor start will eventually
separate the plant from offsite source as the
voltage will not recover to reset the DVR.
The 1977 NRC letter states that voltage and
time setpoints shall be determined from an
analysis of the voltage requirements of the
safety related loads. Safety related (Class
1E) equipment, particularly large motors,
have starting and running voltage
requirements.
When grid voltages are degraded (such as
resulting in Class 1E bus voltages down
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
10
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
ISSUES - 1.
DVR Setting
Design
Calculations -
Page 6
Exelon
Generation
Company,
Under Offsite/Onsite Design Interface Calculations,
Guidelines for voltage drop calculations item
2, i), the acceptance criteria for demonstrating
voltage adequacy would appear to be DVR
Maximum Pickup (the voltage required to assure
relay reset) and not component level voltage
values.
Agree.
RIS Section 2 (i) will be modified to state: For
each case evaluated, the calculated voltages
on each safety bus should demonstrate
adequate voltage at the safety bus and down
to the component level. It is based on Class
1E component terminal voltage
requirements.
23
SUMMARY OF
ISSUES - 2.
Offsite/Onsite
Design Interface
Calculations
Exelon
Generation
Company,
Item 2 (Offsite/Onsite Design Interface
calculations) appears to be additional requirements
for those sites licensed to the Standard Review
Plan (NUREG 0800) Chapter 8 Appendix A Branch
Technical Position PSB-1, "Adequacy of Station
Electric Distribution System Voltages."
Disagree
RIS highlights the guidelines provided in GL
79-36 and NUREG 0800, Chapter 8
Appendix A Branch
Technical Position PSB-1, "Adequacy of
Station Electric Distribution System
Voltages.". There are no new requirements
24
SUMMARY OF
ISSUES - 2.
Offsite/Onsite
Design Interface
Calculations
Exelon
Generation
Company,
Page 7- The phrase "...all operating configurations
of transmission network and plant
systems..." appears in a few sentences. The station
interface agreement with the transmission
provider integrates the considerations among the
transmission network, the operability of the
off site sources, and the voltage regulation (drop)
calculations. The calculations identify certain
controlling parameters for the transmission
network. These controlling parameters are then
incorporated into the Bases for the operability of
the offsite source(s). If the plant configuration
or transmission network parameters are not
bounded by the calculations, then the operability of
the offsite sources needs to be examined. In most
cases, the plant operator has no control over
the "configuration" of the transmission network, but
does have agreements with the transmission
system operator that normal operating voltages
and post unit trip contingency voltages are
controlled within established bounds. Add a few
sentences detailing that the intent of the
phrase "all operating conditions of the transmission
network" means that the controlling
parameters from the transmission network that are
used in the calculations are consistent with
those utilized in the Bases for operability of the
offsite sources.
Disagree.
This is addressed in RIS Section 2 a.
As discussed in GL2006-02 Grid Reliability
and Impact on Plant Risk and the Operability
of Offsite Power, licensees are required to
provide the transmission system operator
(TSO), the operating voltage parameters
required by the plant during all modes of
operation. The analyses discussed in this
comment should be the bases for the
information provided to the TSO.
The DVR protects the safety related
equipment when a perturbation in the grid
system results in degraded voltage
conditions and the normal operating
parameters cannot be restored immediately
to protect safety related equipment. The
DVR setpoint evaluation should be a
separate analysis.
25
SUMMARY OF
ISSUES - 2.
Offsite/Onsite
Design Interface
Calculations
Exelon
Generation
Company,
Page 7, item a) - Change the last sentence to read
"... include voltage drop due to all
transmission system contingencies that are a direct
result of the transient or accident being
analyzed (typically this will include tripping of the
nuclear power unit)."
Disagree.
A transient grid perturbation may be a result
in a plant trip. The DVR setpoint should be
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
ISSUES - 2.
Offsite/Onsite
Design Interface
Exelon
Generation
Company,
Page 7, item a) - Either add to a) or add another
section immediately after a). The transmission
system controlling parameters are assumed to
remain unchanged throughout the initial stages of
Agree.
RIS will be revised to state a): The unit trip
11
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
Calculations
the event with the exception of those effects
resulting from the event (contingency due to the
loss of the unit). For purposes of the calculation,
the Unit trip contingency can be coincident with the
accident, or at a later time consistent with the
assumptions in the plant accident sequence
analyses."
grid contingency voltage drop value should
be used in the accident cases in accordance
with the plant accident analyses since a unit
trip occurs with an accident.
27
SUMMARY OF
ISSUES - 2.
Offsite/Onsite
Design Interface
Calculations
Exelon
Generation
Company,
Page 7, item b) - Delete the tabulation of sources
of power to the emergency buses and replace with
a simple statement of "all credited sources of offsite
power to the emergency buses."
Disagree.
The recommendation does not change the
intent of the tabulation. To maintain
consistency with GL 79-36, it is preferable to
maintain the tabulation.
28
SUMMARY OF
ISSUES - 2.
Offsite/Onsite
Design Interface
Calculations
Exelon
Generation
Company,
Page 8, item c) - Change to read: "...(1) an
accident in the unit being analyzed and shutdown
of all other units at the station consistent with the
licensing basis of the station; ... in the unit
being analyzed (e.g., unit trip) and shutdown of all
other units at that station consistent with the
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.
Disagree.
The RIS is consistent with GL 79-36.
The licensing basis of multi-unit sites has to
be uniquely considered.
29
SUMMARY OF
ISSUES - 2.
Offsite/Onsite
Design Interface
Calculations
Exelon
Generation
Company,
It is recommended that the NRC provide a positive
statement for allowing the credit for voltage
controlling equipment external to the class IE
equipment for this calculation. Licensees perform
LOCA load sequencing under this section of the
calculations and take credit for LTCs (or other
voltage regulating devices) to demonstrate the
adequacy of the offsite sources. In addition,
please clarify if MOVs are to be modeled during
this scenario, even though it appears from the
RIS that MOVs and other equipment like contactors
are to be evaluated with voltages obtained from the
Degraded Voltage Relaying Design Calculations
with voltage just above the lowest set point of DGV
relays.
Disagree.
Use of LTCs is acceptable for regulating
voltage during normal plant operation. LTCs
do not afford protection during a transient
degraded voltage condition that can affect
operation of redundant equipment.
The following changes will be incorporated in
the RIS: Add the following in section 2
(general) and d.
All actions the electric power system is
designed to automatically initiate or control
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
ISSUES - 2.
Offsite/Onsite
Design Interface
Calculations
Exelon
Generation
Company,
Under Item a), for units with LTCs, please clarify if
the analysis is to be performed with the grid
at minimum expected voltage, maximum expected
voltage, or at both.
Disagree
See Question 29.
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
Generation
In general the clarifications contained in the draft
RIS appear to be more restrictive and
Disagree.
12
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
Company,
prescriptive than the cited historical regulatory
documentation, and do not support plant unique
design and current licensing bases that have been
developed and accepted in previous licensing
activities. Unique design and licensing bases that
have previously been accepted and
approved that may not be strictly aligned with the
clarifications in the draft RIS may include use
of an inverse time under voltage relay set between
the DVR and LVR relays (such as .875 to
0.70 PU for a maximum of 60 seconds). Some
sites may not provide coincident logic to
preclude spurious trips; rather, the logic may
include alternate design features to conform to the
intent of the requirements of BTP PSB-1.
Unique designs that may have been
previously accepted should have
appropriate justification with NRC approval of
the licensing documents. Typically, detailed
calculations have not been reviewed as part
of Technical Specification changes. The staff
has relied on licensee correspondence
stating adequacy of DVR setpoint to approve
license amendment requests. Onsite
inspections are used to verify analytical
methods used to meet regulations.
Alternate methods used to demonstrate
conformance may be acceptable provided
they meet the intent of BTP PSB-1 to protect
safety related equipment
32
General
Southern
Nuclear
Operating
Company
Include a definition of key terms (ex. Normal grid
operation, sustained degraded voltage)
Agree
Additional clarifications will be provided in
the RIS wherever appropriate.
33
General
Southern
Nuclear
Operating
Company
The RIS does not address completely the specific
requirements in the PSB-1 (ADAMS Accession No.
ML052350520), Arkansas Nuclear One (ADAMS
Accession No.ML0311801180), and Millstone
(ADAMS Accession No. ML093521388)
documents. In some cases specific positions in
the above documents were omitted from the RIS.
Proposed resolution: include missing positions
especially those related to determining minimum
expected offsite system voltages and testing.
Disagree
The RIS covered the key topics intended to
address inspection findings.
The reference documents should be
reviewed for more details.
The expected offsite system voltages can
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
Nuclear
Operating
Company
The RIS lacks adequate guidance to perform the
requested calculation(s) without additional
interpretations by the licensee and auditors as to
the intent of the provided guidance.
Proposed resolution: Provide a guideline with
examples on how to perform the calculation(s)
including expected assumptions, other
considerations, and criteria to be used for
acceptance.
Disagree
The RIS is consistent with GL 79-36. The
licensee is responsible for performing
calculations, in accordance with industry
engineering practices, with properly
supported inputs and assumptions that
demonstrate compliance in accordance with
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
Nuclear
Operating
Company
The RIS provides some examples of plants that
have NRC reviewed and approved analyses and
goes on to point out that "backfit rule" was applied
because the staff believed the sites were not in
compliance with regulations even though they had
approved the analysis. How is a licensee who has
an NRC approved or acceptable analysis supposed
to know that their analysis is no longer acceptable?
The RIS needs more clarification with regard to
individual plant licensing bases if it is to be useful
to licensees.
Some plants have installed degraded grid alarm
systems and, at the staff request, included them in
Unit Operating Technical Specifications. Required
Disagree
The licensee must be in compliance with all
regulations pertaining to onsite and offsite
power systems. Unless, licensees are
exempted from meeting certain regulatory
requirements, changes to the design and
licensing bases are required to meet the
regulations in accordance with 10 CFR
50.109. The RIS identified some of the
recent inspection findings.
Operator actions could be used only to
13
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
operator actions related to degraded grid
conditions are specified in the bases and
procedures. The RIS does not discuss this
approach.
There are a number of plants that have URIs
related to this issue. Issuance of this RIS could be
used by inspectors to close the URIs to violations
without regard to plant specific licensing bases,
resulting in regulation by inspection.
supplement the automatic DVR scheme by
providing alarm in the control room when grid
voltage is below nominal operating range.
Operator actions cannot be substituted for
protecting the safety related equipment from
degraded voltage. 10 CFR 50.55a(h)(2)
requires all protective actions to be
automatic.
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
Issues", pg. 6,
Item 1. "Degraded
Voltage Relaying
Design
Calculations",
Line 5.
Southern
Nuclear
Operating
Company
The RIS states "The Class 1 E buses should
separate from the offsite power system within a few
seconds if an accident occurs coincident with
sustained degraded voltage conditions."
GDC 17 describes the requirements for onsite and
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".
These analyses assume Loss of Offsite Power
simultaneous with the event. They do not require
assuming degraded grid voltage condition prior to
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 not believed to be credible.
Proposed resolution: Remove or clarify this
statement since proper offsite system design and
operation renders such simultaneous postulated
events as incredible.
Disagree.
The RIS correctly states that if an accident
signal is received during sustained
degraded grid conditions, it may be prudent
to separate from the grid as :
1.
The duration of degraded
conditions on the grid is unknown
2.
It precludes other complications
such as double sequencing.
Chapter 15 Accident Analyses assumes
Loss of Offsite Power as a limiting case for
safe shutdown in view of the limited power
and resources available from the onsite
power sources. The preferred power source
for all operating modes and accident related
safe shutdown is the offsite source. The
DVR provides assurance that the plant
shutdown capability is not compromised
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
Issues", pg. 6,
Item 1. "Degraded
Southern
Nuclear
Operating
The RIS states "During normal plant operation, the
Class 1 E safety related buses should
automatically separate from the power supply
Disagree.
14
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
Voltage Relaying
Design
Calculations",
Line 7
Company
within a short interval (typically less than 60
seconds) if sustained degraded voltage conditions
are detected.
During normal plant operation (i.e. non LOCA), the
degraded grid relay settings may be overly
conservative. Therefore automatic separation from
the preferred power supply may not be desired.
Proposed resolution: Transmission Operators
should be allowed time to correct the degraded
voltage condition while Plant Operators monitor the
safety bus voltages for adequate voltage.
See staffs response to Comment No. 9
Voltages down at the DVR level should be
well below the normal grid voltage levels.
Transmission operators will be taking actions
when voltages fall below the normal low level
(or post contingency low) well above the
DVR value (assuming the plant design is
proper given the grid operating voltage
range).
The plant electrical distribution system
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
Issues, Last
paragraph of
"DVR criteria for
time delay
selections.
Setting Design
Calculations", pg.
7
Southern
Nuclear
Operating
Company
The discussion on time delays does not provide
adequate criteria for time delay selections.
Proposed Resolution: Clarify.
The RIS states 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. For the
purposes of this calculation, no t credit should be
taken for voltage controlling equipment external to
the Class 1E distribution system such as automatic
load tap changers and capacitor banks.
Proposed Resolution: This statement needs to be
clarified to allow reasonable assumptions for the
status of equipment external to the Class 1E
distribution system. For example it is unclear how
to perform motor starting calculations without
taking credit for some Non 1E voltage controlling
equipment. Additionally, normal Transmission grid
switching to prepare for the next contingency to
maintain minimum expected transmission system
voltages should be allowed.
Disagree
The acceptable level of time delay is based
on plant specific accident analyses and
electrical systems designs.
Different time delays may be selected for
different plant designs. BTP PSB-1 suggests
two relays with different settings to
accommodate motor starts and sustained
degraded conditions. Typically, chapter 15
accident analyses assume 10 second time
delay for onsite power sources to energize
safety busses. DVR time delay of 10seconds
or less may be acceptable to demonstrate
that the assumptions in the accident
analyses remain valid.
External voltage regulating equipment (tap
changers) does not afford timely protection
for safety related equipment during sudden
grid perturbations and therefore cannot be
credited for DVR settings including transient
voltages during motor starts.
See staffs response to Comment Nos. 76
and 118.
39
Summary of
Issues, Guidelines
for voltage drop
calculations, item
(f) pg. 8
Southern
Nuclear
Operating
Company
After paragraph (f) the RIS leaves out the guidance
in GL 79-36 concerning minimum expected values
(item 6 of enclosure 2) was omitted from the RIS
guidance.
Proposed resolution: Add item 6 of enclosure 2 in
Disagree
Item 6 was addressed in item(a) in the RIS
40
General
NEI 3/2/11
NEI contends that RIS represents new positions
15
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
Letter
77 Letter and PSB-1 Not consistent. Degraded
voltage event and accident coincident vs.
subsequent. Also, starting and running voltage vs.
just running (term sustained in PSB-1)
Disagree
PSB just states how the scheme should
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
Letter
77 Letter and 79 GL are generic communications
not requirements
Both communications provide guidance on
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
Letter
Multi-unit sites, accident and simultaneous
shutdown vs. orderly shutdown and cooldown per
Disagree
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
Letter
During normal plant operation, a degraded voltage
for 60 seconds resulting in separation doesnt give
the operator enough time (implied by PSB-1) to
take actions
Disagree
See NRC response to .Comment No. 9.
45
(46- 48
not
used)
Also
see
Nos.
86-96
for this
NEI
Letter
NEI 3/2/11
Letter
Offsite/Onsite design interface calculations doesnt
allow for manual actions
Disagree
This means that manual actions for the
purposes of analyses of the offsite power
system voltages should not be assumed.
See GL 79-36 for more details.
PSB-1 allows manual actions after first alarm
at higher voltage.
49
Page 6, DVR
Setting Design
Calculations
Section DVR Setting Design Calculations states:
At the DVR dropout setting ...... 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.
BTP PSB-1 states, "sustained", implying a steady
state voltage condition and not a transient voltage
condition that exists during a motor starting event.
The original 1977 NRC Letter, the later PSB-1, or
GL 79-36 does not require plants to demonstrate
the ability to start motors at the DVR settings.
Requiring the ability to start motors at the DVR
dropout setting does not accomplish anything or
make sense, since starting a motor at this voltage
will ensure a resultant voltage below the DVR
dropout; result in not being able to be reset the
relay, and as a result causing a spurious
disconnection from offsite power and transferring to
Disagree
See Responses to Questions 1 & 2.
Sustained in the context that the loss of
voltage relay was designed for momentary
dip in voltage or complete loss of voltage.
Steady state operation is expected at normal
operating band.
The 1977 NRC letter states that voltage and
time setpoints shall be determined from an
analysis of the voltage requirements of the
safety related loads. Safety related
equipment, particularly large motors, have
starting and running voltage requirements.
This second level of protection should
address these voltage requirements.
Sustained degraded voltage, as discussed in
the 1977 NRC letter, refers to grid voltage
below the expected low value given normal
grid operation and grid post contingency
16
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
the emergency diesel generator. Having a
requirement to be able to start motors at the DVR
dropout setting will result in the raising the DVR
settings to a higher value and is more likely to
result in spurious separation from the grid which is
in direct conflict with PSB-1.
(Single, N-1). Thus, when grid voltages are
degraded beyond the minimum voltage
assured by the grid operator(such as
resulting in SR bus voltages down close to
where DVRs are set based on SR equipment
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
Bottom
In the RIS section of recent inspection findings for
Peach Bottom, it was stated that since the load tap
changers are not safety-related and are subject to
operational limitations and credible failures, they
cannot be relied on.
Agree
The finding is correct.
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)
In the Offsite/Onsite Design Interface Calc section
(C) it states: an accident in the unit being analyzed
and simultaneous shutdown of all other units at the
station.
RG 1.81 states: The Regulatory staff has
determined that, because of the low probability of a
major reactor accident, a suitable design basis for
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".
Disagree
For electrical system, the statement in the
RIS is consistent with GDC 17, GL 79-36,
and IEEE Standard 308-1971, Class IE
Electrical Systems, Section 8, Multi-Unit
Station Considerations.
52
Page 6, DVR
Setting Design
Calculations
Larry
Nicholson,
Nexterra
This paragraph could be interpreted to require the
LOCA sequence to be modeled at the DVR dropout
setting. LOCA sequencing modeled at the DVR
Disagree
17
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
dropout setting would result in separation of the
buses from the Preferred Power Source (off-site
power) as the voltage would not recover above the
DVR reset value.
Proposed Resolution: Clarify the intent 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.
Clarify that LOCA sequencing is evaluated using
minimum switchyard voltage as the starting point.
See response to questions 1&2
The sustained degraded voltage conditions
are not expected to last the total sequencing
process. The analyses should demonstrate
that the largest load (limiting case) for
accident mitigation can be successfully
started under degraded grid conditions and
the loads that are normally operating will not
be adversely impacted. It is expected that
the grid will either recover to nominal voltage
and reset the DVR for the rest of the
sequencing process or the DVR will separate
the plant from source that cannot support
53
Page 6, DVR
Setting Design
Calculations
Larry
Nicholson,
Nexterra
Having a sustained degraded voltage just above
the LVR voltage setling (70%) is not practical
without grid collapse and does not exist in Branch
Technical Position #1 (PSB-1).
Proposed Resolution: Clarify degraded voltages to
be analyzed to a credible level.
Agree
Although undervoltage protection (first level)
is not within the scope of the RIS, the
licensees analysis should ensure that the
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
Setting Design
Calculations
Larry
Nicholson,
Nexterra
The statement that the DVR ensures adequate
operational (starting and running) is the first time in
NRC correspondence that starting equipment at
the DVR setpoint is expected. The example letter
sent to Peach Bottom in June 1977 did not require
starting of equipment at the DVR setpoint. This
requirement should be removed from the RIS since
it is not possible to start equipment at the DVR
setpoint and not subsequently separate from offsite
power. If the equipment starts at the DVR setpoint,
the voltage will dip during the transient and must
then recover above the reset point to avoid
separation from offsite power. Since the reset point
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.
Disagree
The NRC 1977 letter states that voltage and
time setpoints shall be determined from an
analysis of the voltage requirements of the
safety related loads. Safety related (Class
1E) equipment, particularly large motors,
have starting and running voltage
requirements. This second level of
undervoltage protection should address
these voltage requirements
55
Page 7, DVR
Setting Design
Calculations
Larry
Nicholson,
Nexterra
It is agreed that no credit is to be taken for voltage
controlling equipment external to the Class 1 E
distribution system for the establishing the
degraded voltage relay (DVR) settings; however, it
should be clarified that for credit may be taken for
minimum switchyard voltage/voltage drop
calculations (or the Offsite/Onsite Design Interface
Calculations).
Proposed Resolution: Clarify that credit must be
taken for automatic load tap changers and/or
capacitor for minimum switchyard voltage/voltage
drop calculations (or the Offsite/Onsite Design
Interface Calculations).
Agree
Grid Interface calculations can take credit for
voltage correction equipment. The DVR
setpoint should be set independent of
voltage correction equipment that cannot
operate in a timely manner to protect 1E
equipment. RIS will be clarified regarding
crediting voltage controlling equipment
external to the 1E system for Offsite/Onsite
Design Interface.
56
Page 8, c)
Larry
Nicholson,
Nexterra
NRC Generic Letter 79-36, Enclosure 2, Item 2
states 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 at the
station; or (2) an anticipated transient in the unit
being analyzed (e.g., unit trip) and simultaneous
18
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,
with an attempt to clarify "anticipated transient" by
adding in parenthesis "(anticipated operational
occurrence)" immediately afterwards. It is not clear
what the added parenthetical statement is meant to
convey, other than unit trip (which already exists in
GL 79-36).
Proposed Resolution: It is recommended that this
either be removed, or stated "anticipated transient
per station licensing basis".
Disagree
Anticipated Operational Transient is a more
general term for operational events per the
design except for design basis accidents
This is consistent with the term used in GDC
17.
57.
Page 8, c)
Larry
Nicholson,
Nexterra
NRC should clarify "simultaneous shutdown" with
consideration to:
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
"Capacity describes this as a "concurrent safe
shutdown on the remaining units".
Proposed Resolution: The wording for the
proposed RIS, sub clause 2.c should be revised to
indicate orderly or controlled safe shutdown of the
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.
Disagree
This statement is consistent with GDC 17,
GL 79-36, and IEEE Standard 308-1971,
Class IE Electrical Systems, Section 8,
Multi-Unit Station Considerations.
58
Page 8, e) and f)
Larry
Nicholson,
Nexterra
These guidelines seem contradictory that you
cannot credit procedurally controlled operator
actions to reduce load but you have to assume the
actions will be carried out when load is added.
Proposed Resolution: Delete e) Manual load
shedding should not be assumed or add
allowance to credit procedurally controlled operator
actions to decrease load.
Disagree
Adding loads manually per procedure is
conservative in terms of maximum loading,
but not for load reductions. Plant design
should not depend on manual load shedding.
This is not conservative. That was the point
of item e).
But
not59
General
NEI 3/18/11
Letter / 1
The RIS should identify that plant compliance with
the regulation (GDC 17) is by each plant operating
within its Licensing Basis.
Agree
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
Letter / 2
Definitions vary for the same words used with
respect to this topic.
The RIS should include definitions for key terms,
e.g. normal grid operation, sustained degraded
voltage, etc.
Agree
Clarifications will be provided in the RIS
wherever appropriate.
19
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
61
General
NEI 3/18/11
Letter / 3
There are various documents that address the
Adequacy of Station Electric Distribution System
Voltages. These documents have differences in the
methodology, terminology, and level of detail. Such
differences challenge the users of these guidance
documents when they conflict.
Attachment 2 to the NEI supplemental comment
letter provides a table that shows the differences
between GL 79-36, (BTP) PSB-1, IEEE 741, and
the draft RIS.
The RIS should identify the guidance document
differences, establish the NRC position on each
conflicting topic, and provide the basis for each
change in previously accepted guidance.
Disagree
The RIS provides adequate clarifications to
the existing guidance.
The staff positions described in the NRC
1977 letter is are guidance for all licensees
as to how to meet GDC 17 or applicable
plants principle design criteria. This letter
focuses on the need for a second level
undervoltage protection scheme to protect
the Class 1E electrical components from the
consequences of sustained degraded
voltage conditions.
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
paragraph
NEI 3/18/11
Letter / 4
"The U.S. Nuclear Regulatory Commission (NRC)
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:
'The U.S. Nuclear Regulatory Commission (NRC)
is issuing this Regulatory Issue Summary (RIS) to
clarify the NRC staff's technical position on
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.'
Agree
The RIS will be revised to incorporate this
comment.
63
Page 2,
Paragraph a)
NEI 3/18/11
Letter / 5
"The selection of voltage and time delay setpoints
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
"sustained voltage" which by definition would be
steady state voltage, running voltage, or voltage
held at a constant value
(BTP) PSB-1 and BTP 8-6 are silent on
operating/running voltage in the DVR settings
section; however, running is implied by using the
Disagree in part.
Voltage requirements - all voltage
requirements of SR equipment
The term operating voltage is being
removed from the RIS as recommended.
20
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
term "sustained" in the Time Delay settings section.
To be consistent with (BTP) PSB-1, NRC letter,
and BTP 8.6 remove the word "operating".
The term sustained voltage used in the 1977
letter and the BTPs is referring to the voltage
condition on the grid, not steady state
voltage
64
Page 3, Arkansas
Nuclear One
NEI 3/18/11
Letter / 6
"...assuming all onsite sources of AC power are not
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',
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
paragraph of page 6.
Agree
The term operating voltage is being removed
from RIS.
65
Page 6, 1st
Paragraph
NEI 3/18/11
Letter / 7
"...components are provided adequate voltage
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
operating configuration", since experience shows
that a component is most limiting.
Reword the RIS to:
'...components are provided adequate voltage
based on the design of the Class 1E distribution
system in the plant.'
Agree.
RIS will be revised to state the following:
"... all safety related components are
provided adequate voltage based on the
design of the plant power distribution system
(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.
66
Page 6, 1st
paragraph
NEI 3/18/11
Letter / 8
"The Offsite/Onsite Design Interface Calculations
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
LOCA sequence to be modeled at the DVR dropout
setting. LOCA sequencing modeled at the DVR
dropout setting would result in separation of the
buses from the referred Power Source (off-site
power) as the voltage would not recover above the
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.
Disagree
This sentence is not referring to calculations
for setting the relays but referring to
Offsite/Onsite Design Interface Calculations.
67
Page 6, 1st
paragraph
NEI 3/18/11
Letter / 9
"This interface calculation establishes operating
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
21
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
non-safety load voltages need to be evaluated.
The statement that the DVR ensures adequate
operational (starting and running) is the first time in
NRC correspondence that starting equipment at
the DVR setpoint is expected.
Having a sustained degraded voltage just above
the LVR voltage setting (70%) is not practical
without grid collapse and does not exist in Branch
Technical Position #1 ((BTP) PSB-1).
Typically, large motors (like reactor coolant pumps)
need to be evaluated for starting impact on the
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
lowest levels of distribution.
Technical compliance with determining the
degraded voltage relay setpoint would not be
achievable because the RIS requires the DVR
dropout setpoint to be based on the starting voltage
required for motors. Basing the DVR setpoint
(dropout setting) on starting voltage requirements
(rather than steady-state operating voltage)
appears to be a new NRC requirement/position. It
is technically flawed in that it would not actually
provide the required protection for the Class1lE
loads. It also appears to disagree with the intended
purpose of the existing regulations (1977 NRC
Letters on degraded voltage protection and (BTP)
PSB-1).
The letter sent to Peach Bottom in June 1977 did
not require starting of equipment at the DVR
setpoint. This new requirement should be removed
from the RIS, since it is not possible to start
equipment at the DVR setpoint and not
subsequently separate from offsite power. If the
equipment starts at the DVR setpoint, the voltage
will dip during the transient and must then recover
above the reset point to avoid separation from
offsite power. Since the reset point will always be
above the DVR dropout point, it will be impossible
to reset the relay.
Disagree
See response to questions 1&2 for
clarification.
The 1977 NRC letter refers to SR equipment
voltage requirements.
NSR items are being removed from this
sentence in the RIS. However, non safety
related loads should be modeled to the
extent that their operation can affect safety
bus/equipment voltage.
DVR setpoints are based on low voltages
that can occur due to sustained grid
perturbations and can potentially degrade
capability of onsite safety related equipment.
The NRC 1977 letter states that voltage and
time setpoints shall be determined from an
analysis of the voltage requirements of the
safety related loads. Safety related
equipment, particularly large motors, have
starting and running voltage requirements.
This second level of protection should
address these voltage requirements.
Sustained degraded voltage, as discussed in
the 1977 letter, refers to grid voltage below
the expected low value given normal grid
operation and grid post contingency (Single,
N-1). Thus, when grid voltages are degraded
(such as resulting in SR bus voltages down
close to where DVRs are set based on SR
equipment requirements), separation from
the grid is appropriate. The 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
due to sequencing or block loading of loads
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
paragraph
NEI 3/18/11
Letter / 10
"The staff considers degraded voltage conditions
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
excess of GDC 17.
The RIS concludes that the staff considers
degraded voltage conditions coincident with a
postulated design basis accident to be a credible
Disagree
The 1977 NRC letter staff positions guidance
(implementation second level undervoltage
protection scheme) apply applies to all
22
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
event; however, this is not consistent with GDC 17.
The RIS should identify that plant compliance with
the regulation (GDC 17) is by each plant operating
within its Licensing Basis that was developed from
available NRC and industry guidance.
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.
operating plants and provides guidance on
how to meet regulatory criteria..
The 1977 NRC letter indicates that the DVR
scheme time delays should support accident
analysis assumptions which ties degraded
event with an accident.
69
Page 6, 2nd
paragraph
NEI 3/18/11
Letter / 11
"The Class 1E buses should separate from the
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
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".
These analyses assume Loss of offsite Power
simultaneous with the event. They do not require
assuming degraded grid voltage condition prior to
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
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
of this position. It does not apply to Section 2,
"Offsite/Onsite Design Interface Calculations" of the
RIS. This is confirmed in the draft RIS on Page 8,
Paragraph d) which states all electric system action
occur "as designed". It would be beneficial to clarify
the scope limitations associated with this or any
revised position.
Disagree
The NRC 1977 letter indicates that the DVR
scheme time delays should support accident
analysis assumptions which ties degraded
event with an accident.
See NRC response to Comment No. 36
Section 2 is not about DVR schemes and
separation during a degraded voltage
condition. It is about operation of the plant
during normal, abnormal and accident
conditions and assuming the normal
operation of the grid (including the bounding
N-1 contingency and the trip of the unit for
the accident cases).
70
Page 6, 2nd
paragraph
NEI 3/18/11
Letter / 12
... Position (BTP) PSB-1 (revised later to become
BTP-6), is to protect Class 1E safety related buses
23
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
clear what is being protected: the components
(MOV, motor, etc.) or the class 1E function or
something else.
The word "coincident" should read "subsequent to"
or "followed by", per (BTP) PSB-1 and BTP 8-6.
Disagree
Protect means guard or defend safety
related components against the
consequence of sustained degraded voltage
conditions.
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
paragraph
NEI 3/18/11
Letter / 13
"The Class 1E buses should separate from the
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:
'The Class 1E buses should separate from the
offsite power system immediately if an accident
occurs subsequent to a sustained degraded
voltage condition.'
Agree.
RIS will be revised to incorporate the
comment.
72
Page 6, 2nd
paragraph
NEI 3/18/11
Letter / 14
... Class 1E safety related buses should
automatically separate from the power supply
within a short interval (typically less than 60
seconds)..."
There is no basis for "typically less than 60
seconds". In the original context of the time delay
section, it was sufficient time for an operator to
intervene" which is much greater than 60 seconds.
Remove the parenthesis section of the sentence.
With this guidance during normal plant operation,
the degraded grid relay settings may be overly
conservative; automatic separation from the
preferred power supply may occur under conditions
where this action is inappropriate.
The RIS should allow Transmission Operators time
to correct the degraded voltage condition while
Plant Operators monitor the safety bus voltages for
adequate voltage.
Agree
RIS will be revised to delete the parenthesis
section of the sentence as suggested.
Disagree
10 CFR 50.55a(h(2)) requires all protective
actions to be automatic. Operator
intervention is probably not possible when
voltage gets down to the DVR setpoint (grid
voltage is well below normal).
Operator action takes minutes. Operation at
degraded voltage conditions can degrade
equipment performance capabilities within
seconds.
73
Page 6, 3rd
paragraph
NEI 3/18/11
Letter / 15
DVR Setting Design Calculations
This section would be a good place to describe this
type of analysis as having a "bottom-up" approach.
Such calculations would prevent confusion of
crediting anything above the DVR voltage sensors'
values.
In the context of DVR Setting Design Calculations,
using a steady state or sustained voltage analysis
Disagree
The approach is already described in this
paragraph.
24
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
is the only way that can result in a voltage 'setting'
requirement.
The term sustained voltage used in the NRC
1977 letter and the BTPs is referring to the
voltage condition on the grid, not steady
state voltage
74
Page 6, 3rd
paragraph
NEI 3/18/11
Letter / 16
"... DVR ensures adequate operational (starting
and running) voltage..."
The "operational voltage" cannot define both
starting and running voltages.
The only place "operational voltage" is referenced
in the associated documents is in the tap setting
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.
Reword the RIS to:
'... DVR ensures adequate sustained voltage...'
Agree
RIS will be revised to state DVR ensures
adequate voltage (start and run
conditions)
Disagree
The term sustained voltage used in the NRC
1977 letter and the BTPs is referring to the
voltage condition on the grid, not steady
state voltage
75
Page 6, 3rd
paragraph
NEI 3/18/11
Letter / 17
"Licensee voltage calculations should provide the
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
starting a motor. The 0.9 per unit in this context
refers to the typical running voltage requirement of
a motor; whereas, 0.85 per unit is typical for a
starting voltage requirement.
The RIS should identify that voltages other than
90% voltage are common based on detailed plant
analysis.
For example, motors below 90% voltage continue
to have plenty of margin in torque but may
encroach on long time thermal limits. Unless a
motor is fully into its service factor (typicallyl.15)
and below 90% voltage, operation will be
acceptable.
Agreed
This sentence is being reworded
Starting requirements for motors have been
observed over a range of 0.75 to 0.85. It
depends on the particular plant and how the
motors were procured. Either way, the
voltage requirements must be preserved
(starting and running). However, there could
be other components that are more sensitive
to voltage for operation.
0.9 per unit voltage was mentioned as an
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.
25
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
76
Page 6, 3rd
paragraph
NEI 3/18/11
Letter / 18
"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."
The draft RIS suggests the DVR dropout setpoint
to be based on the starting voltage required for
motors.
Basing the DVR dropout setpoint on starting
voltage requirements rather than steady-state
operating voltage appears to be a new NRC
requirement/position. It also appears to disagree
with the intended purpose of the existing
requirements and guidance (1977 NRC Letters on
degraded voltage protection and (BTP) PSB-1).
As suggested, the approach incorrectly implies that
the load(s) should start from the lowest DVR
dropout setting. A specific example for illustration is
as follows: If the initial voltage value is at the lowest
possible value above dropout actuation, starting a
load will cause the DVR dropout. Since the new
steady state voltage will be lower than the initial
value because of the added loads, the DVR reset
will never occur.
Many utilities use the ABB 27N with harmonic filter
which has a minimum 0.5% reset. Thus, with a
setting of 93.6% +/- 0.9%, the dropout value could
be as low as 92.7%. For motors causing more than
0.5% voltage dip at initial start, even if the voltage
at the beginning of the event was 93.2% and a load
was started, the DVR will dropout and never reset.
This will lead to a grid separation.
Reword the RIS to remove "(starting and running)"
Disagree
The NRC 1977 letter states that voltage and
time setpoints shall be determined from an
analysis of the voltage requirements of the
safety related loads. Safety related (Class
1E) equipment, particularly large motors,
have starting and running voltage
requirements. This second level of
undervoltage protection should address
these voltage requirements. Sustained
degraded voltage, as discussed in the NRC
1977 letter as well, refers to grid voltage
below the expected low value given normal
grid operation. Thus, when grid voltages are
degraded (such as resulting in Class 1E bus
voltages down 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. 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 due to sequencing
or block loading of loads during a design
basis event.
Also, see response to questions 1 & 2.
77
Page 6, 3rd
paragraph
NEI 3/18/11
Letter / 19
"For the purposes of this calculation, no credit
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
the DVR setpoint(s) protect against the potential
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
initial bus voltage corresponding to the DVR
setpoint and no voltage regulation capability.
Actually, the calculation should be performed with
the DVR monitored bus voltage at the TS limit, not
the DVR setpoint. All that is required is that under
motor starting conditions, separation from offsite
power occurs before starting loads trip on overload.
The intent could be conveyed in more detail.
The RIS should allow reasonable assumptions for
the status of equipment external to the Class 1E
distribution system. For example it is unclear how
to perform motor starting calculations without
taking credit for some Non 1E voltage controlling
equipment. Additionally, normal transmission grid
switching should be allowed to prepare for the next
Disagree
This sentence is being re-worded
The point being made is that calculations for
the DVR voltage settings should have cases
at voltages just above the DVR voltage
settings (well below what would be based on
normal grid operations and voltage
controlling equipment if applicable) to
demonstrate that the settings enforce the SR
equipment voltage requirements.
This is covered in Offsite/Onsite Design
Section calculations (not DVR calculations
section)
The offsite source is the preferred source of
power for plant shutdown. The DVR should
not separate the plant from the grid for motor
starts. In the event that grid conditions
degrade beyond an acceptable point and an
accident signal is actuated, BTP PSB-1
recommends separation from the grid.
26
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
paragraph
NEI 3/18/11
Letter / 20
"Voltage-time settings for DVR's should be selected
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
historical position stated on Page 2, Item (c)(3).
Either the DVR protection scheme favors ESF-
equipment-protection or connectivity-to-offsite-
power. Otherwise, this position would result in a
mutually exclusive requirement. The prevention of
spurious separation is addressed by coincident
logic channels (Page 2, Item (b)), not the setpoint.
If no credit is to be taken for voltage controlling
equipment external to the Class 1E distribution
system for the establishing the degraded voltage
relay (DVR) settings, then the RIS should state that
credit may be taken for minimum switchyard
voltage/voltage drop calculations (Offsite/Onsite
Design Interface Calculations).
Disagree
This sentence is being re-worded
DVR setting always enforce SR equipment
voltage requirements. Offsite/Onsite Design
should ensure that there is proper margin
between where voltage is in the plant during
normal grid operation as compared to
voltages in the plant when the DVRs actuate.
Credit for voltage controlling equipment in
the Offsite/Onsite Design Section
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
paragraph
NEI 3/18/11
Letter / 21
"These DVRs should disconnect the Class 1E
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
undervoltage condition persist.
(BTP) PSB-1 was written before the application of
voltage regulating devices within the nuclear power
plant offsite power circuit boundary. The RIS
should clarify that if the calculations necessary to
support RIS positions in Section 1, "Degraded
Voltage Relaying Design Calculations" and Section
2, "Offsite/Onsite Design Interface Calculations"
demonstrate completion of ESF functions within
accident analysis assumptions, then immediate
separation per (BTP) PSB-1, Section B(1)(b)(i), is
no longer the preferred NRC position.
"The subsequent occurrence of a safety injection
actuation signal (SIAS) [after an undervoltage
condition longer than a motor starting transient]
should immediately separate the Class 1E
distribution system from the offsite power system."
The RIS should state that this (BTP) PSB-1
position is not included in the draft RIS because it
provides no added protection in terms of
establishing the DVR setpoint(s) or in establishing
operability of the offsite power interface. To the
contrary, this (BTP) PSB-1 increases the
probability of separation from offsite power.
Disagree
The BTP PSB-1 offers an option to set a
higher voltage alarm level to support
corrective action to restore voltage to normal
operating band.
Since offsite power is the preferred source of
power to mitigate design basis event, it is
important that the Offsite/Onsite Design
Interface calculations ensure the capacity
and capability of the offsite power is
adequate to sequence or block load during
design basis events without actuating DVRs
with sufficient margin available at the safety
buses. Separation of the safety buses from
the grid is only appropriate when the DVR
relays actuate indicating that SR equipment
voltage requirements are not being met (not
able to protect or provide adequate voltage
to the terminals of the SR limiting
components at the plant).
Analyses to determine such setpoints always
should have included modeling the plant
power distribution system such that proper
voltages throughout the plant system can be
calculated in all operating and accident
27
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
conditions.
80
Page 7,
paragraph 2 a)
NEI 3/18/11
Letter / 22
"Guidelines for voltage drop calculations
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
Paragraph 2(a) be deleted. It is covered by
Paragraph 2(b), as the switchyard is the "power
source" for the offsite power circuits.
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
circuit is individually capable of performing its
design function is based on a postulated post-trip
switchyard voltage for the present grid
configuration and operating level (i.e. RIS
Paragraph 2(b)).
As written, it is conceivable that a reader of this
paragraph could conclude that the transmission
"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.
Disagree
Enclosure 2 of GL 79-36 provides guidelines
for voltage drop calculations.
Accident cases consider the unit trip grid
contingency since a trip is assumed to occur
coincident with an accident. However, if the
unit trip is not the most limiting grid
contingency (not the largest grid voltage
drop), the cases which assess normal and
abnormal operation (non-accident) need to
assume the bounding grid contingency
(normal grid operating range)
81
Page 8,
paragraph 2 c)
NEI 3/18/11
Letter / 23
"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 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
"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'.
Most multi-unit stations 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, sub-clause 8.1.1
"Capacity" describes this as a "concurrent safe
shutdown on the remaining units".
Disagree
Wording is the same as provided in GL 79-
36
This statement is consistent with GDC 17,
GL 79-36, and IEEE Standard 308-1971,
Class IE Electrical Systems, Section 8,
Multi-Unit Station Considerations.
28
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,
paragraph 2 d)
NEI 3/18/11
Letter / 24
"All actions the electric power system is designed
to automatically initiate should be assumed to
occur as designed..."
This statement is consistent with GDC 17 in that
the presumption is the onsite AC sources are lost.
The postulation of concurrent malfunctions in both
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.
Agree.
No change to this sentence.
83
Page 8,
paragraph 2 e) &
f)
NEI 3/18/11
Letter / 25
"e) Manual load shedding should not be assumed.
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)
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
considered in the maximum load.
The RIS should delete "e) Manual load shedding
should not be assumed" or add allowance to credit
procedurally controlled operator actions to
decrease load.
Disagree
This guidance is consistent GL 79-36.
Adding loads manually per procedure is
conservative in terms of maximum loading,
but not for load reductions. Plant design for
maximum load should not depend on manual
load shedding (not conservative). That was
the point of item e).
84
Page 8,
paragraph 2 f)
NEI 3/18/11
Letter / 26
Omission
After paragraph 2 f), the RIS leaves out the
guidance in GL 79-36 concerning minimum
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
Disagree
It was not omitted. This is covered in item a)
29
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
studies)."
85
Page 8,
paragraph 2 j)
NEI 3/18/11
Letter / 27
"To provide assurance that actions taken to assure
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"
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
actions depends on how high actual voltage
reaches and is addressed by procedures.
Analyses of high grid voltage with light plant load
are standard and provide insights as to what the
grid voltage upper limit should be or what
compensating activities might be required for light
load operations (refueling).
The RIS should provide examples of typical
responses to high grid voltages. For example: in
those cases where unit trip can result in a step
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.).
Disagree
The Offsite/Onsite design should address all
grid operating conditions to prevent
overvoltages from occurring.
The point here is that if a design problem is
identified such as overvoltage conditions,
immediate actions should be taken
(compensatory and/or permanent design
changes) to address the design problem
rather than taking actions after it occurs.
It is not the intent of the RIS to highlight
reasons for voltage perturbations.
86
General
NEI 3/2/11
Letter
Page 2, Paragraph 1
While NEI supports efforts to obtain greater clarity
with respect to the staff's technical position in this
important area, the draft RIS greatly oversimplifies
the regulatory and licensing aspects of the
degraded grid voltage protection issue. As a result
of this oversimplification, the draft RIS
inappropriately combines several generic
communications and guidance documents that
affected the licensing bases of individual plants in
different ways, and fails to adequately address the
significant backfitting concerns that arise when
attempting to eliminate licensing basis variability
via a RIS (or any other guidance document).
Disagree
NRC Staffs position is that the RIS is
intended to clarify the requirements and
associated existing staff positions guidance
which would apply to all plants.
Any inspection findings that questions the
plant-specific licensing bases will be
reviewed by the NRR staff in accordance
NRCs TIA process.
87
General
NEI 3/2/11
Letter
Page 2, Paragraph 2
Unless it is revised, the draft RIS will unnecessarily
increase the potential for loss of the preferred off-
site power source and, consequently, increase
reliance on emergency diesel generators. NEI
believes that the use of emergency diesel
generators more frequently than necessary is
inconsistent with GDC 17 and results in an
unnecessary loss of defense-in-depth.
Disagree
Proper design of the plant electrical
distribution system, given the operating
range of the grid and the proper selection of
DVR settings (based on the voltage
requirements of the 1E equipment), should
provide more than adequate operating
margin, preventing unnecessary separation
from offsite power.
88
General
NEI 3/2/11
Letter
Page 2, Paragraph 3, Comment I
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection
The only generic obligation or legally binding
Agree in part.
As a general matter, NRC staff positions are
guidance, as are, among other things,
30
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
requirement mentioned in the Discussion section of
regulatory guides and Interim Staff Guidance
(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
Letter
Page 3, Paragraph 2, Comment I (Cont.)
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection
Although these letters resulted in changes to the
licensing bases of the nuclear power plants that
received them, they do not function the same way
as generally applicable regulatory requirements.
That is, these generic communications were only
received by plants that were licensed at the time
the communications were issued. Operating
licenses for the current fleet were issued during a
period that ranged from the late 1960s through the
1990s. Thus, not all operating plants received and
responded to the generic communications issued in
1977 and 1979.
Disagree.
The 77 Letter is a staff position 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).The 1977 letter,
as well as other staff guidance, were made
available or sent to all operating plants at
that time. In addition, the NRCs regulatory
practice-which has been understood by the
industry-is that staff guidance represents the
staff position until subsequently modified or
withdrawn. While the staff recognizes that
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
Letter
Page 3, Paragraph 2, Comment I (Cont.)
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection
Further, the Branch Technical Position (BTP)
described in the draft RIS BTP PSB-1, Rev. 0,
"Adequacy of Station Electric Distribution System
Voltages"--was issued in 1981. BTP PSB-1 and the
Standard Review Plan in which it is included were
"prepared for the guidance of the Office of Nuclear
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.
Disagree.
The 1977 Letter is a 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).
31
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
91
General
NEI 3/2/11
Letter
Page 3, Paragraph 3, Comment I (Cont.)
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection
For example, the draft RIS makes several
recommendations that may be inconsistent with the
approved licensing bases for operating plants,
including:
The draft RIS proposes "Degraded
voltage conditions coincident with a
postulated design basis accident." BTP
PSB-1 says "subsequent occurrence."
Disagree.
NRC Staff asserts that coincident degraded
grid and accident is specified in the 77 Letter
and the BTP approach supports that
position.
See also staff response to Comment No. 36.
92
General
NEI 3/2/11
Letter
Page 4, First Bullet, Comment I (Cont.)
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection
The draft RIS proposes "DVR Dropout setting
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.
Disagree.
The staff position is consistent with 1977
letter and BTP PSB-1.
93
General
NEI 3/2/11
Letter
Page 4, Second Bullet, Comment I (Cont.)
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection
The draft RIS proposes "separate 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."
GDC 5 says:
"... in the event of an accident in one unit, an
orderly shutdown and cooldown of the remaining
units."
Disagree
This statement is consistent with GDC 17,
GL 79-36, and IEEE Standard 308-1971,
Class IE Electrical Systems, Section 8,
Multi-Unit Station Considerations.
94
General
NEI 3/2/11
Letter
Page 4, Third Bullet, Comment I (Cont.)
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
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.
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.
Disagree
See staffs response to Comment No. 37.
Manual actions for the purposes of reducing
load for the design of the plant electrical
distribution system should not be assumed.
This is not precluding load shedding as part
of normal operation when there is sufficient
time to do so to support adequate voltage.
See GL 79-36 for more details.
32
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
95
General
NEI 3/2/11
Letter
Page 5, Paragraph 2, Comment I (Cont.)
The Draft RIS Fails to Adequately Consider
Licensing Basis Variability in the Area of
Degraded Grid Voltage Protection
In addition to the specific examples provided
above, the draft RIS states "[t]he staff considers
degraded voltage conditions coincident with a
postulated design basis accident to be a credible
event." It is unclear what exactly the staff intended
with this statement.
Disagree
.
The 77 Letter indicates that the DVR circuits
should be designed assuming coincident
sustained degraded grid voltage and
accident events. Upon the onset of the
coincident accident and degraded grid event,
the time delay for the DVR circuit should
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
Letter
Page 6-7, Comment II
The Backfit Discussion Provided in the Draft
RIS is Inadequate
Given the complex regulatory and licensing history
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
consistent with the aforementioned
regulations [GDC 17] and generic
communications [1977 letter, Generic
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
distribution system design.19
This analysis misses the point. First, GDC 17 (like
most GDC) is cast in broad, general terms;
therefore, the fact that the specific positions
discussed in the draft RIS are "consistent with" the
design criteria does not necessarily mean that they
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
Disagree
The details of the inspection findings and
enforcement actions are discussed in detail
in the applicable inspection reports and TIAs,
if applicable referenced in the RIS.
NRC staff asserts that the regulations and
staff positions articulated in the RIS are
consistent with the regulations and existing
guidance documents and therefore do not
constitute new or different positions with
respect to the backfit rule (50.109).
See the supplemental response attached at
the end of this Comment/Response Table.
33
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
Page 1-2, First Bullet
Contrary to the stated intent, PPL believes that the
RIS does transmit new requirements and staff
positions. Specific comments applicable to
Susquehanna Steam Electric Station (SSES) are
as follows:
The RIS introduces the need to consider
both "starting and running" conditions
during all operating configurations while
maintaining the offsite power supply
connected to the plant electrical
distribution system. The establishment of
a degraded voltage relay (DVR) to detect
a "sustained" degraded voltage condition
challenges the relay's basis for
"protection" if its actuation (dropout)
setpoint must accommodate both starting
and running voltage conditions. The term
"sustained degraded voltage" implies a
steady state degraded voltage condition,
and excludes starting voltage
consideration
Disagree
The NRC 1977 letter states that voltage and
time setpoints shall be determined from an
analysis of the voltage requirements of the
safety related loads. Safety related
equipment, particularly large motors, have
starting and running voltage requirements.
This second level of protection should
address these voltage requirements.
Sustained degraded voltage, as discussed in
the 1977 letter, refers to grid voltage below
the expected low value given normal grid
operation and grid post contingency. Thus,
when grid voltages are degraded (such as
resulting in SR bus voltages down close to
where DVRs are set based on SR equipment
requirements), separation from the grid is
appropriate. The 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 due to sequencing or block loading
of loads during a design basis events.
98
Page 6, Section 1
Page 2, Paragraph 2
RIS 201 1-XX, Page 6, Section 1. "Degraded
Voltage Relaying Design Calculations" contains the
statement "staff considers degraded voltage
condition coincident with a postulated design basis
accident to be a credible event." This statement
implies a requirement to demonstrate capability of
connected loads to start and run at the degraded
voltage relay dropout setting. For Susquehanna,
sequencing of loads from the offsite power source
cannot be demonstrated at the relay dropout
setpoint because operation at this voltage level
would result in separation from the offsite
transmission system. Furthermore, the statement
on page 6 of the RIS is not in agreement with other
regulatory position documents such as GSI 171,
"Engineered Safety Features Failure (ESF) from a
Loss of Offsite Power (LOOP) subsequent to a
Loss of Coolant Accident (LOCA)," which
concluded a degraded voltage condition coincident
with a postulated design basis accident is not a
credible event.
Disagree
NRC Staff asserts that this statement means
that while the events are coincident (which is
important from the standpoint that the time
delay chosen for the DVR must support the
accident analysis), it does not mean that
connected loads must start and run at the
dropout setting. The dropout setting should
be developed based on the voltage
requirements (starting and running) and
therefore to develop values which are
bounding, the studies should be done under
worst starting and loading conditions, which
means the required voltage at the 1E bus
prior to the start or run case would have to
be higher than the setpoint. The main point is
that the setpoint should equate to the limiting
voltage at the limiting component during the
bounding starting or running scenario to
protect the 1E equipment.
99
Page 6, Section 1
Page 2, Paragraph 3
Additional clarification is necessary if starting
transients must be included when determining the
degraded voltage relay (DVR) dropout setpoint.
This condition will increase the probability of
separating from the offsite transmission system
and increase the likelihood of a double sequencing
event, which is a potential nuclear safety concern.
Disagree.
Proper design of the plant electrical
distribution system, given the operating
range of the grid and the proper selection of
DVR settings (based on the voltage
requirements of the 1E equipment), should
provide more than adequate operating
margin, preventing unnecessary separation
from offsite power.
100
Page 8, Section c)
Page 2, Paragraph 3
The RIS requires performance of analyses for an
accident in the unit being analyzed and
simultaneous shutdown of all other units at the
station. This is not consistent with the present
Susquehanna design and licensing basis, which is
an accident on one unit followed by the safe
shutdown of the second (non-accident) unit. The
safe shutdown of the non-accident unit is
Disagree.
This statement in the RIS is consistent with
GDC 17, GL 79-36, and IEEE Standard
308-1971, Class IE Electrical Systems,
Section 8, Multi-Unit Station Considerations.
34
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
Page 2-3, First Bullet
The draft RIS attempts to clarify the requirements
for setting the DVRs based on the criteria
established in the following three main documents:
1) NRC letters to licensees dated June 2 & 3, 1977,
2) Branch Technical Position (BTP), PSB-1
Revision 0,
3) Generic Letter 79-36, "Adequacy of Station
Electric Distribution Systems Voltages"
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.
Disagree.
The 1977 Letter is a 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). In addition, NRCs staff position is that
while the BTPs go into some more detail,
they are consistent with the 77 Letter.
102
General
Page 3, First Bullet
The lack of regulatory clarity in the RIS could result
in revising the degraded voltage setpoint for a
plant's DVR thus increasing the possibility of
premature separation from the offsite circuit (i.e.,
undervoltage relay actuation). This relay operation
could lead to an increase in the likelihood of a
double sequencing event, which has the potential
to create a nuclear safety concern.
Disagree.
Proper design of the plant electrical
distribution system, given the operating
range of the grid and the proper selection of
DVR settings (based on the voltage
requirements of the 1E equipment), should
provide more than adequate operating
margin, preventing unnecessary separation
from offsite power.
103
General
Page 3, Second Bullet
The RIS introduces the need for two sets of
calculations, one to establish the DVR relay
setpoint and one for the interface with the offsite
transmission system. The RIS should not specify
the number of calculations that are necessary for a
plant to meet a regulatory requirement.
Disagree.
The RIS is primarily identifying that different
types of calculations are necessary to
address different requirements. DVR setting
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
Page 3, Third Bullet
The condition the DVR is required to "protect"
needs to be specifically defined along with the
applicable relay setting, (i.e., relay minimum
dropout, maximum dropout, or reset). If the DVR is
installed to provide a level of protection then the
analysis must demonstrate that the safety related
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.
Disagree.
The DVRs function is specified in that it
ensures that 1E equipment is supplied with
adequate voltage in accordance with its
design requirements.
105
General
Page 3, Fourth Bullet
A clarification of the term "sustained" is needed to
determine if "sustained" refers to a steady state
voltage condition (i.e., no equipment starting
voltage effects) for which the DVR setting is to be
established.
Disagree.
Sustained degraded voltage, as discussed in
the NRC 1977 letter as well, refers to grid
voltage below the expected low value given
35
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
normal grid operation.
106
General
Page 3, Fifth Bullet
The guidance in the RIS is too general when
referring to operating voltages. The specific voltage
requirements need to be specified instead of
implied by a general term. The RIS needs to clarify
that the impact of the nuclear unit trip on the
transmission system voltage must be considered in
the plants voltage analysis.
Disagree.
The term voltage requirements used in the
RIS is defined in terms of equipment
manufacturer design requirements. NRC
Staff feels that this terminology is sufficiently
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
Page 3, Sixth Bullet
The time delays suggested are not consistent with
PSB-l. The PSB established one time delay to
allow for operator action. The RIS does not
address this requirement.
Disagree.
While there may be differences, the BTPs
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
Page 4, First Bullet
The RIS also lacks any acknowledgement of
preventative measures the licensees have taken to
minimize the potential for a degraded voltage
condition. Advancements in plant Ioadflow
analyses and measures to increase the reliability of
the offsite transmission system are industry
improvements that have occurred since the
degraded voltage events that occurred 35 years
ago.
Disagree.
Regardless of improvements made in terms
of grid operation and understanding of grid
operations impact on plant voltages, the
plant design has always had to properly
address grid operating parameters and their
impact on plant voltages in all modes of
operation. This point was properly
emphasized in the RIS as was in the original
regulations and guidance.
109
General
Page 2, 1.
The draft RIS asserts that there is a simple and
singular set of design criteria that have been
applied universally to the industry. Over the years
the degraded voltage performance requirements
have changed, as a specific issue, and on a
component basis (e.g., motor operated valves and
contactors), for individual nuclear power plants. As
a result, each nuclear power plant has specific
licensing bases, and there is no singular set of
requirements that have been applied universally to
the industry.
Disagree.
The 77 Letter provides staff positions on the
design criteria in that the voltage
requirements for the 1E equipment has to be
ensured by the DVR circuits by automatic
separation from offsite and transfer to the
onsite sources.
110
General
Page 2, 2.
The draft RIS asserts that the guidance provided to
the industry to address the Millstone and other
degraded voltage events adequately addresses
this potential common mode failure. The common
mode failure potential is that multiple trains of
safety equipment could be simultaneously
negatively impacted if off-site power is degraded.
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.
Disagree.
The DVR circuits will automatically separate
the 1E circuits from offsite power when
voltage requirements are not met which will
prevent the Millstone type event
automatically.
111
Page 6
Page 2, 3.
The draft RIS (page 6 of 10) states:
"The staff considers degraded voltage conditions
coincident with a postulated design basis accident
to be a credible event."
Disagree
The point being made in the RIS is that
setting of the DVR should include
consideration of a coincident accident signal
in that the time delay chosen for the DVR
36
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
It is our understanding that the established staff
interpretation is that this is not a credible event, as
discussed and supported by analysis in NUREG-
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.
should support the accident analysis
assumptions consistent with the NRC1977
letter.
112
General
Page 5, 4.
The recent licensing actions in the industry which
have mandated setpoint changes for the degraded
voltage relays (DVRs) and loss of voltage relays
(such as the one cited in the draft RIS for Fermi-2)
only serve to increase the probability of the 'ESF
Failure from LOOP Subsequent to a LOCA' event
discussed in NUREG/CR-6538 without providing
an advantage for any credible scenario. As such,
these changes may increase core damage
frequency (CDF).
It is APS's understanding that a comprehensive
review of guidance related to degraded grid voltage
has not been performed using the cost-benefit and
risk criteria of 10 CFR 50.109 (backfit rule), nor is it
apparent that risk insights have been used to
inform this guidance.
Disagree
Setting the DVRs in accordance with the
voltage requirements of the 1E equipment
coupled with a properly designed plant
electrical distribution system (and based on
the grids allowable voltage range) must
provide adequate voltage margin to preclude
offsite separation.
Disagree
NRC staff asserts that the regulations and
positions guidance articulated in the RIS are
consistent with the existing regulatory
requirements and NRC staff positions
guidance therefore do not constitute new or
different positions with respect to the backfit
rule (50.109).
113
General
Page 5, 5.
The draft RIS does not address the implication of
the Branch Technical Position (BTP) PSB-1
requirement that "The Class 1 E bus load shedding
scheme should automatically prevent shedding-
during sequencing of the emergency loads to the
bus." A large variety of voltage conditions could
exist during the sequencing period while the
shedding is blocked, and no analytical methods are
discussed that could demonstrate that equipment
damage or malfunction would not occur.
Disagree.
The design of the plant electrical distribution
system and the onsite sources should
provide for adequate voltage to all 1E
equipment in all normal, abnormal and
accident conditions.
Typical designs do not block the DVR or the
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
Page 5-7, 6.
In light of the summary of the resolution of GSI-
171, the draft RIS statement (page 7 of 10) that
"the time-delays(s) chosen for DVRs during
accident conditions should meet the accident
analysis assumptions..." does not seem
appropriate. The degraded voltage condition could
occur at various times during the initial energization
of the accident mitigation equipment, and the relay
time delay value only affects the additional time
Disagree.
Degraded voltage conditions can be
postulated to occur at anytime. The DVR
setpoint should accommodate the limiting
case for equipment protection. If the
recommendations of BTP PSB-1 are
followed, then a separation from the
degraded grid coupled with accident signal is
37
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
Page 7-8, 7.
It is not feasible for multi-unit nuclear plants to
successfully demonstrate that voltage from the
offsite circuits would be adequate, as described in
the draft RIS (page 8 of 10), for:
"(1) an accident in the unit being analyzed and
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."
North American Electric Reliability Corporation
(NERC) Standard TPL-004 recognizes that the
design and operating constraints of the
transmission network are such that the loss of all
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.
Disagree
This wording in the RIS is the same as was
used in GL 79-36.
TPL-004 requires transmission planning to
address simultaneous multiple transmission
contingencies.
The requirements of TPL-004 are not within
the scope of RIS.
116
Page 6
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."
This is not currently a design or licensing
requirement for all existing plants. As such the RIS
process would not be the appropriate method to
communicate a new regulatory position.
Disagree.
As a result of these Millstone events, the
NRC requested that all licensees implement
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).
38
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
117
Page 6
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
conservative, has the net effect of increasing the
frequency of delayed LOOP events during
transients, even when the subsequent sustained
voltage condition is not degraded (see Comment
4), with resulting adverse effects as discussed in
the resolution of GSI-171. It also neglects
consideration of the voltage levels that must be
maintained in the event of a unit trip and coincident
accident to prevent delayed LOOP events. Finally,
the draft RIS is silent on the particulars of the
voltage studies that would be acceptable to use to
determine the optimum time delay (such as the
plant operating conditions and voltage profile).
Disagree
The voltage studies done for evaluating
offsite power/onsite power interface should
use minimum expected voltage at the
plant/grid interface node, demonstrating
adequate voltage for starting and running of
plant components during normal, abnormal
and accident conditions. The voltage studies
for the DVR setpoints should require
plant/grid interface node voltages well below
the minimum expected values (including post
grid contingency).
118
Page 6
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,
the described model is of a nondegraded voltage
scenario that does not result in DVR actuation.
Therefore, it does not demonstrate that "required
safety related components are provided adequate
voltage" for accidents with degraded voltage
scenarios. That conclusion could only be
demonstrated by modeling degraded voltage
scenarios that involve DVR actuation. However, in
all cases involving degraded voltage coincident
with postulated accidents, such models would
result in delayed LOOP scenarios as discussed in
GSI-171.
Also, it reflects a non-conservative voltage profile.
If the voltage at the DVR monitored bus was at the
DVR dropout setting prior to starting a motor, it
would be lower than that during and after starting
the motor, and the voltage at the motor terminals
would be correspondingly lower, as well, compared
to the results using the constant bus voltage
methodology described in the draft RIS.
Disagree
The DVR dropout setting should be
developed based on the voltage
requirements (starting and running) and
therefore to develop values which are
bounding, the studies should be done under
worst starting and loading conditions which
means the required voltage at the 1E bus
prior to the start or run case would have to
be higher than the setpoint. The main point is
that the setpoint should equate to the limiting
voltage at the limiting component during the
bounding starting or running scenario to
protect the 1E equipment.
The RIS does not impose any new
requirements. It provides clarification on
existing requirements.
The DVR setpoint should be optimized for
motor starting transient and protection of
safety related equipment.
119
Page 5
Page 10, 11.
The draft RIS discussion asserts that the NRC
Office of Nuclear Reactor Regulation (NRR) Task
Disagree
39
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
Interface Agreement (TIA) response (TIA 2010-05)
"concluded ... the time delay to transfer from a
degraded offsite source to the standby power
source to support the emergency core cooling
equipment operation must be consistent with
accident analysis time assumptions, as required by
BTP PSB-1 (NUREG 0800)." This statement is not
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
process and was specifically addressed in the NRC
and APS correspondence (NRC Letter dated June
14, 1999, and APS letter dated July 16, 1999,
Question 13). The subject matter of the TIA did not
include the time delay element of the design, with
regard to the accident analysis time assumptions,
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.
The point being made in the RIS is that
setting of the DVR should include
consideration of a coincident degraded grid
and accident in that the time delay chosen
for the DVR should support the accident
analysis assumptions consistent with the
NRC1977 letter.
Task Interface Agreement 2010-005
(ADAMS Accession No. ML102800340)
provides more details regarding Palo Verde
degraded voltage inspection finding.
Plant specific findings are not in the scope of
the RIS.
120
Page 5
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.
The prevention strategy was implemented to
preclude a concurrent degraded voltage condition
and design basis accident because the PSB-1 type
design is not capable of adequately coping with
such an event. All such events would result in
delayed LOOP/double sequencing scenarios, as
described in GSI-171, for which there is no viable
analytical approach.
Disagree.
The point being made in the RIS is that
setting of the DVR should include
consideration of a coincident degraded grid
and accident in that the time delay chosen
for the DVR should support the accident
analysis assumptions consistent with the
NRC1977 letter.
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
Page 11, 13.
PVNGS originally implemented the second level
degraded voltage protection design consistent with
NRC letter Qualification Review of the PVNGS
Disagree
40
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
PVNGS does not reflect the historic licensing basis
for PVNGS.
The licensees analysis must show that
degraded voltage trip setpoint adequately
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
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
scenario, which is a malfunction of an SSC with a
different result than previously evaluated pursuant
to 10 CFR 50.59, for PVNGS. It could result in core
damage regardless of the time delay value at which
the DVR actuation (delayed LOOP) occurs. This is
the reason APS precludes such an event by
establishing appropriate initial conditions, with TS
LCO 3.8.1, Condition G, through license
amendment 123.
Disagree.
The point being made in the RIS is that
setting of the DVR should include
consideration of a coincident degraded grid
and accident in that the time delay chosen
for the DVR should support the accident
analysis assumptions consistent with the
NRC1977 letter.
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
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
in the GSI-171 resolution document to suggest that
Disagree
41
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
a shorter time delay (e.g., delayed LOOP occurring
sooner) would be of any benefit in preventing the
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.
This is a plant specific issue. The issue will
be reviewed through the ROP.
124
Page 5
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
PVNGS license amendment 123, which states:
"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.
Disagree.
See staffs response to Comment No. 123
125
Page 6
Nextera 1
This paragraph could be interpreted to require the
LOCA sequence to be modeled at the DVR dropout
setting. LOCA sequencing modeled at the DVR
Disagree
42
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
dropout setting would result in separation of the
buses from the Preferred Power Source (off-site
power) as the voltage would not recover above the
DVR reset value.
Clarify 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.
Clarify that LOCA sequencing is evaluated using
minimum switchyard voltage as starting point.
The dropout setting should be developed
based on the voltage requirements (starting
and running) and therefore to develop values
which are bounding, the studies should be
done under worst starting and loading
conditions which means the required voltage
at the 1E bus prior to the start or run case
would have to be higher than the setpoint.
The main point is that the setpoint should
equate to the limiting voltage at the limiting
component during the bounding starting or
running scenario to protect the 1E
equipment.
126
Page 6
Nextera 2
Having a sustained degraded voltage just above
the LVR voltage setting (70%) is not practical
without grid collapse and does not exist in Branch
Technical Position #1 (PSB-1).
Clarify degraded voltages to be analyzed to a
credible level.
Agree.
The DVR setpoints are calculated based only
on the voltage requirements of the 1E
equipment, not based on whether the grid
can sustain voltage at levels that result in
such conditions.
127
Page 6
Nextera 3
The statement that the DVR ensures adequate
operational (starting and running) is the first time in
NRC correspondence that starting equipment at
the DVR setpoint is expected. The example letter
sent to Peach Bottom in June 1977 did not require
starting of equipment at the DVR setpoint. This
requirement should be removed from the RIS since
it is not possible to start equipment at the DVR
setpoint and not subsequently separate from offsite
power. If the equipment starts at the DVR setpoint,
the voltage will dip during the transient and must
then recover above the reset point to avoid
separation from offsite power. Since the reset point
will always be above the DVR dropout point it will
be impossible to reset the relay.
Remove starting of equipment at the DVR setpoint
as a requirement.
Disagree
The dropout setting should be developed
based on the voltage requirements (starting
and running) and therefore to develop values
that are bounding, the studies should be
done under worst starting and loading
conditions, which means the required voltage
at the 1E bus prior to the start or run case
would have to be higher than the setpoint.
The main point is that the setpoint should
equate to the limiting voltage at the limiting
component during the bounding starting or
running scenario to protect the 1E
equipment.
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
distribution system for the establishing the
degraded voltage relay (DVR) settings; however, it
should be clarified that for credit may be taken for
minimum switchyard voltage/voltage drop
calculations (or the Offsite/Onsite Design Interface
Calculations).
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).
Agree
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 if the
response time will support normal operation.
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
43
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,
with an attempt to clarify "anticipated transient' by
adding in parenthesis "(anticipated operational
occurrence)" immediately afterwards. It is not clear
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".
Disagree
The reference added is the wording used in
GDC 17 (for consistency).
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
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
"Capacity' describes this as a "concurrent safe
shutdown on the remaining units".
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".
Disagree
This wording in the RIS is the same as was
used in GL 79-36.
TPL-004 requires transmission planning to
address simultaneous multiple transmission
contingencies.
The plant licensing basis provides basis for
analyses related to multi unit sites.
131
Page 8
Nextera 7
These guidelines ( e) and f) )seem contradictory
that you cannot credit procedurally controlled
operator actions to reduce load but you have to
assume the actions will be carried out when load is
added.
Delete "e) Manual load shedding should not be
assumed" or add allowance to credit procedurally
controlled operator actions to decrease load.
Disagree.
These guidelines are not contradictory in that
one is considering load shedding (not
conservative) for design of system based on
maximum load, while the other is about load
additions that occur per procedure
(conservative for maximum loading design).
132
General
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
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
Voltage sensing equipment cannot provide a
predictive function without crediting the capacity or
capability of the upstream system, since it cannot
determine the capacity or capability provided
during a transient condition such as a motor start.
Since the existing regulatory framework for
Disagree
The DVR dropout setting should be
developed based on the voltage
requirements (starting and running) and
therefore to develop values that are
bounding, the studies should be done under
worst starting and loading conditions, which
means the required voltage at the 1E bus
44
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
degraded voltage protection was based on use of
voltage relaying, it is not clear how the existing
relaying equipment could be used to demonstrate
compliance with an adequate motor starting
demonstration requirement.
2) A Degraded Voltage Relay Protection Setpoint
Based on Starting Voltage Does Not Provide Motor
Protection
This method could potentially be calculated but
would mean that the DVR setpoint would have
been determined during the starting of the most
limiting Class-I E motor. A degraded voltage relay
setpoint based on a motor starting would not
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.
prior to the start or run case would have to
be higher than the setpoint. The main point is
that the setpoint should equate to the limiting
voltage at the limiting component during the
bounding starting or running scenario to
protect the 1E equipment. In addition, the
time delay would be determined based on
the limiting starting transient duration only
(not based on allowing time for operator
action). In this manner, if the voltage drops
below expected values during starting
(based on the 1E equipment limits) and
prolongs the start transient, then the DVR
will timeout and separation will occur
(providing low starting voltage protection).
133
General
Progress
Energy 1
Background: The draft NRC Regulatory Issue
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
degraded voltage operation on nuclear plant safety
related / accident mitigating electrical equipment.
"Additional Conservatisms" from this point of view
tends to mean that the degraded voltage relaying
will actuate earlier in a degraded voltage event time
line - meaning it would be set to actuate at a higher
degraded voltage and/or with a shorter time delay.
In conflict with the NRC's desire to impose
additional conservatisms on degraded voltage
protection at nuclear power plants, the North
American Electric Reliability Corporation (NERC),
is developing a national standard for Frequency
and Voltage Excursion Ride- Through Performance
(PRC-024) for all generating stations in North
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
Disagree.
Additional conservatism should not be added
for the sake of adding conservatism.
Conservatism is typically added to
compensate for assumptions that cannot be
accurately verified or proven (e.g. cable
impedances when actual pull lengths are not
known).
.
NERC and FERC guidelines are reviewed by
NRC staff for applicability to nuclear plant
operation.
The DVR and loss of voltage relay settings
should not be in conflict with NERC or FERC
recommended guidelines for grid operations.
45
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
Energy 2
Background: The use of on-load automatic load tap
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
nuclear plants to use and take credit for on load
automatic load tap changing transformers for
nuclear plant offsite power supplies to prevent
degraded voltage events and improve the voltage
transient ride through capability of the nuclear
plants.
Agree
Load tap changers help improve voltage
regulation for normal plant operation. Load
tap changers do not help protect safety
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
Energy 3
Comment: Please also consider the italicized
changes below:
DVR Setting Design Calculations
Licensee voltage calculations should provide the
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.
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
Agree
Italicized changes suggested were
considered along with other similar
comments received from other stakeholders
and clarified in the RIS revision.Italicized
changes were not incorporated in the RIS
since the comments were not consistent with
the staffs existing guidance for DVR
settings.
46
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
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.
Contacts was deleted.
47
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
136
General
Progress
Energy 3
Comment: Please also consider the italicized
changes below:
Offsite/Onsite Design Interface Calculations
The offsite power source is the preferred source of
power to safely shut down the plant during design
basis accidents, abnormal operational occurrence,
and reactor trips. The licensee's voltage
calculations should provide the basis for proper
operation of the plant safety related electrical
distribution system, when supplied from the offsite
circuit (from the transmission network). These
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.
Agree
Italicized changes suggested were
considered along with other similar
comments received from other stakeholders
and clarified in the RIS revision.Italicized
changes were not incorporated in the RIS
since the comments were not consistent with
staffs existing guidance for offsite/onsite
design interface calculations.
137
General
Progress
Energy 3
Comment: Please also consider the italicized
changes below:
Guidelines for voltage drop calculations
a) The plant voltage analysis, while supplied from
the transmission network, should be based on the
operating voltage range of the transmission
network connection. This transmission
owner/operator supplied voltage range should
address normal, startup, shutdown, accident
mitigation, and alternate authorized transmission
network and plant system operating configurations
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.
Agree
Italicized changes suggested were
considered along with other similar
comments received from other stakeholders
and clarified in the RIS revision.Italicized
changes were not incorporated in the RIS
since the comments were not consistent with
staffs guidance provided in Generic Letter 79-36.
48
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
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.
3. Licensees were required to change their
operating license because the staff position.
However, this in and of itself, does not change the
licensees' Current Licensing Basis.
4. The "1977" letters apply only to addressees, i.e.,
plants licensed before 1977.
Adequacy of Station Electric Distribution System
Voltages
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.
Disagree
NRC staff does not agree with this position.
As a result of these Millstone events, the
NRC requested that all licensees implement
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).
49
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
position on existing regulatory requirements," the
RIS seeks to supersede the NRC reviewed and
approved Current Licensing Basis for many
licensees.
Disagree
The purpose of the RIS is to clarify the NRC
staffs 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. 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
Reimers
(DCCP)
The issue I am concerned about is the regulatory
conflict created by requiring the DVR setpoint to
preclude spurious actuation of the undervoltage
protection function.
The NRC draft RIS 2011-XXX discusses spurious
separation at least three times.
1.
The first occurrence is an accurate
restatement of the 1977 NRC position
that "The voltage protection shall include
coincidence logic to preclude spurious
trips of the offsite power source" (See
RIS Page 2, Item (b)).
2.
The second occurrence is in the
"Degraded Voltage Relay Design
Calculations" section. Specifically, the
second sentence of the first paragraph
on Page 7 reads "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." This introduces the DVR
Agree
NRC Staff agrees with commenters position
on use of the term spurious with respect to
the design of the DVR scheme to prevent
false actuations due to DVR component
failures or miss-operations
The RIS will be revised to remove spurious
from this section. The NRC Staff position is
that the settings are to be selected based on
the voltage requirements of the 1E
equipment such that when compared with
the minimum expected grid voltages, there
should be sufficient margin ensure that
separation from the grid would not be
expected during normal, abnormal or
50
No.
Section of RIS
Originator
Specific Comment
NRC Resolution
voltage and time setpoint interaction with
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
cannot predict future operating conditions.
Consequently, the DVR can't distinguish between
voltage transients that are expected to recover and
those that are not. Therefore, in the context of the
original NRC position (i.e. Point #1), I believe the
term spurious was in the context of false signals
from within the DVR instrumentation and not any
group of bus voltage transients. The IEEE 308
requirement that RIS Page 8, Item (i) is trying to
convey is "The preferred power supply shall be
capable of starting and operating all required
loads."
accident conditions.
NRC Staff agrees with commenters position
on use of the term spurious with respect to
the design of the DVR scheme to prevent
false actuations due to DVR component
failures of miss-operations
NRC Staff agrees with the commenters
position that the intent of item i) is to specify
that the preferred power supply is able to
start and run all required 1E equipment in
accordance with its voltage requirements
while not separating
141
Backup power
options
Brian Wilson,
CA
Why are there not back-up power sources located
on the roof of the fuel cell tanks with electric lines
Disagree
51
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
or propane and Ipg. A remote control panel from a
distant site would provide a safe environment to
control a dangerous situation safely.
This comment is not related to the RIS
regarding Adequacy of Station Electric
Distribution System Voltage. Therefore,
staff did not address the comment.
52
Supplemental Response to NEI Comment No. 96 in Comment/Resolution Table (corrected)
RESPONSES TO NEI 3-2-2-11 BACKFITTING COMMENTS
RIS On Adequacy of Station Electric Distribution Voltages
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
(and licensee1) resources.
The NRC believes a more sensical approach is: 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 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
has reviewed its records and believes that (with one exception which is being dealt with in a plant-specific manner2 ),
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.
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
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.
For this RIS, the NRC reviewed the regulatory guidance and NRC practice in this area, and concluded that the
guidance in the RIS is consistent with the NRCs regulations and staff guidance, as well as the NRCs approval of
1 If the NRC prepared a detailed discussion of a plants licensing bases to support the issuance of guidance, then it
would be likely that the licensee would be required to verify the NRCs discussion.
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
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
backfit appeal (ML112730194).
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53
electrical systems in various licensing actions (with one exception which is being dealt with in a plant-specific
manner3). 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
that licensee to invoke backfitting protection when the NRC proposes to impose that NRC position on that licensee.3
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).
54
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.
55
Alarm setpoint
significantly
higher than DVR
setpoint
100%
DVR set point based on minimum
voltage required at equipment terminal
to operate all safety related equipment
(BTP PSB-1) DVR-2 Protection with short time
delay to rideout the motor starting transients.
Ensures minimum voltage required for all
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;
Degraded Voltage Set Points
Figure 1
(BTP PSB-1) DVR -1 Protection - Existence of a sustained degraded
voltage condition
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.
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,
56
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.