ML110960591
| ML110960591 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 03/31/2011 |
| From: | Krich R M Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| IR-10-005 | |
| Download: ML110960591 (16) | |
See also: IR 05000390/2010005
Text
Valley Authority 1101 Market Street, LP 3R Chattanooga, Tennessee
37402-2801
R. M. Kirich Vice President Nuclear Licensing March 31, 2011 10 CFR 2.201 ATTN: Document Control Desk U.S. Nuclear Regulatory
Commission
Washington, D.C. 20555-0001
Watts Bar Nuclear Plant, Unit 1 Facility Operating
License No. NPF-90 NRC Docket Nos. 50-390 Subject: Response to Non-Cited
Violations05000390/2010005-03, "Failure to Use Worst Case 6900 VAC Bus Voltage in Design Calculations" References:
1. Letter from NRC (Eugene F. Guthrie) to TVA (R. M. Krich),"Watts Bar Nuclear Plant -NRC Integrated
Inspection
Report 05000390/2010005," dated January 28, 2011 2. Letter from TVA (R. M. Krich) to NRC "Request for Extension
for Time to Respond to Non-Cited
Violations
05000327,05000328/2010005-03, 'Failure to Use Worst Case 6900 VAC Bus Voltage in Design Calculations'
and 05000390/2010005-03, 'Failure to Use Worst Case 6900 VAC Bus Voltage in Design Calculations,"'
dated February 25, 2011 In Reference
1, the Nuclear Regulatory
Commission (NRC) issued Non-Cited
Violation (NCV)05000390/2010005-03, "Failure to Use Worst Case 6900 VAC Bus Voltage in Design Calculations," regarding
Watts Bar Nuclear Plant (WBN), Unit 1. The NCV addressed
issues regarding
the design basis of the degraded voltage protection
equipment.
In Reference
2, TVA requested
that the response date for potential challenge
of the Reference
1 NCV be extended to March 31, 2011.TVA has completed
a review of the concerns and issues regarding
the design of degraded voltage protection
at WBN, Unit 1, expressed
by the NRC in Reference
1.printed on recycled paper .1III-JIII
U.S. Nuclear Regulatory
Commission
Page 2 March 31, 2011 the NRC Enforcement
Policy, TVA contests Non-Cited
Violation
05000390/2010005-
03, "Failure to Use Worst Case 6900 VAC Bus Voltage in Design Calculation." The basis for TVA's denial of the subject NCV is provided in the Enclosure.
There are no commitments
associated
with this letter.If you have any questions
in this matter, please contact me at 423-751-3628.
Respectfully, Enclosure cc: NRC Director, Office of Enforcement
NRC Regional Administrator
-Region II NRC Senior Resident Inspector
-Watts Bar Nuclear Plant
ENCLOSURE TENNESSEE
VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNIT I NRC INTEGRATED
INSPECTION
REPORT 05000390/2010005
REPLY TO NOTICE OF A NON-CITED
VIOLATION
VALLEY AUTHORITY WATTS BAR NUCLEAR, PLANT UNIT 1 NRC INTEGRATED
INSPECTION
REPORT NO. 05000390/2010005
REPLY TO NOTICE OF A NON-CITED
VIOLATION I. Introduction
In a letter dated January 28, 2011, the Nuclear Regulatory
Commission
issued Non-Cited Violation (NCV)05000390/2010005-03, "Failure to Use Worst Case 6900 VAC Bus Voltage In Design Calculations," regarding
Watts Bar Nuclear Plant (WBN), Unit 1. The NCV addressed issues regarding
the design basis of the degraded voltage protection
equipment.
In the letter, the NRC stated: If you contest any NCV in this report, you should provide a response within 30 days of the date of this inspection
report, with the basis for your denial, to the Nuclear Regulatory
Commission, ATTN: Document Control Desk, Washington
DC 20555-0001;
with copies to the Regional Administrator, Region II; the Director, Office of Enforcement, United States Nuclear Regulatory
Commission, Washington, DC 20555-0001;
and the NRC Resident Inspector...
Based on the issuance date of the letter, the 30-day response date to contest the NCV was February 27, 2011. By letter dated February 25, 2011, the Tennessee
Valley Authority (TVA)requested
an extension
of the date for contesting
the NCV until March 31, 2011.TVA has conducted
a review of the NCV and of apparent NRC concerns regarding
the design of the degraded voltage protection
at WBN, Unit 1. Based on that review, TVA contests the NCV for the reasons discussed
in detail in this response.In addition to contesting
that the NRC's apparent position regarding the adequacy of the design and licensing
basis for degraded voltage protection
at WBN, Unit 1 is unjustified.
The NRC's concerns in this regard, although not clearly expressed
in the inspection
report 05000390/2010005, were reflected
in the discussions
between NRC inspectors
and TVA staff during the 2010 WBN Component
Design Basis Inspection (CDBI), by NRC management
during the CDBI inspection
exit teleconference
on December 16, 2010, and during a public meeting between NRC and TVA on degraded voltage protection
issues in Rockville, Maryland on March 11, 2011.TVA is firmly committed
to fully understanding
and resolving
NRC's concerns regarding degraded voltage protection.
However, TVA's position is that the issues should be resolved in a manner that implements
both existing regulatory
requirements (such as 10 CFR Part 50, Appendix B, Criterion
III, Design Control) and existing regulatory
processes (such as the reactor oversight
process, enforcement
process, and backfit process) in a credible manner that allows for a clear understanding
of the technical
issues and the associated
regulatory
framework.
TVA's response to the NCV and the apparent underlying
technical
and regulatory
concerns are presented
below in two separate elements:* Disputes the use of 10 CFR Part 50, Appendix B, Criterion
III, "Design Control" as the basis for the NCV, and* Disputes the NRC's apparent concerns regarding
the adequacy of the WBN current licensing
basis for degraded voltage protection.
E-1
II. Disputing
the Use of 10 CFR Part 50, Appendix B, Criterion
III, "Design Control" as the Basis for Non-Cited
Violation 05000390/2010005-03
Restatement
of Non-Cited
Violation A restatement
of the subject NCV from NRC Inspection
Report 05000390/2010005
is as follows.Enforcement:
10 CFR 50, Appendix B, Criterion
Ill, Design Control, states, in part, that measures shall be established
to assure that applicable
regulatory
requirements
and the design basis for structures, systems, and components
are correctly
translated
into specifications, drawings, procedures, and instructions.
This appendix also states in part that measures shall be established
for the selection
and review for suitability
of application
of processes
that are essential
to the safety-related
functions
of the structures, systems, and components.
Watts Bar TS Section 3.3.5-1, "Loss of Power Diesel Generator
Start Instrumentation," table 3.3.5-1, item 2 specifies
the 6900 VC emergency
bus undervoltage (degraded)
relay trip setpoints
to be as follows: "Allowable
Value, >_6570 VAC, Trip Setpoint, <6606 VAC and >_6593 VA C." Contrary to the above, since at least December 2001, the licensee failed to assure that applicable
regulatory
requirements
for undervoltage (degraded)
voltage protection, including
those prescribed
in TS 3.3.5-1, item 2, were correctly
translated
into design calculation, WBN-EEB-MS-TI-06-0029, "Degraded
Voltage Analysis," Revision 31, which evaluated
motor starting voltages at the beginning
of a design basis loss of coolant accident (LOCA) concurrent
with a degraded grid condition.
Further, the process used by the licensee for the selection
of input voltage value in the design calculation
was non-conservative
with respect to the TS. Specifically, the licensee used the input value of 6672 VAC which was higher than the maximum value of 6606 VAC specified
in TS. This did not result in a loss of function of safety-related
loads.Because this finding is of very low safety significance
and was entered into the licensee's
corrective
action program as PER 296306 this violation
is being treated as a NCV, consistent
with the NRC Enforcement
Policy. This finding is identified
as NCV 05000390, 2010005-:"Failure
to Use Worst Case 6900 VAC Bus Voltage in Design Calculations." URI 05000390/2010008-02, "Worst Case 6900 VAC Bus Voltage in Design Calculations" is closed.Summary of TVA Basis for Denial of Enforcement
Against Criterion
III, "Design Control" TVA disputes the use of 10 CFR Part 50, Appendix B, Criterion
Ill, "Design Control" as the basis for the NCV. TVA's dispute is based on what TVA views as a fundamental
misapplication
of 10 CFR Part 50, Appendix B, Criterion
Ill, "Design Control," to the specific facts regarding
the design of degraded voltage protection
features at WBN and the associated
Technical Specification (TS) values. TVA is specifically
concerned
with the NRC's discussion
of the relationship
between the TS requirements
and the design calculation
referenced
in the NCV.In the NRC's discussion
for the basis of the NCV, the NRC stated: The degraded voltage relay settings at Watts Bar are in accordance
with TS Table 3.3.5-1 which states the values to be as follows: Allowable
Value >_6570 VAC, Trip Setpoint between 6606 VAC and >6593 VAC.In addition, the NRC stated that: E-2
The inspector
reviewed licensee calculation
of record WBN-EEB-MS-TI-06-0029,"Degraded
Voltage Analysis," Rev. 31, which evaluated
motor starting voltages at the beginning
of a design basis loss of coolant accident (LOCA) concurrent
with a degraded grid condition.
This calculation
used the degraded voltage setpoint of 6672 V to analyze post LOCA load motor starting.
This voltage of 6672 VAC used in the calculation
was non-conservative
with respect to the voltage specified
in TS which specified
a maximum value of 6606 VAC.Finally, the NRC concluded:
Contrary to the above, since at least December 2001, the licensee failed to assure that applicable
regulatory
requirements
for undervoltage (degraded)
voltage protection, including
those prescribed
in TS 3.3.5-1, item 2, were correctly
translated
into design calculation, WBN-EEB-MS-TI-06-0029, "Degraded
Voltage Analysis," Revision 31, which evaluated
motor starting voltages at the beginning
of a design basis loss of coolant accident (LOCA) concurrent
with a degraded grid condition.
Further, the process used by the licensee for the selection
of input voltage value in the design calculation
was non-conservative
with respect to the TS. Specifically, the licensee used the input value of 6672 VAC which was higher than the maximum value of 6606 VAC specified
in TS. This did not result in a loss of function of safety-related
loads.With respect to the referenced
requirements
of 10 CFR Part 50, Appendix B, Criterion
Ill,"Design Control," the regulations
state in part: Measures shall be established
to assure that applicable
regulatory
requirements
and the design basis, as defined in § 50.2 and as specified
in the license application, for those structures, systems, and components
to which this appendix applies are correctly translated
into specifications, drawings, procedures, and instructions.
The regulations
in 10 CFR Part 50, Appendix B, Criterion
III create an expectation
that implementing
documents
used at the plant (i.e., specifications, drawings, procedures
and instructions)
should accurately
reflect the design basis for the associated
Structure, System or Component (SSC) and the regulatory
requirements
for the SSC. The NCV confuses the fact that the design basis of the SSC is developed
in part based on regulatory
design requirements
and the fact that the design basis then gives rise to certain operational
limits which may themselves
be established
as regulatory
requirements
via a condition
of the operating
license.For example, the design basis of an SSC may be developed
in part based on applicable
regulatory
requirements
such as those contained
in 10 CFR 50, Appendix A, General Design Criteria (GDC) or other applicable
design oriented sections of 10 CFR Part 50. The GDC in such instance as this example are binding regulatory
requirements
with regard to the design of the facility.In addition to regulatory
requirements
related to the design of the SSC, 10 CFR 50.36 requires the development
of TS which "will be derived from the analyses and evaluation
included in the safety analysis report, and amendments
thereto..." Within the TS, 10 CFR 50.36 requires the inclusion
of limiting conditions
for operation
which are "the lowest functional
capability
or performance
levels of equipment
required for safe operation
of the facility." The analyses from which the TS are derived include analyses which form, or support, the design basis (since 10 CFR 50.34 requires that the final safety analysis report include information
that presents the design bases). The TS are binding regulatory
requirements
insofar as these are imposed as an Appendix of the facility operating
license and they are in addition to the regulatory
requirements
on which the design was based.E-3
The NRC appears to have mischaracterized
the relationship
between the design basis for the degraded voltage protection
system at WBN (which incorporates
design-related
regulatory
requirements)
and the TS Allowable
Values which are derived from the design basis. Simply stated, the NCV could be read to imply that TVA should have used values which are derived from the design basis (i.e., the TS Allowable
Values) as input requirements
to the design basis calculation
-which is itself the basis from which the TS are derived. TVA views this essentially
circular logic as inconsistent
with the purpose of 10 CFR Part 50, Appendix B, Criterion
Ill,"Design Control." Alternatively, the NRC's NCV discussion
may be read to imply that TVA should have used the TS Allowable
Values as input to a portion of the calculation
whose purpose was to confirm that, for circumstances
not linked to specific expected post-accident
conditions
and for stressed grid voltage conditions
not specifically
linked to any particular
failure mode, individual
loads powered from the auxiliary
power system would have sufficient
voltage to start without tripping protective
devices and without causing the degraded voltage protection
system to transfer from the preferred
offsite power system to the onsite AC power system. As discussed
below, it is difficult to understand
the logic of evaluating
the ability to start motors under conditions
in which the expected effect will be to cause degraded voltage relays to dropout and, with some likelihood, cause the Auxiliary
Power System to transfer to the onsite power source.Relationship
of Calculation
WBN-EEB-MS-TI06-0029, Revision 31 to TS 3.3.5 Allowable Values The discussion
in NRC Inspection
Report 05000390/201005
associated
with the NCV refers to the WBN calculation
WBN-EEB-MS-TI06-0029, Revision 31, "Degraded
Voltage Analysis." WBN-EEB-MS-TI06-0029
is a design calculation
and, as demonstrated
below, is related to the TS values referenced
in the NCV in that it provides an input (analytical
limit) to calculations
that specifically
derive the TS values. From the standpoint
of 10 CFR Part 50, Appendix B, Criterion Ill, "Design Control," WBN-EEB-MS-TI06-0029, and related calculations
discussed
below, are the means by which the regulatory
requirements
regarding
the design of the degraded voltage protections
system are translated
into specifications
-in this case, the TS themselves.
TVA first issued WBN-EEB-MS-TI06-0029
in 1992 and the purpose of WBN-EEB-MS-TI06-0029
is stated in Section 1 of Revision 31 of the calculation:
1.1 The purpose of this calculation
is to demonstrate
that the Watts Bar Auxiliary Power System complies with NRC Branch Technical
Position PSB-1 and to establish the basis for the degraded and loss of voltage relay setpoints
and their associated
time delays.1.2. Ensure that the voltage level is adequate to allow required safety electrical
equipment and devices to successfully
complete their safety function.1.3. Ensure that the duration of the degraded voltage at a given voltage level does not result in thermal degradation
or damage of any equipment.
WBN-EEB-MS-TI06-0029
is the design calculation
for the degraded voltage protection
system at WBN. As the design basis calculation, it captures the design related regulatory
basis for the degraded voltage protection
scheme. For degraded voltage protection
design, there are no explicit requirements
in 10 CFR Part 50, nor is there illuminating
guidance in an existing regulatory
guide. Thus, the reference
to NRC Branch Technical
Position (BTP) PSB-1 constitutes
the effective
regulatory
design requirements
for this system. The relationship
between WBN-EEB-MS-TI06-0029, Revision 31 and the TS Table 3.3.5-1 Allowable
Value and E-4
Trip Setpoints
is found in Section 5.1 of the calculation "Second Level Undervoltage (Degraded Voltage) Relay Dropout Setpoint." A lower boundary should be established
for the dropout setting of the degraded voltage relay.The lower boundary should be greater than the minimum safety bus voltage established
below. The nominal setpoint of the dropout setting should be equal to the lower boundary plus all tolerances
from potential
transformer (PT) ratio and burden errors, setpoint drift errors, and any other errors in accordance
with TVA Technical
Instruction
TI-28 (reference
2.13).The minimum safety bus voltage is selected by evaluating
operation
of the auxiliary
power system under steady-state (running)
conditions, with the 6.9kV Shutdown Boards voltages as low as possible while still keeping all connected
safety-related
loads within their rated operating
voltage range (within ANSI C84.1 utilization
voltages, range "B", reference
2.14).5.1.1. The loads evaluated
are safety-related
loads required for Unit I design basis event, safety injection
phase A or B. which would be used in normal operation
and/or initiated
upon an accident signal. Load types considered
are motor loads including
vendor package loads, 120VAC motor control circuits, and MCC 120VAC distribution
panel loads. The acceptance
criteria and means of evaluation
is as follows: 5.1.1.1. The lowest possible voltage at the 6.9kV Shutdown Boards without tripping (actuating)
the Degraded Voltage Relay is 6555 volts. (Sec. 3.9). The voltage at the various 480V class 1E boards is evaluated
at 6555V based on this relay setting at the 6.9kV Shutdown Board and the maximum steady state loading (Normal Operation
loading)conditions
to ensure that the minimum steady-state
running voltage requirements
to distribution
board and connected
equipment
are maintained.
Starting of motors is evaluated at voltage based on the upper reset setpoint operational
limit of the degraded voltage relays. Starting at a lower voltage could result in dropping out the degraded voltage relays and not being able to reset them prior to disconnection
and transfer to the emergency diesel generators.
This approach is consistent
with the guidelines
given in IEEE 741-1997, reference
2. 12.The degraded voltage protection
relay system operates to provide two distinct functions.
One function of the degraded voltage protection
relays is to ensure that the Auxiliary
Power system does reliably transfer from the offsite power supply to the onsite power supply under conditions
which are indicative
of a sustained
degraded voltage condition
on the offsite power supply. The design requirement
to have the Degraded Voltage Relay dropout setting linked to the Minimum Operating
Voltage ensures that the Auxiliary
Power System does not transfer from the offsite power supply to the onsite power supply except under voltage conditions
which are indicative
of a sustained
degraded voltage condition, that is, to ensure it does not prematurely
transfer from an offsite power supply that is sufficiently
reliable to supply safety related loads under normal and accident conditions.
Because the Degraded Voltage Relays are equipment
required for the safe operation
of the plant, values associated
with the Degraded Voltage Relay dropout setting are included in the TS. The TS Table 3.3.5-1 values referred to in the NCV are the 6.9 kV Emergency
Bus Undervoltage (Degraded
Voltage) Bus Undervoltage
Allowable
Value L>6570V) and Trip Setpoints
(> 6593 and <6606V). These values are associated
with the Degraded Voltage Relay dropout. These values do not include the reset setpoint for the Degraded Voltage Relays. The current TS Trip Setpoints
were calculated
in TVA calculation
WBPE2119202001.
The TS voltage values (Allowable
Value and Trip Setpoint)
are unchanged
from the values issued in the TS when the WBN, Unit 1 Operating
License was issued on February 7, 1996.E-5
As illustrated
by the above discussion, from a 10 CFR Part 50, Appendix B, Criterion
Ill, consideration, WBN-EEB-MS-TI06-0029, Revision 31, and related calculation
WBPE2119202001
are the means by which the regulatory
requirements
regarding
the design of the degraded voltage protections
system are translated
into specifications.
Relationship
of TS Table 3.3.5-1 Allowable
Values and Calculation
WBN-EEB-MS-TI06-
0029, Revision 31 Analysis of Motor Starting In the NCV, the NRC stated that TVA: failed to assure that applicable
regulatory
requirements
for undervoltage (degraded)
voltage protection, including
those prescribed
in TS 3.3.5-1, item 2, were correctly translated
into design calculation, WBN-EEB-MS-TI-06-0029, "Degraded
Voltage Analysis," Revision 31, which evaluated
motor starting voltages at the beginning
of a design basis loss of coolant accident (LOCA) concurrent
with a degraded grid condition.
Further, the process used by the licensee for the selection
of input voltage value in the design calculation
was nonconservative
with respect to the TS. Specifically, the licensee used the input value of 6672 VAC which was higher than the maximum value of 6606 VAC specified
in TS. This did not result in a loss of function of safety-related
loads.TVA's position is that, while the TS are regulatory
requirements, they are not design related requirements
and, in the case of the analysis of motor starting capability
in Section 6.2 of WBN-EEB-MS-TI06-0029, use of the TS values as inputs to the design analyses would be inconsistent
with the stated purpose of that analysis.The analysis in Section 6.2 of WBNEEBMSTI060029, Revision 31 provides insight to TVA as to the ability of the Auxiliary
Power System to provide sufficient
starting capacity for safety related motors for circumstances
in which the offsite power supply may be under sustained
degraded voltage conditions (whether post design basis accident or for other, unspecified
circumstances).
The approach to the analysis in Section 6.2 is identified
in Section 5.2 which states: The safety bus voltages associated
with the upper boundary of the reset setting are evaluated
to ensure adequate operation
of the auxiliary
power system under steady state (starting)
conditions
and recovery (running)
voltage conditions.
This portion of the calculation
was added in WBN-EEB-MS-TI06-0029, Revision 30, which was issued in January 2001. The "post LOCA" motor starting analysis referred to in the NCV was a confirmatory
evaluation
performed
over and above the design basis which existed when the NRC had previously
found the degraded voltage protection
scheme in conformance
with PSB-1 (see discussion
of NUREG-0847
and supplements
in Section II). Revision 31 of WBN-EEB-MS-
T106-0029
explicitly
indicates
the reason for not performing
the motor starting confirmatory
calculation
at voltage values similar to those of the degraded voltage setpoint (dropout):
Starting of motors is evaluated
at voltage based on the upper reset setpoint operational
limit of the degraded voltage relays. Starting at a lower voltage could result in dropping out the degraded voltage relays and not being able to reset them prior to disconnection
and transfer to the emergency
diesel generators.
To elaborate, the circumstances
under which the voltages may have degraded so far below the minimum grid operational
voltage are varied. If the bus voltage is degraded in a situation
where E-6
a safety injection
signal and block start of accident loads have caused the voltage to drop below the degraded voltage dropout setpoint (but not below the loss of voltage setpoint)
but then, due to unspecified
failure mechanisms, recover to a level well below 100% nominal, the Degraded Voltage Relay may or may not have reset. Evaluating
motor starting under such a non-mechanistic
scenario (i.e., by evaluating
at a voltage below the relay reset setpoint)
provides very little useful confirmatory
insight into the capability
of the auxiliary
power system. For that reason, TVA elected to use the value 6672 V AC (which bounds the Degraded Voltage Relay reset of 6681 V AC) rather than a lower value, which as stated in the calculation, would likely simply demonstrate
that the Degraded Voltage Relays would dropout.Furthermore, there is no conflict between TVA's minimum starting voltage analysis and Appendix B, Criterion
III because there are no specific requirements
in the NRC regulations
regarding
the method for evaluating
the competing
degraded voltage protection
system requirement
to provide protection
to safety related equipment
under degraded voltage conditions
and to support an onsite and offsite power system that meets the performance
requirements
of GDC-1 7, "Electric
Power Systems." Thus, TVA's design basis for the degraded voltage protection
scheme, including
the methodology
and assumptions
for a minimum starting voltage analysis, is that which is presented
in WBN-EEB-MS-TI06-0029, Revision 31.As a result, TVA's position is that the NRC's assertion
that TVA failed to properly incorporate
TS requirements
into the design calculation (i.e, specifications, instructions, drawings or procedures)
is unjustified.
Ill. Disputing
the NRC's apparent concerns regarding
the adequacy of the WBN current licensing
basis for degraded voltage protection
During the course of the WBN Component
Design Basis Inspection, including
the pre-exit and exit meetings and at a public meeting with the NRC on March 11, 2011, the NRC referred to concerns regarding
various aspects of the current licensing
basis for degraded voltage protection
at WBN. In addition to the NRC concerns regarding
the methodology
for performing
minimum starting voltage analyses discussed
above, the NRC expressed
concerns about the analytical
consideration
given to the installed
automatic
high-speed
Load Tap Changers associated
with the Common Station Service Transformers.
TVA is presenting
an evaluation
of the degraded voltage protection
current licensing
basis to address what appear to be NRC's underlying
concerns.
Reinforcing
TVA's perspective
that the NRC's concerns are fundamentally
grounded in the current licensing
basis was a discussion
provided by the NRC at the 2011 Regulatory
Information
Conference (RIC) on March 9, 2011.At the RIC technical
session on degraded voltage issues, the NRC confirmed
a recent.trend
in enforcement
actions regarding
degraded voltage protection
systems (consistent
with the recently issued draft Regulatory
Issue Summary (RIS) 201 1-XX, "Adequacy
of Station Electrical
Distribution
Systems," dated January 12, 2011). In discussing
the background
for the enforcement
trend, the NRC acknowledged
that, because degraded voltage protection
requirements
are not grounded in typical regulatory
hierarchy, the licensing
basis from plant to plant is quite variable.
The NRC did acknowledge
that understanding
the degraded voltage protection
requirements
and commitments
for any one plant requires a thorough review of the plant specific current licensing
basis. The NRC indicated
that a thorough review of any one plant's current licensing
basis documentation
can be time consuming.
TVA agrees that a thorough review of the current licensing
basis for a specific issue can be resource intensive
in many cases. However, TVA's position is that such a review is warranted in this case in order to establish
the clearest shared understanding
between the NRC and TVA of the complex technical
and regulatory
issues related to the NCV. TVA's analysis of the degraded voltage protection
system licensing
history at WBN is presented
below.E-7
WBN, Unit I Degraded Voltage Protection
Current Licensing
Basis As discussed
in the recently issued draft Regulatory
Issue Summary (RIS) 201 l-XX, "Adequacy of Station Electrical
Distribution
System," dated January 12, 2011, the NRC's regulatory
actions associated
with degraded voltage protection
essentially
began in the wake of the degraded voltage event at Millstone
Station in July 1976. The history of all of the NRC's regulatory
actions between the Millstone
event and the development
of the current WBN degraded voltage protection
licensing
basis in the early 1990's is not presented
here. The draft RIS provides an overview of the early years of generic regulatory
oversight
for this issue, including
reference
to the issuance of Branch Technical
Positions (BTP) of the Standard Review Plan, PSB-1, Revision 0, which was issued in July 1981.Development
of TVA Degraded Voltage Protection
Design Calculations
In the early 1990's, TVA developed
a methodology
for selecting
Degraded Voltage Relay (DVR)setpoints
based on the recommended
guidelines
that were issued as DMEDS 9211-01/HEE, EDS Clearinghouse
Recommendations
and Guidance Concerning
Settings of Second Level Undervoltage
Protection, dated November 20, 1992 (hereafter
referred to as EDS Clearinghouse).
The EDS Clearinghouse
guidance was developed
by the industry in response to NRC issues during Electrical
Distribution
System Functional
Inspections
in the late 1980s and early 1990s. The EDS Clearinghouse
guidance summarized
existing NRC requirements
for degraded voltage protections
as follows: Branch Technical
Position PSB-1 is a principal
source of NRC Staff technical
guidance in this area. The document is included as a "reference" in the NRC Temporary
Instruction
for EDSFIs TI 2515/107, pg. 4. The only NRC regulation
referenced
in PSB-1 is 10 CFR 50, Appendix A, General Design Criteria (GDC) 17, "Electric
Power Systems." However, GDC 17 does not specifically
address degraded grid voltage issues.With regard to motor starting voltage under accident conditions, the EDS Clearinghouse
stated: "Licensees
should be able to establish
adequate terminal voltages of accident loads under auxiliary
system transient
conditions (such as motor starting transients)
without tripping of protective
devices such as overcurrent
relays, thermal overloads, circuit breakers and fuses. We recommend
that licensees
perform analyses to ensure that during worst case motor transients
with bus voltage equivalent
to the minimum switchyard
voltage during anticipated
worst case system operation; (i) the bus voltage will not drop below the dropout setting of the relay during the transient
or (ii) if the bus voltage drops below the dropout setting of the relay during the transient, it will recover above the reset setting of the relay prior to the relay timing out." With regard to proper setting of degraded voltage relay setpoints, specifically, relay dropout setpoints, the EDS Clearinghouse
guidance stated: "When considering
the dropout setting of the relay, we recommend
lower and upper boundaries
be established.
The lower boundary is the value that is equivalent
to the minimum voltage at the safety related buses to ensure adequate downstream
terminal voltage for steady state operation
of accident loads. In our view, transient
conditions
of accident loads need not be considered
to establish
the lower boundary of the relay setting.If the bus is operating
at a voltage level that is at the lower boundary of the dropout setting, then any transients
applied to the bus, such as a motor start, will: a. cause the relay to dropout, b. begin the time delay to separate the safety bus from the grid, E-8
c. where the bus voltage is already below the reset setting of the relay, it will not recover to a sufficient
level to reset the relay following
the motor transient.
Therefore, the bus will separate from the grid, and d. energize the EDGs, initiate load shedding and resequencing
accident loads onto the bus.When establishing
the lower boundary of the dropout setting, we recommend
that the"enveloping
component
of the accident loads be identified.
Once the enveloping
component is identified, the minimum bus voltage to supply adequate terminal voltage to the enveloping
component
for the worst case "steady state" operating
scenario should be calculated.
When the lower boundary for steady state operation
of accident loads has been determined, we recommend
sufficient
margin be added to this value to establish
the lower end of the relays allowable
tolerance
band for technical
specification
purposes." With regard to the factors to be considered
when identifying
the minimum voltage for starting motors, the EDS Clearinghouse
provided the following
recommendations
on this subject: "In our view, transient
conditions
of accident loads need not be considered
to establish the lower boundary of the relay setting. If the bus is operating
at a voltage level that is at the lower boundary of the dropout setting, then any transients
applied to the bus, such as a motor start, will ... cause the relay to dropout...""Licensees
should be able to establish
adequate terminal voltages of accident loads under auxiliary
system transient
conditions (such as motor starting transients)
without tripping of protective
devices ... We recommend
that licensees
perform analyses to ensure that during worst case motor transients
with bus voltage equivalent
to the minimum switchyard
voltage during anticipated
worst case system operation: (i) the bus voltage will not drop below the dropout setting of the relay during the transient
or (ii) if the bus voltage drops below the dropout setting of the relay during the transient, it will recover above the reset setting of the relay prior to the relay timing our." The EDS Clearinghouse
recommended
the lower boundary of the DVR Dropout setting to be calculated
based on: "...the value that is equivalent
to the minimum voltage at the safety related buses to ensure adequate downstream
terminal voltage for steady state operation
of accident loads." TVA applied the recommendations
of the EDS Clearinghouse
by developing
plant specific design calculations.
WBN-EEB-MS-TI06-0029
as the Degraded Voltage Protection
Design Basis For WBN, TVA developed
calculation
WBN-EEB-MS-TI06-0029, Revision 0 which was issued in March 1992. Unlike SQN, TVA's calculation
for degraded voltage analysis for WBN did not reference
the EDS Clearinghouse
guidance as a source document.
Rather, WBN-EEB-MS-
T106-0029, Revision 0 cited NRC Branch Technical
Position (BTP) PSB-1 as a source of design input. Between the issuance of Revision 0 of WBN-EEB-MS-TI06-0029
in March 1992 and the issuance of NRC's NUREG-0847, "Safety Evaluation
Report related to the Operation
of Watts Bar Nuclear Plants, Units 1 and 2," Supplement
20 in February 1996, WBN-EEB-MS-T106-0029
remained the design basis calculation
for the degraded voltage protection
scheme although it was revised from time to time to account for plant design changes during construction.
Of particular
note relative to NCV 05000390/2010005-03, WBN-EEB-MS-TI06-0029, Revision 0 does not contain an explicit evaluation
of single motor starting capability
as currently
exists in Revision 31. The section of WBN-EEB-MS-TI06-0029 (Section 5.2) which states E-9
"The safety bus voltages associated
with the upper boundary of the reset setting are evaluated
to ensure adequate operation
of the auxiliary
power system under steady state (starting)
conditions
and recovery (running)
voltage conditions." was added in Revision 30 of the calculation
issued in January 2001. The motor starting analysis referred to in the NCV was a confirmatory
evaluation
performed
over and above the design basis which existed when the NRC found the degraded voltage protection
scheme in conformance
with PSB-1. However, as listed below, the NRC found the degraded voltage protection
approach at WBN in conformance
with Branch Technical
Position PSB-1.NRC Licensing
Conclusions
Regarding
TVA Degraded Voltage Methodology
As part of the its review of TVA's application
for an operating
license for WBN, the NRC issued NUREG-0847, "Safety Evaluation
Report related to the Operation
of Watts Bar Nuclear Plants, Units 1 and 2," including
numerous supplements.
In NUREG-0847
and supplements, the NRC found TVA's degraded voltage protection
scheme in conformance
with PSB-1: WBN 1982 SER: "...in IEEE Standard 308-1974, which states that preferred
offsite and the standby onsite emergency
power supplies shall not have a common mode failure between them. The positions
that the staff have developed
are being used in the evaluation
of electrical
power designs for operating
plants, and CP and OL applications.
The applicant
was made aware of these positions, which have been incorporated
into SRP Appendix 8A as BTP PSB-1. The applicant
documented
that the Watts Bar design will be modified as shown on FSAR Figure 040.62-1 to meet BTP PSB-1. By letter dated October 9, 1981, the applicant
provided additional
descriptive
information
to support the conclusion
that the Watts Bar design, once modified, will be in conformance
with positions
B-I and B-2 of BTP PSB-1. The staff concludes
that the proposed design meets BTP positions
and is acceptable.
In addition, design implementation
will be verified as part of the site visit/drawing
review. In regard to positions
3 and 4, the applicant
has documented
that the auxiliary
power system meets these positions
and that the analyses will be verified in the preoperational
testing program. This meets the staff positions
and is acceptable, pending verification
of the analyses.
The staff will verify the test results." WBN 1982 SER, SER Supplement
7, dated September
1991: "In the SER, the staff stated that it would verify the adequacy of the applicant's
analysis regarding
compliance
with Branch Technical
Position (BTP) PSB-1 once the preoperational
test was completed.
The staff noted that the preoperational
test has shown that the Watts Bar design conforms with BTP PSB-1 (see Inspection
Report 50-390/84-90, dated February 11, 1985). The staff is still evaluating
the status of this issue and will update the status in a future SSER." WBN 1982 SER, SER Supplement
13, dated April 1994: "In the SER, the staff stated that it would verify the adequacy of the applicant's
analysis regarding
compliance
with BTP PSB-1 once the preoperational
test was completed.
The staff had confirmed
that a preoperational
test had shown that the Watts Bar design conforms with BTP PSB-1 (see Inspection
Report 50-390/84-90, dated February 11, 1985). Hence, Confirmatory
Issue 28 was resolved.
However, due to design changes, the results obtained from the previous test are no longer valid and the applicant
is performing
preoperational
tests again. The staff will review this issue when it inspects the applicant's
preoperational
test program." E-10
14, dated December 1994: "The material that follows revises the discussion
in SSER 13.(1) Allowable
Technical
Specification
Limits for the Inverse Time Delay Relay. In SSER 13, the staff stated that Technical
Specifications
should require, for example, that the capability
of the relays not to trip when subjected
to a voltage of 75 percent for 30 seconds be demonstrated.
The staff implied that this had been included in the draft Technical
Specifications.
This statement
was wrong. Instead, the staff required that the setpoints
and allowable
values for the load-shed
and diesel start relays be included in the plant's Technical
Specifications
to resolve the concerns." WBN 1982 SER, SER Supplement
20, dated February 1996: In SSER 13, the staff stated that Confirmatory
Issue 28 was resolved on the basis of a preoperational
test documented
in Inspection
Report 50-390/84-90, dated February 11, 1985. However, the staff stated that the results obtained from that test were no longer valid since TVA was reperforming
the preoperational
tests. The preoperational
test was conducted
by TVA and reviewed by the staff in Inspection
Reports 50-390/95-22 (September
8, 1995) and 50-390/95-77 (December
6, 1995). This update does not change the staff's conclusion
regarding
Confirmatory
Issue 28.Additional
Licensing
Basis Documents
Regarding
WBN Degraded Voltage Protection
WBN License Amendment
36 -2002 Since issuance of the Facility Operating
License in 1996, TVA has obtained only one amendment
to the TS related to the degraded voltage protection
scheme for WBN. The license amendment
is relevant to concerns expressed
by the NRC during the WBN CDBI inspection
and at the March 11, 2011 public meeting on degraded voltage protection
issues. The particular
concern, as best understood
by TVA, relates to the "crediting" of automatic
load tap changers in analyses related to degraded voltage protections
designs. This Enclosure
does not include a detailed technical
discussion
regarding
the appropriate
consideration
for any feature of the non-safety related offsite power system including
automatic, high speed load tap changers.However, it should be noted that to the extent that voltage recovery following
a voltage transient is influenced
by many features of the offsite power system, the performance
of automatic
load tap changers in establishing
Degraded Voltage Relay time delay settings cannot be separated from the performance
of those tap changers in evaluating
dynamic voltage performance
on the distribution
system.By letter dated May 14, 2001, TVA requested
an amendment
to the WBN TS to revise the Trip Setpoint and Allowable
Value for Table 3.3.5-1, Function 2(b), 6.9 kV Emergency
Bus Undervoitage (Degraded
Voltage) -Time Delay, as follows:* The Trip Setpoints
were revised from: > 5.84 sec and < 6.16 sec to> 9.73 sec and <10.27 sec.* The Allowable
Value was revised from > 5.7 sec and < 6.3 sec to > 9.42 sec and <10.49 sec.As reason for the change, TVA stated: E-11
WBN design modification (DCN D-50565-A)
would change the setpoint of the degraded voltage relay timers from a nominal 6 seconds to 10 seconds to relax the offsite power criteria.
The primary purpose of this change is to provide the plant additional
operating margin by allowing additional
time for the automatic
load tap changers (LTCs) on the Common Station Service Transformers
C and D (0-XFMR-200-C/CSST
and 0-XFMR-200-
D/CSST) to compensate
for postulated
degraded voltage conditions
on the WBN 161 kV Off-Site Power Grid.In the application, TVA presented
additional
information
regarding
the analyses of on-site distribution
system performance
that had been conducted
to support the proposed amendment, including
consideration
of the role of the automatic
load tap changers.On January 23, 2002, the NRC issued Amendment
36 to the WBN TS and approved the proposed changes. In the safety evaluation (SE) accompanying
the amendment, the NRC stated: The design modification
would change the setpoint of the degraded voltage relay timers from a nominal 6 seconds to 10 seconds to relax the offsite power criteria.
The primary purpose of this change is to provide the plant additional
operating
margin by allowing additional
time for the automatic
LTCs on the CSSTs. C and D to compensate
for postulated
degraded voltage conditions
on the 161 kV off-site power grid.Recent analysis of the offsite grid indicates
that due to future grid loading projections (within about 2 years), voltage fluctuations
on the grid could unnecessarily
challenge
the EDGs and associated
equipment
if the current time delay settings are maintained.
The longer time delay setpoint would relax the present offsite power criteria by allowing a more severe worst case degraded voltage condition
on the 161 kV grid to be accommodated
by the CSST C and D LTCs. This extended time delay would eliminate
an unnecessary
electrical
associated
with the automatic
transfer from the preferred
offsite power supply to the EDGs when a degraded voltage condition
of less than 10 seconds is experienced.
Consequently, challenges
to equipment
associated
with the actuation
of breakers, shedding of loads, starting of the EDGs, etc., would also be reduced or eliminated.
The analysis was performed
using the Electrical
Analyzer Program. The software includes the capability
to analyze the electrical
auxiliary
power system for loading, short-circuit
currents, running voltages, and starting voltages.
The calculations
demonstrated
the ability of the offsite power system to start and operate all required loads for a worst case DBE without transferring
to the EDGs. Increasing
the delay time from 6 to 10 seconds will not change the voltage recovery profile. The lower boundary dropout and the upper reset setpoint of the degraded voltage relays remains unchanged.
Analyses have shown that operating
equipment, such as motors, would not be damaged and would accelerate
back to rated speed, thus ensuring their continued
availability
to perform their intended safety function.
Specifically, the analysis demonstrated
that the required safety-related equipment
in operation
at the time a degraded voltage condition
occurred would continue to operate throughout
the 10-second
delay. If the degraded voltage condition cleared during this time period, the voltage would return to nominal levels and be available for equipment
required to perform safety functions.
Calculations
demonstrated
that the automatic
LTCs remain capable of regulating
the 6.9 kV shutdown board voltage within the present voltage relay setpoints.
The LTCs will restore 6.9 kV shutdown board voltage for a safety injection
signal with a simultaneous
worst case grid drop before the degraded E-12
voltage relays actuate to transfer power supply to the EDGs. Engineered
safeguard motors will have sufficient
voltage available
at the terminals
to ensure proper starting and operation, when supplied by offsite power. Maximum loading on transformers, distribution
system cables, and 6900 V and 480 V boards is bounded by current analyses and remains below component
ratings. If the degraded condition
still existed at the end of the 10-second
time period, transfer to the EDGs would occur and the voltage would recover to an acceptable
level. In either case, acceptable
voltage levels would be available
for equipment
to respond in a timely manner if called upon to perform a safety function.IV. Conclusion
As discussed
in Section II of this enclosure, TVA's position is that the NRC's use of 10 CFR Part 50, Appendix B, Criterion
III, Design Control in issuing NCV 50-390/2010005-03
is not consistent
with essential
purpose of that important
regulatory
requirement
and is not consistent
with the facts associated
with TVA's control of the design of the degraded voltage protection
scheme at WBN, Unit 1. Accordingly, TVA denies the subject NCV.As discussed
in Section III of this enclosure, TVA understands
that the NRC is currently
giving additional
focus to degraded voltage protection
issues at nuclear power plants. While the NRC has attempted
to explain its current concerns through numerous vehicles including
through individual
inspection
activities, a limited number of plant specific backfits, and issuance of draft RIS 2011 -XX, the technical
and regulatory
concerns of the NRC remain unclear.To respond to the NRC's expectations
in the NCV that TVA evaluate post LOCA motor starting using the Degraded Voltage Relay dropout value rather than the 6672 V AC currently
in the design, TVA would have to modify the fundamental
design documents
of the facility which constitutes
a change to the facility design. To the extent that such a modification
would be made in response to the NRC's position in the NCV which conflicts
with the NRC's previous position in NUREG-0847
and related supplements
regarding
conformance
of the design to Branch Technical
Position PSB-1, TVA's position is that such a modification
would constitute
a backfit. Accordingly, should the NRC seek to pursue the issues regarding
the adequacy of the WBN degraded voltage protection
system design, TVA requests that the NRC treat the matter in accordance
with the provisions
of 10 CFR 50.109, "Backfitting." TVA does not take any position in this denial of the subject NCV regarding
whether such a backfit would be eligible for the exceptions
to the backfit analysis and documentation
provisions
of 10 CFR 50.109. However, TVA notes that in the draft RIS, the NRC uses language regarding degraded voltage analysis requirements
and limitations
that do not exist in any previous regulatory
requirement
or guidance documents
on this subject and which now directly conflict with NRC reviewed provisions
of the WBN current licensing
basis.E-13