ML110960591

Watts Bar Nuclear Plant, Unit 1, Response to Non-Cited Violations 05000390-10-005-03, Failure to Use Worst Case 6900 VAC Bus Voltage in Design Calculations

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

Tennessee Valley Authority1101 Market Street, LP 3RChattanooga, Tennessee 37402-2801R. M. KirichVice PresidentNuclear LicensingMarch 31, 201110 CFR 2.201ATTN: Document Control DeskU.S. Nuclear Regulatory CommissionWashington, D.C. 20555-0001Watts Bar Nuclear Plant, Unit 1Facility Operating License No. NPF-90NRC Docket Nos.50-390Subject: Response to Non-Cited Violations05000390/2010005-03, "Failure toUse 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 Report05000390/2010005," dated January 28, 20112. Letter from TVA (R. M. Krich) to NRC "Request for Extension forTime to Respond to Non-Cited Violations 05000327,05000328/2010005-03, 'Failure to Use Worst Case 6900 VAC BusVoltage in Design Calculations' and 05000390/2010005-03, 'Failureto Use Worst Case 6900 VAC Bus Voltage in Design Calculations,"'dated February 25, 2011In Reference 1, the Nuclear Regulatory Commission (NRC) issued Non-Cited Violation(NCV)05000390/2010005-03, "Failure to Use Worst Case 6900 VAC Bus Voltage inDesign Calculations," regarding Watts Bar Nuclear Plant (WBN), Unit 1. The NCVaddressed issues regarding the design basis of the degraded voltage protectionequipment. In Reference 2, TVA requested that the response date for potentialchallenge of the Reference 1 NCV be extended to March 31, 2011.TVA has completed a review of the concerns and issues regarding the design ofdegraded voltage protection at WBN, Unit 1, expressed by the NRC in Reference 1.printed on recycled paper .1III-JIII U.S. Nuclear Regulatory CommissionPage 2March 31, 2011the NRC Enforcement Policy, TVA contests Non-Cited Violation 05000390/2010005-03, "Failure to Use Worst Case 6900 VAC Bus Voltage in Design Calculation." Thebasis 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,Enclosurecc:NRC Director, Office of EnforcementNRC Regional Administrator -Region IINRC Senior Resident Inspector -Watts Bar Nuclear Plant ENCLOSURETENNESSEE VALLEY AUTHORITYWATTS BAR NUCLEAR PLANT, UNIT INRC INTEGRATED INSPECTION REPORT 05000390/2010005REPLY TO NOTICE OF A NON-CITED VIOLATION TENNESSEE VALLEY AUTHORITYWATTS BAR NUCLEAR, PLANT UNIT 1NRC INTEGRATED INSPECTION REPORT NO. 05000390/2010005REPLY TO NOTICE OF A NON-CITED VIOLATIONI. IntroductionIn a letter dated January 28, 2011, the Nuclear Regulatory Commission issued Non-CitedViolation (NCV)05000390/2010005-03, "Failure to Use Worst Case 6900 VAC Bus Voltage InDesign Calculations," regarding Watts Bar Nuclear Plant (WBN), Unit 1. The NCV addressedissues 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 thedate of this inspection report, with the basis for your denial, to the Nuclear RegulatoryCommission, ATTN: Document Control Desk, Washington DC 20555-0001; with copies tothe Regional Administrator, Region II; the Director, Office of Enforcement, United StatesNuclear Regulatory Commission, Washington, DC 20555-0001; and the NRC ResidentInspector...Based on the issuance date of the letter, the 30-day response date to contest the NCV wasFebruary 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 ofthe degraded voltage protection at WBN, Unit 1. Based on that review, TVA contests the NCVfor the reasons discussed in detail in this response.In addition to contesting the NCV, TVA considers that the NRC's apparent position regardingthe adequacy of the design and licensing basis for degraded voltage protection at WBN, Unit 1is unjustified. The NRC's concerns in this regard, although not clearly expressed in theinspection report 05000390/2010005, were reflected in the discussions between NRCinspectors and TVA staff during the 2010 WBN Component Design Basis Inspection (CDBI), byNRC management during the CDBI inspection exit teleconference on December 16, 2010, andduring a public meeting between NRC and TVA on degraded voltage protection issues inRockville, Maryland on March 11, 2011.TVA is firmly committed to fully understanding and resolving NRC's concerns regardingdegraded voltage protection. However, TVA's position is that the issues should be resolved in amanner 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 reactoroversight process, enforcement process, and backfit process) in a credible manner that allowsfor 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 arepresented below in two separate elements:* Disputes the use of 10 CFR Part 50, Appendix B, Criterion III, "Design Control" as thebasis for the NCV, and* Disputes the NRC's apparent concerns regarding the adequacy of the WBN currentlicensing basis for degraded voltage protection.E-1 II. Disputing the Use of 10 CFR Part 50, Appendix B, Criterion III, "Design Control" asthe Basis for Non-Cited Violation 05000390/2010005-03Restatement of Non-Cited ViolationA 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, thatmeasures shall be established to assure that applicable regulatory requirements and thedesign basis for structures, systems, and components are correctly translated intospecifications, drawings, procedures, and instructions. This appendix also states in partthat measures shall be established for the selection and review for suitability of applicationof 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 StartInstrumentation," 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, TripSetpoint, <6606 VAC and >_6593 VA C."Contrary to the above, since at least December 2001, the licensee failed to assure thatapplicable regulatory requirements for undervoltage (degraded) voltage protection,including those prescribed in TS 3.3.5-1, item 2, were correctly translated into designcalculation, WBN-EEB-MS-TI-06-0029, "Degraded Voltage Analysis," Revision 31, whichevaluated motor starting voltages at the beginning of a design basis loss of coolantaccident (LOCA) concurrent with a degraded grid condition. Further, the process used bythe 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 6672VAC which was higher than the maximum value of 6606 VAC specified in TS. This did notresult in a loss of function of safety-related loads.Because this finding is of very low safety significance and was entered into the licensee'scorrective 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." URI05000390/2010008-02, "Worst Case 6900 VAC Bus Voltage in Design Calculations" isclosed.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 basisfor the NCV. TVA's dispute is based on what TVA views as a fundamental misapplication of 10CFR Part 50, Appendix B, Criterion Ill, "Design Control," to the specific facts regarding thedesign of degraded voltage protection features at WBN and the associated TechnicalSpecification (TS) values. TVA is specifically concerned with the NRC's discussion of therelationship 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 withTS 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 thebeginning of a design basis loss of coolant accident (LOCA) concurrent with a degradedgrid condition. This calculation used the degraded voltage setpoint of 6672 V to analyzepost 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 valueof 6606 VAC.Finally, the NRC concluded:Contrary to the above, since at least December 2001, the licensee failed to assure thatapplicable regulatory requirements for undervoltage (degraded) voltage protection,including those prescribed in TS 3.3.5-1, item 2, were correctly translated into designcalculation, WBN-EEB-MS-TI-06-0029, "Degraded Voltage Analysis," Revision 31, whichevaluated motor starting voltages at the beginning of a design basis loss of coolantaccident (LOCA) concurrent with a degraded grid condition. Further, the process used bythe 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 6672VAC which was higher than the maximum value of 6606 VAC specified in TS. This did notresult 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 thedesign basis, as defined in § 50.2 and as specified in the license application, for thosestructures, systems, and components to which this appendix applies are correctlytranslated into specifications, drawings, procedures, and instructions.The regulations in 10 CFR Part 50, Appendix B, Criterion III create an expectation thatimplementing documents used at the plant (i.e., specifications, drawings, procedures andinstructions) should accurately reflect the design basis for the associated Structure, System orComponent (SSC) and the regulatory requirements for the SSC. The NCV confuses the factthat the design basis of the SSC is developed in part based on regulatory design requirementsand the fact that the design basis then gives rise to certain operational limits which maythemselves 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 applicableregulatory requirements such as those contained in 10 CFR 50, Appendix A, General DesignCriteria (GDC) or other applicable design oriented sections of 10 CFR Part 50. The GDC insuch instance as this example are binding regulatory requirements with regard to the design ofthe facility.In addition to regulatory requirements related to the design of the SSC, 10 CFR 50.36 requiresthe development of TS which "will be derived from the analyses and evaluation included in thesafety analysis report, and amendments thereto..." Within the TS, 10 CFR 50.36 requires theinclusion of limiting conditions for operation which are "the lowest functional capability orperformance levels of equipment required for safe operation of the facility." The analyses fromwhich the TS are derived include analyses which form, or support, the design basis (since 10CFR 50.34 requires that the final safety analysis report include information that presents thedesign bases). The TS are binding regulatory requirements insofar as these are imposed as anAppendix of the facility operating license and they are in addition to the regulatory requirementson which the design was based.E-3 The NRC appears to have mischaracterized the relationship between the design basis for thedegraded voltage protection system at WBN (which incorporates design-related regulatoryrequirements) and the TS Allowable Values which are derived from the design basis. Simplystated, the NCV could be read to imply that TVA should have used values which are derivedfrom the design basis (i.e., the TS Allowable Values) as input requirements to the design basiscalculation -which is itself the basis from which the TS are derived. TVA views this essentiallycircular 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 theTS 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 gridvoltage conditions not specifically linked to any particular failure mode, individual loads poweredfrom the auxiliary power system would have sufficient voltage to start without tripping protectivedevices and without causing the degraded voltage protection system to transfer from thepreferred offsite power system to the onsite AC power system. As discussed below, it is difficultto understand the logic of evaluating the ability to start motors under conditions in which theexpected 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 AllowableValuesThe discussion in NRC Inspection Report 05000390/201005 associated with the NCV refers tothe 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 theTS values referenced in the NCV in that it provides an input (analytical limit) to calculations thatspecifically derive the TS values. From the standpoint of 10 CFR Part 50, Appendix B, CriterionIll, "Design Control," WBN-EEB-MS-TI06-0029, and related calculations discussed below, arethe means by which the regulatory requirements regarding the design of the degraded voltageprotections 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-TI06-0029is stated in Section 1 of Revision 31 of the calculation:1.1 The purpose of this calculation is to demonstrate that the Watts Bar AuxiliaryPower System complies with NRC Branch Technical Position PSB-1 and to establishthe basis for the degraded and loss of voltage relay setpoints and their associatedtime delays.1.2. Ensure that the voltage level is adequate to allow required safety electrical equipmentand devices to successfully complete their safety function.1.3. Ensure that the duration of the degraded voltage at a given voltage level does notresult in thermal degradation or damage of any equipment.WBN-EEB-MS-TI06-0029 is the design calculation for the degraded voltage protection systemat WBN. As the design basis calculation, it captures the design related regulatory basis for thedegraded voltage protection scheme. For degraded voltage protection design, there are noexplicit requirements in 10 CFR Part 50, nor is there illuminating guidance in an existingregulatory guide. Thus, the reference to NRC Branch Technical Position (BTP) PSB-1constitutes the effective regulatory design requirements for this system. The relationshipbetween WBN-EEB-MS-TI06-0029, Revision 31 and the TS Table 3.3.5-1 Allowable Value andE-4 Trip Setpoints is found in Section 5.1 of the calculation "Second Level Undervoltage (DegradedVoltage) 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 establishedbelow. The nominal setpoint of the dropout setting should be equal to the lower boundary plusall tolerances from potential transformer (PT) ratio and burden errors, setpoint drift errors, andany 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 powersystem under steady-state (running) conditions, with the 6.9kV Shutdown Boards voltages aslow as possible while still keeping all connected safety-related loads within their ratedoperating 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 uponan accident signal. Load types considered are motor loads including vendor package loads,120VAC motor control circuits, and MCC 120VAC distribution panel loads. The acceptancecriteria 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 various480V class 1E boards is evaluated at 6555V based on this relay setting at the 6.9kVShutdown Board and the maximum steady state loading (Normal Operation loading)conditions to ensure that the minimum steady-state running voltage requirements todistribution board and connected equipment are maintained. Starting of motors is evaluatedat voltage based on the upper reset setpoint operational limit of the degraded voltagerelays. Starting at a lower voltage could result in dropping out the degraded voltage relaysand not being able to reset them prior to disconnection and transfer to the emergencydiesel 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. Onefunction of the degraded voltage protection relays is to ensure that the Auxiliary Power systemdoes reliably transfer from the offsite power supply to the onsite power supply under conditionswhich are indicative of a sustained degraded voltage condition on the offsite power supply. Thedesign requirement to have the Degraded Voltage Relay dropout setting linked to the MinimumOperating Voltage ensures that the Auxiliary Power System does not transfer from the offsitepower supply to the onsite power supply except under voltage conditions which are indicative ofa sustained degraded voltage condition, that is, to ensure it does not prematurely transfer froman offsite power supply that is sufficiently reliable to supply safety related loads under normaland accident conditions.Because the Degraded Voltage Relays are equipment required for the safe operation of theplant, values associated with the Degraded Voltage Relay dropout setting are included in theTS. The TS Table 3.3.5-1 values referred to in the NCV are the 6.9 kV Emergency BusUndervoltage (Degraded Voltage) Bus Undervoltage Allowable Value L>6570V) and TripSetpoints (> 6593 and <6606V). These values are associated with the Degraded Voltage Relaydropout. These values do not include the reset setpoint for the Degraded Voltage Relays. Thecurrent TS Trip Setpoints were calculated in TVA calculation WBPE2119202001. The TSvoltage values (Allowable Value and Trip Setpoint) are unchanged from the values issued in theTS 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 calculationWBPE2119202001 are the means by which the regulatory requirements regarding the design ofthe 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 StartingIn 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 correctlytranslated into design calculation, WBN-EEB-MS-TI-06-0029, "Degraded VoltageAnalysis," Revision 31, which evaluated motor starting voltages at the beginning of adesign 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 thedesign calculation was nonconservative with respect to the TS. Specifically, the licenseeused the input value of 6672 VAC which was higher than the maximum value of 6606VAC 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 relatedrequirements 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 beinconsistent with the stated purpose of that analysis.The analysis in Section 6.2 of WBNEEBMSTI060029, Revision 31 provides insight to TVA as tothe ability of the Auxiliary Power System to provide sufficient starting capacity for safety relatedmotors for circumstances in which the offsite power supply may be under sustained degradedvoltage 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 areevaluated to ensure adequate operation of the auxiliary power system under steadystate (starting) conditions and recovery (running) voltage conditions.This portion of the calculation was added in WBN-EEB-MS-TI06-0029, Revision 30, which wasissued in January 2001. The "post LOCA" motor starting analysis referred to in the NCV was aconfirmatory evaluation performed over and above the design basis which existed when theNRC 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 confirmatorycalculation 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 operationallimit of the degraded voltage relays. Starting at a lower voltage could result in droppingout the degraded voltage relays and not being able to reset them prior to disconnectionand transfer to the emergency diesel generators.To elaborate, the circumstances under which the voltages may have degraded so far below theminimum grid operational voltage are varied. If the bus voltage is degraded in a situation whereE-6 a safety injection signal and block start of accident loads have caused the voltage to drop belowthe degraded voltage dropout setpoint (but not below the loss of voltage setpoint) but then, dueto unspecified failure mechanisms, recover to a level well below 100% nominal, the DegradedVoltage 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) providesvery little useful confirmatory insight into the capability of the auxiliary power system. For thatreason, TVA elected to use the value 6672 V AC (which bounds the Degraded Voltage Relayreset of 6681 V AC) rather than a lower value, which as stated in the calculation, would likelysimply demonstrate that the Degraded Voltage Relays would dropout.Furthermore, there is no conflict between TVA's minimum starting voltage analysis andAppendix B, Criterion III because there are no specific requirements in the NRC regulationsregarding the method for evaluating the competing degraded voltage protection systemrequirement to provide protection to safety related equipment under degraded voltageconditions and to support an onsite and offsite power system that meets the performancerequirements of GDC-1 7, "Electric Power Systems." Thus, TVA's design basis for the degradedvoltage protection scheme, including the methodology and assumptions for a minimum startingvoltage 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 TSrequirements into the design calculation (i.e, specifications, instructions, drawings orprocedures) is unjustified.Ill. Disputing the NRC's apparent concerns regarding the adequacy of the WBN currentlicensing basis for degraded voltage protectionDuring the course of the WBN Component Design Basis Inspection, including the pre-exit andexit meetings and at a public meeting with the NRC on March 11, 2011, the NRC referred toconcerns regarding various aspects of the current licensing basis for degraded voltageprotection at WBN. In addition to the NRC concerns regarding the methodology for performingminimum starting voltage analyses discussed above, the NRC expressed concerns about theanalytical consideration given to the installed automatic high-speed Load Tap Changersassociated with the Common Station Service Transformers.TVA is presenting an evaluation of the degraded voltage protection current licensing basis toaddress what appear to be NRC's underlying concerns. Reinforcing TVA's perspective that theNRC's concerns are fundamentally grounded in the current licensing basis was a discussionprovided 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 inenforcement actions regarding degraded voltage protection systems (consistent with therecently issued draft Regulatory Issue Summary (RIS) 201 1-XX, "Adequacy of Station ElectricalDistribution Systems," dated January 12, 2011). In discussing the background for theenforcement trend, the NRC acknowledged that, because degraded voltage protectionrequirements are not grounded in typical regulatory hierarchy, the licensing basis from plant toplant is quite variable. The NRC did acknowledge that understanding the degraded voltageprotection requirements and commitments for any one plant requires a thorough review of theplant specific current licensing basis. The NRC indicated that a thorough review of any oneplant'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 beresource intensive in many cases. However, TVA's position is that such a review is warrantedin this case in order to establish the clearest shared understanding between the NRC and TVAof the complex technical and regulatory issues related to the NCV. TVA's analysis of thedegraded voltage protection system licensing history at WBN is presented below.E-7 WBN, Unit I Degraded Voltage Protection Current Licensing BasisAs discussed in the recently issued draft Regulatory Issue Summary (RIS) 201 l-XX, "Adequacyof Station Electrical Distribution System," dated January 12, 2011, the NRC's regulatory actionsassociated with degraded voltage protection essentially began in the wake of the degradedvoltage event at Millstone Station in July 1976. The history of all of the NRC's regulatoryactions between the Millstone event and the development of the current WBN degraded voltageprotection licensing basis in the early 1990's is not presented here. The draft RIS provides anoverview of the early years of generic regulatory oversight for this issue, including reference tothe 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 CalculationsIn 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 LevelUndervoltage Protection, dated November 20, 1992 (hereafter referred to as EDSClearinghouse). The EDS Clearinghouse guidance was developed by the industry in responseto NRC issues during Electrical Distribution System Functional Inspections in the late 1980s andearly 1990s. The EDS Clearinghouse guidance summarized existing NRC requirements fordegraded voltage protections as follows:Branch Technical Position PSB-1 is a principal source of NRC Staff technical guidance inthis area. The document is included as a "reference" in the NRC Temporary Instruction forEDSFIs 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, GDC17 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 underauxiliary system transient conditions (such as motor starting transients) without tripping ofprotective devices such as overcurrent relays, thermal overloads, circuit breakers andfuses. We recommend that licensees perform analyses to ensure that during worst casemotor transients with bus voltage equivalent to the minimum switchyard voltage duringanticipated worst case system operation; (i) the bus voltage will not drop below the dropoutsetting of the relay during the transient or (ii) if the bus voltage drops below the dropoutsetting of the relay during the transient, it will recover above the reset setting of the relayprior to the relay timing out."With regard to proper setting of degraded voltage relay setpoints, specifically, relay dropoutsetpoints, the EDS Clearinghouse guidance stated:"When considering the dropout setting of the relay, we recommend lower and upperboundaries be established. The lower boundary is the value that is equivalent to theminimum voltage at the safety related buses to ensure adequate downstream terminalvoltage for steady state operation of accident loads. In our view, transient conditions ofaccident 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 recoverto a sufficient level to reset the relay following the motor transient.Therefore, the bus will separate from the grid, andd. energize the EDGs, initiate load shedding and resequencing accident loads onto thebus.When establishing the lower boundary of the dropout setting, we recommend that the"enveloping component of the accident loads be identified. Once the enveloping componentis identified, the minimum bus voltage to supply adequate terminal voltage to theenveloping component for the worst case "steady state" operating scenario should becalculated. When the lower boundary for steady state operation of accident loads has beendetermined, we recommend sufficient margin be added to this value to establish the lowerend of the relays allowable tolerance band for technical specification purposes."With regard to the factors to be considered when identifying the minimum voltage for startingmotors, the EDS Clearinghouse provided the following recommendations on this subject:"In our view, transient conditions of accident loads need not be considered to establishthe lower boundary of the relay setting. If the bus is operating at a voltage level that is atthe lower boundary of the dropout setting, then any transients applied to the bus, suchas a motor start, will ... cause the relay to dropout...""Licensees should be able to establish adequate terminal voltages of accident loadsunder auxiliary system transient conditions (such as motor starting transients) withouttripping of protective devices ... We recommend that licensees perform analyses toensure that during worst case motor transients with bus voltage equivalent to theminimum switchyard voltage during anticipated worst case system operation: (i) the busvoltage will not drop below the dropout setting of the relay during the transient or (ii) ifthe bus voltage drops below the dropout setting of the relay during the transient, it willrecover 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 becalculated based on:"...the value that is equivalent to the minimum voltage at the safety related buses toensure adequate downstream terminal voltage for steady state operation of accidentloads."TVA applied the recommendations of the EDS Clearinghouse by developing plant specificdesign calculations.WBN-EEB-MS-TI06-0029 as the Degraded Voltage Protection Design BasisFor WBN, TVA developed calculation WBN-EEB-MS-TI06-0029, Revision 0 which was issued inMarch 1992. Unlike SQN, TVA's calculation for degraded voltage analysis for WBN did notreference 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 designinput. Between the issuance of Revision 0 of WBN-EEB-MS-TI06-0029 in March 1992 and theissuance of NRC's NUREG-0847, "Safety Evaluation Report related to the Operation of WattsBar Nuclear Plants, Units 1 and 2," Supplement 20 in February 1996, WBN-EEB-MS-T106-0029remained the design basis calculation for the degraded voltage protection scheme although itwas revised from time to time to account for plant design changes during construction. Ofparticular note relative to NCV 05000390/2010005-03, WBN-EEB-MS-TI06-0029, Revision 0does not contain an explicit evaluation of single motor starting capability as currently exists inRevision 31. The section of WBN-EEB-MS-TI06-0029 (Section 5.2) which statesE-9

"The safety bus voltages associated with the upper boundary of the reset setting areevaluated to ensure adequate operation of the auxiliary power system under steadystate (starting) conditions and recovery (running) voltage conditions."was added in Revision 30 of the calculation issued in January 2001. The motor startinganalysis referred to in the NCV was a confirmatory evaluation performed over and above thedesign basis which existed when the NRC found the degraded voltage protection scheme inconformance with PSB-1. However, as listed below, the NRC found the degraded voltageprotection approach at WBN in conformance with Branch Technical Position PSB-1.NRC Licensing Conclusions Regarding TVA Degraded Voltage MethodologyAs part of the its review of TVA's application for an operating license for WBN, the NRC issuedNUREG-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 NRCfound 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 onsiteemergency power supplies shall not have a common mode failure between them. Thepositions that the staff have developed are being used in the evaluation of electricalpower designs for operating plants, and CP and OL applications. The applicant wasmade aware of these positions, which have been incorporated into SRP Appendix 8A asBTP PSB-1. The applicant documented that the Watts Bar design will be modified asshown 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 thatthe Watts Bar design, once modified, will be in conformance with positions B-I and B-2of BTP PSB-1. The staff concludes that the proposed design meets BTP positions and isacceptable. In addition, design implementation will be verified as part of the sitevisit/drawing review. In regard to positions 3 and 4, the applicant has documented thatthe auxiliary power system meets these positions and that the analyses will be verified inthe 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 analysisregarding compliance with Branch Technical Position (BTP) PSB-1 once thepreoperational test was completed. The staff noted that the preoperational test hasshown 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 issueand 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 analysisregarding compliance with BTP PSB-1 once the preoperational test was completed. Thestaff had confirmed that a preoperational test had shown that the Watts Bar designconforms 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 isperforming preoperational tests again. The staff will review this issue when it inspectsthe applicant's preoperational test program."E-10 WBN 1982 SER, SER Supplement 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 SSER13, the staff stated that Technical Specifications should require, for example, that thecapability of the relays not to trip when subjected to a voltage of 75 percent for 30seconds be demonstrated. The staff implied that this had been included in the draftTechnical Specifications. This statement was wrong. Instead, the staff required that thesetpoints and allowable values for the load-shed and diesel start relays be included inthe 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 apreoperational 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 longervalid since TVA was reperforming the preoperational tests. The preoperational test wasconducted 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 notchange the staff's conclusion regarding Confirmatory Issue 28.Additional Licensing Basis Documents Regarding WBN Degraded Voltage ProtectionWBN License Amendment 36 -2002Since issuance of the Facility Operating License in 1996, TVA has obtained only oneamendment to the TS related to the degraded voltage protection scheme for WBN. The licenseamendment is relevant to concerns expressed by the NRC during the WBN CDBI inspectionand at the March 11, 2011 public meeting on degraded voltage protection issues. The particularconcern, as best understood by TVA, relates to the "crediting" of automatic load tap changers inanalyses related to degraded voltage protections designs. This Enclosure does not include adetailed 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 transientis influenced by many features of the offsite power system, the performance of automatic loadtap changers in establishing Degraded Voltage Relay time delay settings cannot be separatedfrom the performance of those tap changers in evaluating dynamic voltage performance on thedistribution system.By letter dated May 14, 2001, TVA requested an amendment to the WBN TS to revise the TripSetpoint and Allowable Value for Table 3.3.5-1, Function 2(b), 6.9 kV Emergency BusUndervoitage (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 degradedvoltage relay timers from a nominal 6 seconds to 10 seconds to relax the offsite powercriteria. The primary purpose of this change is to provide the plant additional operatingmargin by allowing additional time for the automatic load tap changers (LTCs) on theCommon 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 kVOff-Site Power Grid.In the application, TVA presented additional information regarding the analyses of on-sitedistribution 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 theproposed changes. In the safety evaluation (SE) accompanying the amendment, the NRCstated:The design modification would change the setpoint of the degraded voltage relay timersfrom a nominal 6 seconds to 10 seconds to relax the offsite power criteria. The primarypurpose of this change is to provide the plant additional operating margin by allowingadditional time for the automatic LTCs on the CSSTs. C and D to compensate forpostulated 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 theEDGs and associated equipment if the current time delay settings are maintained. Thelonger time delay setpoint would relax the present offsite power criteria by allowing a moresevere worst case degraded voltage condition on the 161 kV grid to be accommodated bythe CSST C and D LTCs. This extended time delay would eliminate an unnecessaryelectrical transient associated with the automatic transfer from the preferred offsite powersupply to the EDGs when a degraded voltage condition of less than 10 seconds isexperienced. Consequently, challenges to equipment associated with the actuation ofbreakers, shedding of loads, starting of the EDGs,etc., would also be reduced or eliminated.The analysis was performed using the Electrical Transient Analyzer Program. Thesoftware includes the capability to analyze the electrical auxiliary power system forloading, short-circuit currents, running voltages, and starting voltages. The calculationsdemonstrated the ability of the offsite power system to start and operate all required loadsfor a worst case DBE without transferring to the EDGs. Increasing the delay time from 6 to10 seconds will not change the voltage recovery profile. The lower boundary dropout andthe upper reset setpoint of the degraded voltage relays remains unchanged. Analyseshave shown that operating equipment, such as motors, would not be damaged and wouldaccelerate back to rated speed, thus ensuring their continued availability to perform theirintended safety function. Specifically, the analysis demonstrated that the required safety-related equipment in operation at the time a degraded voltage condition occurred wouldcontinue to operate throughout the 10-second delay. If the degraded voltage conditioncleared during this time period, the voltage would return to nominal levels and be availablefor equipment required to perform safety functions. Calculations demonstrated that theautomatic LTCs remain capable of regulating the 6.9 kV shutdown board voltage withinthe present voltage relay setpoints. The LTCs will restore 6.9 kV shutdown board voltagefor a safety injection signal with a simultaneous worst case grid drop before the degradedE-12 voltage relays actuate to transfer power supply to the EDGs. Engineered safeguardmotors will have sufficient voltage available at the terminals to ensure proper starting andoperation, when supplied by offsite power. Maximum loading on transformers, distributionsystem cables, and 6900 V and 480 V boards is bounded by current analyses andremains below component ratings. If the degraded condition still existed at the end of the10-second time period, transfer to the EDGs would occur and the voltage would recover toan acceptable level. In either case, acceptable voltage levels would be available forequipment to respond in a timely manner if called upon to perform a safety function.IV. ConclusionAs discussed in Section II of this enclosure, TVA's position is that the NRC's use of 10 CFRPart 50, Appendix B, Criterion III, Design Control in issuing NCV 50-390/2010005-03 is notconsistent with essential purpose of that important regulatory requirement and is not consistentwith the facts associated with TVA's control of the design of the degraded voltage protectionscheme 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 givingadditional focus to degraded voltage protection issues at nuclear power plants. While the NRChas attempted to explain its current concerns through numerous vehicles including throughindividual inspection activities, a limited number of plant specific backfits, and issuance of draftRIS 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 startingusing the Degraded Voltage Relay dropout value rather than the 6672 V AC currently in thedesign, TVA would have to modify the fundamental design documents of the facility whichconstitutes a change to the facility design. To the extent that such a modification would bemade in response to the NRC's position in the NCV which conflicts with the NRC's previousposition in NUREG-0847 and related supplements regarding conformance of the design toBranch Technical Position PSB-1, TVA's position is that such a modification would constitute abackfit. Accordingly, should the NRC seek to pursue the issues regarding the adequacy of theWBN degraded voltage protection system design, TVA requests that the NRC treat the matter inaccordance 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 abackfit would be eligible for the exceptions to the backfit analysis and documentation provisionsof 10 CFR 50.109. However, TVA notes that in the draft RIS, the NRC uses language regardingdegraded voltage analysis requirements and limitations that do not exist in any previousregulatory requirement or guidance documents on this subject and which now directly conflictwith NRC reviewed provisions of the WBN current licensing basis.E-13