ML13092A347
| ML13092A347 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 04/02/2013 |
| From: | Tennessee Valley Authority |
| To: | Division of Reactor Safety II |
| References | |
| IR-12-008 | |
| Download: ML13092A347 (8) | |
Text
Watts Bar Nuclear Plant 2012008*004 Degraded Vonage Relay Issue Response This position paper provides additional clarification of TVA's response to a concern raised by the NRC during the 2012 Component Design Basis Inspection (CDBI) at Watts Bar (WBN). Inspection Report 05000390/2012008'" documents the concern as Unresolved Item (URI)0500039012012008-04, "Effect of System Harmonics on Degraded Voltage Relay Function."
NRC concern:
The team was concerned that harmonics on the 6.9 kV system could cause the degraded voltage relays to fail to actuate at the setpoint specified by technical specifications. Persistent harmonics can be produced by factors external to the nuclear site or by internal phenomena. A typical internal source of harmonics at nuclear power plants is motor defects. The team was also concemed that transient harmonics could cause the relays to spuriously reset during an actual degraded voltage event. thereby delaying the protective function beyond the 10 seconds stipulated in technical specification limiting condition for operation 3.3.5. SpeCifically, the degraded voltage relay's design features an
'instantaneous' reset characteristic that could allow reset of the degraded voltage relay in less than two cycles in the presence of harmonics, thereby reinitiating the external 10 seconds timer. The reset function of the existing degraded voltage relays is identical to the tripping fUnction of the ovelVoltage relays that actuated due to transient harmonics in 1993. In 1993, transient harmonics were measured at levels of greater than 10% total harmonic distortion during the troubleshooting for PER 930397 versus the 0.3% distortion deemed acceptable by the relay vendor. The transient harmonics documented in PER 930397 were attributed to events that included the trip of the nearby Sequoyah generating station, and to breaker operations at the Watts Bar station. The team noted that similar conditions could exist during an accident scenario when proper performance of the degraded voltage scheme time delay would be critical with respect to satisfying the response time assumptions in the accident analysis.
In response to the team's concerns, the licensee provided information regarding condition monitoring of large motors that consisted of periodic measurement and analysis of motor bearing vibration from which various defects that may produce harmonics could be identified. The team noted, however, that there was no written guidance or acceptance criteria for these tests that would prompt engineering to investigate whether suspected motor defects could produce harmonics that would adversely affect the accuracy of degraded voltage relays. Specifically, there was no recognition in design or maintenance documents regarding the susceptibility of the degraded voltage relays to harmonic distortion, or the need to investigate suspected motor defects with respect to this susceptibility. The team further noted that during nonnal bus voltage conditions when voltage is above the degraded voltage relay reset setpoint, harmonics would shift system peak voltage away from the degraded voltage relay operating setpoint rather than closer to it, and so the presence of harmful harmonics would not 'self-reveal' by spurious actuations. The ovelVoltage relays are now equipped with harmonic filters so they will also not reveal the presence of either transient or persistent harmonics. Based on the team's observations, the licensee has entered these concerns into their corrective action program as PER 515413 and PER 546072.
The team determined that additional review of information recently received from the licensee regarding Watts Bar's design and licensing bases was necessary to determine if the licensee's performance constituted a violation of NRC regulatory requirements. Additionally, the team determined that additional cc>nsultation with the Office of Nuclear Reactor Regulation was warranted before reaching a final disposition of the unresolved item. This unresolved item is open pending (1) the review of additional infonnation from the licensee regarding the design and licensing basis of the degraded voltage relays and (2) consultation with the Office of Nuclear Reactor Regulation: URI 05000390/2012008*04, "Effect of System Harmonics on Degraded Voltage Relay Function.
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Watts Bar Nuclear Plant 2012008-004 Degraded Voltage Relay Issue Response Based on conference calls with Region-II staff on March 28,2012, April 9, 2012, April 30, 2012. and May 11, 2012, TVA understands the NRC's concern as follows:
Actuations of the 6.9kV shutdown board overvoltage relays occurred in 1993 during preoperational testing. Harmonic distortion and voltage spikes were observed during these events, although the primary cause of these actuations was likely the use of an inappropriate relay calibration source. The inspectors questioned whether the degraded voltage relays could be reset by voltage spikes since the degraded voltage relays (DVRs) currently installed at WaHs Bar are of similar design to the overvoHage relays that were installed in 1993. Resetting the degraded voltage time delay could adversely affect WBN's ability to transfer power supplied to the 6.9kV shutdown boards from offsite to the Emergency Diesel Generators (EDGs) within accident analysis assumptions.
WATTS BAR DEGRADED VOLTAGE RELAY SCHEME The WBN DVR scheme is a voltage monitoring scheme designed to General Design Criteria (GOC) 17(2) and the NRC guidance of Branch Technical Position (BTP) PSB-1(3) (hereafter referred to as "BTP PSB-1"). The design basis scenario described in BTP PSB-1 is a degraded voltage event followed by a subsequent Loss of Coolant Accident (LOCA)_ In the BTP PSB-1 scenario, the LOCA occurs after the DVR time delay, and power to the 6_9kV shutdown boards is immediately transferred from offsite power to the onsite EDGs. It shourd be noted that even though the WBN DVR scheme is designed in accordance with BTP PS8-1, which allows a time delay of limited duration for operators to restore adequate voltage, it will transfer power to the 6.9kV shutdown boards immediately (after the nominal10-second time delay) even without a LOCA occurring_ This design conservatively favors transfer to the EDGs for sustained degraded voltage events. If a lOCA occurs during the 10-second time delay period, the WBN design would also transfer the source of power to the 6.9kV shutdown boards immediately after the 10-second time delay.
The WBN DVR scheme is purposely designed to reset immediately anytime voltage recovers above the reset setpoint in order to favor offsite power, which is the preferred power supply in accordance with Chapter 8 of the WBN Updated Final Safety Analysis Report (UFSAR){4,.
Section 8.1.1, "Utility Grid and Interconnections," of the WBN UFSAR states, "Preferred power is supplied from the existing Watts Bar Hydro 161-kV Switchyard over two radial lines located entirely on TVA property."
Section 8.3.1, "AC Power System," of the WBN UFSAR states, *Power continuity to the 6.9 kV shutdown boards is maintained by switching the preferred (normal or alternate offsite) sources, and the standby (onsile source)_~
Section 8.3.1 also states, 'When the preferred (offsite) power is not available, each Shutdown board is energized from a separate standby diesel generator. Each 6_9 kV shutdown board is equipped with loss-of-voltage and degraded-voltage relaying. The loss-of-voltage and degraded-voHage relays initiate a transfer to the standby diesel generator."
Section 8.2.2, "Analysis," of the WBN UFSAR states, "The degraded-voltage relays (27 DAT, DBT, OCT) have a voltage setpoint of 96% of 6.9kV (nominal, decreasing). These relays are arranged in a lwo-out-of-three coincidence logic (Figure 8.3-5A) to initiate a 10-second (nominal) time delay. If the voltage is still low at the end of 10 seconds, an alarm will be enunciated in the Control Room, a trip of the 6.9kV shutdown-board supply breaker will occur, load shedding from the 6.9kV and 480V shutdown boards and diesel generator start will be initiated, and the 480V shutdown-board current-limiting reactor-bypass breaker will close_"
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Watts Bar Nuclear Plant 2012008-004 Degraded Voltage Relay Issue Response The WBN DVR scheme conforms to a typical industry design that does not introduce any purposeful time delay into the reset function. Neither industry standard IEEE 741-2007(51 nor NRC BTP PSB-1 call for any reset time delay. The DVR scheme at most nuclear stations would reset in this manner in order to prevent an unnecessary Loss of Off site Power (LOOP), which is a major contributor to core melt risk.
A degraded grid event Will likely produce voltage fluctuations which are caused by automatic protection in the transmission system (e.g. line switching, load shedding, generator tripping, etc.). These voltage fluctuations may be indicative of a recovering grid or they may be short-lived and only last a few seconds or less. Other than introducing an arbitrary time delay in the reset function of the DVR, there is no available state-of-the-art technology that could be used to distinguish an actual degraded grid recovery from a momentary voltage fluctuation above reset (RMS voltage or voHage transienUspike).
The proposed scenario in URI 05000390/2012008-04 is not a part of the WBN Licensing Basis, however, the WBN DVR scheme is consistent with GDC-17 and BTP PSB~1, as described in the WBN Updated Final Safety Analysis Report (UFSAR) and NRC Safety Evaluation Reports (SERs) described below.
WATTS BAR SAFETY EVALUATION REPORTS NUREG 0847, "Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Units 1 and 2,"
and its supplements document the NRC's evaluation of WBN's degraded voltage analysis and preoperational testing. Specifically, NUREG 0847 confirmed thatWBN complies with GDC-17 and BTP PS8-1 through safety analysis and verification through inspection.
SER, June 198216) - The NRC evaluated the WBN electrical power design with respect to degraded grid voltage conditions in section 8.3.1.2, "Low anellor Degraded Grid Voltage Condition." The NRC concluded that the WBN design met the BTP PSB-1 positions, and is therefore acceptable. The design and analysis were confirmed during subsequent site visits and inspections during preoperational testing.
SER Supplement 2(1) - During a July 12~14, 1982 site visit, the NRC verified the design implementation of the previously approved WBN deSign.
SER Supplement 13(81 - The NRC confirmed that a preoperational test had shown that the Watts Bar design conforms with BTP PSB-1. However, due to design changes, the NRC did not consider the results of these preoperational tests valid, and revisited this issue during subsequent inspections of WBN's preoperational test program (Inspection Report 50-390/84-90(9).
SER Supplement 20(11). - Preoperational tests were performed again to validate deSign changes, as discussed in SER Supplement 13. The NRC reviewed the preoperational tests and confirmed that Watts Bar design conforms with BTP PSB-1 (Inspection Reports 50-390/95-22(11) and 50-390/95-77(121).
DEGRADED VOLTAGE PROTECTION - HISTORICAL PERSPECTIVE 10 CFR Part 50, Appendix A, GDC Electric Power Systems (hereafter referred to as GDC 17) contains the requirements for providing sufficient electric power to structures, systems, and components important to safety such that: (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents.
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Watts Bar Nuclear Plant 2012008-004 Degraded Voltage Relay Issue Response However. GDC 17 does not specifically address DVR requirements.
Subsequent to the event at Millstone Unit 2, the NRC staff issued a generic request to individual licensees dated August 12 or 13, 1976, requesting them to provide analyses and results conceming the vulnerability of their pJant{s) to similar degraded grid voltage conditions. By letters dated June 2 or 3, 1977 to individual Iicensees(13), the NRC staff indicated it was reviewing the onsite emergency power systems of all plants and that in the meantime it had completed its review of the responses to its earlier letter in August 1976. The June 1977 letters contained enclosures for each plant entitled "Safety Evaluation and Statement of Staff Positions Relative to the Emergency Power Systems for Operating Reactors." In addition, the Staff requested licensees to compare their plants to the Staffs position and propose any modifications necessary to satisfy the position.
The Staff arso stated that "we require certain technical specifications to be incorporated into all facility operating licenses." Model technical specifications were prOVided.
The Position 1 of June 1977 letters entitled, "Second Level of Under-or-Over Voltage Protection with a TIme Delay," provided the following criteria regarding the DVR issue:
We reqUire that a second level of voltage protection for the onsite power system be provided and that this second level of voltage protection shall satisfy the following criteria:
a) The selection of voltage and time set points shall be detennined from an analysis of the voltage requirements of the safety-related loads at all onsile system distribution levels; b) The voltage protection shall include coincidence logic to preclude spurious trips of the offsite power source; c) The time delay selection shall be based on the following conditions:
(1) The allowable time delay, including margin, shall not exceed the maximum time delay that is assumed in the final safety analysis report (FSAR) accident analysis (2) The time delay shall minimize the effect of short duration disturbances from reducing the availability of the off site power source(s); and (3) The allowable consideration of a degraded voltage condition at all distribution system levels shall not result in failure of safety system components; d) The voltage monitors shall automatically initiate the disconnecfjon of oftsite power sources whenever the voltage set point and time delay limits have been exceeded; e) The voltage monitors shall be designed to satisfy the requirements of /EEE Std. 279-1971. "Criteria for Protection Systems for Nuclear Power Generating Stations H
- and f)
The Technical Specifications shall include limiting conditions for operation, surveillance requirements, trip setpoints with minimum and maximum limits, and allowable values for the second-level voltage monitors.
Following a degraded voltage event at Arkansas Nuclear One on September 16, 1978. the NRC issued Information Notice (IN) 79-04, "Degradation of Engineered Safety Features," on February 14, 1979(14) and Generic Letter (GL) 79-36, "Adequacy of Station Electric Distribution Systems Voltages," on August 8, 1979(15).
In GL 79-36, the NRC staff stated:
"... we must now confirm the acceptability of the voltage conditions on the station electric distribution systems with regard to both (1) potential overloading due to transfers of either safety or non-safety loads, and (2) potential starting transient problems in addition to the concerns expressed in our June 2, 1977 correspondence with regard to degraded voltage conditions due to conditions originating on the grid.
"... the NRC is requiring ai/licensees to review the electric power systems at each of their nuclear power plants to detennine analytically if, assuming all onsite sources of AC power are not ava#able, the 4
Watts Bar Nuclear Plant 2012008*004 Degraded Voltage Relay Issue Response off site power system and the onsite distribution system is of sufficient capacity and capabifity to automatically staTt as well as operate all required safety loads. Within their required voltage ratings in the event of (1) an anticipated transient (such as unit trip) or (2) an accident (such as a LDCA) regardless of other actions the electric power system is designed to automatically initiate and without the need for manual shedding of any electric loads. Protection of safety loads from undervoltage conditions must be designed to provide the required protection without causing voltages in excess of maximum voltage ratings of safety loads and without cBusing spurious separations of safety buses from offsite power. NRC should be informed of any required sequential/oading of any portion of the offsite power system or the onsite distribution system which is needed to assure that power provided to all safety loads is within required voltage limits for these safety loads. Guidance on evaluating the performance of electric power systems with regard to voltage drops is provided in Enclosure 2. "
In Enclosure 2 of GL 79-36, the staff stateS:
"VoitagfJ~time settings for undervo/tage relays shall be selected so as to avoid spurious separation of safety buses from off site power during plant startup, normal operation and shutdown due to startup and/or operation of electric loads. U In July 1981, the NRC staff issued Branch Technical Specification BTP PSB~1. "Adequacy of Station Electric Distribution System Voltages (PSB-1)." In STP PSB-1, the staff established criteria for a second set of undervoltage protection. With respect to these starting voltage issues, BTP PSB-1 Position 1 states:
"In addition to the undeNoltage scheme provided to detect loss of oft site power at the Class 1 E buses, a second level of undeNoltage protection with time delay should also be provided to protect the Class 1 E equipment; this second level of undelVoltage protection shall satisfy the fol/owing criteria:
a) The selection of undervoltage and time delay setpoints shall be determined from an analysis of the voltage requirements of the Class 1E loads at all onsite system distribution levels; b) Two separate time delays shall be selected for the second level of undervoltage protection based on the following conditions:
- 1) The first time delay should be of a duration that established the existence of a sustained degraded grid condition (i.e., something longer than a motor starting transient).
Following this delay, an alarm in the control room should alert the operator to the degraded condiUon. The subsequent occurrence of a safety injection actuation signal (S/AS) should immediately separate the Class 1 E distrlbut;on system from the offsite power system.
- 2) The second time delay should be of a limited duration such that the permanently connected Class 1E loads will not be damaged. Following this delay, ;fthe operator has failed to restore adequate voltages, the Class 1 E distribution system should be automatically separated form the oftsite power system. Bases and justification must be provided in supporl of the actual delay chosen. "
Subsequently,licensees used guidance provided in the BTP PS8-1 to establish DVR protection in their licensing basis. In addition, the NRC staff used BTP PSB-1 during its review of applications for facility operating licenses and amendments to facility operating licenses. As noted above, the NRC accepted the WBN degraded voltage design in NUREG 0847, uSafety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Units 1 and 2," and its Supplements through safety analysis and verification through inspection.
Additional operating experience with regard to plant performance under degraded voltage conditions was provided by the NRC in GL 88-15, "Electric Power Systems -Inadequate Control Over DeSign Processes,"
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Watts Bar Nuclear Plant 2012008-004 Degraded Voltage Relay Issue Response dated September 12, 1988(16) and IN 89-83, "Sustained Degraded Voltage on the Offsite Electrical Grid and Loss of Other Generating Stations as a Result of a Plant Trip, ~ dated December 11, 1989(17).
2012 COMPONENT DESIGN BASIS INSPECTION AT WATTS BAR During the 2012 CDBI at WBN, the NRC ~uestiOned the degraded voltage protection scheme during the inspector's review of WBN PER 93039i18, which described a WBN preoperational testing issue where the 6.9kV shutdown board overvoltage relays had been actuated erroneously on several occasions in 1993. Note that the overvoltage relay setpoint had not been exceeded during any of the overvoltage alarms, as verified by voltage recordings during this time. There were mo different types of voltage monitoring relays (ITE27N and ITE59H) that were discussed in WBN PER 930397. The vendor of the ITE27N undervoltage relays recommended that a Doble test source certified to have less than 0.3% Total Harmonic Distortion (THO) be used to calibrate the relays. For consistency and repeatability, the ITE59H overvoltage relays were calibrated with the same source. The "loW harmonic distortion," test source lowered the setpoint of all but one ITE59H relay by 0.7 to 2.68 volts. This was the most likely cause of the 6.9kV shutdown board overvoltage relay actuations which occurred despite the overvoltage relay setpoint not being reached.
Several of the ITE59H overvoltage relay actuations occurred concurrent with rises in THD. The lower overvoltage relay setpoints may have made the relays more likely to operate during transients that cause harmonic distortion events. In addition, several voltage spikes were recorded on all four B.9kV shutdown boards concurrent with varying levels of THO, although the overvoltage relays did not operate during all of these events.
The NRC questioned whether similar THD or voltage spikes could reset the ITE27N degraded voltage relays when the relays have been actuated and are within their nominal10-second time delay. After a degraded voltage condition has been present for 10 seconds, the degraded voltage relays are designed to transfer power to the 6.9kV shutdown boards from offsite power to the EOGs. The NRC's concern is that resetting the DVR time delay could adversely affect WBN's ability to transfer power supplied to the 6.9kV shutdown boards from offsite to the EDGs within accident analysis assumptions.
RESPONSE TO NRC CONCERN The WBN DVR scheme is designed in accordance with the guidance provided in BTP PSS-1. When in a degraded voltage condldon, the DVRs transfer power from offslte to the EDGs after a 1 O-second tIme delay. A degraded grid event will likely produce voltage fluctuations which are caused by automatic protection in the transmission system (e.g. line switching, load shedding, generator tripping, etc.). These voltage fluctuations may be indicative of a recovering grid or they may be short-lived and only last a few seconds or less. The voltage spike scenario proposed during the 2012 CDSI at WBN would have the same effect as RMS voltage fluctuations due to a degraded voltage event; namely, to reset the DVRs. Note that the NRC concerns with potential sources of persistent harmonic distortion have been resolved and WBN has entered the NRC's concerns into the Corrective Action Program under PER 515413(19), PER 515413 addressed the affects of persistent harmonic distortion and provided reasonable assurance of continued functionality of the degraded voltage relays. Specifically, there are no large contributors to harmonics connected to the plant 6.9kV Shutdown Power system. A broken rotor bar in an induction would not be expected to cause a synchronous harmonic due to the motor slip. In addition, Watts Bar previously addressed the effects of harmonics associated with the relay calibration source, 6
Watts Bar Nuclear Plant 2012008-004 Degraded Voltage Relay Issue Response The WBN DVR scheme is purposely designed to reset immediately anytime voJtage recovers above the reset setpoint in order to favor offsite power, which Is the preferred power supply In accordance with Chapter 8 of the WBN UFSAR, as discussed abOve.
This is a typical industry design that does not introduce any purposeful time delay into the reset function.
Neither industry standard I!;EE: 741~2007 nor NRC BTP PSB-1 call for any reset time delay. The DVR scheme at most nuclear stations would reset in this manner in order to prevent an unnecessary LOOP, which is a major contributor to core melt risk.
The design basis scenario described in BTP PSB-1 is a degraded voltage event fallowed by a subsequent L.OCA. In this scenario, the LOCA occurs after the OVR time delay, and power to the 6.SkV shutdown boards is immediately transferred from ot'fsite power to the onsite EDGs. It should be noted that even though the WBN DVR sCheme is deSigned in accordance with BTP PS8-1, which allows a time delay of limited duration for operators to restore adequate voltage, it will transfer power to the 6.9kV shutdown boards immediately (after the nominal10-second time delay) even without a lOCA occurring. This design conseJVatively favors transfer to the EDGs for sustained degraded grid voltage events. If a LOCA occurs during the 1Q-second time delay period, the WBN design would also transfer the source of power to the 6,9kV $hutdown boards Immediately after the 10-second time delay.
CONCLUSION The WBN DVR scheme protects the 6.9kV shutdown boards and supplied loads from sustained degraded grid voltage conditions in accordance with GDC-17 and BTP PSB-1. The DVRs are designed to reset immediately anytime voltlilge recovers above the reset setpoint in order to favor offsite power, which is the preferred power supply. During a degraded voltage event, power to the 6.SkV shutdown boards will immediately transfer 'rom offsite power to the E:DGs after the 1 O-second time delay (whether or not a LOCA has occurred), protecting the 6.9kV shutdown boards and suppfied loads from sustained grid voltage conditions. In addition, although the proposed scenario in URI 0500039012012008-04 is not a part of th" WBN L.icensing BaSiS, the WBN DVR scheme is consistent with GDC-17 and BTP PSB-1, as described in theWBN UFSAR and NRC SERs.
1Z.. A.,",,\\LOuD 0 Watts Bar Site Licensing Manager, Watts Bar SHe Licensing 7
Sr. Program Manager, Aux Power Design & AnalysiS Man~ca~
Engineering Date 4
Date Date Date 7
Watts Bar Nuclear Plant 2012008-004 Degraded Voltage Relay Issue Response REFERENCES
- 1. NRC Inspection Report 05000390/2012008, dated June 13, 2012
- 2. 10 CFR 50, Appendix A, General Design Criteria 17, "Electric Power Systems"
- 3. NRC Branch Technical Position PSB-1, "Adequacy of Station Electrical Distribution System Voltages"
- 5. IEEE 741-2007, "IEEE Standard Criteria for the Protection of Class 1 E Power Systems and Equipment in Nuclear Generating Stations"
- 6. NUREG-0847, "Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Units 1 and 2°
- 7. Supplement 2 to NUREG-0847, "Safety Evaluation Report Related to the Operation of watts Bar Nuclear Plant, Units 1 and 2," dated January 1984
- 8. Supplement 13 to NUREG-0847, "Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Units 1 and 2," dated April 1994
- 9. NRC Inspection Report 05000390184-90, dated February 11, 1985
- 10. Supplement 20 to NUREG-Q847, *Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Units 1 and 2," dated February 1996
- 11. NRC Inspection Report 05000390195-22, dated September 8, 1995
- 12. NRC Inspection Report 05000390/95-77, dated December 6, 1995
- 13. Letter dated June 3, 1977 from NRC to TVA regarding NRC review of onsite emergency power systems of all operating nuclear power facilities
- 14. Information Notice 79-04, "Degradation of Engineered Safety Features," dated February 14, 1979
- 15. Generic Letter 79-36, ~Adequacy of Station Electric Distribution Systems Voltages,~ dated August 6, 1979 Ie. Gt:!lIeric Lt:!llt:ll S8-*15, MElel.:LJic.: Power Systems -Inadequate Control Over DesIgn Process," dated September 12, 1988
- 17. Information Notice 89-83, ~Sustained Degraded Voltage on the Offsite Grid and Loss of Other Generating Stations as a Result of a Plant Trip," dated December 11, 1989
- 18. WBN Problem Evaluation Report (PER) 930397
- 19. PER 515413 8