05000305/LER-2012-001
| Docket | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 47707 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 3052012001R00 - NRC Website | |
Event Description:
On February 22, 2012, while developing Design Change KW-12-01049 to address inadequate Appendix R reactor coolant system high/low interface valve cable [CBL4] separation issues for pressurizer power operated relief valves (PORVs) PR-2A [PCV] and PR-2B [PCV], additional concerns were raised regarding the condition originally identified in Condition Report CR109107 and discussed in LER 2008-001-00. This additional concern was identified in Condition Report CR 463680 and entered in the Station's Corrective Action Program. Subsequently, on February 28, 2012, additional Appendix R fire spurious operation concerns (related to a postulated fire in the relay room and 480 volt Bus 51/52 area) were identified for the pressurizer [PZR] and reactor [RCT] head vent valves [FSV] PR-33A, RC-45A, and RC-46 than as originally identified in Condition Report CR114404 and discussed in LER 2008-001-00. This condition was identified in Condition Report CR464393 and also entered into the Corrective Action Program.
Per 10 CFR 50, Appendix R, and NRC guidance for reactor coolant system (RCS) [AB] high/low pressure interfaces, components required to achieve and maintain safe shutdown conditions must be capable of performing their required safe shutdown function in the event of a fire in either the dedicated or alternate fire area. The Kewaunee Power Station (KPS) licensing basis term for fire area is fire zone. The KPS licensing basis fire event is a single worst case fire in either fire zone, but not simultaneously in both fire zones.
Condition Report CR109107, Pressurizer PORV PR-2B Control Cable Appendix R Spurious Operation Concern, discussed that the current routing of PORV PR-2B alternate solenoid SV33731 control cables 1S6C1228 and 1S6C1229 did not meet current NRC guidance and could lead to the spurious opening of this valve during an Appendix R fire event in the relay room. The extent of condition evaluation previously performed for the other PORV control circuits, specifically, PR-2B normal solenoid SV33113 and. PR-2A solenoid SV33114, concluded that the same issue did not apply. However, while developing a resolution for the solenoid SV33731 cables, the other circuits were re-evaluated and the extent of condition conclusion was questioned, as documented in Condition Report CR463680.
As discussed in Operability Determination 0D000194, the control cabling is routed in dedicated conduit [CND] from the control room to containment. This is true for SV33114 cable 1S6C1426 and SV33113 cables 1S5C1545 and 1S5C1542. However, additional cables within these circuits are not routed in dedicated conduit and do not appear to have been evaluated in the original Condition Report. The effects from a fire event on these additional cables associated with PR-2B are evaluated as follows:
1S5C380 (PR-2B normal solenoid SV33113): Routed in cable tray between terminal cabinet [CAB] TC12S5 and relay rack RR142 within the relay room. An intra-cable short circuit between conductors 3311302 and 331130 will bypass the normally open contacts from relays [RLY] PC429/FX and PC430/BX. This combined with normally closed contact 2L of control switch 46414 creates a conductive path from the positive leg of the circuit to the solenoid.
1S5C887 (PR-2B normal solenoid SV33113): Routed in cable tray between fuse [FU] panel [PL] RR171 and Mechanical Control Console [CSL] C. This cable consists of seven conductors, two of which support this control circuit. Four of the remaining five conductors support two 125VDC non-Appendix R control circuits (valves NG-107 and RC-507). Step A1.a.1 of procedure OP-KW-AOP-FP-002, Fire in Alternate Fire Zone, de-energizes the. SV33113 circuit however the other two circuits are not de-energized. Two intra-cable hot shorts of proper polarity, combined with the intra-cable short discussed for cable 1S5C380, could lead to spurious opening of PR-2B for a fire in the relay room.
The "Response Not Obtained" (RNO) portion of step Al .a of OP-KW-AOP-FP-002, Fire in Alternate Fire Zone, has operator's open the supply breaker [BKR] to fuse panel RR171, which de-energizes all of the circuits discussed above. However, since PR-2B is designated as a high/low pressure interface valve in the Appendix R program and cable 1S5C887 is routed in the same cable tray, two inter-cable hot shorts of proper polarity must be assumed to occur (according to NRC guidance). This again, combined with the intra-cable short on 1S5C380, could lead to spurious opening of PR-2B.
A review of the control circuits associated with PR-2A concluded the following:
Cable 1S6C290 (PR-2A solenoid SV33114): Routed in cable tray between Mechanical Control Console C in the Control Room and relay rack RR147 in the relay room. An intra-cable short circuit between conductors 3311402 and 331140 will bypass the normally open contacts from relays PC431/BX and PC449/BX. This, combined with normally closed contact 2L of control switch 46415, creates a conductive path from the positive leg of the circuit to the solenoid.
Cable 1S6C806 (PR-2A solenoid SV33114): Routed in cable tray between fuse panel RR176 and Mechanical Control Console C. This cable consists of seven conductors, two of which support this control circuit. Four of the remaining five conductors support two 125VDC non-Appendix R control circuits (valves MG(R)-504 and MG(R)-510). Step Al .c.3 of OP-KW-AOP-FP-002, Fire in Alternate Fire Zone, de-energizes the SV33114 circuit; however, the other two circuits are not de-energized. Two intra-cable hot shorts of proper polarity, combined with the intra-cable short discussed for cable 1S6C290, could lead to spurious opening of PR-2A for a fire in the Relay Room.
The RNO portion of step Al .c of OP-KW-AOP-FP-002, Fire in Alternate Fire Zone, has operators open the supply breaker to panel RR176, which de-energizes all of the circuits discussed above. However, since PR 2A is designated as a high/low pressure interface valve in the Appendix R program and cable 1S6C806 is routed in a cable tray, two inter-cable hot shorts of proper polarity must be assumed to occur (according to NRC guidance). This again, combined with the intra-cable short on 1S6C290, could lead to spurious opening of PR-2A.
While reviewing pressurizer and reactor head vent valve (PR-33A/B, RC-45A/B, RC-46 and RC-49) control circuit cable routings for Reasonable Assurance of Safety, RAS000199, it was discovered that an Appendix R fire event could also cause spurious operation of PR-33A, RC-45A, and RC-46.
Condition report CR114404 identified that a fire event in an Alternate fire zone could cause spurious operation of PR-33B, RC-45B and RC-49. Operability Determination 0D000211, was created from this condition report. The extent of condition performed in OD000211 focused on the opposite train valves (PR 33A, RC-45A and RC-46) and stated, "For a fire in an Alternate fire zone (such as the Relay Room) the valves could spuriously open. For a fire in the Alternate area, the spurious opening of these valves would be mitigated by OP-KW-AOP-FP-002, Rev. 2, and Step A6.a, which directs the operator to place all DSP local/remote switches to LOCAL. This step will isolate the head vent valve circuits from the Alternate area, which will de-energize the circuit and the valves will close.
This latest review for Reasonable Assurance of Safety RAS000199 focused on a fire event in a dedicated fire zone (such as the 480V Bus 51/52 Room, where the Dedicated Shutdown Panel (DSP) is located). For a fire in this, or any other dedicated fire zone, valves PR-33A, RC-45A and RC-46 are to remain closed. The Appendix R Design Description (ARDD), sections 4.1.3.3 and 5.5, credits RC-46 for remaining closed by removing its power and routing its solenoid valve cable in dedicated conduit to prevent spurious operation from a hot short. However, the following scenario could cause a fire induced spurious operation of RC-46 (and PR-33A and RC-45A).
Cables 1S5C1295 and 1S5C1279 share cable tray routing point 1TT104S5 and support all three valves. An intra-cable short on 1S5C1279 bypasses the normally open contact from the Control Room control switches. This, combined with a proper polarity inter-cable hot short on 1S5C1295, could cause any or all of these valves to spuriously open. Turning off the power supply to RC-46, or the entire SD-101 panel as prescribed in step B2.b of procedure OP-KW-AOP-FP-003, Fire in Dedicated Zone, will not prevent this from occurring as other 125VDC circuits exist in cable tray 1TT104S5 that could provide the external hot short.
Immediate notification of the above described conditions was previously provided in Event Notification#47686 on February 22, 2012 and Event Notification#47707 on February 28, 2012 per 10 CFR 50.72(b)(3)(ii)(B), for any event or condition that results in the nuclear plant being in an unanalyzed condition that significantly degrades plant safety.
This event is hereby being reported pursuant to 10 CFR 50.73(a)(2)(ii)(B) for any event that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
Event and Safety Consequence Analysis:
An assessment was made of the combined risk of the four scenarios discussed in this report. This assessment use the methodology from NUREG/CR-6850 to determine the frequency for fires that could affect the cables in question, the severity factors for these fires, and the probability of the hot shorts required to produce a loss of reactor coolant inventory. The assessment conservatively assumed that the loss of inventory would result directly in core damage. Given that methodology, the sum total for all of these scenarios is below the threshold for very low risk (green) in the Significance Determination Process.
Cause:
An insufficient extent of condition (EOC) was performed from Operability Determination OD000194 in September 2008, associated with the protection of specific circuits/cables for pressurizer PORVs PR-2A and PR-2B from fire induced spurious operation.
Enforcement discretion for the pertinent 2008 Triennial Fire Protection Inspection violation was in effect because the Station had committed to resolve the issue through NFPA 0805 (July 21, 2008 commitment letter), if required to do so. Since further analysis work was already part of the NFPA 0805 transition project scope, and reanalysis of Appendix R was not warranted (because there were no consequences), both the Operability Determination (OD) and the Reasonable Assurance of Safety (RAS) were updated without further EOC analysis. Therefore, management and administrative process decision-making did not consider any additional or immediate actions as necessary.
Corrective Actions:
The current compensatory measure for an hourly fire watch that was already in place for the previous issue was expanded to include the 480 volt Bus 51/52 area. This measure satisfies KPS Fire Protection Program Plan requirements for non-functional fire protection systems, barriers, and Appendix R safe shutdown systems.
Corrective Action CA228092 initiated actions to perform an evaluation and extent of condition review of outstanding 10 CFR 50, Appendix R, issues for the station including the pressurizer and head vent valves PR-33A/B, RC-45A/B, RC-46, and RC-49 potential spurious operation concerns.
Design change KW-12-01049 has been created to address the Appendix R spurious valve operation concerns for pressurizer PORVs PR-2A and PR-2B.
Similar Events:
Operation Concern.