05000482/LER-2008-009
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 10-21-2008 |
|---|---|
| Report date: | 12-19-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4822008009R00 - NRC Website | |
PLANT CONDITIONS PRIOR TO EVENT:
MODE — 1 Power — 100
EVENT DESCRIPTION:
Wolf Creek Nuclear Operating Corporation (WCNOC) conducted a self assessment of the WCNOC Post-Fire Safe Shutdown Analysis (PFSSDA). As a result of this self assessment, in August 2008, WCNOC identified a concern with the mitigation strategy for fire induced spurious opening of a pressurizer power operated relief valve (PORV) [EIIS Code: AB RV] in the event of a fire outside the control room. The mitigation strategy is to de-energize DC power to the valve to close the valve. In addition, since the Wolf Creek Generating Station (WCGS) license requirements require WCNOC to consider proper polarity cable-to-cable hot shorts when analyzing the PORV circuits, it is necessary to fail DC power to all potential sources running in a common raceway with the PORV circuits.
Therefore, it is necessary to de-energize DC control power to the entire DC bus. This causes a loss of DC control power to several components.
The corrective action evaluation, from the self assessment, stated that this mitigation strategy is used in 15 fire areas where one pressurizer PORV could open. In 14 of the 15 areas, the Train of DC power that is being de-energized is the same Train that is affected by the fire and the credited Train of equipment remains available. However, in 1 fire area (area A-27) the Train of DC power [EIIS Code: EJ] that is being de-energized is the same Train that is credited to achieve safe shutdown. The evaluation further explained that spurious opening of a PORV is not likely based on the regulatory position in Regulatory Issue Summary (RIS) 2004-03, Rev. 1 which states that cable-to-cable hot shorts involving thermoset cables is substantially less likely than intra cable shorting.
During the Triennial Fire Potection Inspection, it was discovered that the evaluation inappropriately used RIS 2004-03 to consider risk insights in the decision to not establish compensatory measures. RIS 2004-03 is not a licensing document. Therefore, the WCGS license basis that requires consideration of all possible hot shorts, open circuits, and shorts to ground should have been applied to establish compensatory measures.
A concern was also raised that the mitigation strategy to close a failed open PORV could lead to unwanted consequences. These consequences include loss of breaker function due to the loss of 125 VDC control power to a number of 4,160 VAC switchgear breakers and 480 VDC load center feeder breakers. This would both prevent operation of the associated equipment from the control room and prevent the breaker from opening if a fault occurred on an energized conductor. In addition, since Wolf Creek credits primarily Train B components if a fire occurs in area A-27, isolation of Train B Class 1 E 125 VDC power could result in a number of credited components not operating.
BASIS FOR REPORTABILITY:
Appendix R to 10 CFR 50, Section II.B states: "A fire hazards analysis shall be performed by qualified fire protection and reactor systems engineers to ... (2) determine the consequences of fire in any location in the plant on the ability to safely shut down the reactor ...". In addition,Section III.G of Appendix R provides the requirements for fire protection of safe shutdown capability and provides options deemed acceptable to the NRC for protecting redundant post-fire safe shutdown cables or equipment. In addition,Section III.G of Appendix R provides the requirements for fire protection of safe shutdown capability and provides options deemed acceptable to the NRC for protecting redundant post-fire safe shutdown cables or equipment.
A fire in area A-27 could cause a pressurizer power operated relief valve to spuriously open. The manual actions established to mitigate this condition would cause a loss of Class 1 E 125 VDC power to a number of Train B components that are required for safe shutdown if a fire occurs in area A-27. This condition was self identified by Wolf Creek but no compensatory measures were established at that time.
Because compensatory measures were not established, Wolf Creek made an eight hour Emergency Notification System call in accordance with 10 CFR 50.72(b)(3)(ii)(B). This condition is also reportable pursuant to 10 CFR 50.73 (a)(2)(ii)(B) for any event of condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
ROOT CAUSE:
An action from an evaluation in 2002 was to proceduralize operator manual actions credited for fires outside the control room. WCNOC did not have a detailed analysis of the design basis for all of the Fire Areas at that time. The manual action to de-energize the bus was added to the procedure without being sufficiently evalluated to identify the unwanted consequences. When the information for Fire Area A-27 was formalized and issued through a Design Change Package, the thought process of the workers was already established to de-energize DC control power to the entire DC bus. Therefore the action to establish an hourly fire watch was not performed.
CORRECTIVE ACTIONS:
An hourly fire watch is in place in fire area A-27 and will remain in place until this condition is resolved.
WCNOC's design change process was verified to be sufficiently robust. Engineering personnel were counseled against having a preconceived mind-set when performing a design change.
A review of all manual actions credited for fires outside the control room will be conducted, to ensure the manual action does not result in a secondary impact that could adversely impact the ability to achieve and maintain safe shutdown. This review will be completed by December 31, 2009.
A modification will be made to ensure the pressurizer PORVs can be closed from the control room. This modification will be completed prior to plant startup following Refueling Outage 18.
SAFETY SIGNIFICANCE:
This condition is considered low safety significance because a fire of sufficient magnitude to cause the postulated damage is not considered to be credible due to the low combustible loading in fire area A-27. This condition causes no equipment operability or plant reliability problems. There are no nuclear safety, environmental stewardship or reactivity management issues associated with this condition.
OPERATING EXPERIENCE/PREVIOUS EVENTS:
None.