05000266/LER-2007-006
| Docket Number Rev | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 43487 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2662007006R00 - NRC Website | |
Event Description:
On July 12, 2007, a review of abnormal operating procedure (A0P)-10A, Safe Shutdown-Local Control identified that fire damage to the pressurizer [PZR] power-operated relief valve (PORV) and block valve circuits as a result of a fire in the cable spreading room could also result in simultaneous damage to a block valve circuit and spurious actuation of a PORV. There are actions in AOP-10A to provide reasonable assurance that positive control of reactor coolant system (RCS) inventory is maintained. However, these steps do not account for a simultaneous failure of the block valve circuit and spurious operation of a PORV before the manual actions are completed in the control room. This postulated chain of events could result in RCS depressurization. An 8-hour non-emergency report, EN 43487, was made on July 12, 2007, pursuant to 10 CFR 50.72(b)(3)(ii)(B), as an unanalyzed condition. This event is not a safety system functional failure.
Event Analysis:
Potential spurious actuations and circuit failures in an area are considered and mitigated, consistent with the priorities established by the functional requirements analysis. The Point Beach Nuclear Plant (PBNP) functional requirements analysis contained in the Fire Protection Evaluation Report (FPER) was performed to evaluate response to fire failures such as the simultaneous, instantaneous fire damage to PORV and block valve circuits for a fire in the cable spreading room or control room. As discussed in FPER Section 5.2.5.3, the magnitude, duration or complexity of a fire cannot be foreseen to the extent that circuit failure timing and quantity can be predicted. Spurious actuations from fire-induced circuit failures were not postulated to occur simultaneously unless the failures are common mode resulting from fire damage to a single related circuit. The functional requirements analysis established limiting timeframes that could result from fire-induced equipment failures and spurious component actuations. These analyses were performed using conservative assumptions based on worst case initial conditions and limited component availability to mitigate the consequences of a fire event. The timeframes that resulted from these analyses and calculations were not considered absolute values for acceptability of operator action or system performance. Instead, these results, in conjunction with considerations on the likelihood of the event, were used to help prioritize the order in which manual operator actions are performed in response to a fire.
In the case of the PORV and block valves, circuit failure has been postulated and evaluated. Actions to close the block valves and initiate containment isolation early are contained in the abnormal operating procedure (A0P)-10A, Safe Shutdown-Local Control. This is followed with removing power to the block valves at their respective motor control center to ensure they remain closed and removing instrument air (IA) from the PORVs to ensure they remain shut. Initiation of containment isolation early in the procedure provides additional assurance that a PORV will not spuriously operate or stay open if operated because this action isolates the IA supply to containment. Therefore, the only available IA would be the residual pressure in the containment IA piping.
FACILITY NAME (1) DOCKET NUMBER (21_, LER NUMBER (6) PAGE (3) � SEQUENTIAL� REVISION
Safety Significance:
This Licensee Event Report (LER) documents a fire event which could result in block valve circuit failure and spurious operation of a PORV prior to establishment of positive control room actions to mitigate the consequences of the fire. The scope of this issue is limited to the ability to achieve and maintain hot safe shutdown in the event of a severe fire. Based on the functional requirements analysis contained in the FPER, mitigation of a potential spurious operation of a PORV is given a high priority in AOP-10A. Thus, actions to ensure closure of the block valves are prioritized. The initiation of containment isolation early in the procedure provides additional assurance that a PORV will not spuriously operate or stay open if operated because this action isolates the IA supply to containment. Therefore, the only available IA would be the residual pressure in the containment IA piping. These failures must also occur prior to, or shortly after, initiation of containment isolation for there to be sufficient residual air pressure in the containment IA piping to open and maintain open a PORV. Some of the cables that can potentially cause the condition described are thermoset-type cables. Although this does not ensure that they will not fail, they generally exhibit better high temperature properties and are less susceptible to failure when compared to thermoplastic-type cables.
Based on the above and the analysis of the cable spreading room in the Fire Hazards Analysis Report, the likelihood of a fire that affects these circuits prior to performance of proceduralized mitigative actions is believed to be low. This condition will be evaluated under the NFPA 805 transition project and the safety significance will be determined at that time.
Cause:
PBNP used a probabilistic rather than deterministic approach to the issue of multiple, simultaneous failures.
The approach outlined in the FPER states that the magnitude, duration or complexity of a fire cannot be foreseen to the extent of predicting the timing and quantity of circuit failures. However, each potential spurious operation can be addressed and its effects can be individually mitigated. Signals originating from fire-induced circuit failures may occur as a result of a given fire. They are not expected to occur simultaneously unless the spurious operations are caused by the fire-induced failure in circuits that can affect multiple components. As an example, the FPER states that although control circuits for essentially all plant safety systems are routed through the control room, it is not considered credible that the plant must be able to sustain the simultaneous, instantaneous spurious actuation and failure of all these circuits. The spurious opening of a PORV was individually evaluated and actions were developed to mitigate the postulated failure. The first action is to close the respective block valves and shortly thereafter remove power from them. The normally shut PORV's are maintained shut with the additional action of bleeding air from these air-operated valves.
FACILITY NAME (1)_ DOCKET NUMBER (2)_ LER NUMBER (6)_ PAGE (3) Corrective Action:
Twice-per-shift fire rounds in the control room and cable spreading room were established as a compensatory measure. Resolution of potential fire effects on PORV and block valve circuitry will be performed by the NFPA 805 Transition Project.
Previous Occurrences:
August 19, 1999.
Additional Information:
None