|Cooper Nuclear Station|
|Reporting criterion:||10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident|
|ENS 52813||10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident|
|2982017004R00 - NRC Website|
|Person / Time|
Nebraska Public Power District (NPPD)
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17236A167 (4)|
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V:ICY-4) Cooper Nuclear Station 05000- 298 NUMBER NO.
2017 -04 - 00
Cooper Nuclear Station (CNS) was in Mode 1, Power Operations, 100 percent power, at the time of discovery; i.e., June 19, 2017.
The function of the Torus to Drywell Vacuum Breaker [EIIS:VACB] is to relieve vacuum in the drywell.
There are 12 internal vacuum breakers which allow air and steam flow from the suppression chamber (torus) to the drywell when the drywell is at a negative pressure. Torus to Drywell Vacuum breakers prevent an excessive negative differential pressure across the drywell boundary. Each vacuum breaker is a self-actuating valve [EllS:V], similar to a check valve, which can be remotely operated for testing purposes.
A negative differential pressure across the drywell wall is caused by rapid depressurization of the drywell.
Design Bases Accident (DBA) analyses assume the vacuum breakers to be closed initially and to remain closed and leak tight, until the suppression pool is at a positive pressure relative to the drywell. The requirement that the vacuum breakers be closed ensures that there is no excessive bypass leakage should a Loss of Coolant Accident (LOCA) occur.
The function of the primary containment [EIIS:NH] is to isolate and contain fission products released from the Reactor Primary System following a design basis LOCA and to confine the postulated release of radioactive material. The safety design basis for the primary containment is that it must withstand the pressures and temperatures of the limiting DBA without exceeding the design leakage rate. The leakage from the drywell to the suppression chamber must be limited to ensure the pressure suppression function is accomplished and the suppression chamber pressure does not exceed design limits.
The vacuum breaker has a safety function in the closed position to limit the amount of bypass flow to ensure proper containment response on a postulated LOCA event and an open safety function post LOCA to limit negative differential pressure between the drywell and the suppression chamber. With the valve partially open, the LOCA containment response cannot be assured.
On June 19, 2017, during performance of surveillance 6.PC.207, "Torus to Drywell Vacuum Breaker Operation," the control switch for vacuum breaker PC-AOV-NRV21 was cycled open, then closed. When the control switch was taken to close, the vacuum breaker failed to indicate closed. As such, Operations declared primary containment and PC-AOV-NRV21 inoperable and entered Technical Specification (TS) Limiting Condition for Operation (LCO) 18.104.22.168 Condition A and LCO 22.214.171.124 Condition B at 21:15 hours.
In addition, TS LCO 126.96.36.199 Condition B was entered at 22:15 hours due to PC-AOV-NRV21 still indicating intermediate.
control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Cooper Nuclear Station 05000- 298 NUMBER NO.
The control switch for PC-AOV-NRV21 was cycled open, then closed a second time. At this time, PC- AOV-NRV21 indicated closed. Operations declared primary containment operable at 23:11 hours and exited TS LCO 188.8.131.52, Condition A and Condition B, and TS LCO 184.108.40.206 Condition B.
The event is currently under investigation. CNS will provide additional event details, the safety significance, cause, corrective actions, and previous events in a supplement to this Licensee Event Report.
BASIS FOR REPORT
CNS is reporting this event under 10 CFR 50.73(a)(2)(v)(D) as a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
CNS subsequently performed an engineering evaluation which concluded that PC-AOV-NRV21 would have completed its closure under forces resulting from reactor blowdown. As such, this event is not considered a Safety System Functional Failure.
Other criteria, if determined to be applicable, will be provided in the supplemental report.