05000298/LER-2016-007

LER-2016-007, Isolation of Shutdown Cooling due to Relay Maintenance Results in a Loss of Safety Function

Cooper Nuclear Station
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
Initial Reporting
ENS 52327	10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat
2982016007R00 - NRC Website

LER 16-007-00 for Cooper Nuclear Station Regarding Isolation of Shutdown Cooling due to Relay Maintenance Results in a Loss of Safety Function

ML16365A009
Person / Time
Site:	Cooper
Issue date:	12/19/2016
From:	Limpias O A Nebraska Public Power District (NPPD)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NLS2016077 LER 16-007-00
Download: ML16365A009 (5)

Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

PLANT STATUS

Cooper Nuclear Station (CNS) was in Mode 5, Refueling, at 0 percent power, at the time of the event.

BACKGROUND

The safety objective of the Residual Heat Removal (RHR) system [EllS:BO] is to provide core cooling, in conjunction with other Emergency Core Cooling Systems, and to provide containment cooling as required during abnormal operational transients and postulated accidents. The RHR system consists of two heat exchangers [EllS:HX], four main system pumps [EIIS:P], and associated piping, valves [EIIS:V], controls and instrumentation. Irradiated fuel in the shutdown reactor core generates heat during the decay of fission products and increases the temperature of the reactor coolant. The decay heat must be removed to reduce the temperature of the reactor coolant to less than or equal to 212 degreeS Fahrenheit (F) in preparation for performing Refueling or Cold Shutdown maintenance operations, or the decay heat must be removed for maintaining the reactor in Hot Shutdown condition.

Shutdown Cooling (SDC) is a subsystem of RHR and is placed in operation during a normal reactor shutdown and cooldown. Reactor Coolant is pumped by the RHR main system pumps from recirculation loop 'A' through the RHR heat exchangers prior to returning to the reactor vessel through connections to their respective recirculation loop.

The relays used in the Primary Containment Isolation System (PCIS) logic are model CR120A industrial relays [EllS:RLY] manufactured by General Electric. The CR120A relay has a max voltage rating of 300 volts and is a small multi-circuit AC control relay. The relays are popular due to their small size and long mechanical life. CNS currently performs predictive maintenance plans on the relays to replace the coils on a 10-year frequency.

EVENT DESCRIPTION

On October 28, 2016, SDC isolation valves RHR-MO-17 and RHR-MO-18 were open with RHR Loop A in RHR SDC flush lineup, with an intention of placing RHR Loop A in SDC. The Alternate Decay Heat Removal (ADHR) system was maintaining Reactor Pressure Vessel (RPV) and Spent Fuel Pool temperature.

Work orders were created to replace twenty-seven PCIS relay coils, including relay coil for PCIS-REL-K27, in Refueling Outage (RE29). During testing after completion of the work order, it was identified that the PCIS-REL-K27 relay did not actuate as expected (delayed response). This led to a revision of the work order to replace the entire relay instead of just a coil replacement. This required more wires to be lifted and the relay to be removed from the DIN rail and replaced.

SDC was placed in service at 08:49 hours on October 28, 2016. Subsequently, during replacement of the PCIS-REL-K27 relay, the action of installing a new relay onto the shared plastic DIN rail disturbed the mounting rail in a manner that caused the 1-2 contact of the adjacent relay, PCIS-REL-K30, to open. This caused RHR-MO-17 to close, which actuated the logic to trip the running 'A' RHR pump. Operations declared A RHR SDC subsystems inoperable at 09:24 hours and entered Technical Specification (TS) Cooper Nuclear Station 05000- 298 2016

3. LER NUMBER

-007 -00 Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

05000- 298 Cooper Nuclear Station -007 -00 2016 Limiting Condition for Operation (LCO) 3.9.7, Condition A, Required Action A.1, "Verify an alternate method of decay heat removal is available within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and once per 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> thereafter;" entered TS LCO 3.9.7, Condition C, Required Action C.1, "Verify reactor coolant circulation by an alternate method within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> from discovery of no reactor coolant circulation and once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> thereafter;" and also entered TS LCO 3.9.7, Condition C, Required Action C.2, "Monitor reactor coolant temperature hourly.

ADHR remained in service throughout the event and the plant remained aligned for natural circulation.

Spent fuel pool weir temperature monitoring was commenced to verify natural circulation. No increase in RPV temperature was observed and there was no impact to plant operations.

While SDC was out of service, PCIS relay K27 work was completed. SDC was declared operable at 05:30 hours on October 29, 2016, and TS LCO 3.9.7, Condition A was exited. The plant remained in TS LCO 3.9.7, Condition C, and aligned for Natural Recirculation, until SDC was placed in service-at 18:30 hours on October 29, 2016, at which time TS LCO 3.9.7, Condition C was exited.

BASIS FOR REPORT

This event is reportable in accordance with 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 remove residual heat. Event Notification 52327 was made to the Nuclear Regulatory Commission Operations Center per 10 CFR 50.72(b)(3)(v).

SAFETY SIGNIFICANCE

This is a Safety System Functional Failure because of the loss of the shutdown cooling function of the RHR system. The plant was in MODE 5 performing refueling activities and conducting an Operations with a Potential for Draining the Reactor Vessel activity. The shutdown cooling function of RHR was required to be operable per TS. The Division 1 RHR system was fully functional except for the isolation of RHR-17MV. During the loss of SDC, decay heat removal was being provided by the ADHR system.

The reactor coolant temperature did not change during the event. There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal due to the limited duration the condition existed and the redundant decay heat removal equipment which remained available.

CAUSE

The root cause is that CNS did not identify the risk from mechanical agitation during PCIS relay installation; therefore, the risk was not adequately evaluated or mitigated.

CORRECTIVE ACTIONS

To prevent recurrence, CNS will revise Procedure 0.50.5 to list the relays or other devices which could impact SDC when in service.

Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

05000- 298 Cooper Nuclear Station - 007 - 00 2016

PREVIOUS EVENTS

On May 30, 2015, steam flashing in the SDC line occurred, creating pressure transients that caused RHR-MO-17 and RHR-MO-18 to close. The steam flashing occurred due to temperature in the SDC line being at or near saturation temperature causing localized boiling then void collapse with coolant being drawn from the reactor vessel thru the reactor recirculation system. This event was reported under LER 2015-004-00, Isolation of Shutdown Cooling Results in a Loss of Safety Function, dated July 29, 2015.

On October 14, 2012, flashing of the hot reactor coolant to steam occurred, causing a pressure spike that exceeded 72 psig. Consequently, RHR-MO-17 and RHR-MO-18 clbsed, isolating the RHR SDC loop. This event was reported under LER 2012-004-00, Isolation of Shutdown Cooling Results in Loss of Safety Function, dated December 6, 2012.