05000373/LER-2009-002

LER-2009-002, Loss of Shutdown Cooling Due to Spurious Closure of the Shutdown Cooling Suction Isolation Valve

Docket Number
Event date:	07-20-2009
Report date:	09-18-2009
Reporting criterion:	10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat
3732009002R00 - NRC Website

A. PLANT AND SYSTEM IDENTIFICATION

General Electric Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power

CONDITION PRIOR TO EVENT

�Unit(s): 1 Event Date: 7/20/09�Event Time: 1448 � Reactor Mode(s): 4 Power Level(s): 000 Mode(s) Name: Cold Shutdown

B. DESCRIPTION OF EVENT

On July 20, 2009, LaSalle Unit 1 was in Mode 4 Cold Shutdown. At 1448 CDT the inboard shutdown cooling suction isolation valve (1E12-F009) unexpectedly closed, causing a trip,of both the 1A and 1B Residual Heat Removal (RHR)[BO] pumps and a loss of shutdown cooling.

Control Room operators entered the appropriate abnormal procedures, reopened the 1E12- F009 valve, and restarted the 1A RHR pump at 1500 CDT, thereby restoring shutdown cooling.

At the time of the event, Unit 1 was operating in shutdown cooling mode with two RHR pumps running to perform dual loop flushes of the RHR suction piping. In order to start the second RHR pump, the shutdown cooling suction high flow isolation had been bypassed in accordance with approved station procedures by connecting a jumper across the contacts of relays 1B21H- K74 (Division 1) and 1B21H-K77 (Division 2). The jumper prevents relays 1B21H-K75 (Division 1) and 1B21H-K29 (Division 2) from dropping out on a trip of relays K74 and K77, respectively, which would result in a high flow isolation of the shutdown cooling suction isolation valves.

Instrument Maintenance (IM) technicians had been dispatched to remove the jumpers as part of restoring from the dual loop flush. When the jumper was removed from the terminal of the 1B21H-K29 relay, the IM technicians observed that the circuit appeared to be under load, as a small arc was drawn and they heard an audible click. The IM technicians went to the front of the panel and verified that the relay had not changed state and was positioned as expected.

The technicians then exited the area. A short time later, other personnel working in the room heard a relay change state, and were subsequently advised by the Main Control Room operators that the 1A and 1B RHR pumps had tripped.

C. CAUSE OF EVENT

The root cause was identified as high resistance on contacts of the 1B21H-K77 relay. Oxidation buildup caused a high relay contact resistance, resulting in downstream relay 1B21H-K29 momentarily dropping out and causing the inboard shutdown cooling suction isolation valve to close. This condition was exacerbated by the installation of the jumper, because while it was installed the jumper decreased the amount of current through the contacts.

D. SAFETY ANALYSIS

The safety consequences of this event were minimal. Shutdown cooling was restored within 12 minutes and reactor coolant temperature rose 12 degrees F. Both Reactor Recirculation (RR)[AD] pumps were running during the event, so forced coolant circulation was maintained.

The event was determined to be reportable in accordance with 10 CFR50.73(a)(2)(v)(B) as an event or condition that could have prevented the fulfillment of the safety function of systems needed to remove residual heat. This event is considered a Safety System Functional Failure.

Corrective Actions:

• The 1B21H-K77 relay was replaced.

• The remaining affected Agastat relays in the SDC isolation logic system were scheduled for replacement in upcoming division work windows.

Corrective Action to Prevent Recurrence:

• A preventative maintenance activity will be created to periodically replace the affected relays.

• A design change will be evaluated for defeating the shutdown cooling isolation logic when not required to be operable by the Technical Specifications.

F. PREVIOUS OCCURENCES

A document search found that LaSalle has not experienced a loss of shutdown cooling due to a spurious closure of the shutdown cooling suction isolation valve within the past five (5) years.

G. COMPONENT FAILURE DATA

Tyco Electronics - Agastat Model E7000 Series Time Day Relay