05000373/LER-2011-004
| Lasalle County Station, Unit 1 | |
| Event date: | 02-02-2011 |
|---|---|
| Report date: | 12-14-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| Initial Reporting | |
| 3732011004R01 - NRC Website | |
LaSalle County Station Units 1 is a General Electric Company Boiling Water Reactor with 3546 Megawatts Rated Core Thermal Power.
A. CONDITION PRIOR TO EVENT:
Unit(s): 1 Event Date: February 2, 2011 Event Time: 16:57 CST Reactor Mode(s): 3 Mode(s) Name: Hot Shutdown Power Level: 0 percent
B. DESCRIPTION OF EVENT:
On February 2, 2011, Unit 1 was in Mode 3 (Hot Shutdown), cooling down to Mode 4 (Cold Sutdown) in order to perform forced outage work following an unplanned reactor scram. At 16:57 hours CST, while in Mode 3, the 1B Residual Heat Removal (RHR)[BO] pump was started in preparation for starting the Shutdown Cooling (SDC) system. When the pump was started, a momentary high pump suction flow signal was received, causing the common pump suction isolation valve 1E12-F009 to close and the 1B RHR pump to trip.
Both SDC trains were declared inoperable, and Technical Specification (TS) 3.4.9, "RHR Shutdown Cooling System — Hot Shutdown," Required Action (RA) A.1 was entered, which requires that action be intitiated immediately to restore the RHR shutdown cooling system to operable status. The control room operators determined that the isolation was spurious and reset the containment isolation logic. At 17:10 hours the 1E12- F009 valve was reopened and the TS RA exited.
This occurrence is reportable under 10 CFR 50.73(a)(2)(v)(B) as an event or condition that alone could have prevented the fulfillment of the safety function of structures or systems that are needed to remove decay heat.
This event constitutes a safety system functional failure.
C. CAUSE OF EVENT:
The cause of the isolation was determined to be a spurious high pump suction flow signal due to a flow transient when the 1B RHR pump was started.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. With Unit 1 in Mode 3 with both the 1A and 1B Reactor Recirculation (RR) pumps running, SDC was not required to be in operation. Had the event occurred with SDC in operation, there was no equipment failure that would have presented a challenge in restarting the system.
E. CORRECTIVE ACTIONS:
- The containment isolation logic was reset and the 1E12-F009 valve was reopened. The containment high suction flow isolation logic was bypassed in accordance with procedure, and the 1B RHR pump was started and placed in operation.
- The time delay on the SDC High Flow Isolation Relays 1(2)B21H-K74 and 1(2)B21H-K 77 were modified from one second to three seconds in order to preclude spurious isolations.
F. PREVIOUS OCCURRENCES:
On Unit 1, on July 20, 2009, with 'A' RHR loop in shutdown cooling, the common RHR pump suction containment isolation valve (1E1 2-F009) unexpectedly closed, causing a loss of shutdown cooling. The cause of this event was determined to be poor contact continuity in time delay relay 1B21H-K77.
G. COMPONENT FAILURE DATA:
Not applicable, as no component failure occurred.