05000373/LER-2011-003
| Lasalle County Station, Unit 1 | |
| Event date: | 03-02-2011 |
|---|---|
| Report date: | 04-29-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| 3732011003R00 - NRC Website | |
LaSalle County Station Units 1 and 2 are General Electric Company Boiling Water Reactors with 3546 Megawatts Rated Core Thermal Power.
A. CONDITION PRIOR TO EVENT:
Unit(s): 1 / 2 Event Date: March 2, 2011 Event Time: 0928 CST Reactor Mode(s): 1 / 5 Mode(s) Name: Power Operation / Refuel Power Level: 100 / 0 percent
B. DESCRIPTION OF EVENT:
On March 2, 2011, Unit 1 was in Mode 1 at 100% power and Unit 2 was in Mode 5 in a refueling outage, with fuel moves in progress. At 0928 hours0.0107 days <br />0.258 hours <br />0.00153 weeks <br />3.53104e-4 months <br /> CST, the Unit 1 Reactor Operator reported that the control room indication for secondary containment [NG] vacuum was 0.17 inch of vacuum water gauge. Technical Specification (TS) Surveillance Requirement (SR) 3.6.4.1.1 requires that secondary containment vacuum be greater than or equal to 0.25 inch of vacuum water gauge. Secondary containment was declared inoperable, and a four-hour timeclock to restore secondary containment to operable status was started in accordance with TS 3.6.4.1 Required Action (RA) A.1. In addition, TS RAs C.1, C.2, and C.3 were entered, and fuel moves and core alterations on Unit 2 were immediately suspended. Personnel were dispatched to the Unit 1 Reactor Building and the Refuel Floor ventilation panels to observe local differential pressure readings, where the control room indication was confirmed. No ventilation system operational problems were identified.
Shortly thereafter, secondary containment vacuum indication returned to its normal reading of approximately 0.5 inches of vacuum water gauge. At 0942 hours0.0109 days <br />0.262 hours <br />0.00156 weeks <br />3.58431e-4 months <br /> CST, secondary containment was declared operable and the associated RAs were exited. At 1000 hours0.0116 days <br />0.278 hours <br />0.00165 weeks <br />3.805e-4 months <br />, fuel moves and core alterations were resumed on Unit 2.
This occurrence is reportable under 10 CFR 50.73(a)(2)(v)(C) and (D) as an event or condition that alone could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material and to mitigate the consequences of an accident. This event constitutes a safety system functional failure.
C. CAUSE OF EVENT:
The cause of the event could not be determined; however, it is suspected that both Unit 2 steam tunnel airlock doors were simultaneously opened and held open to transport material during outage demobilization activities.
Interviews with personnel working in the area could not confirm this apparent cause; however, there is no evidence of ventilation system problems that would have caused the differential pressure transient.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. The secondary containment was inoperable for a brief period of approximately 14 minutes, which is significantly less than the four-hour Completion Time to restore the secondary containment to operable status allowed by TS 3.6.4.1 Condition A. The Reactor Building-to-outside differential pressure remained negative throughout the period that the secondary containment was inoperable.
E. CORRECTIVE ACTIONS:
- The interlock magnets and switches will be replaced to assure reliability.
- Plant barrier training will be conducted for all personnel who traverse secondary containment doors.
- New signage regarding plant barrier requirements will be developed and placed on the secondary containment doors.
F. PREVIOUS OCCURRENCES:
A review of Licensee Event Reports submitted by LaSalle over the past ten years found no previous occurrences of an unplanned inoperable secondary containment.
G. COMPONENT FAILURE DATA:
This is not applicable, as no component failure occurred.