05000374/LER-2013-003
| Lasalle County Station, Unit 2 | |
| Event date: | 12-07-2013 |
|---|---|
| Report date: | 02-05-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| Initial Reporting | |
| 3742013003R00 - NRC Website | |
LaSalle County Station Unit 2 is a General Electric Company Boiling Water Reactor with 3546 Megawatts Rated Core Thermal Power.
A. CONDITION PRIOR TO EVENT:
Unit(s): 2 Event Date: December 7, 2013 Event Time: 2355 CST Reactor Mode(s): 1 Mode(s) Name: Power Operation Power Level: 60%
B. DESCRIPTION OF EVENT:
On December 7, 2013, Unit 2 was at 60 percent power during a scheduled load reduction for surveillance testing and control rod sequence exchange. Core monitoring software is used to verify APRM indication is within the requirements of Technical Specification (TS) 3.3.1.1. This software does not provide continuous APRM indication; instead it provides indication only when predetermined setpoints are reached or when demanded by the operator. The APRM system is divided into two groups of channels with three APRM inputs to each trip system.
At approximately 23:36 hours CST the core monitoring software indicated that all three APRM channels in the `A' RPS system were operable as required by TS 3.3.1.1. At approximately 2355 hours0.0273 days <br />0.654 hours <br />0.00389 weeks <br />8.960775e-4 months <br /> CST, following planned control rod movement, evaluation of data from the core monitoring software system determined that all three APRM channels in the `A' RPS trip system were inoperable. TS Surveillance Requirement (SR) 3.3.1.1.2 requires that the absolute difference between an APRM channel output and calculated core thermal power not exceed two percent. All three APRM channels in the 'A' RPS trip system were confirmed to read less than the calculated power, which is non-conservative, by more than two percent (approximately 2.5 percent).
TS 3.3.1.1 requires at least two operable APRM channels per RPS trip system, and at least one APRM channel is required to be operable to maintain the safety function. TS 3.3.1.1 allows delaying entry into the associated Conditions and Required Actions for up to two hours when Functions 2.b and 2.c are inoperable due solely to not meeting the two percent differential limit in the non-conservative direction. The gains were adjusted for all three APRM channels to return indicated power to within the TS SR limit by 0017 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> CST on December 8, 2013; therefore, no TS Required Action time clocks were entered.
This occurrence is reportable under 10 CFR 50.73(a)(2)(v)(A) as an event or condition that could have prevented the fulfillment of the safety function of the structures or systems that are needed to shutdown the reactor and maintain it in a safe shutdown condition. An ENS report was made to the NRC (EN 49615) at 0509 CST on December 8, 2013, pursuant to 10 CFR 50.72(b)(3)(v)(A).
This event constitutes a safety system functional failure for Unit 2.
C. CAUSE OF EVENT:
The cause of the event was an unexpected degree of APRM indication drift, following planned control rod movement. Corrective actions include training and procedure revisions to minimize the possibility of future occurrences.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. The gains were adjusted for all three APRM channels to return indicated power to within the acceptable range within 23 minutes of the condition being discovered. This is significantly less than the two hours allowed by TS 3.3.1.1.
Additionally, the transient analysis assumes APRM trips are initiated approximately 6% above the nominal setpoint used by the station. Therefore, even with APRMs 2.5% non-conservative, a trip would have occurred earlier than that assumed by the transient analysis.
E. CORRECTIVE ACTIONS:
The gains for all three APRMs were promptly adjusted to return indicated power to within the required two percent tolerance.
- Site procedures will be revised to provide enhanced APRM indication monitoring using the plant process computer in addition to the core monitoring software.
- Training will be conducted on the event with Operations personnel.
F. PREVIOUS OCCURRENCES:
A document search identified no reportable events due to APRM gains being out of tolerance in the previous 10 years on either LaSalle Unit.
G. COMPONENT FAILURE DATA:
There were no component failures associated with this event.