05000373/LER-2013-002
| Lasalle County Station, Unit 1 | |
| Event date: | 04-17-2013 |
|---|---|
| Report date: | 01-22-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3732013002R02 - 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: April 17, 2013 Event Time: 1459 CDT Reactor Mode(s): 1 / 1 Mode(s) Name: Power Operation Power Level: 100%
B. DESCRIPTION OF EVENT:
On April 17, 2013, LaSalle Units 1 and 2 were operating in Mode 1 at 100% power, with a severe thunderstorm in progress. At 1457 hours0.0169 days <br />0.405 hours <br />0.00241 weeks <br />5.543885e-4 months <br /> CDT, lightning struck 138KV Line 0112, resulting in a phase-to-ground fault which subsequently cleared. At 1459 hours0.0169 days <br />0.405 hours <br />0.00241 weeks <br />5.551495e-4 months <br />, a second phase-to-ground fault on Line 0112 occurred and all 345 KV oil circuit breakers (OCBs) in the main switchyard (SY)[FK] opened, resulting in a loss of offsite power and reactor scrams on both Units. All emergency diesel generators (DG)[EK] automatically started and loaded onto their respective busses.
Plant systems on both Units responded as expected. All control rods went full in. The main steam isolation valves closed, with decay heat being removed via the safety relief valves. High Pressure Core Spray (HPCS)[BG] automatically started on both Units on low reactor water level; Reactor Core Isolation Cooling (RCIC)[BN] was used for level control. At 1511 hours0.0175 days <br />0.42 hours <br />0.0025 weeks <br />5.749355e-4 months <br />, LaSalle declared an Unusual Event due to a loss of offsite power (LOOP) for greater than 15 minutes.
Primary containment pressure increased as expected, consistent with the loss of containment cooling due to the loss of non-ESF AC power. Primary containment pressure reached the Primary Containment Isolation System (PCIS)[JM] isolation setpoint on April 17, 2013, at 1721 hours0.0199 days <br />0.478 hours <br />0.00285 weeks <br />6.548405e-4 months <br /> on Unit 2 and at 2004 hours0.0232 days <br />0.557 hours <br />0.00331 weeks <br />7.62522e-4 months <br /> on Unit 1.
Offsite power was restored to all ESF busses by 2301 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.755305e-4 months <br /> on April 17, 2013. Containment cooling was by 0814 hours0.00942 days <br />0.226 hours <br />0.00135 weeks <br />3.09727e-4 months <br />. The Unusual Event was terminated at 0814 hours0.00942 days <br />0.226 hours <br />0.00135 weeks <br />3.09727e-4 months <br /> on April 18, 2013.
This occurrence is reportable under 10 CFR 50.73(a)(2)(iv)(A) as an event which resulted in the automatic actuation of the reactor protection system (RPS)[JC], emergency core cooling systems (ECCS), and ESF systems as listed in 10 CFR 50.73(a)(2)(iv)(B). An ENS report was made to the NRC (EN# 48939) at 1559 CDT on April 17, 2013, and was updated as required throughout the event.
This event constitutes an unplanned scram with complications for both LaSalle Units 1 and 2.
C. CAUSE OF EVENT:
The initiating event was a lightning strike on 138KV Line 0112 in the main 345/138 KV switchyard. Line 0112 was inspected in the field and had sustained heavy damage to phase "C" insulators.
The root cause of the event was determined to be degradation of the 138kV switchyard grounding system that allowed a lightning induced fault to flash over onto the DC protective system. The grounding system degradation was due to poor workmanship during original construction. This degradation allowed a fault initiated by a lightning strike on the L0112 C phase capacitance coupled voltage transformer (CCVT) in the 138kV switchyard to damage the shared DC protection system. Another contributor to the event was determined to be inadequate lightning shielding of the 138kV switchyard.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. On the loss of offsite power, all emergency diesel generators automatically started and loaded onto their respective busses. Both reactors automatically scrammed, with all control rods fully inserting. All ESF and ECCS systems were operable at the time of the event. 345 KV Lines 0101 and 0102 from Plano, and 345 KV Lines 0103 and 0104 from Braidwood remained energized during the event.
This event was determined not to be reportable under 10 CFR 50.73(a)(2)(v)(D) as an event that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. NUREG-1022, Revision 3, does not require a LOOP to be reported under this requirement. An engineering review of UFSAR Chapter 6 "Engineered Safety Features" and 15 "Accident Analyses" was performed that validated that the non safety-related offsite power system is not required to mitigate the consequences of an accident. In the event of loss of offsite power, the ESF loads are automatically connected to the EDGs in sufficient time to provide for safe reactor shutdown and to mitigate the consequences of a design basis accident such as a LOCA. Therefore, the event did not constitute a safety system function failure.
E CORRECTIVE ACTIONS:
- Offsite power was restored to the ESF busses on both Units by 2301 hours0.0266 days <br />0.639 hours <br />0.0038 weeks <br />8.755305e-4 months <br /> on April 17, 2013.
- All degraded connections and ground cables in the 138kV switchyard were repaired.
- Lightning shielding in the 138kV switchyard have been improved.
F. PREVIOUS OCCURRENCES:
A search identified no previous occurrences within the past 10 years of a scram or a loss of offsite power at LaSalle County Station resulting from a lightning strike.
G. COMPONENT FAILURE DATA:
Westinghouse PCA-5 type capacitance coupled voltage transformer.