05000374/LER-2013-001
| Lasalle County Station, Unit 2 | |
| Event date: | 04-18-2013 |
|---|---|
| Report date: | 06-17-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| 3742013001R00 - 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: April 18, 2013 Event Time: 1400 CDT Reactor Mode(s): 3 Mode(s) Name: Hot Shutdown Power Level: 0%
B. DESCRIPTION OF EVENT:
On April 18, 2013, Unit 2 was in Mode 3 following a scram and a loss of offsite power that had occurred on both LaSalle Units the previous day. At 1400 hours0.0162 days <br />0.389 hours <br />0.00231 weeks <br />5.327e-4 months <br /> CDT, three pin hole through-wall leaks in the U2 High Pressure Core Spray (HPCS)[BG] minimum flow line piping were discovered. The leaks were on the outside bend of the first elbow downstream of the minimum flow restricting orifice, and appeared to be leaking a total of approximately 0.5 gpm with the HPCS pump not running.
Unit 2 HPCS was declared inoperable and, because the HPCS minimum flow line is in direct communication with the suppression pool, primary containment was also declared inoperable. The 2C Residual Heat Removal (RHR)[BO] loop was inoperable prior to the event, so Technical Specification (TS) 3.5.1 Required Action H.1 was entered, requiring entry in TS 3.0.3 and the plant to be placed in Mode 4 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />. Mode 4 was entered at 0250 hours0.00289 days <br />0.0694 hours <br />4.133598e-4 weeks <br />9.5125e-5 months <br /> CDT on April 19, 2013.
This occurrence is reportable under 10 CFR 50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of a safety function needed to mitigate the consequences of an accident, and 10 CFR 50.73(a)(2)(ii)(A) as an event or condition that resulted in the condition of the nuclear power plant, including its principal safety barriers, being seriously degraded. An ENS notification was made to the NRC (EN 48943) pursuant to 10 CFR 50.72(b)(3)(v)(D) and 50.72(b)(3)(ii)(A) at 2032 hours0.0235 days <br />0.564 hours <br />0.00336 weeks <br />7.73176e-4 months <br /> CDT on April 18, 2013.
This event constitutes a safety system functional failure.
C. CAUSE OF EVENT:
The direct cause of the event was a combination of cavitation and mechanical wear/erosion of the piping wall.
The apparent cause was procedural inconsistencies that allowed operation of the HPCS system in minimum- flow for extended periods.
General Electric Operations and Maintenance Instructions GEK 63041A states that when in minimum flow mode following emergency operation, prompt operator action should be taken to either place HPCS in a test mode (establish flow through the full flow test valve) or in standby mode in order to minimize wear on the HPCS pump impeller.
Also, operation in minimum flow mode had been addressed in the LaSalle response to NRC Bulletin 88-04, "Potential Safety-Related Pump Loss". The response evaluated the adequacy of minimum flow bypass lines for safety-related centrifugal pumps. Based on this event, the caution added to procedures to consider securing ECCS pumps whenever possible rather than operate at minimum flow for extended periods of time will be strengthened.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. Although the system was declared inoperable, HPCS was still available with pin hole leaks in the minimum flow line. Containment leakage from the pin hole leaks in the HPCS minimum flow line, although not quantified, would have been minimal due to their size. The leaks were discovered with the Unit shutdown in Mode 3, and the Unit was placed in Mode 4 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> of discovery.
E. CORRECTIVE ACTIONS:
- The leaking pipe elbow was replaced on Unit 2.
- An extent of condition wall-thickness ultrasonic inspection of piping and elbows downstream of the flow orifice on Unit 1 and Unit 2 HPCS minimum flow lines was performed.
- The first elbow after the flow orifice on the Unit 1 HPCS minimum flow line will be replaced during the next refueling outage.
- Operations procedures will be reviewed and revised as required to provide consistent guidance for minimizing operation of HPCS in minimum flow mode.
- ECCS lesson plans will be updated as needed to incorporate guidance for minimizing operation in minimum flow mode.
F. PREVIOUS OCCURRENCES:
A review identified no previous reportable events within the past ten years caused by piping leakage from erosion and/or corrosion.
G. COMPONENT FAILURE DATA:
Schedule 40, six inch diameter piping.