05000373/LER-2010-003
| Lasalle County Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| Initial Reporting | |
| 3732010003R01 - NRC Website | |
LaSalle County Station (LSCS) Unit 1 is a General Electric Boling Water Reactor with 3546 Megawatts Thermal Rated Core Power. Unit 2 is a General Electric Boiling Water Reactor with 3489 Megawatts Thermal Rated Core Power.
A. CONDITION PRIOR TO EVENT:
� Unit(s): 1 and 2 Event Date: October 28, 2010� Event Time: 1045 CDT � Reactor Mode(s): 1 Mode(s) Name: Power Operation Power Level: 100 percent
B. DESCRIPTION OF EVENT:
Standby Liquid Control (SBLC) [BR] system test tanks was challenged. On October 27, 2010, the operability determination process was initiated, and Engineering began verifying design data and calculations. The SBLC test tanks on both units were drained of water on October 27, 2010. On October 28, 2010, Engineering determined that the historical calculations used the wrong formula in determining the test tank's natural frequency. The consequences of this event is that if the test tank is filled with water, the possibility exists that the test tank could fall over during a seismic event and adversely affect both trains of SBLC. Calculations confirm that with the test tanks empty, the mounting is seismically qualified. Therefore, the Unit 1 and Unit 2 SBLC systems were fully operable upon discovery of the calculation error.
The event was reported to the NRC Operations Center at 1521 (ET) on October 28, 2010 (EN # 46372). The event is reportable in accordance with 10CFR50.73(a)(2)(v)(A)/(C)/(D), an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shutdown the reactor and maintain it in a safe condition, control the release of radioactive material, and mitigate the consequences of an accident. In addition this event is reportable under 10CFR50.73(a)(2)(i)(B), a condition which is prohibited by Technical Specification.
The potential for both trains of SBLC system to be inoperable existed for a period of time greater than allowed by the LSCS Technical Specification. This event is also reportable under 10CFR50.73(a)(2)(vii), an event where a single cause or condition caused two independent trains to become inoperable in a single system. A failure of the SBLC test tank had the potential to render both trains of SBLC inoperable.
C. CAUSE OF EVENT:
The possibility of non-safety-related, non-seismic items falling over during a seismic event was not a consideration in the original design of LSCS. It was identified as an industry issue near the completion of LSCS's construction. An evaluation of the test tank seismic mounting was performed by an outside vendor and documented in a design analysis. In 1981 the analysis demonstrated the adequacy of the fasteners that attached the test tank's legs to the floor but, did not verify the structural adequacy of the legs. Typically, a separate design analysis is performed to address structural integrity of the test tank mounting to withstand hydrodynamic loads resulting from a seismic event. A search of the LSCS's controlled documents did not identify the existence of such an analysis. The existing mounting design analysis was inaccurate and incomplete to support structural integrity of the SBLC test tank. The cause of the event is less than adequate (historical) design analysis and information. .
D. SAFETY ANALYSIS:
The safety significance of this event is minimal. The safety function of the SBLC system is to provide the capability of bringing the reactor, at any time in a fuel cycle, from full power and minimum control rod inventory to a subcritical condition. The probability of a design basis earthquake concurrent with an anticipated transient without scram (ATWS) condition is low. Additionally there is procedural guidance for injecting sodium pentaborate solution using the Reactor Water Clean-up (RWCU) system with the SBLC system unavailable.
E. CORRECTIVE ACTIONS:
This event occurred in 1981 and therefore is historical. An Engineering evaluation was performed to support seismic analysis of the SBLC test tank mounting with the test tank drained. Procedural controls have been put in place to direct the operator to drain the SBLC test tank following surveillance testing that requires use of the test tank.
F. PREVIOUS OCCURRENCES:
A review of LSCS Licensee Event Reports (LERs) for the last three years did not identify any LERs associated with seismic analysis of the SBLC system.
G. COMPONENT FAILURE DATA:
Component failures were not involved with this event.