05000282/LER-2009-005

LER-2009-005, Reactor Trip Due to 12 Circulating Water Pump Trip Caused by Electrical Ground Fault

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
2822009005R00 - NRC Website

At 1304 Central Daylight Time (CDT) on May 18, 2009 while Prairie Island Nuclear Generating Plant Unit 1 was operating at 100 percent power, control room annunciators alerted operators that 12 Circulating Water (CW) pump' locked out. This caused circulating water flow rate through the condensers to be reduced and a subsequent decrease in condenser vacuum. A differential pressure developed between the two condensers which automatically tripped the turbine. The turbine trip automatically tripped Unit 1 reactor at 1305 CDT. Throughout the event, safety systems operated as expected.

The station determined 12 CW pump lockout occurred due to a ground fault in the power supply cable. The cause for the fault has been determined to be age related degradation, accelerated by water intrusion. The cable was purchased in 1972 from the Okonite Company. The cable was rated at 5 kV with a copper conductor and insulated with Okoguard (equivalent to ethylene propylene rubber).

This event is reportable since actuation of several systems to which 10 CFR 50.73(a)(2)(iv)(A) applies occurred. Auxiliary feedwater pumps automatically started on low steam generator level. All control rods fully inserted. Decay heat removal was accomplished via auxiliary feedwater and condenser steam dump. Offsite power was maintained to safeguards and non-safeguards alternating current buses.

EVENT ANALYSIS

Testing of the CW pump cables revealed a single phase fault in the power supply cable. The pump motor was not damaged. This cable was replaced as part of corrective actions. The cable had experienced water intrusion which ultimately reduced the service life of the cable.

The event did not have the potential to prevent the fulfillment of safety functions. The turbine trip resulted in a reactor trip as expected and all other safety-related equipment functioned as designed.

There was no safety system functional failure.

SAFETY SIGNIFICANCE

There were no safety consequences impacting plant or public safety as a result of this event.

The CW pump trip initiated a turbine trip, resulting in a reactor trip. The reactor trip is necessary due to a loss of the turbine as a heat sink for the reactor coolant system. Thus, the actuation of the reactor protection system was valid for this plant condition. During the reactor shutdown, all required safety systems responded appropriately.

There were no releases of radioactivity. There was a small thermal transient in the external circulating water system. The temperature trends were analyzed and judged to be within the bounds of normal variations.

EMS System Identifier: KE 9-200 The reactor trip and safety system actuations were caused by the age-related failure of a power supply cable. Contributing factors were that cable aging management did not identify and replace cables with known susceptibility to accelerated aging. Corrective actions from operating experience evaluations were not effectively implemented in a complete and timely manner.

CORRECTIVE ACTION

Short term corrective actions included replacing the failed cable. Long term corrective actions include replacement and monitoring of other cables that may be vulnerable to similar failures. The cable monitoring program will be fully implemented to identify, test, and inspect cables that are potentially susceptible to failure.

Additionally, industry operating experience identified that the cable manufactured during this timeframe was susceptible to contaminants being introduced into the insulation during the manufacturing process. The cable from the event is being tested to determine if potential contaminants contributed to this failure.

PREVIOUS SIMILAR EVENTS

In September 2008, the site recognized that it had not fully implemented the response to NRC Generic Letter 2007-01 by fully implementing an underground cable monitoring program.