05000282/LER-2009-006
| Prairie Island Nuclear Generating Plant Unit 1 | |
| Event date: | 10-20-2009 |
|---|---|
| Report date: | 04-08-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2822009006R01 - NRC Website | |
A high energy line break (HELB) in the turbine building can result in flooding due to release of condensate/feedwater from the break. Additionally, the broken high energy pipe can cause consequential failure of adjacent piping such as cooling water or fire protection (which have unlimited sources of water). These water sources might eventually fill the turbine building to the point where operability of certain equipment in emergency diesel generator', auxiliary feedwater (AFW)2, cooling water3, safety injection4, station airs, safeguards powers, and DC electrical power' systems could be adversely affected. The staff determined that, within the previous three years, this condition had resulted in the plant being in an unanalyzed condition that significantly degraded plant safety and a postulated condition that could have prevented fulfillment of a safety function.
PINGP staff have been evaluating the potential effects of a postulated HELB in the turbine building, including the impact of flooding due to a HELB. On October 20, 2009, Prairie Island Nuclear Generating Plant (PINGP) Unit 1 was in Mode 5 during a refueling outage. As part of the evaluation of postulated HELB flooding, PINGP staff determined that in the event of flooding in the turbine building due to a HELB, operability of the Unit 1 Emergency Diesel Generators (D1 and D2) may not be assured. Due to being in Mode 5 (which does not have the potential for a HELB), at the time of discovery Unit 1 was not in a condition that significantly degraded plant safety.
EVENT ANALYSIS
This condition was a susceptibility of equipment to a consequential failure from a postulated turbine building HELB. The event is being reported under 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition and 10 CFR 50.73(a)(2)(v)(A) as a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shut down the reactor and maintain it in a safe shutdown condition. The event represents a safety system functional failure for both Units 1 and 2.
Evaluation of potential flooding from a postulated HELB in the turbine building is ongoing. Additional information from the evaluation will be submitted to the NRC via a supplement to this LER.
SAFETY SIGNIFICANCE
This LER is not associated with an event resulting in actual flooding of any portion of the plant. There were no actual consequences to the health and safety of the public as a result of this condition. The evaluation of risk significance of the as-found condition is ongoing and results will be included in a supplement to this LER.
A causal analysis for this issue will be performed based on the results of the engineering analysis.
Northern States Power Company, a Minnesota corporation, will provide a supplement to this LER once the cause and corrective actions have been determined.
Corrective actions that have been implemented or are currently in progress are:
1. A design and licensing basis for internal flooding is being compiled.
2. Rollup doors on the exterior of the turbine building have been opened and potential obstacles removed to allow floodwater to drain unimpeded.
3. Gaps around battery room doors have been decreased.
4. Access covers in the AFW pump room trench cover plates have been fastened down.
5. Flood barriers have been installed to reduce impact upon D1, D2, and the Unit 2 Emergency Diesel Generators (D5 and D6).
PREVIOUS SIMILAR EVENTS
a Postulated High Energy Line Break, Supplement 1, was submitted January 19, 2009. This LER described a condition where both trains of the component cooling water system were susceptible to a single failure caused by a postulated HELB in the turbine building.