05000306/LER-2008-001
| Prairie Island Nuclear Generating Plant Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3062008001R01 - NRC Website | |
On July 31, 2008, Prairie Island Nuclear Generating Plant (PINGP) Unit 2 was operating at 100 percent power. At 1345 CDT Prairie Island staff declared both trains of the Unit 2 component cooling water' (CC) system inoperable due to the discovery that a postulated Unit 2 high energy line break (HELB) in the Turbine Building could fail a CC line that would affect both trains of the Unit 2 CC system. With both trains of CC declared inoperable, PINGP staff entered Technical Specification (TS) Limiting Condition for Operability (LCO) 3.0.3. PINGP staff isolated the CC line in the Turbine Building at 1612 on July 31, 2008, which returned Unit 2 CC to operable status. Unit 2 remained at 100 percent power throughout the event.
EVENT ANALYSIS
The CC system is required to mitigate a HELB, but the CC line that was susceptible to a Unit 2 HELB supplies cooling to Unit 2 sample coolers. While this line can remain isolated at power, there is no automatic isolation of this line (e.g., on a safety injection signal) and any CC inventory loss out of this line would eventually affect both trains of the CC system. This condition was identified in support of turbine building CC system seismic qualification. Turbine building loads can be lined up to either unit CC system, but were aligned to Unit 2 CC at the time of discovery.
This condition was a susceptibility of two trains of Unit 2 CC to a consequential failure from a postulated Unit 2 HELB. A postulated HELB break location was identified that would have been expected to break a nearby CC line to Unit 2 sample coolers. Since a Unit 2 HELB could directly result in the loss of both trains of Unit 2 CC (a system that is required to meet the single failure criterion), in this configuration the Unit 2 CC system did not meet the single failure criterion. Thus, this condition is reportable per 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition. Had a postulated HELB occurred at the specific break location, a consequential CC line break would reasonably be expected to have occurred. Thus, this condition represented a safety system functional failure, so this event is reportable per 10CFR 50.73(a)(2)(v).
SAFETY SIGNIFICANCE
This condition resulted in a potential for a postulated HELB at a specific break location to cause a break in a CC line. There were no actual consequences to the health and safety of the public as a result of this condition. Once a leak in the CC system is isolated or repaired, the system can be refilled and returned to service. Loss of CC due to a pipe rupture is addressed in plant operating procedures, which provide operators immediate and subsequent action steps to mitigate the consequences of a event. However, depending upon the scenario being postulated, it may not be possible to recover the CC system in time to mitigate the postulated event. The NSPM evaluation of risk significance of the as-found condition is ongoing and will be shared with the NRC as part of the significance determination process.
1 EIIS System Code: CC U.S. NUCLEAR REGULATORY COMMISSIONNRC FORM 366A� LICENSEE EVENT REPORT (LER)(9-2007)
CONTINUATION SHEET
Prairie Island Nuclear Generating Plant Unit 2 05000306 YEAR
CAUSE
The as-found condition was the result of an original design issue that was uncovered during walkdowns in support of turbine building CC system seismic qualification. However, Prairie Island staff could have identified this condition earlier. Northern States Power Company, a Minnesota corporation (NSPM) conducted a causal evaluation associated with the as-found condition. This apparent cause evaluation (ACE) looked at why the corrective action process was not used to evaluate operability when this issue was identified in July of 2006. The cause of this oversight was attributed to a lack of operational focus on the part of Engineering.
A subsequent ACE was initiated to evaluate why a number of other previous opportunities to identify HELB and CC system interaction were missed. A vendor study of January 2008 documented an issue with a postulated HELB of a feedwater line damaging a CC line such that the system would be drained in a matter of minutes. A June 2007 draft of this study contained the same issue.
Furthermore, the study referred to a calculation approved in 1995 that had enough information to provide an opportunity to identify this issue. Finally, the calculation referred to a 1987 study that also recognized that there were loads in the turbine building, but failed to identify the potential vulnerability to a HELB in the turbine building. That ACE concluded the cause of the oversight was less than adequate procedure use and adherence on the part of Engineering due to cognitive personnel error.
CORRECTIVE ACTION
The susceptible CC line in the Turbine Building was isolated in the Auxiliary Building.
The planned corrective action is to modify the cooling source to the cold lab and turbine building sample coolers to reroute or eliminate CC lines from the Turbine Building.
PREVIOUS SIMILAR EVENTS
Review of LERs for Unit 1 and Unit 2 since 2006 found no similar events.