ML20149K339
| ML20149K339 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1982-009-00 | |
| Download: ML20149K339 (1) | |
Text
LER 285/82-009 Event
Description:
Three of Four CCW Heat Exchangers Inoperable Date of Event: April 11, 1982 Plant: Fort Calhoun Summary During normal power operation on April 11, 1982, while the component cooling water (CCW) heat exchangers were being exchanged, three of the four outlet valves (HCV-490B, HCV-491B, and HCV-492B) failed to open. (HCV-491B did open partially.) Only one CCW heat exchanger was operational. Within minutes, an operator was dispatched and arrived at the three subject valves.
He manually tapped on the actuator parts of all three valves. Valves HCV-490B and HCV-492B opened after they were tapped. Again, HCV-491B opened only slightly. HCV-491 B was disassembled. No apparent problem could be determined so the valve packing was loosened and the valve was tested for operation. All valves were cycled several times and operated successfully. The CCW system at Fort Calhoun consists of three CCW pumps and four CCW heat exchangers. One pump and at least two heat exchangers are normally operating. Pumps are rotated once a week. The CCW system provides cooling to the high-pressure safety injection (HPSI) pumps, the low pressure safety injection (LPSI) pumps, the shutdown cooling heat exchangers, the control air conditioners, the containment air cooling coils, and the containment spray system pumps. According to the Fort Calhoun individual plant examination document, three of four CCW heat exchangers and one CCW pump are sufficient to provide cooling to these systems for all plant modes, and CCW cooling is only needed during recirculation modes of residual heat removal (RHR) and high-pressure recirculation system (HPR). It was assumed in this analysis that three of the four CCW heat exchangers are needed for decay heat removal. This may be conservative for colder weather.
The licensee event report (LER) states that the CCW, heat exchanger valves are cycled two to three times a week. Thus this event was modeled as an unavailability of three of the four CCW heat exchangers for a period of three days (72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />). To reflect the loss of CCW cooling to the residual heat removal system, containment spray recirculation (CSR), and the high pressure recirculation system, all trains for each system were set to failed and the corresponding non-recovery probabilities for each system were set to 1.0. To determine an estimate of the conditional core damage probability that reflects the ability of the operators to locally recover the CCW valves and therefore recover the failed systems, the estimated conditional core damage probability for the failed systems was multiplied by the non-recovery probability for CCW. The CCW non-recovery probability was assumed to be 0.054, based on recovery times for service water-related failures included in "Faulted Systems Recovery Experience," Nuclear Safety Analysis Center (NSAC)-161, May 1992. The nominal conditional core damage probability given CCW is recovered and all recirculation systems function normally would not contribute significantly to the total estimated conditional core damage probability compared with that of the CCW not being recovered and was therefore not subtracted from the estimate. The increase in core damage probability (CDP), or importance, over the duration of the event is 5.7x10-6. The base-case CDP over the duration of the event is 3.3x10-7 resulting in an estimated conditional core damage probability of 6.0x10-6. The dominant sequence involves a postulated small loss-of-coolant accident (LOCA) with the failure of RHR and HPR.