ML20135H316
| ML20135H316 | |
| Person / Time | |
|---|---|
| Site: | Robinson  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 261-1992-013, LER 261-1992-014, LER 261-1992-018 | |
| Download: ML20135H316 (8) | |
Text
B-49 B.9 LER Number 261/92-013, 261/92-014, and 261/92-018 Event
Description:
Safety Injection Pump Out of Service Date of Event:
June 18, 1992, through August 22, 1992 Plant:
H. B. Robinson, Unit 2 B.9.1 Summary Both safety injection (SI) pumps were out of service for 1.5 h on July 10, 1992, while H. B. Robinson was at 100% power.
The "B" SI pump was rendered inoperable because plastic sheeting material obstructed the pump's recirculation line. The plastic material was believed to have been used during a design modification during the refueling outage that ended on June 18, 1992. The "A" pump was out of service for 1.5 h on July 10, 1992, because of a blown control power fuse in the pump's breaker closing circuit. On August 22, 1992, with the plant operating at 100% power, the plant experienced a total loss of offsite power (LOOP) (See LER 261/92-017). Following the LOOP, on August 24, 1992, the "B" SI pump recirculation line was again found to be obstructed with the plastic sheeting material from the outage modification.
The conditional core damage probability for the 1.5 h that both SI pumps were inoperable (LERs 261/92-013 and -014) is 6.2 x 10-1. This is below the precursor cutoff value of 106. Therefore, this event is not a precursor but is included here since this is when the extended inoperability of the "B" SI pump began.
The conditional core damage probability for the time period when the "B" SI pump was inoperable (LERs 261/92-013 and -018) is 3.5 x 10-i. The relative significance of this event compared to other postulated events at H. B. Robinson Unit 2 is shown in Fig. B. 11.
L13R 261M4013, -014,&.-018 E-7 E-65 1E-4 1E-3 1E-2 1TI*
36OhElP pr rr cutoff..
LOFW + 1 360 h LOOP MTR AFW AFW Fig. B.11.
Relative event significance of LERs 261/92-013, -014, and -018 compared with other potential events at H. B. Robinson 2.
LER NO: 261/92-013, -014, and -018
B-50 B.9.2 Event Description On July 8, 1992, at 2307 hours0.0267 days <br />0.641 hours <br />0.00381 weeks <br />8.778135e-4 months <br />, the "B" SI pump was declared out of service because of low flow on the pump's recirculation line. Plastic sheet material was found in the "B" SI pump minimum flow line.
The plastic material was believed to be from a purge dam that had been fabricated for welding operations for a modification to the minimum flow line for the residual heat removal (RHR) system during the cycle 14 refueling outage. The refueling outage ended on June 18, 1992. It is believed the material was introduced as a result of breakage of one of the 9-in.-diameter purge dam pieces.
A portion of the material was introduced into the RHR system, the refueling water storage tank (RWST), and SI and containment spray (CS) pump suction piping. The debris was removed through system flushing.
On July 9, 1992, at 1839 hours0.0213 days <br />0.511 hours <br />0.00304 weeks <br />6.997395e-4 months <br />, with the plant still at 100% power, an attempt was made to start the "A" SI pump. During this attempt, one of the two control power fuses in the pump's breaker closing circuit blew. The fuses were replaced, and the pump was returned to service 1.5 h later, at 2009 hours0.0233 days <br />0.558 hours <br />0.00332 weeks <br />7.644245e-4 months <br /> on July 9, 1992. The fuse manufacturer concluded that the "fuse was progressively weakened by repeated breaker closures until it opened to clear the circuit."
At 2030 hours0.0235 days <br />0.564 hours <br />0.00336 weeks <br />7.72415e-4 months <br />, on July 9, 1992, a plant shutdown to the hot shutdown condition was initiated because of the continued inoperability of the "B" SI pump. On July 12, 1992, at 0812 hours0.0094 days <br />0.226 hours <br />0.00134 weeks <br />3.08966e-4 months <br />, the "B" SI pump was returned to service following repeated flushing of the SI system.
Operability tests were also performed for the RHR and CS systems. The plant returned to service on July 12, 1992.
On August 22, 1992, with the plant at 100% power, a LOOP occurred at 1007 hours0.0117 days <br />0.28 hours <br />0.00167 weeks <br />3.831635e-4 months <br /> because of the loss of the startup transformer (see LER 261/92-017 in Appendix B). On August 24, 1992, following the LOOP and before plant restart, the "B" SI pump was tested and declared inoperable because of low flow in the recirculation line. The "A" SI pump was also declared inoperable because of reduced flow in its recirculation line. Investigation revealed that additional plastic sheeting, similar to the material found in the line on July 8, had partially blocked the "B" SI pump recirculation line. It was speculated by the licensee that a residual piece from the RHR system modification performed during the cycle 14 refueling outage that was initially too large to enter the recirculation line had been eroded by subsequent use of the SI pumps. The licensee had originally thought that the material was broken into very small pieces from the SI pump and the material would have easily entered the piping during previous flushing of the system.
This was based on the fragments found in the SI pump recirculation line in July. No debris was found in the "A" pump recirculation line, and the flow was within the required limits. Therefore, the "A" line was considered to have been operable throughout the event.
B.9.3 Additional Event-Related Information H. B. Robinson has two RHR pumps, which take suction from the RWST or the containment sump. The system can discharge to the reactor coolant system (RCS) cold legs or to the suction of the SI and CS system pumps. The RHR pump recirculation lines run back to the suction of the pumps.
The SI system uses two pumps that can take suction from the RWST or the RHR pump discharge.
Each pump has a recirculation line to provide pump cooling. The recirculation lines return to the RWST. The RHR,, SI, and CS pumps all share a common suction line from the RWST. The original SI system LER NO: 261/92-013, -014, and -018
B-51 included three pumps; however, one of the pumps has been removed from service for an extended period of time.
B.9.4 Modeling Assumptions These three licensee event reports (LERs) are analyzed together in two separate cases because of the unavailability of the "B" SI pump throughout the entire time period. The root cause of the "B" SI pump inoperability was the plastic sheeting material from the RHR system modification performed during the cycle 14 refueling outage.
The first case was modeled assuming that the two SI pumps were inoperable for 1.5 h. For the second case, it was assumed that the "B" SI pump was inoperable from the time the plant went critical following the completion of the plant outage on June 18, 1992, until the LOOP event occurred on August 22, 1992 (64.5 d).
The failure probability for the "A" SI pump was doubled for Case 2. This was to account for the increased likelihood of "A" pump failure due to recirculation line clogging. Following the failure of the "B" SI pump due to recirculation line plugging, all flow would be through the "A" pump. This increased flow potentially increases the likelihood of failure for the "A" pump from the same cause. For Case 1 the "A" pump was failed because of the fuse failure in the'starting circuit.
The nonrecovery values for the high pressure injection (HPI) and high pressure injection for feed and bleed (HPI(F/B)) were modified for both cases. For Case 1, the HPI nonrecovery was decreased from 0.84 to 0.34. This is based on the assumption that sufficient time would be available to recover the "A" SI pump by locally closing the breaker. In the HPI(F/B) case, the nonrecovery was increased from 0.84 to 1.0, assuming that neither pump would be recoverable in the required time period. For Case 2, the nonrecovery values for both HPI and HPI(F/B) were set to 1.0. This based on the assumption that the dominant failure mechanism would be blockage of the recirculation line by the plastic material and that this would not be recoverable in the required time period.
The system failure probabilities for HPI were modified to include the use of low pressure injection (LPI) in lieu of a failed high pressure injection (HPI) system. This process involves the use of the secondary side to cooldown and depressurize the RCS to below the LPI system injection pressure.
A failure probability of 0.12 was assigned to the cooldown process as this a proceduralized process performed under stress (see Appendix A, Sect. A. 1). This operator failure is dominant and the equipment failure rates are insignificant. The system probabilities for HPI in both cases were modified to include this recovery process.
B.9.5 Analysis Results The conditional core damage probability for the 1.5 h that both SI pumps were inoperable (Case 1, LERs 261/92-013 and -014) is 6.2 x 10-1. This is below the precursor cutoff. Therefore, this event is not a precursor. The conditional core damage probability for the time period when the "B" SI pump was inoperable (Case 2, LERs 261/92-013 and -018) is 3.5 )* 10'. The dominant core damage sequence for this precursor, shown in Fig. B. 12, involves a postulated loss of coolant accident (LOCA) followed by a failure of HPI.
LER NO: 261/92-013, -014, and -018
B-52 I
LOCA R"
I AFW MFw HPI HPR PORV SEQ END NO STATE OK 71 CD 72 CD OK 73 CD 74 CD OK 75 CD 76 CD 77 CD 78 ATWS Fig. B.12.
Dominant core damage sequence for LERs 261/92-013, -014, and -018 (case 2).
LER NO: 261/92-013, -014, and -018
B-53 LER NO: 261/92-013, -014, and -018
B-54 LER NO: 261/92-013, -014, and -018
B-55 LER NO: 261/92-013, -014, and -018
B-56 LER NO: 261/92-013, -014, and -018