ML20135G983
| ML20135G983 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1992-004-00, LER 1992-005-00 | |
| Download: ML20135G983 (6) | |
Text
B-65 B.11 LER Number 269/92-004 and 269/92-005 Event
Description:
Reactor Trip with One Emergency Feedwater Train Inoperable Date of Event:
May 8, 1992 Plant:
Oconee 1 B.11.1 Summary On May 8, 1992, Oconee tripped from 14% power as a result of a pressure transient in the main feedwater (MFW) system. On May 27, 1992, it was discovered that one train of emergency feedwater had been inoperable at the time of the trip on May 8. The conditional core damage probability estimated for this event is 4.0 x 10-.
The relative significance of this event compared to other postulated events at Oconee 1 is shown in Fig. B. 16.
LER 269/92-004 & -005 1&-7 IE-6 M5-5 1E-4 1E-3 IB&2 360h BFW LOOP ll~RWE~w360 h EP Fig. B.16.
Relative event significance of LERs 269/92-004 and -005 compared with other potential events at Oconee 1.
B.11.2 Event Description With Oconee 1 at 14% power, draining of the condenser hotwell was in progress during a plant startup on May 8, 1992.
Because of the low power level, only one MFW pump (the lB MFW pump) was required; the IA MFW pump was idle. When the operator opened the condensate dump line (from the condensate system to the condensate storage tank) to drain the condenser, the decreased flow to the feedwater pumps caused a plant trip on low MFW pump discharge pressure. Following the trip, the emergency feedwater (EFW) system actuated, and the lB MFW pump continued to run. After verifying that the 1B MFW pump was running, the operator manually shut down both the IA and lB EFW pumps.
LER NO: 269/92-004 and -005
B-66 The two motor-driven EFW pumps had run for 43 sec. The turbine-driven EFW pump did not start because the start signal was not present for greater than 15 sec. The remainder of the post-trip recovery was uneventful.
Between May 12 and May 24, 1992, the plant operated at 100% power. On May 24 the plant was shut down to repair a reactor coolant pump seal.
On May 27, 1992, with the plant in hot standby, the quarterly stroke test procedure was conducted on the A steam generator (SG) EFW control valve. The test revealed that the solenoid valve for enabling automatic control of the A SG EFW control valve had failed.
A review of the post-trip data for the May 8, 1992, event revealed that the A EFW train had exhibited no flow during the event. The valve had last been successfully tested on September 22, 1991.
B.11.3 Additional Event-Related Information The condensate pumps, condensate booster pumps, and MFW pumps are arranged in series to provide the SGs with water from the condenser hotwell and secondary side drains. The condensate dump line to the condensate storage tank branches off between the condensate booster pumps and the MFW pumps.
The EFW system consists of three pumps: two motor-driven and one turbine-driven. The pumps start on loss of the MFW pumps as indicated by low discharge pressure or loss of hydraulic oil pressure on both MFW pumps. If the start signal clears within 15 sec, the turbine-driven EFW pump will reset. The three pumps discharge into two lines, each of which is connected to a SG. The A SG EFW flow control valve automatically varies its position to bring the A SG level to a predetermined setpoint following a reactor trip. Failure of the automatic control portion of the system does not prevent manual control of the valve.
A standby shutdown facility (SSF) is located in a separate building on the Oconee site. This facility, which is not normally manned, is capable of providing limited RCS makeup, RCP seal cooling, and steam generator makeup. SSF systems consist of single trains and are therefore not single-failure-proof.
B.11.4 Modeling Assumptions This event was modeled as a reactor trip with one of two EFW trains inoperable. The model normally utilizes pump status for input, and as a result, the existing EFW model is a 1 of 3 system. The EFW system failure probability was calculated using a one of two train success criteria since the component that failed is one of two EFW lines to the SGs. The first train was modeled as failed; the second with a failure probability of 0.1. This results in a system failure probability of 0.1. Consistent with other ASP analyses, the nonrecovery probability for EFW was not revised since the system was observed to be degraded and not failed. The use of the SSF as a alternate source of steam generator feedwater was included in the modeling. A combined operator and equipment failure probability of 0.2 was used for the SSF. This probability is consistent with values developed in the Oconee PRA (NSAC-60) and in the analysis of another event (see LER No. 270/92-004).
LER NO: 269/92-004 and -005
B-67 B.11.5 Analysis Results The conditional probability of core damage estimated for this event is 4.0 x 10-6. The dominant core damage sequence, highlighted on the event tree in Fig. B. 17, involves a postulated failure of EFW and MFW, PORV challenge and reseat, failure of the SSF feedwater function, successful initial feed-and-bleed, and subsequent failure when recirculation is initiated.
LER NO: 269/92-004 and -005
B-68 PORV/ I POV/
I CIAL IRESEATI NO STATE OK OK 11 CD 12 CD OK OK OK 13 CD 14 CD OK OK OK 16 CD 17 CD OK 31 CD 32 CD ATWS 18 Fig. B.17.
Dominant core damage sequence for LER 269/92-004 and -005.
LER NO: 269/92-004 and -005
B-69 LER NO: 269/92-004 and -005
B-70 LER NO: 269/92-004 and -005