ML20156A217
| ML20156A217 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1985-017-00 | |
| Download: ML20156A217 (6) | |
Text
PRECURSOR DESCRIPTION AND ANALYSIS LER No.: 206/85-017 Event
Description:
Effective LOOP and AFW System Unavailability Date of Event: November 21, 1985 Plant: San Onofre 1 EVENT DESCRIPTION Sequence While operators were searching for a ground on 4160-V safety bus 1C, a differential relay actuated because of a ground fault and opened breakers that isolated auxiliary transformer C from the switchyard..
This resulted *in loss of power to bus 2C, the other safety bus, and trip of the main feed pump supplied by that bus. The IC bus, which is normally fed from auxiliary transformer C, was powered by bus 1A from auxiliary transformer A, which was tied to the main generator output.
The operators believed that a loss of offsite power had occurred and manually scrammed the reactor and tripped the turbine. Because safety bus IC was supplied from auxiliary transformer A (tied to the main generator), power was lost to all 4160-V buses.
Both diesel generators started but were not loaded as required (manual action required). Instead, operators began to restore power by backfeeding through the main transformer, which is allowed by plant operating procedures.
Shortly af ter the loss of power to saf ety bus. 2C, a loud bang was heard. The east Nos. 4-5 feedwater heater tubes and shell (a 15- by 2-ft break) had ruptured because of overpressurization from the west main feed pump and failure of the east MEW pump discharge check valve to seat correctly. The continued operation of the west main, feed pump was due to the unusual electrical alignment (power to the west feed pump was lost subsequent to trip of the plant).
Feed flow to all steam generators was terminated when the unit tripped. However, the A, B, and C feedwater regulating valve discharge check valves failed to seat. All three steam generators began blowing down through the stuck-open check valves.
The turbine-driven auxiliary feed pump started as required but was initially prevented from delivering flow because of a pump warm-up period. After 3.5 min the pump began to supply relatively cold ANW to the three MEW lines. Because of the failed check valves, AFW in the feedwater lines flowed toward the feed pumps. After 8.5 min the operators closed the feedwater regulating valves. Continued AFW subsequently encountered steam in a horizontal pipe run, rapid Event Identifier: 206/85-017 E- 15
condensation occurred, and a water slug in the line was accelerated in the direction of the main feed pumps. This resulted in damage to the B feedwater line and supports, damage to the B feedwater regulating valve bypass line check valve top-hat bolts, and extrusion of the check valve gasket, which resulted in a nonisolable leak.
Power was restored to the 4160-V buses ~-4 min into the transient, at which time the motor-driven AEW pump started. AFW flow was set at 25 gal/mmn to each steam generator to limit RCS cooldown.
Subsequently, wide-range level-was observed off scale (low) in all three steam generators, and ANW flow was increased and blowdown secured.
Steam generator B is believed to have gone dry at this time, although some water remained in the other two steam generators. At this point, also, the feedwater regulating valves were closed by the operators, restoring ANW flow to the steam generators.
Corrective Action Investigation into the cause and determination of corrective action were initiated in cooperation with the NRC Incident Investigation Team (see NUREG- 1190).
Plant/Event Data Systems Involved:
Electrical, MFW and condensate, and AFW Components and Failure Modes Involved:
Check valves, transformer, pipes, supports, AFW -fail in operation and on demand Component Unavailability Duration: NA Plant Operating Mode: 1 (60% power)
Discovery Method: Operational event Reactor Age: 18.4 years Plant Type: FWR Comments None Event Identifier: 206/85-017 E- 16
MODELING CONSIDERATIONS AND DECISIONS Initiators Modeled and Initiator Nonrecovery Estimate Unique transient 1.0 (includes LOOP)
Branches Impacted and Branch Nonrecovery Estimate AFW 0.04 Recoverable from the control room AFW given 0.12 Lack of information available and emergency power tension during event failure MFW Base case Loss of flow through the damaged check valve not believed large enough to prevent sufficient feedwater flow had it been demanded Plant Models Utilized Unique to this event Event Identifier: 206/85-017 E-17
CONDITIONAL CORE DAMAGE CALCULATIONS LER Number: 206/85-017 Event
Description:
Effective LOOP and AFW Unavailability Event Date: 11/21/85 Plant:
INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS I.OO0E+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 9. 346E-04 Total 9.346E-04 CV TRANS 1.06SE-03 Total 1.06BE-03 ATWS TRANS 3.600E-05 Total 3.600E-05 DOMINANT SEQUENCES End State: CD Conditional Probability: 1.940E-04 40 TRANS -RT AB.BUS.PNR- DG.PWR.C AFW/DG End State: CV Conditional Probability: 3.35SE-04 11 TRANS -RT -AB.BUS.PWR -C.TO.AB AFW MFW.REC B&F(CH.PUMPS) -SSR/COND End State: ATWS Conditional Probability: 3.600E-05 41 TRANS RT Event Identifier: 206/85-017 E- 18
SEQUENCE CONDITIONAL PROBABILITIES Sequence End State Seq. Prob Non-Recov#*
7 TRANS -RT -AB.BUS.PWR -C.TO.AB AFW MFW.REC -B&F(CH.PUMPS) -P CV ').187E-04 6.260E-04 ORV.OPEN RECIRC -SSR/C0ND 8 TRANS -RI -AB.BUS.PWR -C.TO.AB AFW MFW.REC -B&F(CH.PUMPS) -P CD 1.642E-04 3.225E-04 ORV.OPEN RECIRC SSRICOND q TRANS -RI -AB.BUS.PWR -C.TO.AB AFW MFW.REC -B&F(CH.PUI¶PS) P CV 3.326E-04 6.521E-04 DRY. OPEN -SSR/COND 10 TRANS -RI -AB.BUS.PWR -C.TO.AB AFW MFW.REC -B&F(CH.PUMPS) P CD 1.713E-04 3.357E-04 ORV.OPEN SSR/COND 11 TRANS -RI -AB.BUS.PWR -C.TO.AB AFW MFN.REC B&F(CH.PUMPS) -S CV 3.35BE-04 I3.869E-04 SRICOND 12 TRANS -RI -AB.BUS.PNR -C.TO.AB AFW MFW.REC B&F(CH.PUMPS) S CO 1.730E-04 1.?93E-04 SR/ COND 19 TRANS -RI -AB.BUS.PWR C.TO.AB -DG.PWR.C AFW !IFW.REC -B&F(CH CV 1.277E-05 2.641E-05
.PUMPS) -PORV.OPEN RECIRC -SSR/COND 20 TRANS -RI -AB.BUS.PWR C.IO.AB -D6.PWR.C AFN MFW.REC -B&F(CH CD 6.581E-06 1.361E-05
.PUMPS) -PORV.OPEN RECIRC SSR/COND 21 TRANS -RI -AB.BUS.PWR C.IO.AB -D6.PWR.C AFW MFW.REC -B&F(CH CV I1.333E-05 2.751E-05
.PUMPS) PORV.OPEN -SSR/COND 22 TRANS -RI -AB.BUS.PWR C.IO.AB -DG.PWR.C AFW MFW.REC -B&F(CH CD 6.869E-06 1.417E-05
.PUMPS) PORV.OPEN SSR/COND 23 TRANS -RT -AB.BUS.PWR C.IO.AB -D6.PWR.C AFW MFW.REC B&F(CH CV 1.346E-05 1.633E-05
.PUMPS) -SSR/COND 24 TRANS -RI -AB.BUS.PWR C.IO.AB -D6.PWR.C AFN NFW.REC B&F(CH CD 6.934E-06 8.411E-06
.PUMPS) SSR/COND 26 TRANS -RT -AB.BUS.PWR C.IO.AB D6.PWR.C AFW/D6 CO 1.863E-04 2.093E-04 33 TRANS -RI AB.BUS.PWR -DG.PWR.C AFW MFW.REC -B&F(CH.PUMPS) - CV 1.331E-05 2.751E-05 PORV.OPEN RECIRC -SSR/CDND 34 TRANS -RI AB.BUS.PWR -D13.PWR.C AFW MFW.REC -B&F(CH.PUMPS) - CD 6.856E-06 1.417E-05 PORV.OPEN RECIRC SSR/COND 35 TRANS -RI AB.BUS.PWR -D6.PWR.C AFW MFW.REC -B&F(CH.PURPS) CV 1.389E-05 2.866E-05 PORY. OPEN -SSR/COND 36 TRANS -RI AB.BtJS.PWR -D6.PWR.C AFW NFW.REC -B&F(CH.PUMPS) CD 7.156E-06 1.476E-05 PORV. OPEN SSR/COND 37 TRANS -RI AB.BUS.PWR -D6.PWR.C AFW MFN.REC B&F(CH.PUMPS) - CV 1.402E-05 1.701E-05 SSR/COND 38 TRANS -RI AB.BUS.PWR -DG.PWR.C AFW MFW.REC B&F(CH.PUMPS) CD 7.223E-06 B.761E-06 SSRICOND 40 TRANS -RI AB.BUS.PWR D6.PWR.C AFW/D6 CD 1.940E-04
- 2.180E-04 41 TRANS RI AIWS 3.600E-05
- 1.000E+00 Idominant sequence for end state IInon-recovery credit for edited case Note-Event Identifier: 206/85-017 E- 19
Conditional probability values are differential values which reflect the added risk due to observed failures.
Parenthetical values indicate a reduction inrisk compared to a similar period without the existing failures.
MODEL: A: sanonof .dat DATA.
No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail TRANS I.000E+00 1.000E+00 RI 3.600E-05 1.O00E+00 AB. BUS. PNR 1.OOOE-04 1.000E1+00 4.OOOE-02 C.TO. AB 1.000E-04 1.000E+00 4.OOOE-02 DG.PWR.C 8.OOOE-04 4.300E-01 4.OOOE-02 AFN 1.OO0E+00 4.OOOE-02 AFWIDG 1.O00E+00 1.200E-01 MFW. REC 1.000E+00 3.400E-01 RY. CHALL/-AFW 3.OOOE-01 1.000E+00 RY. CHALL/MFW. REC 1.OOOE+00 1.000E+00 RV. CLOSE 3.OOOE-04 1.OOOE+00 CH. PUMPS 1.200E-03 5.200E-01 B&F (CH. PUMPS) 1.2O0E -03 5.200E-01 4.OOOE-02 PORV. OPEN 2.OOOE -03 1.000E+00 4.OOOE-02 SI (MF.PUMPS) 5.OOOE-03 3.400E-01 RECIRC 2.OOOE-03 1.OOOE+00 4.OOOE-02 SSR/COND 1'.
00E+00 3.400E-01
- I forced Austin O8-13-1986 00:46:07 Event Identifier: 206185-017 E-20