ML20135G747
| ML20135G747 | |
| Person / Time | |
|---|---|
| Site: | FitzPatrick  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 333-1991-006 | |
| Download: ML20135G747 (6) | |
Text
B-312 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.:
Event
Description:
Date of Event:
Plant:
333/91-006 Trip with both LPCI trains inoperable May 7, 1991 Fitzpatrick Summary During a surveillance test, the "A" residual heat removal (RHR) / low-pressure coolant injection (LPCI) outboard containment isolation valve failed to provide required containment isolation. Later, it was found that the valve stem had fractured and the valve disk and seat had sustained severe damage.
Subsequently, the same surveillance procedure was performed on the "B" RHRILPCI train. When the inboard RHR/LPCI containment isolation valve was operated, it opened partially and stopped. Efforts to further open or close it were unsuccessful. Plant operators then reduced power and scrammed the unit so that repairs could be made.
The conditional core damage probability for this event is estimated at 2.0 x 10-5. The relative significance of the event compared to other postulated events at Fitzpatrick is shown below.
_LER 333/91-006 1E-7 IE-6 1E-5 IE-4 1E-3 IE-2 I
1
///
1 1 I 36I P
+ HPCI precursor cutoff __.j
+ RCIC Event Description While preparing to demonstrate operability of the loop "A" RHR/LPCI inboard containment isolation valve, 1OMOV-25A, plant personnel attempted to pressurize the space between the inboard valve and the outboard valve, 1OMOV-27A, to facilitate opening the inboard valve. When these attempts were unsuccessful, it was determined that the outboard LPCI loop "A" isolation valve was incapable of performing its isolation function.
B-3 13 When the "B" train valves were tested, the inboard isolation valve, 1OMOV-25B, opened partially and then failed.
Subsequent attempts to open and close the valve were unsuccessful. Attempts at manual operation of the valve also failed.
Subsequently, power was reduced and the plant was scrammed to allow repairs to the RHRI/LPCI system. RI-R loop "A" was placed in service for shutdown cooling. Testing revealed that it was possible to force approximately 5000 gpm past the defective loop "A" outboard isolation valve.
The loop "B" inboard isolation valve, 1OMOV-25B3, was isolated for repair. Its valve stem threads were found to be worn and broken, and pieces of the threads were found in the fixed valve stem nut. When repairs to the loop "B" inboard isolation valve were completed, shutdown cooling (SDC) was transferred from loop "A" to loop "B".
Investigation revealed that the loop "A" outboard isolation valve had sustained severe seat, disk, and disk guide rib damage. In addition, the valve stem was fractured.
Additional Event-Related Information The definition of LPCI success may vary with circumstances. However, the minimum requirement when LPCI is demanded is that full flow from one pump be provided, approximately 7000 gpm. As the loop "A" flow was determined to be 5000 gpm and only limited flow through loop "B" was possible, it appears that neither LPCI loop was capable of performing its safety function.
ASP Modeling Assumptions and Approach This event was modeled as a scram with LPCI and one train of RHR unavailable.
Although one train of RHR functioned during the event, its injection valve was found to be significantly damaged. A failure probability of 0.5 was assumed for this train.
The Accident Sequence Precursor (ASP) models assume that RHR-suppression pool cooling is more likely to fail if LPCI and RHR-SDC are failed. In this event, the suppression pool cooling function should not have been impacted by the failure of the LPCI isolation valves. Therefore, the failure probability for suppression pool cooling given unavailability of LPCI and RHR(SDC) was reduced to 2.0 x 10-3. This value is consistent with values used elsewhere in the model.
The ASP models also address the potential use of RHR service water (RHRSW) for low-pressure injection, given that LPCI is failed. In this event, the dominant failure mode for LPCI is failure of both injection valves. If these valves fail, RIIRSW is also failed. A failure probability of 1.0 was assumed in this analysis.
B-314 Analysis Results The conditional core damage probability for this event is estimated at 2.0 x 1i-5. The dominant sequence, as highlighted on the following event tree, involves trip, failure of the power conversion system, successful safety/relief valve operation, feedwater success, and failure of both shutdown cooling and suppression pool cooling in the long term.
B-3 15
.pl (Be
- ý"
PENT SHUT on PDC C.WW DOVM CKhL HWA ADS MOCIN) MOM 07"a MEO EM No STArE OK COR DAMAGE 01 01 DOR C1 CK OOE AMG UK CK DOME~MO 01 01 CORE AME C1 01 COEDAMAGE 01 01FI Cl(
COEDAMAGE DMM 01(
01 DOM DAMAGE 01(
01H CK GOR DAMAGE Ox at COOK AW 01 01 OOEDAMAGE COR DAMAGE CAT 31 32 33 31 36 39 Dominant core damage sequence for LER 333/91-006
B-316 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
333/91-006 Event
Description:
Trip with both LPCI trains and one RHR train unavailable Event Date:
05/07/91 Plant:
Fitzpatrick INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator l.OE+00 Probability CD TRAMNS Total ATWS 2.OE-05 2.OE-05 3.OE-05 3.OE-05 TRAMNS Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence 11 trans -rx.shutdown pcs/trans srv.chall/trans.-scram -srv.close
-fw/pcs.trans RHR(SDC) rhr(spcool) /rhr(sdc) 12 trans -rx.shutdown pcs/trans srv.chall/trans.-scram -srv.close fw/pcs.trans -hpci RHR(SDC) rhr(spcool)/rhr(sdc) 21 trans -rx.shutdown pcs/trans srv.chall/trans.-scram srv.close
-fw/pcs.trans RHR(SDC) rhr(spcool) /rhr(sdc) 99 trans rx.shutdown
-~ non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence 11 trans -rx.shutdown pcs/trans srv.chail/trans.-scram -srv.close
-fw/pcs.trans RHR(SDC) rhr(spcool) /rhr(sdc) 12 trans -rx.shutdown pcs/trans srv.chali/trans.-scram -srv.ciose fw/pcs.trans -hpci RHR(SDC) rhr(spcooi(/rhr(sdc) 21 trans -rx.shutdown pcs/trans srv.chall/trans.-scram srv.ciose
-fw/pcs.trans RHN(SDC) rhr (spcool) /rhr(sdc) 99 trans rx.shutdown
-* non-recovery credit for edited case SEQUENCE MODEL:
c:\\asp\\1989\\bwrcseal.cmp BRANCH MODEL:
c:\\asp\\1989\\fitzpatr.sll PROBABILITY FILE:
c:\\asp\\1989\\bwr_csll.pro End State Prob CD 1.6E-05 N Rec**
1 OE-0l CD CD 3.OE-06 3.9E-02 6.2E-07 l.OE-0l 3.OE-05 1.0E4-00 ATWS End State Prob CD 1.6E-05 CD 3.OE-06 CD 6.2E-07 ATWS 3.OE-05 N Rec**
1.OE-01
- 3. 9E-02
- 1. OE-01 1.0E+00 Event Identifier: 333/91-006
B-317 No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch trans loop loca rx. shutdown rx. shutdownfep pcs/trans srv.chall/trans.-scram srv.chall/loop.-scram srv.close emerg.power ep. rec fw/pcs.t rans fw/pcs.loca hpc i rcic crd s rv.ads lpcs LPCI (RHR) ILPCS Branch Model:
l.OF.2 Train 1 Cond Prob:
Train 2 Cond Prob:
RUM (SDC)
Branch Model:
l.OF.2+ser4-opr Train 1 Cond Prob:
Train 2 Cond Prob:
Serial Component Prob:
rhr (sdc) /-lpci rhr (sdc) flpci rhr (spcool) /rhr (sdc) rhr (spcool) /-lpci. rhr (sdc)
Branch Model:
l.OF.l Train 1 Cond Prob:
RHRSW Branch Model:
l.OF.1+opr Train 1 Cond Prob:
System 3.4E-04 1.6E-05 3.3E-06 3.OE-05 3.5E-04 1.7E-01 1.05+00 1.05+00 3.6E-02 2.9E-03 1.6E-01 4.6E-01 1.05+00 2.9E-02 6.0E-02 1.05-02 3.7E-03 3.05-03 l.OE-03 > 5.OE-0l 1.05-02 > Failed 1.05-01 > 5.OE-0l 2.1E-02 > 5.15-01 3.0Z-03 >Failed 3.OE-0l > 5.05-01 2.05-02 2.OE-02 1.05+00 2.05-03 2.05-03 9.3E-02 > 2.05-03 9.35-02 2.OE-02 > 1.OE+00 2.OE-02 > l.OE+00 Non-Recov Opr Fail 1.0E+00 3.6E-01 5.05-01 1.0E+00 1.OE+00 1.0E+00 1.05+00 1.05+00 1.05+00 S.05-01 1.0E+00 3.4E-01 3.4E-01 7.05-01 7.05-01 1.OE+00 7.1E-01 3.4E-01 7.1E-01 > 1.05+00 3.4E-01 3.4E-01 1.05+00 3.4E-01 3.4E-01 1.0E+00 3.4E-01 > 1.OE+00 1.05-02 1.OE-02 1.OE-03 1.OE-03 l.OE-03 2.05-03
- branch model file
- forced Minarick 03-16-1992 18:03:34 Event Identifier: 333/91-006