ML20140A229
| ML20140A229 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 05/19/2020 |
| From: | NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-004-00 | |
| Download: ML20140A229 (5) | |
Text
B-264 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.:
Event
Description:
Date of Event:
Plant:
304/91-004 Main feedwater pump trip with one AFW pump failed June 11, 1991 Zion 2 Summary Failure of a capacitor in the steam generator (SG) level controller power supply caused an SG overfeed during a startup. As a result of the high SG level, a main feedwater (MFW) pump trip and reactor trip occurred. During the startup, one of the auxiliary feedwater (AFW) pumps had failed to start. The conditional core damage probability estimated for this event is 1.0 x 1O-5.
The relative significance of this event compared to other postulated events at Zion 2 is shown below.
LER 304/91-004 1E-7 1E-6 1E-5 1E-4 1E-3 1E-2 I
I JýI I
I TrpII LI6 hE TripMI L
FW+1 I
LO precursor cutoff _
TjF 360 h AFW Event Description During a Zion 2 startup on June 11, 1991, the AFW pumps were being used to feed the SGs. MFW pump 2C had been started and was being aligned to feed the SGs. The bypass feedwater regulating valves (FRVs) were placed in automatic to control the SG level. Failure of a capacitor in the SG level controller power supply circuitry resulted in overfeed of the SGs. MFW pump 2C tripped on a high SG level signal resulting in a turbine and reactor trip. During the startup attempt, one of the AFW pumps had failed to start (reason unspecified). Prior experience with leakage from the FRVs led operators to conclude that the overfeed event had resulted from valve leakage, thereby confounding accurate diagnosis of the event. Several attempts at startup were made over two shifts, and each resulted in a high SG level trip, before the problem was correctly diagnosed.
B-265 Additional Event-Related Information Zion 2 has two motor-driven AFW pumps and one turbine-driven AFW pump. The AFW success criterion for decay heat removal is one pump out of three.
ASP Modeling Assumptions and Approach The event has been modeled as a potentially recoverable loss of feedwater with one motor-driven AFW pump unavailable. The event was analyzed assuming it had occurred at power, although it actually occurred during startup, when decay heat loads were less.
Analysis Results The conditional core damage probability for this event is conservatively estimated at 1.0 x 1O-5. The estimate is conservative because of the analysis assumption that the event occurred at power, when it actually occurred during startup. The dominant core damage sequence, highlighted on the following event tree, involves a reactor trip with unavailable secondary-side cooling and failure of bleed and feed.
B-266 1
PORV/
PORV/
IOR TRANS RT AF MFW SRV SRV HPI HPR OPEN I1 CHAL RESEAT OE SEO END NO STATE OK OK 11 C
12 CD OK OK OK 13 CD 14 CD OK OK 15 CD (1) 16 CD 17 CD 18 ATWS (1)0OK for Class D Dominant core damage sequence for LER 304/91-004
B-267 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
304/91-004 Event
Description:
NFW' pump trip and reactor trip with one Al's pump failed Event Date:
06/11/91 Plant:
Zion 2 INITIATING EVENT NON-RECOVERAB3LE INITIATING EVENT PROBABILITIES TRANS 1.OE+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CO TRANS 1.OE-05 Total 1.OE-05 ATWS TRANS 3.4E-05 Total 3.4E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
17 trans -rt AF's NFl? hpi(f/b)
CD 5.1E-06 7.4E-02 15 trans -rt AFW NFl? -hpi(f/b) -hpr/-hpi porv.open CD 4.8E-06 8.8E-02 16 trans -rt AF's NFl? -hpi(f/b) hpr/-hIpi CO 5.3E-07 8.8E-02 18 trans rt ATNS 3.4E-05 1.2E-01
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
15 trans -rt AF's Ml's -hpi(f/b) -hpr/-hpi porv.open CD 4.8E-06 8.8E-02 16 trans -rt AFW MEN -hpi(f/b) hpr/-hpi CD 5.3E-07 8.8E-02 17 trans -rt AFW NFl? hpi(f/b)
CD 5.1E-06 7.4E-02 18 trans rt ATMS 3.4E-05 1.2E-01
- - non-recovery credit for edited case SEQUENCE MODEL:
c:\\asp\\1989\\pwrbseal.cmp BRANCH MODEL:
c:\\asp\\1989\\zion.sll PROBABILITY FILE:
c:\\asp\\1989\\pwrbsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail Event Identifier: 304/91-004
B-268 trans loop loca rt rt/ loop emerg.power AFW Branch Model:
L.OF.3+ser Train 1 Cond Prob:
Train 2 Cond Prob:
Train 3 Cond Prob:
Serial Component Prob:
afw/emerg.power MFW Branch Model:
1.OF.1 Train 1 Cond Prob:
porv.or. srv.chall porv.or. srv.reseat porv.or. srv. reseat /emerq.power seal. loca ep.rec(sl) ep. rec hpi hpi (f/b) hpr/-hpi porv.open
- branch model file
- forced Minarick 05-26-1992 09: 18 :47 1.5E-04 1.6E-05 2.4E-06 2.8E-04 0.OE+00 5.4E-04 3.8E-04 > 5.3E-03 2.05-02 > Failed l.0E-01 5.05-02 2.8E-04 5.OE-02 2.OE-0l >l.OE+00 2.OE-01 > Unavailable 4.OE-02 2.OE-02 2.0E-02 2.7E-01 5.7E-01 3.1E-02 1.OE-03 l.OE-03 1.5E-04
- 1. 05-02 1.0E+00 5.3E-01 4.3E-01 1.2E-01
- 1. OE+00 8.OE-01 2.6E-01 3.4E-01 3.4E-01 l.OE+00 1.15-02 1.05+00
- 1. 05+00 1.05+00 1.OE+00 8.45-01 B 01 1.OE+00 1.05+00 1.05-02 1.0E-03 4.05-04 Event Identifier: 304/91-004