ML20140A196
| ML20140A196 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 05/19/2020 |
| From: | NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1990-011-00 | |
| Download: ML20140A196 (5) | |
Text
B-26 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.:
Event
Description:
Date of Event:
Plant:
206/90-011l Reactor trip with one train of auxiliary feedwater autostart disabled May 15, 1990 San Onofre 1 Summary The reactor was manually tripped from 9 1.5% power due to a low and decreasing level in one of three steam generators (SGs). The automatic start for one train of auxiliary feedwater (AFW) was disabled to prevent automatic start of that train's pump while corrective maintenance on the pump's valve control circuit was being performed. An inadvertent short caused the closure of the main feedwater (MFW) regulating valve for "C" steam generator. Manual control of the valve was not obtained before a manual scram was required by procedure. The conditional core damage probability estimated for this event is 7.6 x 10-6. The relative significance of this event compared to other postulated events at San Onofre 1 is shown below.
LER 206/90-011 1E-7 1E-6
,[E_5 IE-4 1E-3 1E-2 Tip-360 hAFW-j L 36Oh EFW I
LOIW +1I MTR AFW LOOP precursor cutoff -J Event Description San Onofre 1 was operating at 9 1.5% power when the reactor was tripped due to a low and decreasing level in steam generator "C" resulting from a loss of feedwater flow in loop "C". The loss of feedwater flow occurred during corrective maintenance on an auxiliary feedwater (AFW) train A pump G-10 valve control circuit, which resulted in an inadvertent short circuit. The short caused a brief voltage reduction, which resulted in the transfer of the 120 Vital Buses 3 and 3A from their normal inverter power source to their backup power source. The brief power interruption which occurred during the
B-27 transfer resulted in a spurious SG "C" high level actuation signal, which initiated closure of SG "C" main feedwater regulating valve FCV-458. In accordance with procedures, the control room operator then reset the FCV-458 controls to control the valve manually.
However, the operator was unable to reestablish feedwater flow before SG "C" reached the level at that procedures require the reactor to be manually tripped.
Following the trip, AFW train "A" did not automatically actuate because the channel had been placed in manual to preclude automatic pump start during maintenance.
Additional Event-Related Information The auxiliary feedwater system for this plant consists of two separate trains. Train "A" is provided with a turbine and motor in series to turn a single pump (either motive source will supply 100% of the necessary flow). Train "B" contains a single motor-driven pump (100% capacity).
ASP Modeling Assumptions and Approach The event has been modeled as a reactor trip with degraded AFW. Consistent with other events modeled in the ASP Program, a nominal non-recovery value for AFW was assumed (since the postulated failure mode for the other pump is unknown).
Analysis Results The conditional core damage probability estimated for this event is 7.6 x 10-6. The dominant core damage sequence, highlighted on the following event tree, involves the observed trip with subsequent failure of secondary-side cooling and bleed and feed.
B-28 TRN T
AW W IPSORI I PORV/
HI HR PORV TRANS RV SRV HI PR OPEN L I iCHAL RESEAT III SEQ END NO STATE OK OK CD 12 aM OK OK OK 13 CD 14 CD OK OK 15 CD (1) is CD 17 CD 1 8 ATWS (1) OKfor Class D Dominant core damage sequence for LER 206/90-01 1
B-29 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
206/90-011 Event
Description:
Reactor trip with one train of AFW autostart disabled Event Date:
05/15/90 Plant:
San Onofre I INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.OE+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator CD TRANS Total ATWS TRANS Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Probability 7.6E-06
- 7. 6E-06 3.4E-05 3.4 E-05 Sequence End State Prob 17 15 16 trans -rt AFW mfw hpi(f/b) trans -rt AFW mfw -hpi (f/b) -hpr/-hpi porv.open tra~ns -rt AFW mfw -hpi(f/b) hpr/-hpi CO CD CD 3.7 E-06 3.5E-06 3.9E-07 N Rec**
7.4E-02 S.8E-02 8.8E-02 18 trans rt ATWS 3.4E-05 1.2E-01
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob 15 16 17 1s trans -rt AFW mfw -hpilf/b) -hpr/-hpi porv.open trans -rt AFW mfw -hpi(f/b) hpr/-hpi trans -rt AFW mfw hpi(f/b) trans rt CD CD CO ATWS 3.5E-06
- 3. 9E-07 3.7 E-06 3.4E-05 N Rec**
S.SE-02 8.8E-02 7.4E-02 1.2E-01
- non-recovery credit for edited case SEQUENCE MODEL:
BRANCH MODEL:
PROBABILITY FILE:
c: \\asp\\1989\\pwrbseal.cmp c :\\asp\\1989\\sanonol.sll c :\\asp\\1989\\pwrball.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail Event Identifier: 206/90-011
B-30 trans loop loca rt rt /loop, emerg.power AFW Branch Model:
1.OF.2+ser Train 1 Cond Prob:
Train 2 Cond Prob:
Serial Component Prob:
afw/smerg. power mfw porv.or.srv.chall porv.or.srv. reseat porv~or.srv. reseat/emerg.power seal.loca ep. rec(si) ep. rec hpi hpi (t/b) hpr/-hpi porv~open 1.2E-04 2.OE-05 2.4E-06 2.8E-04 O.OE+OO 2.9E-03 2.3E-03 > 2.OE-02 2.OE-02 1.05-01 > Unavailable 2.8E-04 5.OE-02 1.9E-01 4.OE-02 2.05-02 2.OE-02 3.2E-01 7.6E-01 1.6E-01 1.05-03 1.OE-03 1.5E-04 1.05-02 1.05+00 5.85-01 4.3E-01 1.2E-01 1.05+00 8.05-01
- 2. SE-01 1.05-03 3.4E-01 3.4E-01 1.05+00
- 1. 1E-02 1.05+00 1.05+00 1.0E+O0 1.05+00 8.45-01
- 8. 4-01 1.05+00 1.054+00 1.05-02 1.05-03 4.0OE-04
- branch model file Sforced Minarick 08-06-1991 17:32:39 Event Identifier: 206/90-011