ML20140A197
| ML20140A197 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 05/19/2020 |
| From: | NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-014-00 | |
| Download: ML20140A197 (5) | |
Text
B-24 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No: 206/91-014 Event
Description:
Inoperable volume control tank level transmitters Date of Event: August 7, 1991 Plant: San Onofre 1 Summary The automatic actuation for re-alignment of the charging pumps from the volume control tank (VCT) to the refueling water storage tank (RWST) on low VCT level was disabled.
In the event of a small-break loss-of-coolant accident (LOCA), and if manual realignment failed, the charging pumps would become gas bound due to hydrogen from the VCT.
This condition existed for -'17 h.
The conditional probability of core damage associated with this event is 2.1 x 10-6. The relative significance of the event compared to other postulated events at San Onofre 1 is shown below:
Event Description VCT level transmitter LT- 1100 was exhibiting erratic indication as compared to the opposite train level transmitter (LT-2550). To avoid inadvertent actuation during corrective maintenance on LT-11110, the automatic actuation functions of both transmitters were bypassed. Both transmitters were bypassed for a period of -17 h.
Additional Event-Related Information Level transmitters (LT- 1100 and LT-2550) function to realign the charging pumps from
B-25 the VCT to the RWST when the VCT level becomes low and to provide a protective trip to the charging pumps to avoid the introduction of VCT hydrogen gas to the pump suctions. On a low-low VCT level, each level transmitter (LT-1 110 for train A and LT-2550 for train B) initiates opening of its respective RWST isolation valve; when these valves complete opening, limit switches initiate closure of the associated VCT isolation valves.
On low-low-low VCT level these transmitters also trip the charging pumps. Two trains of automatic charging pump protection are provided on low VCT level to preclude the VCT hydrogen cover gas from gas binding and potentially damaging the charging pump.
The valves controlled by LT-1 100 and LT-2550 were still capable of being repositioned by their safety injection (SI) contacts. However, for certain small LOCAs, the VCT would have drained and the charging pumps would have been damaged before the reactor coolant system depressurized to the SI setpoint.
ASP Modeling Assumptions and Approach The level transmitters were assumed to be unavailable for 17 h. For the purposes of this analysis, all potential small-break LOCAs were assumed to be small enough to drain the VCT before reaching the SI setpoint. Because the operators were aware that VCT level transmitters were unavailable, a non-recovery probability of 0.12 for high-pressure injection (FIPI) was assumed. Feed and bleed was assumed not to be impacted by the unavailability of the level transmitters, since Si is manually actuated when initiating feed and bleed.
Analysis Results The conditional probability of subsequent core damag etmed for this event is 2.1 x 10-6. The dominant core damage sequence, highlighted on the following event tree, involves a postulated LOCA with successful reactor trip and auxiliary feedwater initiation with a failure of HPI.
H-PI HPR PORV I OPEN CSR SEO END NO STATE OK 80 CD 71 CD 72 CD OK a1 CD 73 CD 74 CD OK 82 CD 75 CD 76 CD 77 CD 78 ATWS Dominant core damage sequence for LER 206/91-014
B-27 CONDITIONAL CORE DAMAGEPROBABILITY CALCULATIONS Event Identifier: 206/91-014 Event
Description:
Inoperable Volume Control Tank level transmitters Event Date: 08/07/1991 Plant: San Onofre 1 UNAVAILABILITY, DURATION= 17 NON-RECOVERAB3LE INITIATING EVENT PROBABILITIES TRANS 2 .OE-03 LOOP 1.9E-04 LOCA LSBE-05 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD TRAMNS 2.1E-09 LOOP 5 .9E-09 LOCA 2 .lE-06 Total 2.1E-06 ATWS TRAN S 0 .OE+00 LOOP 0 .OE+00 LOCA 0 .0E+00 Total 0 .OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER) tt Sequence End State Prob N Rec 72 loca -rt -afw fIPI CD 2.1E-06 5.2E-02
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBAB3ILITIES (SEQUENCE ORDER)
Sequence End State P rob N Rec**
72 loca -rt -afw HPI CD 2 . 1E-0 6 5 .2E-02
- non-recovery credit for edited case Note: For unavailabilities, conditional probability values are differential values which reflect the added risk due to failures associated with an event. Parenthetical values indicate a reduction in risk compared to a similar period without the existing failures.
SEQUENCE NODEL: c:\asp\1989\pwrbseal.cmp BRANCH MODEL: c:\asp\1989\sanonol.sll PROBABILITY FILE: c:\asp\1999\pwrbsll.pro No Recovery Limit Event Identifier: 206/91-014
B-28 BRANCH 9'REQUENCIES/PROBABILITIES Branch Syst em Non-Recov Opr Fail trans 1.2E-04 l.OE+00 loop 2 .OE-05 5.8E-01 loca 2.4E-06 4.3E-01 rt 2.8E-04 1.2E-01 rt/loop 0 .OE+00 1. 05+00 emerg.power 2 . 9E-03 A .05-01. 1.OE-03 a fw 2 .3E-03 2.6E-01 afw/emerg.power 5.OE-02 3. 4E-01 mfw 1. 9E-01 3.4E-01 porv.or. srv.chall 4 .OE-02 1. OE+00 porv.or. srv.reseat 2 . OE-02 1.1E-02 porv.or. srv.reseat/emerg.power 2.OE-02 1.0E+00 seal. loca 3.2E-01 1.OE+00 ep. rec (si) 7. 6E-01 1.05+00 ep. rec 1. 6E-01 1.OE+00 HP I 1.05-03 > l.OE+00
- 8.4E-01 > 1.2E-01 Branch Model: l.OF.2 Train 1 Cond Prob: 1.05-02 > Unavailable Train 2 Cond Prob: 1.OE-0l > Unavailable hpi (f/b) 1.05-03 8 .4E-01 1.05-02 hpr/-hpi 1.5E-04 1.05+00 1.05-03 porv.open 1.05-02 1.0E+00 4 .OE-04
- branch model file
- forced Minarick 06-07-1992 21:39:07 Event Identifier: 206/91-014