ML20135H318
| ML20135H318 | |
| Person / Time | |
|---|---|
| Site: | Salem  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-030-00 | |
| Download: ML20135H318 (5) | |
Text
B- 106 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No: 272/91-030 Event
Description:
Both PORVs failed due to leaking actuators Date of Event: September 20, 1991 Plant: Salem 1 Summary The power-operated relief valves (PORVs) at Salem 1 were inoperable because of leakage from the flange bolting area on the air-operated PORV actuators. It is assumed that both PORVs were inoperable for one half of their surveillance period (81 d).
The conditional probability of core damage estimated for this event is 4.4 x 10-6. The relative significance of the event compared to other postulated events at Salem 1 is shown below.
LER 272/91-030 1E-8 1E-7 1E-6 1E- 1E-4 1E-3 Trip]30h precursor cutoff LOF W +1],
Event Description The plant was in Mode 4 (200 0 F < Tavg < 350'F) with a plant shutdown to Mode 5 (Tavg *ý 200'F) in progress to support a maintenance outage. Technical Specifications require that the pressurizer PORVs be used to provide pressurizer overpressure protection when one or more of the reactor cooling system (RCS) cold legs is less than or equal to 3 12OF (except with the reactor head removed). The PORVs were functionally checked and failed to open upon demand. At the time of these functional tests, the control room alarm for PORV accumulator low air pressure actuated. Investigations showed that both the IPRi and 1PR2 valve actuators leaked. The valve actuator diaphragm bolts were observed to be loose, which allowed air leakage from the flange bolting area. The valves successfully stroked after the actuator bolts were tightened.
B-_107 Investigations indicated that the iPR I and lPR2 actuator diaphragms appeared to be in a functional condition. Further assessment showed that the diaphragm material (Buna-N rubber) is subject to "creep," where the diaphragm may change from its original geometry under load and over time. This phenomena can be exacerbated by uneven torquing of the actuator joint. Leak paths may develop that were not present at the time of original installation. The IPRi and 1PR2 actuator diaphragms had been replaced on March 21, 1991, and April 12, 1991, respectively.-
Additional Event-Related Information At Salem, the PORVs and pressurizer spray valves; are used to help mitigate the consequences of a steam- generator (SG) tube rupture. Additionally, the PORVs are used for a total loss of feedwater (LOFW) accident. During a LOFW with failure of auxiliary feedwater, decay heat removal would be provided by utilizing the PORVs and the safety injection (SI) pumps in a "feed and bleed" mode. This is addressed in the station emergency operating procedures (EOPs) in FRHS-1, "Functional Restoration of Heat Sink."
ASP Modeling Assumptions and Approach It was assumed that both PORVs were inoperable for half the period from April 12, 199 1, to September 20, 1991 (81 d). Unavailability of either PORV results in failure of feed and bleed capability. One of the PORVs (1PR2) was assumed to be still capable of performing its pressure relief function (since it apparently lifted partially during testing).
Analysis Results The conditional probability of subsequent core damage estimated for this event is 4.4 x 10-6. The dominant core damage sequence, highlighted on the following event tree, involves a postulated transient with unavailable secondary-side cooling and unavailability of feed and bleed.
B- 108 TRANS RT 1PORV/
SRV -HPI HPR PR OPOEN SEQ END II ICHAL JRESEAT OPE NO STATE OK OK 11 CD 12 CD OK OK OK 13 CD 14 CD OK OK 15 CD (1) 16 CD 17 CD 18 AThYS (1) OK for Class D Dominant core damage sequence for LER 272/91-030
B-109 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 272/91-030 Event
Description:
Both PORVs failed due to leaking actuators Event Date: 09/20/91 Plant: Salem I UNAVAILABILITY, DURATION= 1944 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS -5.2E-01 LOOP 1.7E-02 LOCA 2 .0E-03 SEQUENCE CONDITIONAL PROBAB3ILITY SUMS End State/Initiator Probability CO TRABS 2 .8E-06 LOOP 1.6E-06 LOCA 1.4E-08 Total 4.4E-06 ATWS TRANS 0 .0E+00 LOOP 0. OE+00 LOCA 0 .OE+00 Total 0. 0E+00 SEQUENCE CONDITIONAL PROBAB3ILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
15 trans -rt afw mfw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 2.8E-06 1.8E-02 43 loop -rt/loop -erserg.power afw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 1.6E-06 1.4E-01
- - non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec*a 15 trans -rt afw mfw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 2.8E-06 1.8E-02 43 loop -rt/loop -emerg.power afw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 1.6E-06 1.4E-01
-- 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 failurea.
SEQUENCE MODEL: c:\asp\1989\pwrbaeal.cmp BRANCH MODEL: c:\asp\1989\saleml.sll PROBABILITY FILE: c:\asp\1999\pwr~bsll.pro Event Identifier: 272/91-030
B-110 No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 2 .7E-04 1.0E+00 loop 1.*6E-05 5.3E-01 loca 2 .4E-06 4.3Z-01 rt 2.82E-04 1 .2E-01 rt /loop O.OE+OO 1.02+00 emerg.power 7.5E-03 8.OE-01 a fw 3.8E-04 2. 6E-01 afw/emerg.power 5.02-02 3.4E-01 mfw l.OE+00 7.02-02 porv.or.srv.chall 4.02-02 1.02+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 2.7E-01 1.0E+00 ep. rec (Si) 5 .7E-01 1.0E+00 ep. rec 7.02-02 1.02+00 hpi 1.02-03 8.4E-01 hpi (f/b) 1.6E-03 8. E-01 1.OE-02 hpr/-hpi 1. 5E-04 1.0E+00 1.02-03 P OR V.OPEN l.OE-02 > 1.OE+00 1.0E+00 4.02-04 Branch Model: l.OF.1+Opr Train 1 Cond Prob: 1.02-02 > Failed
- branch model file
- forced OMlnarick 06-07-1992 20:49:22 Event Identifier: 272/91-030