ML20149K483
| ML20149K483 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-004-00 | |
| Download: ML20149K483 (5) | |
Text
B-200 Accident Sequence Precursor Program Event Analysis LER No: 339/88-004 Ri Event
Description:
Bath emergency diesels inoperable Date of Event: May 20, 1988 Plant: North Anna Unit 2 Summary While at 100% power, it was discovered on May 20, 1988, that both emer-gency diesel generators had been unavailable for 38.5 h. The event was caused by the failure of a DC output breaker while the redundant DC was unavailable because of preventive maintenance. The core damage prob-ability estimate for this event has been calculated at 2.5 . 10-5. The relative significance of this event compared with other potential events at North Anna Unit 2 is shown below.
LER 339/88-004 RI IE-8 I1E-7 I1E-6 IE-5 I1E-4 IE-3 IE-2 LOOP L EP Unavail (360 hrs)
Trip Trlip w/1 AEW AFJnavail Train Unavail (360 hrs)
Event Description While at 100% power on May 20, 1988, it was discovered that both emer-gency diesel generators were unavailable for a period of 38.5 h from 0630 on May 18 to 2034 on May 19. This event was caused by the closing spring charging motor for breaker 25H2 of the 2H emergency diesel becoming disengaged from the breaker housing and not charging the breaker closing spring as designed, in combination with a 2J emergency diesel unavailability due to preventive maintenance.
Due to a successful monthly surveillance test of the breaker 25H2 on May 6, operators were unaware of the 2H emergency diesel unavail-ability. Consequently, emergency diesel 2J was removed from service
B-201 between 0630 on May 18 to 2034 on May 19 for preplanned preventive main-tenance. During this maintenance, an additional problem with the 2J emergency diesel was identified and corrected. Plant Technical Specifi-cations require that the remaining diesel generator be demonstrated operable in the event that an EDG becomes inoperable for any reason except preplanned preventive maintenance or testing. Since an addi-tional problem was identified with the 2J diesel, the 2H diesel was tested to satisfy Technical Specifications. During the test, the diesel started properly; however, it did not load to the emergency bus as required. Investigation revealed that the breaker closing springs were not charged due to mounting bolts on the charging motor backing out of the breaker housing, allowing the charging motor to disengage.
Event-Related Plant Design Information Onsite emergency power to North Anna Unit 2 is provided by two 4160-V emergency buses, 2H and 21, with the capability of being powered by the respective 2H and 21 emergency diesel generators.
ASP Modeling Assumptions and Approach This event has been modeled as an unavailability of emergency power for 38.5 h. A nonrecovery likelihood for emergency power of 0.34 was assumed,, based on the potential for local recovery by manual breaker closure.
Analysis Results The conditional probability of core damage is estimated to be 2.5 . 10-5. The dominant sequence for this event (see attached event tree) involves a postulated loss of offsite power during the 38.5 h unavailability period (p = 3.3 x 0-4), failure to recover emergency power in the short term (p = 0.34), a subsequent RCP seal LOCA (p =0.27), and failure to recover AC power prior to core uncovery (p = 0.57).
B-202 PORVI PORV/ SEAL IEPREC LOOP RT/LOOP EP IAFW I SRV ISRV ILOCA (LONG) IHPI IHPR 550 END PORV CR IOPEN Iý NO STATE I I ICHALL RESEAT OK OK 57 CD 41 CD 42 CD OK OK 58 CD 43 CD 44 CD 45 CD OK 59 CD 46 CD 47 co 48 CD OK 49 CD 50 CD OK 60 CD 51 CD 52 CD 53 CD OK 54 CD 55 CD 40 ATWS Dominant Core Damage Sequence for LER 339/88-004 Ri
B- 203 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 339/88-004 Event
Description:
Both emergency diesel generators unavailable Event Date: 05/20/89 Plant: North Anna 2 UNAVAILABILITY, DURATION= 38.5 NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 3. 3E-04 SEQUENCE CONDITIONAL PROBABILITY BUMS End State/Initiator Probability CD LOOP 2 .5E-05 Total 2.5SE-O5 ATWS LOOP 0. 0E+00 Total 0. 0E+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State P rob N Rec**
53 loop -rt/loop EMERG.POWER -afw/emerg.power -porv.or.srv.chall.~ CD I1.6E-05 1 .8E-0l seal.loca ep.rec(sl) 54 loop -rt/ioop EMERG.POWER -afw/emerg.power -porv.or.srv.chall - CO 5.4E-06 1. BE-01 seal.loca ep.rec 55 loop -rt/loop EMERG.POWER afw/emerg.power CD I1.9E-06 6. 1E-02 48 loop -rt/loop EMERO.POWER -afw/emerg.power porv.or.srv.chail - CD 6. 6E-07 1. BE-01 porv.or.srv.reseat/emerg.power seal.loca ep.rec(sl)
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State P rob N Rec-*
48 loop -rt/loop EMERG.POWER -afw/emerg.power porv.or.srv.chall - CO 6. 6E-07 1. 8E-01 porv.or.srv.reseat/emerg.power seal.loca ep.rec(sl) 53 loop -rt/loop EMERG.POWER -afw/emerg.power -porv.or.srv.chall CO 1. 6E-05 1. 8E-01 seal.loca ep.rec(sl) 54 loop -rt/loop EMERG.POWER -afw/emerg.power -porv.or.srv.chall - CD 5.4E-06 1.8BE-01 seat.loca ep.rec 55 loop -rL/loop EMERG.PONER afw/emerg.power CD 1. 9E-06 6. 1E-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 MODEL: c:\asp\seaimod\pwraseal .cmp BRANCH MODEL: c:\asp\sealmod\northan2.sll PROBABILITY FILE: c:\asp\sealmod\pwr~bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Faill Event Identifier: 339/88-004
B- 204 trans 1.9E-05 1.0 8+00 loop 1. 6E-05 5. 3E-01 loca 2.4E-0 6 4.3E-01 rt 2. BE-04 1.2E-01 rt/ loop 0.OE+00 1.0 8+00 B MERG.tPC WER 2.9E-03 > 1.0E+00 8.08-01 > 3.48-01 Branch Model: 1.OF.2 Train 1 Cond Prob: 5.0E-02 > Failed Train 2 Cond Prob: 5.7E-02 > Unavailable a fw 3. 8E-04 2. 6E-01 afw/emerg .power 5.08-02 3.4E8-01 rsfw 1.9E-01 3.48-01 porv.or.srv.chall 4.0 8-02 1. 08+00 pore .or.srv. reseat 3.08-02 1.18-02 porv.or.srv.reseat/emerg.power 3.0OE-02 1.08+00 seal. loca 2.7E-01 1.08E+00 ep. rec (si) 5.7E-01 1.08+00 ep. rec 7.0OE-02 1.08+00 hpi 1.5E-03 8.48-01 hpi (fib) 1.5E-03 8.4E-01 1.08-02 pore .open 1.08-02 1.08+00 4.0E-04 hpr/-hpi 1.5E-04 1.08+00 1.08-03 csr 9.3E-05 1.08+00
- branch model file Sforced Minarick 11-09-1989 11:57:54 Event Identifier: 339/88-004