ML20161A169
| ML20161A169 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 06/09/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-003-00 | |
| Download: ML20161A169 (11) | |
Text
B-52 Accident Sequence Precursor Program Event Analysis LER No: 251/88-003 Event
Description:
Reactor shutdown due to inoperability of three battery chargers Date of Event: February 7, 1988 Plant: Turkey Point Unit 4 Summnary Unit 4 was shut down and cooled down from 100% power as a result of the discovery of a battery charger failure mechanism that affected three of the six battery chargers at Turkey Point. The failure mechanism was discovered during the removal of two of the three battery chargers from service for maintenance. Unavailability of the battery chargers increases the likelihood that a DC bus would be unavailable during miti-gation of a postulated event. If this were the case, the associated safety-related train would also be unavailable, which would increase the likelihood of mitigating system failure. The conditional probability of core damage estimated for the event is 1.6 . 10-5. Its significance compared with other potential events at Turkey Point is shown below.
LER 251/88-003 IE-8 IE-7 1E-6 1E-5 IE-4 IE-3 1E-2 I
Trip LLOOP L EP Unavail (360 hrs)
Trip w/1 AFW AFW Unavail Train Unavail (360 hrs)
Event Description On February 6, 1988, with Unit 4 at 100% power and Unit 3 shut down, the 4S battery charger was taken out of service for preventive maintenance.
When the battery charger was reloaded onto its DC bus 4 h later, smoke was observed coming from the charger. The 4S battery charger was de-energized.
B-5 3 The 4S battery charger was apparently repaired, and at 1410 on February 7, loaded on the 3B DC bus. Upon de-energizing the 3B battery charger, it experienced a failure similar to the one initially experienced by the 4S charger. An inspection of the 4A charger revealed a failed circuit card similar to the ones that failed in the 3B and 4S battery chargers. Based on this observation, Unit 4 shutdown and cooldown was begun.
The battery chargers failed due to the use of a snubber capacitor on the gate filter module card, which was inappropriate for the particular application. The module cards had been replaced by the vendor in 1987. The particular capacitor used was prone to failure when subjected to transients. This, combined with the plant procedures for removing a battery charger from service (opening of the battery breaker followed by opening of the AC input breaker) created large transients that resulted in the immediate or later failure of the snubber capacitor.
During the plant shutdown, battery chargers 4S and 4A were considered inoperable, but were still powering their respective DC buses. The chargers were vulnerable to failure from switching transients, how-ever. The other three chargers O3A, 4B, and 3S) are of a different design and are not subject to the same failures.
The snubber capacitors on the affected gate filter module cards were replaced with modules less susceptible to the observed failures. In addition, the applicable operating procedure was revised to require the charger input to be tripped prior to opening the DC output breaker.
Event-Related Plant Design Information The 4S battery charger is a swing charger for battery chargers 4A and 3B.
ASP Modeling Assumptions and Approach Two analyses were performed to bound the significance of this event: a postulated trip during the shutdown reported in this event and a poten-tial unavailability of one DC bus from the time of the inverter modifi-cations that led to the reported problems.
- 1. Postulated reactor trip during the Feb. 7, 1988, shutdown. For this analysis, it is assumed that, had a trip occurred, switching tran-sients would have resulted in the loss of battery chargers 4S and 4A, requiring one DC bus on both units to be powered from its battery. To simplify the analysis, the loss of the battery chargers has been assumed to be nonrecoverable; it has also been assumed that the impacted bus on each unit would have failed prior to mitigation of the event. This would require operability of the other DC bus
B-5 4 and safety-related equipment associated with that bus to prevent core damage. As a result of the assumed unavailable DC bus, one train of AFW, HPI, and LPI was assumed unavailable. The likelihood of trip during the shutdown was assumed to be 0.1.
- 2. Potential unavailability of one DC bus. For this analysis, it was assumed that, following any initiator that could have occurred after installation of the defective module cards, a 0.1 chance existed that the affected DC bus would be unavailable (the assumption is conservative). An exposure time of 1 'year was also assumed (this time is most likely also conservative).
The estimate for this -case cannot be directly obtained using the ASP code. However, the ASP code can be used to obtain the conditional probabilities needed for the calculation.
For this case, the increase in core damage probability is:
p(cd) zduration *[it*p(cd I int, DC bus unavail)
- p(DC bus unavail)]
+ { nit *p(cd i mit, DC bus avail) * [1 - p(DC bus unavail)])
init
- E [X ini
- p(cd I init)]~, it = trip, LOOP, SLOCA mnit Since unavailability of a DC bus is only one of many contributors to system unavailability, p(cd Imiit, DC bus avail) p(cd i mit), and p(cd) =duration
- E [X.int p(cd I init, DC bus unavail)
- p(DC bus unavail)] EX. *n p(cd i mit) mnit
E X Xinit [p(cd I mnit, DC bus unavail) - p(cd I mnit)]
init
B-55 Analysis Results For case 1, the conditional probability estimate is 4.3 . 10' For case 2, the following values are estimated:
p(cd trip, DC bus unavail) =4.3 x 10-6 p(cd I trip) = 6.4 x1-X(trip) =1.7 x 10-4 /h p(cd LOOP, DC bus unavail) =9.6 x 10-5 p(cd I LOOP) =8.6 x 1-X(LOOP) =6.7 x 1l- 5 /h p(cd ISLOCA, DC bus unavail) =1.2 x 10-2 p(cd I SLOCA) 1.4 x 10-3 X(SLOCA) = 2.4 x 10-6 /h duration =6132 critical hours p(DC bus unavail) =0.1 Combining these as described above results in a conditional probability estimate for case 2 of 6132
- 0.1
- 2.67 x 10-" = 1.6 x 10-5. The dominant sequence for this event is highlighted on the following event tree.
B- 56 LOC1 RT 1AFW IMFW HPI HPR POIRV SEG NO END STATE LIiIi OPEN OK 71 CD 72 OK 73 CD 74 CD OK 75 CD 76 CD 77 CD 78 ATWS Dominant Core Damage Sequence for LER 251/88-003
B- 57 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 251/88-003 Event
Description:
Reactor shutdown due to charger inoperability Event Date: 02/07/88 Plant: Turkey Point 4 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1. SE-Si SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD TRANS 4. 3E-01 Total 4. 3E-07 ATWS TRANS 3.4E-06 Total 3.4E-06 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Nec-17 TRANS -rt AFN mfw NPI(F/B) CO 2. 2E-07 7 .7E-03 16 TRANS -rt AEN mfw -NPI(F/B) HPR/-NPI CD 9. 3E-08 9. 2E-03 15 TRANS -rt AFN mfw -HPI(F/B) -HPR/-NPI porv.open CO 8. 7E-08 9. 2E-03 18 TRANS rt ATWS 3.4E-06 1 . 2E-02
- - non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State P rob N Rec**
15 TRANS -rt AFW mfw -NPI(F/B) -NPR/-NPI porv.open CD 8. 7E-08 9. 2E-03 16 TRANS -rt AFW mfw -NPI(F/B) NPR/-NPI CO 9.3E-08 9.2E-03 17 TRANS -rt AFN mfw NPI(F/B) CO 2 .2E-07 7 .71-03 18 TRANS rt ATWS 3.4E-06 1. 2E-02
-~ non-recovery credit for edited case SEQUENCE MODEL: c:\asp\sealmod\pwrbseal.cmp BRANCH MODEL: c:\asp\sealmod\turkey.sil PROBABILITY FILE: c:\asp\sealmod\pwr bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail TRANS 1.7E-04 > 1 7E-04 l.OE+00 > 1.01-01 Branch Nodel: INITOR Initiator Freq: 1.7E-04 loop 6. 7E-05 1.7E-01 1 oca 2.4 E-06 4 . 3E-01 rt 2. 8E-04 1. 2E-01 rt/ loop 0. OE+00 1. OE+00 emerg .power 2.9E-03 8. QE-Ol AFW 1.5E-03 > 5.05-03 2.7lE-01 Event Identifier: 251/88-003
B- 58 Branch Model: 1.OF.3 Train 1 Cond Prob: 5.02-02 Train 2 Cond Prob: 1.02-01 Train 3 Cond Prob: 3.02-01 > Unavailable afw/emerg.power 1.5E-03 2.7E-01 mfw 1.9E-01 3.4E-01 porv.or.srv.chall 4.02-02 1.02+00 porv.or.srv. reseat 2.0 2-02 1. 1E-02 porv.or.srv.reseat/erserq.power 2.0 2-02 1.02+00 seal. loca 2.6E-01 1.02+00 ep.rec(sl) 6.2E-01 1.02+00 ep. rec 7.6E-02 1.02+00 H21 2.42-03 > 1.9E-02 8 .4E2-01 Branch Model: 2.OF.3 Train 1 Cond Prob: 1.0 2-02 Train 2 Cond Prob: 1.02-01 Train 3 Cond Prob: 3.02-01 > Unavailable 921 (P/B) 2.42-03 > 1.9E-02 8.4 -01 1.02-02 Branch Model: 2.CF.3+opr Train 1 Cond Prob: 1.0OE-02 Train 2 Cond Prob: 1.02-01 Train 3 Cond Prmb: 3.02-01 > Unavailable HPR/-H21 1.5E-04 > 1.02-02 1.02+00 1. 0 -03 Branch Model: 1.OF.2+opr Train 1 Cond Prob: 1.02-02 Train 2 Cond Prob: 1.5E-02 > Unavailable porv.open 1.0OE-02 1.02E+00 4.0E-04
- branch model file
- forced Minarick 11-09-1989 13:15:06 Event Identifier: 251/88-003
B-5 9 CONDITIONAL CORE CAMCADE PROBABILITY CALCULATIONS Event Tdentifier: 251/88-003 Event
Description:
Reactor shutdown due to charger inoperability (calc2(
Event Date: 02/07/88 Plant: Turkey Point 4 INITIATIND EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1. SE+00O LOOP 1.7E-01 LOCA 4. 3E-01 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD TRANS 4. 3E-06S LOOP 9.6SE-OS LOCA 1. 2E-02 Total 1. 2E-02 ATWS TRANS 3.4E-05 LOOP 0. 0E+OO LOCA 1.4E-05 Total 4.8E0-O SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
72 loca -rt -AER B(PI CD 6.9E-03 3.6E-01 71 loca -rt -AFW -((PT NPR/-NPI CD 4.60-03 4.3E-01 18 trans rt AIRS 3.4E0- 1.2E-01 78 boca rt AIRS 1.4E-05 5.2E-02
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Nec**
18 trans rt AIR'S 3.4E-05 1.2E-01 71 lees -rt -AER -((P1 NPR/-NPI CD 4.6E-03 4.3E-01 72 loca -rt -AFR ((PT CD 6.90-03 3.6E-01 78 inca rt AIRS 1.4E0- 5.2E-02
- non-recovery credit for edited ease SEQUENCE NODEL: a: \sealmod\pwrbseal .cmp BRANCH NODEL: a:\sealmod\turkey.s11 PROBABILITY FILE: a:\sealmod\pwr bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Necov Opr Fail trans 1.7E-04 l.OE--OO loop 6.7E-05 1.7E-01 boca 2.4E0- 4.3E-01 Event Identifier: 251/88-003
B- 60
- 2. 8E-04 1.2E-01 rt/ loop 0. OE+O0 1.00+00 emerg. power 2.9E-03 8.00-01 AEW 1.5E-03 > 5.00-03 2.70-01 Branch Model: l.CF.3 Train 1 Cond Prob: 5. OE-02 Train 2 Cond Prob: 1. OE-01 Train 3 Cond Prob: 3.OE-01 > Unavailable afw/emerg.power 1.5E-03 2.70-01 mfw 1.9E-01 3.4E-01 porv.or.srv.chall 4 .0E-02 1.00+00 porv.or.srv. reseat 2.00E-02 1. 10-02 porv.or.srv.reseat/emerg.power 2 . OE-02 1. 0E+00 seal .loca 2.6E-01 1.00+00 ep. rec (si) 6.2E-01 1.00+/-00 ep. rec 7. 6E-02 1.00+00
((P1 2.4E-03 > 1.9E-02 8 .40-01 Branch Model: 2.OF.3 Train 1 Cond Prob: 1.00-02 Train 2 Cond Prob: 1.00-01 Train 3 Cond Prob: 3.OE-01 > Unavailable
((PT(F/B) 2.4E-03 > 1.9E-02 8.40-01 1.00-02 Branch Model: 2.OF.3+opr Train 1 Cond Prob: 1. OE-02 Train 2 Cond Prob: 1. 00-01 Train 3 Cond Prob: 3.00-01 > Unavailable HPR -((PI 1.5E-04 > 1.0E-02 1. 0 E400 1.00-03 Branch Model: 1.OF.2+/-opr Train 1 Cond Prob: 1.0OE-02 Train 2 Cond Prob: 1.50-02 > Unavailable porv. open 1.00-02 1 .OE+00 4 .0E-04
- branch model file
~*forced Minarick 11-14-1989 0 5: 27 :4 2 Event Identifier: 251/88-003
B-6 1 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 251/88-003 Event
Description:
Reactor shutdown due to charger inoperability (calc3)
Event Date: 02/07 /8 8 plant: Turkey Point 4 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1. BE-rOB LOOP 1. 7E-01.
LOCA 4. 3E-01 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability cD TRANS 6.4 E-07 LOOP 8. 6E-05 LOCA 1.4E-03 Total 1. 4E-03 ATWS TRANS 3.4E-05 LOOP 0. 0E-+00 LOCA 1.4E-05 Total 4 .8E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec*
712 loca -rt -afw hpi CD 8.71-04 3.61-01 71 loca -rt -afw -hpi hpr/-hpi CO 4.9E-04 4.3E-01 53 loop -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall CO 6.01-05 1.4E-01 seal.loca ep.rec(sl) 18 trans rt AIMS 3.4E-05 1.2E-01 78 loca rt AIMS 1.4E-05 5.2E-02
ý-non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec-*
18 trans rt ATWS 3.4E-05 1.2E-01 53 loop -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall CO 6.01-05 1.4E-01 seal.loca ep.rec(sl(
71 loca -ri -afw -hpi hpr/-hpi CO 4.9E-04 4.3E-01 72 loca -rt -afw hpi CO 8.71-04 3.61-01 78 loca rt ATWS 1.41-05 5.2E-02
-- non-recovery credit for edited case SEQUENCE MODEL: a:\sealmod\ pwrbseal .cmp BRANCH MODEL: a:\sealmod\ turkey. sli PROBABILITY FILE: a:\sealmod\ pwr bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opt Fail Event Identifier: 251/88-003
B- 62 trans 1.7E-04 1.0t-.-00 loop 6.7E-05 1.7E-01 loca 2.4E-06 4.3SE-0l rt 2.8E-04 1.2E-01 rt/loop 0.08+/-00 1.08+00 emerg.power 2.9E-03 8.08-01 afw 1.5E-03 2.7E-El afw/emerg.povwer 1.58-03 2718E-01 mfw 1.9E-El 3.4E-01 porv.or.srv.chall 4.08-02 1.08+00 porv.or.srv. reseat 2.08-02 1.1E-02 porv.or.srv.reseat/emeerg.power 2.08-02 1.08+00 seal.loca 2.68-0l 1.08*00 ep.rec(sl) 6.2E-01 1.08+00 ep.rec 7.6E-02 1.08+00 hpi 2 .48E-03 8.4E-El hpi(f/b) 2.4E-03 8.4E-01 1.08-02 hpr/-hpl 1.5E-04 1.08+00 1.08-03 porv.open 1.08-02 1.08+00 4.08-04
- branch model file
- forced Minarick 11-14-1989 05:29:51 Event Identifier: 251/88-003