ML20147A534
| ML20147A534 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-014-00 | |
| Download: ML20147A534 (6) | |
Text
B-7 7 Accident Sequence Precursor Program Event Analysis LER No.:
275/88-014 Event
Description:
Degraded diesel generators due to clogged filters Date of Event:
May 5, 1988 Plant:
Diablo Canyon Unit 1 Summnary During the performance of a diesel generator surveillance test, the generator load decreased below specified acceptance criteria.
This was caused by a fungus, which clogged the primary fuel oil filter.
The fungus existed in the day tanks and main fuel storage tanks in units 1 and 2.
For this analysis, the fungus is assumed to render the diesel generators unavailable if required to mitigate a loss of offsite power.
Such a situation would result in a station blackout.
The condi-tional core damage probability estimated for the event is 4.1 x 10-4.
The relative significance of this event compared to other postulated events at Diablo Canyon is shown below.
LER 275/88-014 IE-8 I E-7 I E-6 1 E-5 I E-4 IE-3 I E-2 LOOP ILEP Unavail Trip (30Is Trip w/1 AFW LAFW Unavail Train Unavail (360 hrs)
Event Description During the performance of the 1-1 diesel generator surveillance test, the generator load decreased below specified acceptance criteria.
The cause of the load decrease was due to high differential pressure across the primary fuel oil filter, which was clogged by fungus.
Ten minutes into the test, operations switched to the standby primary fuel oil,.
filter, which caused the load to return to the required value.
Investi -
gation revealed that the primary fuel oil filter had been clogged by fungus and fungus spores from the generator day tank.
This fungus was
B-78 found in the day tanks and main fuel storage tanks for both units.
To prevent recurrence, the day tanks were drained, inspected, and cleaned; and the main fuel storage tanks were suctioned out and biocided.
- Also, a biocide program and an inspection program were to be developed to pre-vent and monitor fungus growth in the diesel fuel storage tanks.
Event-Related Design Information There are five diesel generators serving Diablo Canyon Units 1 and 2.
Two generators are dedicated to each unit, and one generator may swing between units.
Each diesel is equipped with a 550-gal fuel oil day tank that provides about 2.5 h of operation at full load.
Additionally, two 40 000-gal storage tanks provide a 7-day supply of fuel, which feeds the day tanks.
There is a primary and secondary fuel filter between the day tank and the diesel, with redundant filters that permit either filter to be isolated and changed during diesel operation.
Diesel 1-1, which was discovered to be impacted during testing, is dedicated to Unit 1 and provides power to vital 4160-V bus H serving the following equipment:
Redundant Equipment on bus Hequipment available
- 1. Auxiliary feedwater pump No. 12 1 of 2
- 2. Containment spray pump No. 12 1 of 2
- 3. Residual heat removal pump No. 12 1 of 2
- 4. Component cooling water pump No. 13 2 of 3
- 5. Safety injection pump No. 12 1 of 2
- 6. 480-V vital busses ASP Modeling Assumptions and Approach The event has been modeled as a potential loss of offsite power with unavailability of all emergency diesel generators.
Because the fungus was both in the day tanks and in the main fuel storage tanks, all the diesels have an equal opportunity of failing before operators could detect, evaluate, and then respond to the trouble during an actual loss of offsite power. The failure period of the diesels has been assumed to be 360 h (one-half of a 1-month test interval).
Because local recovery of the diesels is possible, a nonrecovery likelihood of 0.34 was assigned.
B-79 Analysis Results Given that the diesel generators were potentially unavailable for half a month, the probability for core damage resulting from diesel unavail-ability without a specific initiating event is 4.1 x 10-4.
This event is considered a significant event from an ASP standpoint.
The dominant core damage sequence (highlighted on the following event tree) is asso-ciated with station blackout:
a postulated LOOP in the vulnerability period (p = 4.1 x 10-3), failure to recover emergency power (p = 0.34),
a subsequent seal LOCA (p = 0.32), and failure to recover AC power prior to core uncovery (p = 0.65).
If the diesels cannot be recovered, then the probability of core damage frequency is increased by a factor of 3 to 1.2.
10-3.
B-8 0 IPORVI PORVI CHALL RESEAT LOCA I(LONG) I I
IOPENI SEQ END NO STATE OK OK 41 CD 42 CD OK OK 43 CD 44 CD 45 CD OK 49 CD 47 CD 48 CD OK 49 CD S0 CD OK 51 CD 52 CD 53 CD OK 54 CD 55 CD 40 ATWS Dominant Core Damage Sequence for LER 275/88-014
B-8i COND ITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
275/88-014 Event
Description:
Degraded diesel generators due to Event Date:
05/05/88 Plant:
Diablo Canyon 1 UNAVAILABILITY, DURATION= 360 NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator clogged filters 4.1E-03 Probability CD LOOP Total ATNS 4 1 E-04 4.1E-04
- 0. OE+ 00
- 0. 0E+00 LOOP Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence 53 loop -rt/loop EMERG.PONER -afw/emerg.power -porv.or.srv.chall seal.loca ep.rec(sl) 54 loop -rt/loop ENERG.POWER -afw/erserg.power -porv.or.srv.chall -
seal.loca ep.rec 55 loop -rt/loop EMERG.PONER afw/emerg.power 48 loop -rt/loop ED4ERO.POWER -afw/erserg.power porv.or.srv.chall -
porv.or.srv.reseat/emerg.power seal.loca ep.rec (sl)
End State CD CD CD CD Prob 2. 7E-04
- 9. BE-O5
- 2. 4E-05 1.1lE-05 N Rec**
- 1. 9E-01
- 1. 9E-01
- 6. 7E-02
- 1. 9E-01 non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State 48 loop -rt/loop ENERG.POWER -afw/erserg.power porv.or.srv.chall -
CD porv.or. srv.reseat/emerg.power seal. inca ep.rec (si) 53 loop -rt/loop ENERG.POWER -afw/erserg.power -porv.or.srv.chall CD seal. loca ep. rec (sl) 54 loop -rt/loop ENERG.PONER -afw/emerg.power -porv.or.srv.chall -
CD seal.loca ep.rec 55 loop -rt/loop ENERG.PONER afwlemerg.power CD
-* non-recovery credit for edited case Note:
For unavailabilities, conditional probability values are differential values added risk due to failures associated with an event.
Parenthetical values indicate compared to a similar period without the existing failures.
SEQUENCE MODEL:
c:\\asp\\sealmod\\pwrbseal.cmp BRANCH MODEL:
c:\\asp\\sealmod\\diablol.sll PROBABILITY FILE:
c:\\asp\\sealmod\\pwr_bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Prob 1.1lE-05
- 2. 7E-04
- 9. 8E-O5
- 2. 4E-05 N Rec**
- 1. 9E-01
- 1. 9E-0l
- 1. 9E-0l 6.7E-02 which reflect the a reduction in risk Opr Fail Event Identifier: 275/88-014
B-8 2 trans I cop loca rt rt/ loop EMERG. POWER Branch Model:
l.OF.3 Train 1 Cond Prob:
Train 2 Cond Prob:
Train 3 Cond Prob:
a fw afw/emerg.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 (f/b) hpr/-hpi porv.open
- 5. 6E-04 2.OE-05 2.4E-06
- 2. 8E-04
- 0. OE+O0 5.4E-04 > l.OE+00 S.OE-02 > Failed 5.7E-02 > Failed 1.9E-01 > Failed
- 3. 8E-04 5.OE-02 l.OE+00 4.0 E-02 3.OE-02
- 3. 0E-02 3.2E-01 6.5E-01
- 1. lE-0l l.OE-03 l.0E-03
- 1. 5E-04 l.OE-02
- 1. OE+00 5.8E-01
- 4. 3E-01 1.2E-01
- 1. OE+00 8.OE-01 > 3.4E-01
- 2. 6E-01 3.4E-0l 7.0 E-02 l.OE+00 1.lE-02 l.OE+00
- 1. OE+00
- 1. OE+00
- 1. OE+00 8.4E-01 8.4E-01
- 1. OE+00 1.OE4-00 1.0E-02 l.OE-03 4.OE-04
- branch model file
- forced Minarick 11-09-1989 13:20:21 Event Identifier: 275/88-014