ML20147A347
| ML20147A347 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 10/16/1988 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-022-00 | |
| Download: ML20147A347 (5) | |
Text
B-374 Accident Sequence Precursor Program Event Analysis LER No: 456/88-022 Event
Description:
Loss of offsite power for approximately 2 h Date of Event: October 16, 1988 Plant: Braidwood 1 Summnary A 2-h loss of offsite power occurred at Braidwood 1. Emergency power was provided as designed, and all other plant responses were as expected. The core damage probability associated with the event is estimated to be 1.8 x 104 The relative significance of this event compared with other potential events at Braidwood 1 is shown below.
SL EP Unavail ITlp Trip w/1 AFW J LOOP (360 hrs)
Train Unavail AFW Unavail (360 hrs)
Event Description The failure of the phase "A" potential transformer for 138-ky line 8604 at Davis Creek caused a current surge that resulted in an attempt to transfer to 138-ky line 2002. However, a breaker took longer to open than designed, which resulted in actuation of a pole disagreement relay, and the transfer was not completed. RCP 1C supply breaker tripped on instantaneous overcurrent, causing a reactor trip. This resulted in a loss of offsite power at Braidwood 1. Offsite power for unit 1 was lost for ~-2 h. The emergency diesel generators started and loaded as designed. Unit 2 was powered by offsite power throughout this event.
The cause of the loss of line 2002 was due to a failure of the phase "A" potential transformer for 138-ky line 8604 at Davis Creek. This resulted in a current surge on the low side of transformer 83, which
13-37 5 caused its sudden pressure relay to actuate. This caused a transfer trip signal to be sent to the 345-ky breakers associated with line 2002 at both Braidwood Station and Davis Creek.
The cause of this loss of power to the station auxiliary transformers was improper time between opening for the different phase poles for 345-ky OCBs 4-7 and 7-8, which resulted in a pole disagreement actua-tion. This caused the local breaker backup system to open 345-ky ACB 3-4, which resulted in removing 345-ky power from the high side of station auxiliary transformers 142-1 and 142-2.
The cause of the reactor trip was the result of the 1C RCP supply breaker on 6.9-ky bus 158 tripping on instantaneous overcurrent. This was caused by a piece of cardboard inserted in the 1C RCP breaker instantaneous overcurrent relay, bypassing the 5-6 cycle time delay. It was suspected that the cardboard was inserted during the last mainte-nance on the relay as a relay block. This was considered by the utility to be a programmatic deficiency in that no mechanism existed to ensure that relay blocks were removed following maintenance activities. (Addi-tional administrative controls on the use of relay blocks were sub-sequently issued.)
ASP Modeling Assumptions and Approach The event has been modeled as a plant-centered loss of offsite power.
Analysis Results The conditional probability of severe core damage estimated for this event is 1.8 x 104 This event is considered significant from an ASP standpoint.
The dominant sequence (highlighted on the following event tree) involves a failure of emergency power (p = 2.3 x 10-3) following the LOOP (a station blackout), a subsequent seal failure (p =0.2-3), and fciilure to recover AC power prior to core uncovery (p = 0.48).
B-376 LOP RT/LOOP EP AW AFWPORV/ S PORV/ EL ýE PORV SEQ END A EPEE HPI HPR j CHALL RvsV RESEAT LOG (LONG) OPEN NO STATE OK OK 41 CD 42 CD OK OK 43 CD 44 CD 45 CD OK 46 CD 47 CD 48 CD OK 49 CD 50 CD OK 51 CD 52 CD 53 CD OK 54 CD 55 CD 40 ATWS Dominant Core Damage Sequence for LER 456/88-022
B3-37 7 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 456/88-022 Event
Description:
Loss of offslte power for approximatetly two hours.
Event Dare: 10/16/88 Plant: Braidwood 1 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 5.O5 -Il SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD LOOP 1. 8E-04 Total 1. 8E-04 ATWS LOOP S. SE+/-SS Total 0 . OE 005 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec-53 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv~chall CD 1. 2E-04 4. OE-01 SEAL.LOCA EP.REC (SL) 54 LOOP -rt/loop erserg.power -afw/emserg.power -porv.or.srv.chall - CO 3. GE-I5 4 OE-01 SEAL.LOCA EP.REC 55 LOOP -rt/locp emnrg.powor afw/emerg.powcr CD 1. 9E-05 1.4F-0l 48 LOOP -rt/loop emerg.power -afw/emerg.power porv.o~r.srv.chall - CO 4.8E-0 6 4. SE-01 porv.or.srv.reseat/emerg.power SEAL.LOCA EP.REC(SL)
-non-recovery credit for edited case SEQUENCE CONDITIONIAL PROBABILITIES (SEQUENCE ORDER)
Sequence End S-ato pro!) N Rec-48 LOOP -rt/loop emerg.power -afw/emerg.po~wer porv.or.srv.chait - CD 4. 8E-06 4 OE5-01 porv.or.srv.reseat/emerg.power SEAL.LOCA EP.REO(SL) 53 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall CO 1.2E-04 4 OE-S1 SEAL.LOCA FP.RECMS) 54 LOOP -rt/ioop emrsor.power -afw/emerg.po~or -porv.or.srv.o~hall - 3. GE-05 4 CE-01 SEAL.LCCA EP.REC CD 55 LOOP -rt/loop cmrsor.powor afw/omerg.powcr 1. 9E-05 1 .4E-0l
-* non-recovery credit for edited case SEQUENCE MODEL: a:\sealrsod\pwrbseal.cmp BRANCH MODEL: a:\soalrnod\braid.stl PROB3ABILITY FILE: a:\seaimod\pwr~bsll.pro No Recovery Limit BRANCH FREQUE.NCIEI/PROBABILITIES Branch System Non-oRe o v Opr Fail trans 4.3E-04 1.OE+00 Loop 1.6E-05 > 1.6E-05 5.3E-01 >5.0P,-0i Branch ModolI: INITUR Event Identifier: 456/88-022
B- 378 Initiator Freq: 1.60-05 O oca 2.40-06 4. 3E-01 rt 2.80-04 1.2E-01 rt/ loop 0.004-00 1 .00+00 ene rg .power 2.9E-03 B .0OE-01 afw 1. 3E-03 2. 60-01 afw/emerg .power 5.0 E-02 3.4E0-01 mfw 1.00+/-00 7.00-02 porv.or.srv.chall 4.0E-02 1.00+/-00 po rv .or .sarv . rccat 2.00-02 1. 10-02 porv.or.arv.reaeat/emerg.power 2.00E-02 1.00+00 S 0AL . LaCA 2.70-01 > 2.30-01 1 .00*00 Branch Model: 1.0F.1 Train 1 Cond Prob: 2.70-01 > 2.30-01 00.000 (SL) 5.80-01 > 4.80-01 1.00+00 Branch Model: 1.00.1 Train 1 Cond Prob: 5.80-01 >4.80-01 OP. EEC 1.80-02 > 4.30-02 1.00-00 Branch Model: 1.00.1 Train 1 Cond Prob: 1.8E-02 > 4.30-02 hpi, 1.00-03 8.4E-Cl hpi (fib) 1. OE-0 3 8.40-01 1.00-02 hpr/-hpi 1. 50-04 1 .00+00 1.00-0 3 porv .open 1.00-02 1.00+00 4.00-04
- branch model file Sforced Minarick 11-12-198 9 2 1: 35:2 9 Event Identifier: 456/88-022