ML20156A134
| ML20156A134 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1983-007-00 | |
| Download: ML20156A134 (5) | |
Text
B.16-1 B.16 LER No. 293/83-007 Event
Description:
LOOP During Shutdown Date of Event: February 13, 1983 Plant: Pilgrim B.16.1 Summary On February 13, 1983, a loss of offsite power (LOOP) occurred following a load rejection and scram caused by salt buildup on insulators in the switchyard. The LOOP occurred during the process of washing down portions of the switchyard to remove the salt deposits that had accumulated during a heavy ocean storm. The conditional core damage probability estimated for the event is 9.7 x 10-5.
B.16.2 Event Description On February 13, 1983, during a shutdown condition resulting from a load reject, a LOOP occurred. The load reject occurred when a heavy ocean storm caused a salt buildup on switchyard insulators, creating arcing to ground and the subsequent opening of breakers. During the process of washing down the isolated portion of the switchyard, melting ice and salt deposits on the remaining inservice portion of the switchyard created a separate ground that caused the inservice breakers to open, resulting in a LOOP. The emergency diesel generators (EDGs) started and other safety-related equipment functioned as designed. A secondary offsite power source was available as backup to the EDGs. After completion of the washdown, power was restored to the startup transformer and preparations for startup commenced.
B.16.3 Additional Event-Related Information Pilgrim has two safety-related 4160 V ac buses. Both of these buses can be powered from the unit auxiliary transformer (UAT) or the startup transformer (SUT). Upon loss of the UAT following a reactor trip, the safety-related buses are transferred to the SUT. If the SUT is lost, the EDGs are started to power safety-related loads. If an EDG fails, the 23-kV secondary offsite source automatically powers the bus.
B.16.4 Modeling Assumptions This event was modeled as a severe weather-induced loss of offsite power with all equipment available to respond to the event. The probabilities of failing to recover offsite power in the short term and before battery depletion were modified using the models described in Revised LOOP Frequency and PWR Seal LOCA Models, ORNL/CRC/LTR-89/1 1, August 1989.
LER No. 293/83-007
B.16-2 The 23-kV line is unusual because it is used following the failure of the EDGs to start. The Pilgrim IPE indicates that 18 failures of the 345-kV lines occurred between September 13, 1975, and February 21, 1989.
Of these 18 LOOPs, 7 were caused by severe weather. In three of these severe-weather-induced LOOPs, the 23-kV line was also lost. Therefore, the conditional probability that the 23-kV line is lost, given that the 235-kV lines are lost due to a severe-weather-induced LOOP, was set to 0.43 (3/7). Because the 23-kV line would close in automatically following the failure of the EDGs, the EDG nonrecovery value was modified to include the probability that the 23-kV line would be unavailable. Breaker failures and control system failures were assumed to be not significant, given the high unavailability of the line under these conditions.
The probabilities of failing to recover offsite power in the short term and before battery depletion were set to 0.9 and 5.5E-2, respectively.
B.16.5 Analysis Results The estimated conditional core damage probability for the severe weather-induced LOOP is 9.7 x 10'. The dominant sequence, highlighted on the event tree in Figure BA:6.1, involves a LOOP initiating event, successful reactor shutdown, failure of the emergency power system, and failure to restore offsite power before battery depletion.
LER No. 293/83-007
B.16-3 No HPI CR0 I4S LOP a~T.
E ~ SNs or R# SRV&/ PUMPS HS DOWN SHUT.RCC I ISTATE RCI AD (IM) LPCS LPCI (INJ RHR END SE.
NO.
201 OK 202 CD 203 4 vinOK 204 205 OK 206 CD 207 OK 208 CD 20 210 OK 211 212 CD 213 OK 214 CD 215 OK 216 CO 217 OK 218 CD 219 220 E: OK 221 CD 222 223 OK 224 CO 225 OK 226 Co 227 CD 228 CD 229 OK 230 openCD 231 1 .2
- OK 232 E4 Cc OK 233 234 CD 235 CD 236 OK 237 CD 235 OK 239 cc 240 CD 241 cc 242 cc 243 Cc 244 On 245 Figure B. 16.1 Dominant core damage sequence for LER 293/83-007 LER No. 293/83-007
B.16-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 293/83-007 Event
Description:
Loop during shutdown Event Date: February 13, 1983 Plant: Pilgrim INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 9.OE-01 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOOP 9.7E-05 Total 9.7E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
244 LOOP -rx.shutdown EP EP.REC CD 5.2E-05 3.3E-01 245 LOOP rx.shutdown CD 3.1E-05 9.OE-02 202 LOOP -rx.shutdown -EP srv.ftc.<2 -hpci rhr CD 1.1E-05 1.4E-02
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
202 LOOP -rx.shutdown -EP srv.ftc.<2 -hpci rhr CD 1.1E-05 1.4E-02 244 LOOP -rx.shutdown EP EP.REC CD 5.2E-05 3.3E-01 245 LOOP rx.shutdown CD 3.1E-05 9.OE-02
- non-recovery credit for edited case SEQUENCE MODEL: d:\asp\models\bwrc8283.cmp BRANCH MODEL: d:\asp\models\pilgrim.82 PROBABILITY FILE: d:\asp\models\bwr8283.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail LER No. 293/83-007
B.16-5 trans 1.2E-03 1.OE+OO LOOP 2.OE-05 > 2.OE-05 4.3E-01 > 9.OE-O1 Branch Model: INITOR Initiator Freq: 2.OE-05 loca 3.3E-06 6.7E-01 rxshutdown 3.5E-04 1.OE-O1 pcs 1.7E-01 1.OE+OO srv.ftc.<2 1.OE+OO 1.OE+OO srv.ftc.2 1.3E-03 1.OE+OO srv.ftc.>2 2.2E-04 1.OE+00 mfw 2.9E-01 3.4E-01 hpci 2.9E-02 7.OE-01 rcic 6.OE-02 7.OE-O1 srv.ads 3.7E-03 7.OE-01 1.OE-02 crd(inj) 1.OE-02 1.OE+OO 1.OE-02 cond 1.OE+0O 3.4E-01 1.OE-03 1pcs 2.OE-03 1.OE+OO Ipci 1.1E-03 1.QE+0O rhrsw(inj) 2.OE-02 1.OE+OO 1.OE-02 rhr 1.5E-04 1.6E-02 1.OE-O5 rhr.and.pcs.nrec 1.5E-04 8.3E-03 1.OE-05 rhr/-Ipci O.OE+OO 1.OE+OO 1.OE-05 rhr/lpci 1.OE+OO 1.OE+0O 1.OE-05 rhr(spcool) 2.1E-03 1.OE+0O 1.OE-03 rhr(spcool)/-Ipci 2.OE-03 1,OE+00 1.OE-03 EP 2.9E-03 > 2.9E-03 8.7E-01 > 3.7E-01 Branch Model: 1.OF.2 Train 1 Cond Prob: 5.OE-02 Train 2 Cond Prob: 5.7E-02 EP.REC 3.1E-02 > 5.5E-02 1.OE+OO Branch Model: 1.OF.1 Train 1 Cond Prob: 3.1E-02 > 5.5E-02 rpt 1.9E-02 1.OE+OO slcs 2.OE-03 1.OE+OO 1.OE-02 ads.inhibit O.OE+OO 1.OE+OO 1.OE-02 man.depress 3.7E-03 1.OE+0O 1.OE-02
- branch model file
- forced LER No. 293/83-007