ML20140A198
| ML20140A198 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 05/19/2020 |
| From: | NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-008-00 | |
| Download: ML20140A198 (6) | |
Text
B-479:
ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.:
Event
Description:
Date of Event:
Plant:
443/91-008 Loss of offsite power June 27, 1991 Seabrook Summary Seabrook underwent a loss of offsite power (LOOP) on June 27, 1991. Following the LOOP, the main turbine generator tripped, causing the reactor to scram from 1 0O%,
power. Both emergency diesel generators (EDGs) started and energized their respective buses and loads. Normal power was restored to the plant's emergency buses 36 min after the LOOP. The plant was stabilized in Operational Mode 3, Hot Standby, within 1
- h. The conditional core damage probability estimated for this event is 4.4 x 1O-5. The relative significance of this event compared to other postulated events at Seabrook is shown below.
LER 443/91-008 IE-7 1E-6 1E-5 I.E-4 IE-3 1E-2 Trip]30 precursor cutoff.. j
+I (oinl MTh F Event Description Seabrook was operating at 100% of rated power at 1334 hours0.0154 days <br />0.371 hours <br />0.00221 weeks <br />5.07587e-4 months <br /> on June 27, 1991, when two 345-ky switchyard circuit breakers tripped open while returning a relay to service following preventative maintenance. The relay had two break-before-make switches instead of one bre ak-before-make and one make -before -break as required.
Consequently, the two switchyard breakers opened without generating a signal to open the unit auxiliary transformer (UAT) supply breakers for onsite buses, 1-4, E5, and E6.
This prevented the automatic transfer to the reserve auxiliary transformer (RAT). Both EDGs automatically started and energized their respective buses and loads. The opening of the switchyard breakers caused a turbine trip followed by a reactor scram. When the
B-480 turbine tripped, the turbine control valves fast-closed causing a steam line high-pressure spike. This, in turn, generated a high-high steam generator (SG) level signal, which isolated feedwater. The actual SG level never approached the high-high level setpoint, but the loss of feedwater (LOFW) caused an emergency feedwater (EFW) actuation. After the LOOP, the shift superintendent confirmed within 5 min with the load dispatcher that power was available to the RAT. Buses 1-4 were energized from offsite sources within 20 min of the LOOP. Following the trip, the atmospheric steam dump valves (ASDV) opened to limit steam line pressure. When the operators started reactor coolant pump (RCP) C to establish forced coolant flow, ASDV C did not modulate to control pressure; consequently, the SRV on SG C lifted. Vital buses E5 and E6 were energized from offsite sources within 36 min of the LOOP, and the EDGs were secured within 45 min. The plant was stabilized in Operational Mode 3, Hot Standby, within 1 h of the LOOP.
Additional Event-Related Information Seabrook is supplied 345-ky from three offsite sources distributed between two buses that, in turn, supply the two RATs. The RATs are the alternate supply for the 4.1 6-ky emergency buses, E5 and E6. The normal supply for E5 and E6 comes from the UATs, which receive power from either a 345-ky offsite source or the unit main generator via a generator step-up transformer connection. Each UAT and RAT is a three-phase, three-winding transformer, with one wye-connected 13.8-ky output winding and one delta-connected 4.16-ky output winding. The 4.16-ky windings supply buses 3, 4, E5, and E6. Buses E5 and E6 supply vital, 4.16-ky safety-related loads and are backed up with emergency power from the EDGs. Buses 3 and 4 supply 4. 16-ky nonsafety-related loads.
ASP Modeling Assumptions and Approach The event has been modeled as a plant-centered LOOP. Probabilities for AC power nonrecovery following an RCP seal loss-of-coolant accident (LOCA) and prior to battery depletion, and for a seal LOCA, were revised to reflect values associated with a plant-centered LOOP (see ORNL/NRC/LTR-89/1 1, Revised LOOP Recovery and PWR Seal LOCA Models, August 1989).
Since power was available to the RAT, the LOOP nonrecovery probability used in the analysis was revised from that assumed for a nominal plant-centered LOOP at Seabrook Ito 0.12, to reflect burdened recovery in the control room had the EDGs failed.
Analysis Results The conditional probability of core damage estimated for this event is 4.4 x 10-5. The dominant core damage sequence, highlighted on the following ev.ent tree, involves a
B-481 LOOP, failure of emergency power, an RCP seal LOCA, and failure to recover AC power before core uncovery.
B-482 I1 V
P O R V I P O R V I S A ILOOPRT,1LOOP EP I AFW SRV SRVIEyF HPI IHPR IPO"']
I ICHALL RESEAT LOCA (LONG)
OPEN SEQ END NO STATE OK OK 41 CO 42 CO OK OK 43 CD (1) 44 CO 45 CD OK 46 CO 47 CO 48 CO OK 49 CO 50 CO OK 51 CO 52 CO 53 CO OK 54 CO 55 CO 40 ATWS (1) OK for O~ass D Dominant core damage sequence for LER 443/91-008
B-483 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
443/91-008 Event
Description:
Loss of offsite power Event Date:
06/27/91 Plant:
Seabrook 1 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBAB3ILITIES LOOP SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator 1.2E-01 Probability CD LOOP Total ATWS 4.4E-05 4.4E-05 0.OE+00 0.0E+00 LOOP Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER) 53 54 55 48 Sequence LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall SEAL.LOCA EP.REC(SL)
LOOP -rt/loop emerg.power -afw/emerq.power -porv.or.srv.chall -
SEAL.LOCA EP.RZC LOOP -rt/loop emerg.power afw/emerg.power LOOP -rt/loop erserg.power -afw/emerg.power porv.or.srv.chall -
porv.or.srv.reseat/emnerg.power SEAL.LOCA EP.REC(SL)
End State CD CO CO CD P rob
- 2. 9E-05 8.5E-06 4.7E-06 1.2E-06 N Rec**
9.5E-02 9.5E-02 3.3E-02 9.5E-02 non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence 48 LOOP -rt/loop emerg.power -afw/emerg.power porv.or.srv.chali porv.or.srv.reseat/emerg.power SEAL.LCCA EP.REC(SL) 53 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall SEAL.LOCA EP.REC(SL) 54 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall -
SEAL.LOCA EP.REC 55 LOOP -rt/loop emerg.power afw/emerg.power non-recovery credit for edited case SEQUENCE MODEL:
c:\\asp\\1989\\pwrbseal.cmp BRANCH MODEL:
c:\\asp\\1989\\seabrook.sll PROBABILITY FILE:
c:\\asp\\1989\\pwr~bsil.pro End State Prob CD 1.2E-06 CD 2.9E-05 CD 8.5E-06 CD 4.7E-06 N Rec**
9.5E-02 9.5 E-02 9.5E-02 3.3E-02 Event Identifier: 443/91-008
B-484 No Recovery Limit BRANCH FR2QUENCIES/PROBABTLTTIES Branch trans LOOP Branch Model:
INITOR Initiator Freg:
loca rt rt/loop emerq.power a fw afw/emerg.power mfw porv.or. srv.chall porv.or. srv. reseat porv.or. srv.reseat/emerq.power SEAL.-LOCA Branch Model:
l.OF.1 Train 1 Cond Prob:
Branch Model:
1.OF.1 Train 1 Cond Prob:
EP.REC Branch Model:
l.OF.1 Train 1 Cond Prob:
hpi hpi (f/b) hpr/-hpi porv.open
- branch model file
- forced Minarick 06-06-1992 14:34:37 System 5.3E-04 1.6E-05 > 1.6E-05
- 1. 6E-05 2.4E-06 2.92-04 0. 0+00
- 2. 9E-03 1.3E-03 5.02-02 1.0E+00 4.02-02 2.02-02 2.02-02 2.72-01 > 2.3E-01 2.72-01 > 2.32-01 5.72-01 > 4.92-01 5.72-01 > 4.92-01 7.02-02 > 4.3E-02 7.02-02 > 4.32-02
- 1. 02-03 1.02-03 1.5E-04 1.02-02 Non-Recov Opr Fail
- 1. 02+00 5.32-01 > 1.22-01 4.32-01 1.2E-01 1.0E+00 9.02-01
- 2. 6E-01 3.4E-0 1 7.02-02
- 1. 02+00 1.1E-02 1.02+00 1.02+00 1.02+00 1.02+00 8.4E-01 8.4E-01 1.02+00 1.02+00 1.02-02 1.02-03 4.02-04 Event Identifier: 443/91-009