ML20135G729
| ML20135G729 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-018-00 | |
| Download: ML20135G729 (5) | |
Text
B-301 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.:
Event
Description:
Date of Event:
Plant:
325/91-018 Loss of feedwater with -degraded HPCI. system July 18, 1991 Brunswick 1 Summary The reactor scrammed from 100% power during surveillance on a reactor water level transmitter. A spurious low water level signal resulted in main steam isolation valve (MSIV) closure and initiated the high-pressure coolant injection (HPCI) and react~or core isolation cooling (RCIC) systems, which provided reactor vessel makeup. HPCI had an oil leak that would have degraded HPCI performance if left uncorrected. The conditional core damage probability estimated for this event is 6.0 x 10-5. The relative significance of this event compared to other postulated events at Brunswick is shown below.
LER 325/91-018 IE-7 IE-6 1E-5 1E-4 1E-3 1E-2
[Trip
%6h L360 hEP I
HPCI IOW precursor cutoff _.j RCIC LIMC (nominal)
Event Description Brunswick 1 scrammed from 100% power on July 18, 199 1. The scram occurred while preparing to return a reactor vessel level transmitter to service. Leakage by the instrument manifold isolation valve for the transmitter resulted in a pressure transient on the common instrument variable leg header, which serves instruments for both reactor protection divisions, when the transmitter's drain valve was opened. The erroneous level signal closed all MSIVs and actuated the HPCI, core spray (CS), RCIC, and emergency power systems. The MSIV closure generated a full reactor scram, and all control rods fully inserted. The reactor power decrease and MSIV closure caused an actual momentary reactor water level decrease due to steam void collapse. Safety relief valves (SRVs) operated as designed to control reactor pressure. During the event, an ~-0.4 gpm oil leak
B-302 on the HPCI turbine oil filter inlet pressure gage drained -10 gal of oil before being isolated by closing a manual isolation valve. HPCI operation was not affected by the loss of oil (78 of 88 gals of oil were still available). However, if the oil leak had not been detected and isolated, HPCI would have been rendered unavailable.
Additional Event-Related Information The IIPCI system is a high-pressure injection system designed for small-break loss-of-coolant accidents (LOCAs) that do not depressurize the reactor. HPCI maintains sufficient reactor vessel inventory during plant shutdown until the vessel is depressurized.
HPCI is an independent system, uses a turbine-driven pump, and automatically initiates on reactor low water level. HPCI can deliver 4250 gpm of makeup water to the vessel through the feedwater piping. There are two sources of water for the HPCI system. Initially, the system uses demnineralized water from the condensate storage tank (CST). When the CST reaches a low level, the system automatically transfers to the suppression pool.
ASP Modeling Assumptions and Approach In this event, if the IIPCI oil leak had not been isolated, the oil-operated trip valve would have closed due to low oil pressure, and the HPCI turbine would have stopped prior to bearing damage. However, at the leakage rate for this event, it would take more than 60 min for the oil level in the reservoir to drain down to the low level alarm setpoint and more than 1 h and 20 min for the level to drop to the point where the oil pressure was low enough to close the trip valve. This assumes that nothing is done by the operators or that the HPCI room is inaccessible. This event was modeled as a loss of feedwater (due to MSIV closure) with HIPCI inoperable but with a nonrecovery factor of 0.34.
Analysis Results The conditional probability of core damage estimated for this event is 6.0 x 10-5. The dominant core damage sequence, highlighted on the following event tree, involves a loss of feedwater, a stuck-open SRV (transient-induced LOCA), failure of HPCI, and failure to depressurize using the automatic depressurization system to allow use of the low-pressure injection systems.
B-303
'gENT' SHUT
~OR 1
R CTf0 1
Rv/UC~
.0a0CLI KO ODC No STATE Ox OK 00, CORE CAMAEO OK OK CORE DAMAEO DOMEs DAMAEO C'
DORE DAMAEO DORE DAMAEO CORE DAMAEO DORE DAMAES WERE DAMAEO al, COME DAMAEO WORE DAMAEO WASE DAMAEO aco CK ECOEECMO aC CWEECMO C'
WOEECMO WOEECMO COME DAMAEO C'
Gl(
WORE DAMAEO OK C'
WERE DAAG CK a'
CORE DAMAEO CK O,
DORE DAMAEO OK CK WEE DAMAEO a'
01C CORE DAMAEO CK CWE DAMAEO CK W0EE DAMAEO OGEECMO WGRE DAMAEO ATWS Dominant core damage sequence for LER 325/91-018
B-304 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
325/91-018 Event
Description:
Loss of feedwater with degraded HPCI system Event Date:
07/18/91 Plant:
Brunswick 1 INITIATING EVENT NON-RECOVERABLE IN ITIATING EVENT PROBABILIT IES TRANS SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator 1.OE+00 Probability CD TRANS Total1 ATWS 6.OE-05 6.OE-05 3.OE-05 3.OE-05 TRANS Total SEQUENCF CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence 28 trans -rx.shutdown PCS/TRANS srv.chall/trans.-scram srv.close FN/PCS.TRANS HPCI srv.ads 11 trans -rx.shutdown PCS/TRANS srv.chall/trans.-scram -srv.close
-FW/PCS.TRANS rhr(sdc) rhr(spcool) /rhr(sdc) 99 trans rx.shutdown, non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence 11 trans -rx.shutdown PCS/TRANS srv.chall/trans.-scram -srv.close
- 4W/PCS.TRANS rhrisdc) rhr(spcool)/rhr(sdc) 28 trans -rx.shutdown PCS/TRANS srv.chall/trans.-scrarn srv.close FW/PCS.TRANS HPCI srv.ads 99 trans rx.shutdown End State CD CD ATWS End State CD CO ATWS P rob 5.3E-0 5 3. 5E-0 6 N Rec**
8.2E-02
- 7. 6E-02 3.OE-05 1.OE+00 P rob 3.5E-06 5.3E-05 3.OE-0 5 N Rec**
- 7. 6E-02 8.2E-02
- 1. OE+00
- non-recovery credit for edited case SEQUENCE MODEL:
c:\\asp\\1989\\bwrcseal.cmp BRANCH MO0DEL:
c: \\asp\\1989\\brunswck.sll PROBABILITY FILE:
c:\\asp\\1989\\bwr_csll.pro No Recovery Limit B3RANCH FREQUENCIES/PROBABILITIES Event Identifier: 325/91-018
B-305 Branch trans loop loca rx. shutdown rx. shutdown/ep PCS/TRANS Branch Model:
1.OF.1 Train 1 Cond Prob:
srv.chall/trans.-scram srv.chali/loop.-scram srv.close emerg.power ep. rec FW/PCS.TBANS Branch Model:
1.OF.1 Train 1 Cond Prob:
fw/pcs.loca I4PCI Branch Model:
1.OF.1 Train 1 Cond Prob:
rcic crd srv.ads lpcs lpci (rhr) /lpcs rhr (sdc) rhr (sdc) /-lpci rhr(sdc) /lpci rhr (spcool) /rhr (sdc) rhr (spcool) /-lpci. rhr (sdc) rhr(spcool) /lpci.rhr(sdc) rhrsw System 2.3E-04 1.6E-05 3.3E-06 3.02-05 3.5E-04 1.7E-01 > 1.02+00 1.72-01 > Unavailable 1.02+00 1.02+00 3.6E-02 1.4E-03 1.6E-01 4.62-01 > 1.02+00 4.62-01 Unavailable 1.02+00 2.9E-02 > 1.02+00 2.92-02 > Failed 6.02-02 1.02-02 3.*7E-03 3.02-03 1.02-03 2.1E-02 2.02-02 1.02+00 2.02-03 2.02-03 9.3E-02 2.02-02 Mon-Recov 1.02+00 3.6E-01 5.02-01 1.02+00 1.02+00 1.02+00 1.02+00 1.02+00 1.02+00 9.02-01 1.02+00 3.4E-01 3.4E-01 7.02-01 > 3.4E-01 7.02-01 1.02+00 7.1E-01 3.4E-01 7.1E-01 3.4E-01 3.42-01
- 1. 02+00 3.4E-01 3.4E-01
- 1. 02+00
- 3. 4E-01 Opr Fail
- 1. 02-02 1.02-02 1.02-03 1.02-03 1.02-03 2.02-03 branch model tile
-**-forced Minarick 06-07-1992 21:55:18 Event Identifier: 325/91-018