ML20147A353
| ML20147A353 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1983-074-00 | |
| Download: ML20147A353 (5) | |
Text
B.7-1 B.7 LER No. 260/83-074 Event
Description:
Trip with HPCI Inoperable Date of Event: November 10, 1983 Plant: Browns Ferry 2 B.7.1 Summary Unit 2 was operating at approximately 98% power when a reactor scram occurred. Reactor vessel level dropped sufficiently to provide an auto-initiation signal to the high pressure coolant injection (HPCI) system.
HPCI started and immediately isolated when a turbine exhaust rupture diaphram ruptured, rendering HPCI inoperable. The conditional core damage probability estimated for the event is 3.2 x i 0".
B.7.2 Event Description On November 10, 1983, while operating at essentially full power, Unit 2 experienced a scram. Reactor vessel level dropped sufficiently to result in HPCI auto-initiation; however, HPCI immediately isolated when its turbine exhaust rupture diaphragm ruptured.
The cause of the failure was not determined with certainty. An exhaust diaphragm rupture which occurred during testing five days earlier had been attributed to inadequate draining of condensate from HPCI steam lines. Apparently, the November 5 rupture disk failure may have been caused by the impact of a slug of water which accelerated in the steam exhaust line after the turbine started. While the disk rupture patterns were found to be similar in both events, the November 10 failure was tentatively attributed to control system problems. Testing conducted later, in February of 1984, suggested that improper HPCI control system behavior could lead to exhaust line pressure fluctuations, perhaps great enough to cause failure of the rupture disk. Adjustments were made to the control system to minimize these fluctuations.
B.7.3 Additional Event-Related Information High-pressure makeup sources at Browns Ferry include the turbine-driven main feedwater pumps, HPCI, the reactor core isolation cooling system (RCIC) and the control rod drive (CRD) pumps. For events involving isolation of the reactor vessel, only HPCI can provide high flow-rate (5,000 gpm) makeup to the reactor.
B.7.4 Modeling Assumptions This event was modeled as a scram with HPCI assumed unavailable and not recoverable. Because the HPCI auto-initiation reported indicates that reactor vessel level had dropped to -51.5 inches below instrument zero, it can be assumed that the main steam isolation valves (MSIVs) isolated, causing an initial loss of main feedwater and power conversion systems. The nonrecovery probability for the power conversion system (PCS) was revised to 0.017 to reflect initial assumed closure of main steam isolation valves.
LER No. 260/83-074
B.7-2 B.7.5 Analysis Results The conditional core damage probability estimated for this event is 3.2 x 10'. The dominant core damage sequence, highlighted on the event tree in Figure B.7. 1, involves the observed trip, unavailability of the power conversion system, failure of two safety relief valves (SRVs) to close, unavailability of HPCI, and failure of the automatic depressurization system (ADS).
LER No. 260/83-074
B.7-3 00 00 0 0 0 0 U 0 0 0 0 0 -
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IL U) 0 CO)
Figure B.7.1 Dominant core damage sequence for LER 260/83-074 LER No. 260/83-074
B.7-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 260/83-074 Event
Description:
Scram with HPCI inoperable Event Date: November 10. 1983 Plant: Browns Ferry 2 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.OE+O0 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 3.2E-05 Total 3.2E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
138 trans -rx.shutdown PCS srv.ftc.2 HPCI srv.ads CD 1.6E-05 7.OE-01 103 trans -rx.shutdown PCS srv.ftc.<2 -MFW RHR.AND.PCS.NREC CD 6.6E-06 1.8E-04 119 trans -rx.shutdown PCS srv.ftc.<2 MFW HPCI rcic srv.ads c CD 3.6E-06 1.7E-O1 rd(inj) 107 trans -rx.shutdown PCS srv.ftc.<2 MFWHPCI -rcic RHR.AND.PC CD 3.3E-06 9.1E-05 S.NREC 414 trans rx.shutdown rpt CD 6.7E-07 1.OE-O1
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
1.8E-04 103 trans -rx.shutdown PCS srv.ftc.<2 -MFW RHR.AND.PCS.NREC CD 6.6E-06 107 trans -rx.shutdown PCS srv.ftc.<2 MFW HPCI -rcic RHR.AND.PC CD 3.3E-06 9.1E-05 S.NREC 119 trans -rx.shutdown PCS srv.ftc.<2 MFW HPCI rcic srv.ads c CD 3.6E-06 1.7E-O1 rd(inj) 138 trans -rx.shutdown PCS srv.ftc.2 HPCI srv.ads CD 1.6E-05 7.OE-01 414 trans rx.shutdown rpt CD 6.7E-07 1.OE-01
- non-recovery credit for edited case SEQUENCE MODEL: c:\asp\1982-83\bwrc8283.cmp BRANCH MODEL: c:\asp\1982-83\brown2.82 PROBABILITY FILE: c:\asp\1982-83\bwr8283.pro LER No. 260/83-074
B.7-5 No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 2.OE- 03 1.OE+00 loop 1.6E-05 2.4E-01 loca 3.3E -06 6.7E -01 rx.shutdown 3.5E-04 1,OE-01 PCS 1.7E-01 > 1.OE+00 1.OE+00 Branch Model: 1.OF.1 Train 1 Cond Prob: 1.7E-01 >1.OE+00 srv.ftc.<2 1.0E+00 1.OE+00 srv.ftc.2 1.3E-03 1.OE+00 srv.ftc.>2 2.2E- 04 1.OE+00 MFW 4.6E-01 > 1.OE+00 3.4E-01 Branch Model: I.OF.1 Train 1 Cond Prob: 4.6E-01 > 1.OE+00 HPCI 2.9E-02 > 1.OE+00 7.OE-01 > 1.OE+00 Branch Model: 1.OF.1 Train 1 Cond Prob: 2.9E-02 > 1.0E+00 rcic 6.OE-02 7.OE-01 srv.ads 3.7E-03 7.OE-01 1.OE-02 crd(inj) 1.OE-02 1.OE+00 1.GE-02 cond 1.OE+00 3.4E-01 1.OE-03 1pcs 1.7E-03 1.OE+00 1pci 1.1E-03 1.OE+00 rhrsw(inj) 2.OE-02 1.OE+0O 1.OE-02 rhr 1.5E-04 1.6E-02 1.OE-05 RHR.AND.PCS.NREC 1.5E-04 > 1.5E-04 8.3E-03 > 2.7E-04 1.OE-05 Branch Model: 1.OF.4+opr Train 1 Cond Prob: 1.OE-02 Train 2 Cond Prob: 1.OE-01 Train 3 Cond Prob: 3.OE-01 Train 4 Cond Prob: 5.OE-01 rhr/-lpci 0.OE+00 1.OE+00 1.OE-05 rhr/lpci 1.OE+00 1.OE+00 1.OE-05 rhr(spcool) 2.1E-03 1.0E+00 1.OE-03 rhr(spcool)/-Ipci 2.OE-03 1.OE+00 1.OE-03 ep 7.5E-03 8.7E-01 ep.rec 1 .4E-01 1.0E+00 rpt 1.9E-02 1.OEO00 slcs 2.OE-03 1.OE+00 1.OE-02 ads.inhibit 0.OE+O0 1.0 E+00 1.OE-02 man.depress 3.7E-03 1.QE+00 1.OE-02
- branch model file
- forced LER No. 260/83-074