ML20149K359
| ML20149K359 | |
| Person / Time | |
|---|---|
| Site: | Hatch  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1982-081-00 | |
| Download: ML20149K359 (5) | |
Text
B.41-1 B.41 LER No. 366/82-081 Event
Description:
Scram, Isolation, RCIC Failure, SRV Tailpipe Vacuum Relief Failed Date of Event: August 25, 1982 Plant: Hatch 2 B.41.1 Summary Hatch 2 was operating at power when a main steam isolation valve (MSIV) failed closed, causing a scram and isolation. Multiple problems and equipment failures, including failure of the reactor core isolation cooling (RCIC), leakage of reactor coolant into the drywell, and leakage of reactor coolant into the reactor building, complicated the scram recovery. The conditional core damage probability estimated for the event is 1.4 x 10'.
B.41.2 Event Description During power operation at Hatch Unit 2, a valve disk in an MSIV in steamline C separated from its valve stem and closed suddenly. The sudden isolation of one of four main steam lines caused a reactor pressure increase and void collapse. The increased moderator density resulting from the void collapse caused a sudden power increase which in turn caused a scram. Increased flow in the three open main steamlines also caused a high-steam flow group 1 isolation, closing the rest of the MSIVs.
With the MSIVs closed, reactor pressure began increasing and two safety/relief valves (SRVs) opened. The high-pressure coolant injection (HPCI) system and RCIC system started, then tripped on reactor high level.
Operators opened the MSIV bypasses in preparation for resetting the isolation and restarted RCIC to provide makeup. The RCIC system flow rate was inadequate to maintain reactor inventory and HPCI was restarted.
B.41.3 Additional Event Information During a scram, reactor coolant is used to hydraulically drive pistons to which control rods are attached. Water from nitrogen-charged accumulators is routed beneath the pistons and water from the over-piston spaces is drained to a tank, the scram discharge volume (SDV). The SDV is normally kept vented and drained. A scram signals the vents and drains to close, to protect reactor coolant system integrity. Water from the over-piston spaces and control rod drive seal leakoff collects in the SDV until the scram is reset, at which time the SDV inventory is aligned to drain to the reactor building equipment drain sump (RBEDS).
The SDV drain valve failed to fully close during this event. This opened a direct path between the reactor coolant system and the RBEDS, outside of containment. Flow was limited by the control rod drive seals and the 2-inch SDV drain line, but reactor coolant drained to the sump continuously. Enough inventory transferred to the sump that steam began flowing from an uncapped drain connection in the RCIC space. The steam release in the RCIC space caused actuation of the fire protection system and the combined effects of the steam and fire protection spray caused RCIC instrumentation to fail, and the system shut down.
LER No. 366/82-081
B.41-2 At the same time, an SRV tailpipe vacuum relief valve failed. SRVs relieve the suppression pool via "tailpipes." After SRV operation, steam in the tailpipes condenses, creating a partial vacuum. The vacuum can then draw water into the tailpipe from the suppression pool. Subsequent operation of the associated SRV will accelerate the water in the tailpipe, which can result in damage to the tailpipe or the attached quencher.
To prevent this, each tailpipe is equipped with vacuum relief valves. During this event, one of the tailpipe relief valves failed open, allowing steam to vent directly to the drywell and causing a drywell high-pressure accident/scram signal. This scram signal could not be reset until drywell pressure was reduced. At the same time, it was difficult to reduce drywell pressure because the drywell chillers were isolated by the accident signal.
Operators aligned a reactor feedpump to provide makeup and the steam was dumped to the main condenser.
The high drywell pressure accident/scram signal was jumpered out and a drywell chiller was restarted. Drywell pressure was reduced and the scram was reset, finally isolating the flow path from the reactor to the RBEDS.
B.41.4 Modeling Assumptions This event was modeled as a transient and isolation with RCIC inoperable. The long-term nonrecovery probability for the power conversion system (PCS) was revised to 0.017 to reflect the initial MSIV isolation.
Accordingly, for sequences involving potential residual heat removal (RHR) or PCS nonrecovery, the nonrecovery estimate was revised to 0.016 x 0. 11, or 2.7E-4 (see Appendix A).
B.41.5 Analysis Results The conditional core damage probability estimated for this event is 1.4 x10 5 . The dominant core damage sequence, shown in Figure B.41.1, involves the observed transient, failure of PCS, recovery of main feedwater, and failure of RHR.
LER No. 366/82-081
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Figure B.41.1 Dominant core damage sequence for LER 366/82-081 LER No. 366/82-081
B.41-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 366/82-081 Event
Description:
Scram. isolation. RCIC and SRV vacuum relief valve failure Event Date: August 25. 1982 Plant: Hatch 2 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.OE+O0 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 1.4E-05 Total 1.4E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
103 trans -rx.shutdown PCS srv.ftc.<2 -MFWRHR.AND.PCS.NREC CD 6.6E-06 1.8E-04 105 trans -rx.shutdown PCS srv.ftc.<2 MFW -hpci RHR.AND.PCS.NREC CD 3.3E-06 9.1E-05 119 trans -rx.shutdown PCS srv.ftc.<2 MFWhpci RCIC srv.ads c CD 1.7E-06 1.7E-01 rd(inj) 414 trans rx.shutdown rpt CD 6.7E-07 1.OE-O1 413 trans rx.shutdown -rpt slcs CD 4.1E-07 1.OE-O1 412 trans rx.shutdown -rpt -slcs PCS ads.inhibit CD 3.4E-07 I.OE-OI 138 trans -rx.shutdown PCS srv.ftc.2 hpci srv.ads CD 3.3E-07 4.9E-01
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
103 trans -rx.shutdown PCS srv.ftc.<2 -MFW RHR.AND.PCS.NREC CD 6.6E-06 1.8E-04 105 trans -rx.shutdown PCS srv.ftc.<2 MFW-hpci RHR.AND.PCS.NREC CO 3.3E-06 9.1E-05 119 trans -rx.shutdown PCS srv.ftc.<2 MFW hpci RCIC srv.ads c CD 1.7E-06 1.7E-01 rd(inj) 138 trans -rx.shutdown PCS srv.ftc.2 hpci srv.ads CD 3.3E-07 4.9E-01 412 trans rx.shutdown -rpt -slcs PCS ads.inhibit CO 3.4E-07 I.OE-OI 413 trans rx.shutdown -rpt slcs CD 4.1E-07 1.OE-01 414 trans rx.shutdown rpt CD 6.7E-07 I1,E-01
- non-recovery credit for edited case SEQUENCE MODEL: c:\asp\1982-83\bwrc8283.cmp BRANCH MODEL: c:\asp\1982-83\hatch2.82 LER No. 366/82-081
B.41-5 PROBABILITY FILE: c:\asp\1982-83\bwr8283.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 1.5E-03 1.OE+00 loop 1.6E-05 3.6E-01 loca 3.E-06 6.7E-01 rx.shutdown 3.5E-04 I.QE-01 PCS 1.7E-01 > l.OE+OO 1.OE+00 Branch Model: I.OF.1 Train 1 Cond Prob: 1.7E-01 > 1.OE+OO srv.ftc.<2 1.OE+0O 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+OO 3.4E-01 Branch Model: 1.OF.1 Train 1 Cond Prob: 4.6E-01 > 1.OE+OO hpci 2.9E-02 7.OE-01 RCIC 6.OE-02 > 1.OE+OO 7.OE-Ol > 1.OE+OO Branch Model: I.OF.1 Train 1 Cond Prob: 6.OE-02 > 1.OE+OO srv.ads 3.7E-03 7.OE-01 -02 I1.OE crd(inj) 1.OE-02 1.OE+0O 1.OE-02 cond 1.OE+00 3.4E-01 1.OE-03 1pcs 2.OE-03 1.OE+00 1pci 1.1E-03 1.OE+00 rhrsw(inj) 2.OE-02 1.OE+00 1.OE-02 rhr 1.5E-04 1.6E-02 i.QE-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 0.0 E+OO 1.OE+00 1.OE-05 rhr/lpci 1.OE+00 1.OE+00 1.OE-05 rhr(spcool) 2.1E-03 1.OE+00 1.OE-03 rhr(spcool)/-lpci 2.OE-03 1.OE+00 1.OE-03 ep 2.9E-03 8.7E-01 ep.rec 1.6E-01 1.OE+00 rpt 1.9E-02 1.OE+00 slcs 2.OE-03 1.OE+00 1.OE-02 ads inhibit 0.0 E+OO 1.OE+00 1.-OE
-02 man.depress 3.7E-03 1.OE+00 1.OE-02
- branch model file
- forced LER No. 366/82-081