ML20147A348
| ML20147A348 | |
| 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 1983-006-00 | |
| Download: ML20147A348 (5) | |
Text
B.6 LER No. 259/83-006 and -007 Event
Description:
Scram, MSRV and its Vacuum Breaker Fail Open Date of Event: February 5, 1983 Plant: Browns Ferry 1 B.6.1 Summary Unit 1 was operating at approximately 100% power when a reactor scram occurred. A main steam relief valve (MSRV) was opened manually to control reactor pressure but when operators attempted to close it, they were unable to do so. The MSRV tailpipe vacuum relief valve failed open during the event, venting steam into the drywell instead of directing it to the suppression pool. The conditional core damage probability estimated for the event is 4.4 x 10-5.
B.6.2 Event Description On February 5, 1983, Unit 2 was operating at 100% power, when the operator began a surveillance test of the main turbine overspeed trip system. At that time something, possibly a turbine trip, occurred which caused a reactor scram. MSRV 1-1-22 was manually opened to control reactor pressure, but operators were unable to close it.
At the same time, the vacuum relief valve for the MSRV 1-1-22 tailpipe was stuck open. The main steam relief valves at Browns Ferry sit on stub headers attached to the main steam lines. Each MSRV's exhaust is routed via a tailpipe to a quencher submerged in the suppression pool. After MSRV operation, steam in the tailpipe will condense, drawing a vacuum. This tends to draw a slug of water up from the suppression pool into the tailpipe. Subsequent operation of the associated MSRV could propel this water slug into the quencher, causing damage. To prevent this, each tailpipe is equipped with two vacuum breakers which open after MSRV operation to limit negative pressure in the tailpipe.
At the time of the event, the disk in a vacuum breaker for MSRV 1-1-22 was partially separated from its hinge arm and jammed in the open position, which allowed steam to vent continuously to the drywell. The peak drywell pressure attained and the specific leak rate were not noted in the licensee event report for this event, but it was noted that leakage into the drywell exceeded a Technical Specification limit of 5 gpm. Therefore, it may be assumed that steam leakage into the drywell was in excess of 200 cubic feet per minute, possibly substantially in excess. It is unclear why the drywell pressure did not exceed the 2.45 psig loss-of-coolant accident (LOCA) setpoint during this event.
The failure of the MSRV was initially attributed to its pilot valve so, after the unit was shut down, the pilot valve on 1-1-22 was replaced. The vacuum relief valve for the MSRV was found stuck open and was replaced also. Four other vacuum relief valves were also found to be damaged and were replaced.
LER No. 259/83-006 and -007
B.6-2 Three days after the event described, the unit was restarted. The intention was to test the repaired MSRV when reactor pressure reached 250 psig. However, when reactor pressure reached 178 psig, MSRV 1-1-22 opened spontaneously and could not be closed until the reactor was shut down again.
In addition, the movement indicating arm attached to the tailpipe vacuum breaker for MSRV 1-1-22 was apparently bent out of its normal alignment, causing the vacuum breaker to again stick open. Once again, an unspecified quantity of steam leakage greater than 5 gpm entered the drywell until the unit was shut down.
This time, MSRV 1-1-22 was replaced. More rigorous inspection of the MSRV found that the pilot valve inlet tube mounting bracket had broken, and a piece of debris had lodged under the seat of the main valve.
The position indicating arms were removed from all vacuum relief valves to prevent further valve failures.
B.6.3 Additional Event-Related Information A safety evaluation report provided with licensee event report 259/83-007 indicates that this event, involving a stuck-open SRV relieving directly to the drywell instead of through quenching headers to the suppression pool, may be represented as a loss-of-coolant accident (LOCA) with a break area of approximately 0.15 square feet.
B.6.4 Modeling Assumptions This event was modeled as a LOCA, and it was assumed that the initiating event was nonrecoverable. A break area of 0.15 square feet is defined as an intermediate break in Browns Ferry analyses. To represent the medium-break LOCA, the low flow-rate makeup systems, control rod drive (CRD) and reactor core isolation cooling (RCIC), were set to failed, since they were assumed to be inadequate to make up losses.
The second event occurred three days after shutdown, with a reduced decay heat load and at temperature and pressure conditions only slightly above those which would permit alignment of the residual heat removal (RHR) system. Presumably, given a stuck-open relief valve, reactor pressure was sufficiently reduced to allow resumption of shutdown cooling within a short time. The second event was therefore considered to be little different from a routine trip during startup, and the event was not analyzed.
B.6.5 Analysis Results The conditional core damage probability estimated for this event is 4.4 x 10'. The dominant core damage sequences, highlighted on the event tree in Figure B.6.1, involve a failure to trip following the LOCA [this anticipated transient without scram (ATWS) sequence is not developed in the model], and the observed LOCA, failure of the power conversion system (PCS), the feedwater system (FW), high-pressure coolant injection (HPCI), and the automatic depressurization (ADS). Excluding the potentially conservative ATWS sequence, a conditional core damage probability of 8.6 x 106 is estimated.
LER No. 259/83-006 and -007
B.6-3 0*
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B.6-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 259/83-006 and -007 Event
Description:
Scram. MSRV and its vacuum breaker fail open Event Date: February 5, 1983 Plant: Browns Ferry 1 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOCA 1.OE+O0 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOCA 4.4E-05 Total 4.4E-05
.SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
320 LOCA rx.shutdown CD 3.5E-05 1.OE-01 319 LOCA -rx.shutdown pcs mfw hpci RCIC srv.ads CRD(INJ) CD 6.8E-06 1.7E-01 303 LOCA -rx.shutdown pcs -mfw rhr CD 1.8E-06 1.4E-02
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
303 LOCA -rx.shutdown pcs -mfw rhr CD 1.8E-06 1.4E-02 319 LOCA -rx.shutdown pcs mfw hpci RCIC srv.ads CRD(INJ) CD 6.8E-06 1.7E-01 320 LOCA rx.shutdown CD 3.5E-05 I.0E-01
- non-recovery credit for edited case SEQUENCE MODEL: d:\asp\models\bwrc8283.cmp BRANCH MODEL: d:\asp\models\brownl.82 PROBABILITY FILE: d:\asp\models\bwr8283.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail LER No. 259/83-006 and -007
B.6-5 trans 1.7E-03 1.E -00 loop 1.6E-05 2.4E -01 LOCA 3.3E-06 > 3.3E-06 6.7E-01 > 1.OE+00 Branch Model: INITOR Initiator Freq: 3.3E-06 rx.shutdown 3.5E-04 1.CE-0l pcs 1.7E-01 1.OE+00 srv.ftc.<2 1.OE+O0 L.OE+00 srv.ftc.2 1.3E-03 1.OE+00 srv.ftc.>2 2.2E-04 1.CE+00 mfw 4.6E-01 3.4E-01 hpci 2.9E-02 7.OE-0l RCIC 6.0E-02 > LOE+OO 7.OE-01 > 1.OE+00 Branch Model: 1.OF.1 Train 1 Cond Prob: 6.0E-02 > 1.OE+O0 srv.ads 3.7E-03 7.OE-01 1.OE-02 CRD(INJ) 1.OE-02 > 1.OE+O0 1.OE+C0 l.OE-02 Branch Model: 1.OF.l+opr Train 1 Cond Prob: 1.OE-02 > 1.OE+O0 cond 1.OE+O0 3.4E-01 1.OE-03 1pcs 1.7E-03 1.CE+00 1pci 1.1E-03 1.OE+00 rhrsw(inj) 2.OE-02 L.OE+00 l.OE-02 rhr 1.5E-04 1.6E-02 1.OE-05 rhr.and.pcs.nrec 1.5E-04 8.3E-03 1.CE-05 rhr/-lpci O.OE+O0 L.OE+00 1.OE-05 rhr/lpci 1.OE+O0 L.OE+00 1.OE-05 rhr(spcool) 2.1E-03 L.OE+00 L.CE-03 rhr(spcool)/-Ipci 2.OE-03 1.OE+00 1.CE-03 ep 7.5E-03 8.7E-01 ep.rec 1.4E-01 1.OE+C0 rpt 1.9E-02 1.OE+00 slcs 2.0E-03 1.OE+00 L.OE-02 ads.inhibit O.OE+O0 1.OE+00 1.OE-02 man.depress 3.7E-03 1.OE+00 L.OE-02
- branch model file
- forced LER No. 259/83-006 and -007