ML20156A211
| ML20156A211 | |
| Person / Time | |
|---|---|
| Site: | Salem  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1983-001-00 | |
| Download: ML20156A211 (5) | |
Text
B.21-1 B.21 LER No. 311/83-001 and 311/82-072 Event
Description:
Transient with One Automatic Trip Breaker Failing to Open Date of Event: January 6, 1983 Plant: Salem 2 B.21.1 Summary On January 6, 1983, while Salem 2 was operating at 46% power, the reactor tripped due to low level in the number 21 steam generator. Following the trip, the operator noticed that trip breaker A had failed to open on the trip signal, but trip breaker B had opened and de-energized the rod drive mechanisms, resulting in a shutdown. It was later determined that trip breaker A undervoltage relay had malfunctioned due to dirt or corrosion which interfered with proper relay operation. A similar breaker failure occurred on August 20, 1982 during a surveillance test. The conditional core damage probability estimated for the reactor trip on January 6, 1983 is 4.4 x 10".
B.21.2 Event Description On January 6, 1983, while Salem 2 was operating at 46% power, the reactor tripped due to low level in the number 21 steam generator. Following the trip, the operator noticed that trip breaker A had failed to open on the trip signal, but trip breaker B had opened and de-energized the rod drive mechanisms, resulting in a shutdown. It was later determined that the trip breaker A undervoltage relay had malfunctioned due to dirt or corrosion which interfered with proper relay operation. This dirt or corrosion resulted from the infrequent operation of the breaker, which led to insufficient self-cleaning of the relay. The debris accumulated and caused a mechanical binding of the undervoltage relay.
On August 20, 1982, during a surveillance test with the plant at 82% power, reactor trip breaker B failed to trip as required. Trip breaker A was operable. Investigation revealed that'the cause of the B trip breaker was binding of the undervoltage coil. The coil was replaced and trip breaker B was reinstalled and satisfactorily tested.
B.21.3 Additional Event-Related Information The Salem 2 reactor protection system (RPS) uses independent channels and trains which consist of sensors, transmitters, relays and trip breakers to detect and protect against unsafe plant conditions. When an unsafe plant condition occurs, the RPS signals the trip breakers to open and de-energize the rod drive mechanisms, resulting in a reactor shutdown. The reactor trip breakers are ac circuit breakers positioned in series. When either trip breaker is tripped open, holding power to the control rods is lost and the rods drop into the core.
At the time of this event, one mechanism, de-energization of the undervoltage coils, could open the trip breaker. A second mechanism for tripping open the breakers was installed after the February 1983 Salem anticipated transient without scram (ATWS). This mechanism energizes the shunt trip coil. The shunt trip coils, once energized, will open the breakers.
LER No. 311/83-001 and 311/82-072
B.21-2 B.21.4 Modeling Assumptions The August 20, 1982 surveillance test failure was considered incidental to the event of interest. The January 6, 1983 reactor trip was modeled as a trip with the reactor trip system degraded. One train of the reactor trip (RT) system was set to failed. Manual scram capability was not affected by the failure of the reactor trip breakers so the RT nonrecovery probability (which models the manual scram capability of the RT system) was not modified. The potential for common cause failure exists, even when a component is failed. Therefore, the conditional probability of a common cause failure of the other train of the reactor trip system was included in the analysis.
B.21.5 Analysis Results The conditional core damage probability for this event is 4.4 x 10'. The dominant sequence is a postulated ATWS sequence involving a failure to trip, success of auxiliary feedwater (AFW) given ATWS, and failure of emergency boration, and is highlighted on the event tree shown in Figure B.2 1.1.
LER No. 311/83-001 and 311/82-072
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Figure B.21.1 Dominant core damage sequence for LER 311/83-001 and 311/82-072 LER No. 311/83-001 and 311/82-072
B.21-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 311/83-001 and 311/82-072 Event
Description:
Transient with one automatic trip breaker failing to open Event Date: January 6, 1983 Plant: Salem 2 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.0E+O0 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 4.4E- 04 Total 4.4E- 04 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
507 trans RT -prim.press~limited -afw/atws emrg.boration CD 1.9E-04 I.OE-01 509 trans RT prim.press.limited CD 1.7E-04 I.OE-OI 508 trans RT -prim~press~limited afw/atws CD 8.1E-05 I.0E-OI
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
507 trans RT -prim.press.limited -afw/atws emrg.boration CD 1.9E-04 1.0E-01 508 trans RT -prim.press.limited afw/atws CD 8.1E-05 1.OE-01 509 trans RT prim.press.limited CD 1.7E-04 1.0E-01
- non-recovery credit for edited case SEQUENCE MODEL: c:\aspcode\models\pwrb8283.cmp BRANCH MODEL: c:\aspcode\models\salem2.82 PROBABILITY FILE: c:\aspcode\models\pwr8283.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 2.1E-03 I.OE+O0 LER No. 311/83-001 and 311/82-072
B.21-5 loop 1.6E-05 5.3E-01 loca 2.4E-06 5.4E-O1 sgtr 1.6E-06 1.OE+OO RT 2.8E-04 > 1.9E-01 1.OE-01 Branch Model: 1.OF.2 Train 1 Cond Prob: 1.5E-03 > Failed Train 2 Cond Prob: 1.9E-O1 rt(loop) O.OE+O0 1.OE+OO afw 3.8E-04 4.5E-01 afw/atws 4.3E-03 1.OE+0O afw/ep 5.OE-02 3.4E-01 mfw 2.OE-O1 3.4E-O1 1.OE-03 porv.chall 4.OE-02 1.OE+OO porv.chall/afw I.OE+O0 1.OE+OO porv.chall/loop I.OE-O1 1.OE+OO porv.chall/sbo 1.OE+O0 1.OE+OO porv.reseat 2.OE-02 1.1E-02 porv.reseat/ep 2.OE-02 1.OE+OO srv.reseat(atws) I.OE-O1 1.OE+OO hpi 1.OE-05 8.9E-01 feed.bleed 2.OE-02 1.OE+OD 1.OE-02 emrg.boration O.OE+O0 1.OE+OO 1.OE-02 recov.sec.cool 2.OE-O1 1.OE+OO recov.sec.cool/offsite.pwr 3.4E-01 1.OE+OO rcs.cooldown 3.OE-03 1.OE+OO 1.OE-03 rhr 2.2E-02 7.OE-02 1.OE-03 rhr.and.hpr 1.OE-03 1.OE+OO 1.OE-03 hpr 4.OE-03 1.OE+0O 1.OE-03 ep 5.4E-04 8.9E-01 seal.loca 2.7E-01 1.OE+OO offsite.pwr.rec/-ep.and.-afw 2.2E-01 1.OE+OO offsite.pwr.rec/-ep.and.afw 6.7E-02 1.OE+0O offsite.pwr.rec/seal.loca 5.7E-01 1.OE+0O offsite.pwr.rec/-seal.loca 7.OE-02 1.OE+0O sg.iso.and.rcs cooldown I.OE-02 1.OE-O1 rcs.cool.below.rhr 3.OE-03 1.OE+OO 3.OE-03 prim.press.limited 8.8E-03 1.OE+00
- branch model file
- forced LER No. 311/83-001 and 311/82-072