ML20147A489
| ML20147A489 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter Office of Nuclear Reactor Regulation |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1985-001-00 | |
| Download: ML20147A489 (5) | |
Text
PRECURSOR DESCRIPTION AND ANALYSIS LER No.: 318/85-001 Event
Description:
Stuck-Open Atmospheric Dump Valve Date of Event: April 25, 1985 Plant: Calvert Cliffs 2 EVENT DESCRIPTION Sequence At 0933 h on April 25, 1985, Unit 2 was manually tripped while operating in Mode 1 (full power) at 100% power. This trip was caused by a rapid degradation of the 21A RCP shaft seal. Both MFW pumps were under manual control at this time. The core scram and turbine trip caused a loss of MFW. No. 21 MFW pump tripped on high pressure, and No. 22 NFW pump was manually tripped. No. 21 RCP was tripped 60 s after the scram. AFW was initiated. No. 21 atmospheric dump valve stuck open, contributing to a RCS cooldown to 517*F. The dump valves malfunctioned because of a fault in the valve positioner. The positioners have a tendency for their internal levers to interfere with one another, causing the levers to disengage, resulting in failure of the valves to close.
By 1000 h the reactor plant was stable, and boration to shutdown had commenced. Subsequently, the plant was cooled down to affect RCP seal replacement.
Two days before the trip, temperature fluctuations (over a 5-h period) in the CCW system had begun to cause large pressure oscillations between the stages of the 21A RCP seal that continued until it degraded and a manual trip was initiated. The 21A seal had exhibited some pressure oscillations in the past.
The seal pressure/temperature fluctuations began when the No. 21A SW heat exchanger was removed from service for maintenance. This removal caused a heat imbalance in the salt-water cooling system. The SW system and the CCW system are both cooled by the salt-water system and operate in parallel to each other. All interfacing systems experienced tem-perature transients; the CCW experienced the greatest fluctuations.
Efforts to control the CCW pressure oscillation (300 to 500 psi AP) were unsuccessful.
Corrective Action The RCP seal was replaced, and Unit 2 was restored to operation on May 6, 1985. The SW heat exchanger was returned to service.
Event Identifier: 318/85-001 E-1 51
An investigation has been initiated to determine the effect of SW and CCW system transients on RCP operation. Number 21 atmospheric dump valve was repaired, and an inspection program that should prevent similar failures in the future was, begun.
Plant/Event Data Systems Involved:
SW, salt water, CCW, RCP seal, atmospheric steam dump, MFW Components and Failure Modes Involved-SW heat exchanger - out for maintenance CCW - temperature and pressure transients in operation ASD valve - failed to open in operation RCP seals - degrade in operation MFW pumps - trip in transient Component Unavailability Duration: NA Plant Operating Mode: 1 (100% power)
Discovery Method: Operational event Reactor Age: 8.4 years Plant Type: PWR Comments Potential for an RCP seal LOCA to occur except plant was aware of RCP seal problems MODELING CONSIDERATIONS AND DECISIONS Initiators Modeled and Initiator Nonrecovery Estimate Transient 1.0 Nonrecoverable Branches Impacted and Branch Nonrecovery Estimate Secondary-side 0.34 Base-case value release terminated Secondary-side 0.34 Base-case value release terminated; given NFW success MFW 0.34 Base-case value; manually tripped pump was recoverable Plant Models Utili'zed PWR plant Class G Event Identifier: 318/85-001 E-1 52
CONDITIONAL CORE DAMAGE CALCULATIONS LER Number: 318/B5-001 Event
Description:
Stuck Open Atmospheric Dump Valve Event Date: 4/25/85 Pl ant: Calvert Cliffs 2 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.000E+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CV TRANS 5.609E-05 Total 5.609E-05 CD TRANS 1.795E-06 Total 1.795E-06 AIWS TRANS 3.OOOE-05 Total 3.OOOE-05 DOMINANT SEQUENCES End State: CV Conditional Probability: 5.OIOE-05 104 -TRANS_-Ri -7AFW -POPiV..UR..SRV..CHALL. SS.RELEAS. TERM HPI End State. CD Conditional Probability: 1.232E-06 119 TRANS -RT AFW MFW HPIfF/B) -SS.DEPRESS CONDIMFW End State: ATWS Conditional Probability: 3.OOOE-05 121 TRANS RT Event Identifier; 318/85-001 E-1153
SEQUENCE CONDITIONAL PROBABILITIES Sequence End State Seq. Prob Non-ReCDV0 101 TRANS -RT -AFW PORV.OR.SRV.CHALL -PORV.OR.SRV.RESEAT SS.RELE CV 2.119E-06 1.766E-01 AS.TERM HPI 102 TRANS -RI -AFW PORV.OR.SRV.CHALL PORV.OR.SRV.RESEAT -HPI HP CD 3.998E-08 5.000E-02 RI-HPI 104 TRANS -RT -AFW -PORV.OR.SRV.CHALL SS.RELEAS.TERM HPI CV 5.09DE-05 I 1.76BE-01 113 TRANS -RT AFW MFW -HPIIFIB) -HPRI-HPI PORV.OPEN -SS.DEPRESS CD 2.942E-07 2.996E-02 COND/MFW 118 TRANS -RT AFW MFW HPI(F/B) -SS.DEPRESS -COND/MFW CV 2.392E-06 2.527E-03 119 TRANS -RI AFW MFW HPI(F/B) -SS.DEPRESS COND/MFW CD 1.232E-06 t 1.302E-03 120 TRANS -RT AFW MFW HPI(F/B) SS.DEPRESS CD 1.353E-07 3.829E-03 121 TRANS RT ATWS 3.OOOE-05 I 1.200E-01 t dominant Sequence for end state 0*non-recovery credit for edited case Note:
Conditional probability values are differential values which reflect the added risk due to observed failures.
Parenthetical values indicate a reduction in risk compared to a similar period without the existing failures.
MODEL: b:pwrgtree. cap DATA. b-.cal vprob. cmp No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail TRANS 1.030E-03 1.000E+00 LOOP 2.2BOE-05 3.400E-01 LOCA 2.5bOE-02 3.400E-01 RT 2.500E-04 1.200E-01 RT/ LOOP 0.OOOE+00 1.OOOE+00 EMERG. POWER 5.415E-04 5.100QE-01 AFN 1.020E-03 2.700E-01 AFW/EMERB. POWER 5.OOOE-02 3.400E-01 MFW 2.000E-01 > 1.00OE+00 3.400E-01 Branch Model: l.OF.1 Train 1 Cond Prob: 2.OOOE-01 > Failed PORY. OR. SRV. CHALL 4.000E-02 1.OO0E+00 PORY. OR. SRV. RESEAT 2.000E-02 5.OOOE-02 PORV. OR. SRV. RESEAT/EMERG.POWER 2.006OE-02 5. 000E-02 SS.RELEAS. TERM 1.500E-02 ) 1.000E+00 3.400E-01 Branch Mode]: 1.OF.i Event Identifier; 318/85-001 E-154
Train 1 Cond Prob: 1.500E-02 > Failed SS. RELEAS. TERM/-PIFW 1.500E-02 > 1.OOOE+00 3.400E-01 Branch Model: I.GF.1 Train I Cond Prob: 1.500E-02 > Failed SS. DEPRESS 3.600E-02 1.000E+00 COND/MFW 1.000E+00 3.400E-01 HPI 3.OOOE-04 5.200E-01 HP! (F/B) 3.OOOE-04 5.200E-01 4.OOOE-02 PORY. PEN 1.000E-02 1.OOOE+00 HPR/ -HPI 1.ODOE-03 1.OOOE+00 CSR 2.OOOE-03 3.400E-01 111 forced JD HARRIS 10-07-1986 14:50:,06 Event Identifier: 318/B5-001 E-155