ML20147A528
| ML20147A528 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1984-003-00 | |
| Download: ML20147A528 (6) | |
Text
PRECURSOR DESCRIPTION SHEET LER No.: 346/84-003 Event
Description:
SFRCS Initiation Leads to Stuck-Open MSRV and Trip Date of Event: March 28, 1984 Plant: Davis-Besse 1 EVENT DESCRIPTION Sequence At 1220 h on March 2, Davis-Besse 1 was operating at -99% of full power. The plant was in full automatic control. During periodic steam and feedwater rupture control system (SFRCS) surveillance testing-, a previously undetected SFRCS channel failure resulted in closure of the loop 2 MSIV. This caused an increase in feedwater to the other SG, which overcooled that side of the reactor. Flux increased because of the negative moderator coefficient. The reactor tripped on high flux ~-13 s after MSIV 2 closed.
The cause of the MSIV closure was a failed optical isolator in a relay driver card for a relay in SFRCS Channel 4. This failure, unde-tected and in conjunction with normal testing on another channel, resulted in a close signal to MSIV 2. During troubleshooting, a wiring anomaly was found in the circuitry for MSIV 2. This anomaly, due to a circuit modification made in 1979, was the reason that the failed relay driver card had not been detected. Both an equipment failure and an installation and construction error were associated with the MSIV closure.
Following the reactor trip, steam pressure on SG 2 did not stabilize as would normally be expected. Local observation determined that main steam SRV, SP17A4, with a set pressure of 1070 psig, had not fully closed on the No. 2 steam line.
Subsequent investigation by the licensee determined that the cause of the valve sticking open was the failure of a cotter pin holding a nut in place on the valve stem. The nut rotated when the valve opened and prevented the safety valve from closing.
Manual actuation of SFRCS isolated SG 2. After AFW isolation, SG 2 boiled dry and depressurized to atmospheric pressure in -5 min. Thi s excessive depressurization rate caused the RCS to exceed the normal cooldown limits for a short time. Plant cooldown was conducted with SG 1, which was unaffected. At -340*F RCS temperature, the failed MSRV was replaced. When operators attempted to restore level in SG 2, the AFW valve (AF599) failed to open. It was opened manually, and SG 2 level was restored to operable status. The plant was then cooled down to cold shutdown.
Event Identifier: 346/84-00'3 185
-D-
It was determined later that an additional MSRV (SP17A1) on the No. 2 steam line hdd failed to open when it should have.
Corrective Action MSRV SP17A4 was replaced. The cotter pins in all other MSRVs were replaced with stainless steel pins, and maintenance procedures were modified to ensure new pins are used after any maintenance or testing in the future.
MSRV SP17AI was declared inoperable and gagged. It was to he 're-paired or replaced during the next refueling outage. The cause of its failure to lift was still unknown at the time. The RPS high-flux trips must be set <99.69% of rated thermal power with this safety inopera'ble.
The effects of the SG No. 2 boiling dry were analyzed, and it was concluded that the transient was within SG design limits. The effects of high main steam flow from SG 1 when MSIV 2 closed was analyzed. It was concluded that some (-1400) tubes may have been unstabl1e for a period of seconds. As corrective action, the suspect tubes were to be eddy-current tested during the next refueling outage.
The AFW valve AF599 problem was investigated. ,The motor operator torque switch settings were changed from 1.5 open and close to 1.0 close and 1.5 open. This was to prevent the valve disk from being jammed into its seat. This change was also made to AF608, the AFW valve for SG 1.
The faulty relay driver board in SFRCS Channel 4 was replaced. The wiring anomaly was corrected and verified not to exist in the circuitry for l4SIV 1.
Plant/Event Data_
Systems Involved:
Steam and feedwater rupture control, main steam relief, AFW Components and Failure Modes Involved:
SFRCS Channel 4 relay card - failed in test MSRV - failed to close in operation AFW regulating valve - failed to open on demand Component Unavailability Duratipn: NA Plant Operating Mode: 1 (99% power)
Discovery Method: During testing Reactor Age: 6.6 years Plant Type: PWR Event Identifier: 346/84-003 D-186
Comments
Reference:
-Abnormal Occurrence Reports to Congress, NUREG-0090, 1(1), 36.
MODELING CONSIDERATIONS AND DECISIONS Initiators Modeled and Initiator Nonrecovery Estimate Transient 1.0 No recovery Branches impacted and Branch Nonrecovery Estimate MFW Base case Unavailable due to SFRCS trip AFW Base case Degraded (one train unavailable)
Plant Models Utilized PWR plant Class B (because it has low-head HPI, it is grouped with
,Class B and not Class D)
Event Identifier: 346/84-003 D- 187
CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 346/84-003 Event
Description:
SFRCS Initiation Leads to Stuck Open MSRV and Trip Event Date: 3/28/84 Plant: Davis-Besse INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS I.0E+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CV TRANS 2.6E-04 Total 2.6E-04 CD TRANS 1.5E-04 Total 1.5E-04 ATWS TRANS 3.OE-05 Total 3.OE-05 DOMINANT SEQUENCES End State: CV Conditional Probability: 1.2E-04 125 TRANS -RT AFW MFW HPI (F/B) -SS.DEPRESS -COND/MFW End State: CD Conditional Probability: 6.1E-05 126 TRANS -RT AFIW MFWHPI (F/B) -SS.DEPRESS COND/MFW End State: ATWS Conditional Probability: 3.OE-05 Event Identifier: 346/84-003
)- 188
128 TRANS RT SEQUENCE CONDITIONAL PROBABILITIES Sequence End State Prob N Rec**
119 TRANS -RT AFW MFW -HPI (F/B) -HPR/-HPI PORV.OPEN -SS.DEPRESS CV 2.7E-05 6.1E-02
-COND/MFW 120 TRANS -RT AFW MFW -HPI (F/B) -HPR/-HPI PORV .OPEN -SS .DEPRESS CD 1.4E-05 3.1E-02 COND/MFW 122 TRANS -RT AFW MFW -HPI (F/B) HPR/-HPI -SS.DEF'RESS -GOND/MFW Cv I .2E-04 3 .4E-02 123 TRANS -RT AFW MFW -HPl(F/B) HPR/-HPI -SS.DEPRESS COND/MFW CD 6 .OE-05 1.7E-02 124 TRANS -RT AFW MFW -HPI(F/B) HPR/-HPI SS.DEPRESS CD 6.6E-06 5. 1E-02 125 TRANS -RT AFW MFWHPI(F/B) -SS.DEPRESS -COND/MFW CV 1.2E-04
- 3.2E-02 126 TRANS -RT AFW MFWHPI(F/B) -SS.DEPRESS COND/tIFW CD 6.1E-05 *. 1.6E-02 127 TRANS -RT AFW MFWHPI(F/B) SS.DEPRESS CD 6 .7E-06 4.8E-02 128 TRANS RT ATWS 3.OE-05
- 1.2E-01
- dominant sequence for end state
- non-recovery credit for edited case MODEL: b:\pwrb tree .cmp DATA: b:\davispro.cmp No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail TRANS 1.OE-03 1.0E+00 LOOP 2.3E-05 3.,4E-01 LOCA 4 .2E-06 3.4E-01 RT 2 .5E-04 1.2E-01 RT/ LOOP 0 .OE+00 1.OE+00 EMERS .POWER 2 .9E-03 5.1lE-01 AFW 5.OE-03 > 5.OE-02 2 .7E-01 Branch Model: 1.OF.2 Train I Cond Prob: S.OE-02 Train 2 Cond Prob: I.0E-01 > Unavailable AFW/EMERG .POW'ER S.OE-03 2 .7E-01 MFW 2.0E-01 > 1.0E+00 3.4E-01 Branch Model: 1.DF.1 Train 1 Cond Prob: 2.OE-01 > Unavailable PORY .OR .SRV .CHALL B.0E-02 I .OE+00 PORV .OR .SRV .RESEAT 1.OE-02 5.OE-02 PORV .DR.SRV .RESEAT/EMERG .POWER 1.0E-02 S.OE-02 SS .RELEAS .TERM 1.5E-02 3.4E-01 SS .RELEAS .TERM/-MFW 1.5E-02 3.4E-01 Event Identifier: 346/84-003 1)-189
HPI 1.OE-03 5.2E-01 HP1 (FIB) I.OE-03 .5.2E-01 4 OE-02 HPRI-HPI 3.OE-03 5.6E-01 4.OE-02 PORY .OPEN 1.OE-02 1.OE+00 9SSDEPRESS 3.E-02 1.OE+00 CDND/14FW 1.OE+00 3.E-01 LPI/HPI 1 O0E-03 3.E-01 LPR/-HPI .HPR 6.E-01 I.OE+00 LPR/HPI I.0E-03 1.OE+00
- forced Minarick 04-12-1987 10:44:10 Event Identifier: 346/84-003 D~-190