ML20156A196
| ML20156A196 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1985-025-00 | |
| Download: ML20156A196 (6) | |
Text
PRECURSOR DESCRIPTION SHEET LER No.: 312/85-025 Event
Description:
Los's of Integrated Control System and Loss of Fee dwa te r Date of Event: December 26, 1985 Plant: Rancho Seco EVENT DESCRIPTION Sequence At 0414 h on December 26, 1985, the plant was operating at 76% power, when a loss of integrated control system (ICS) dc power occurred because of a single failure. The plant had just finished a 2-d outage to repair essential valves. The plant computer was out of service, and the core decay heat level was still very low. The plant was operating normally, otherwise.
The ICS failure was a result of a failed power supply monitor that opened switches S1 and S2 to the 120V-dc ICS. The loss of dc power to the ICS (a non-safety-related system) caused a number of feedwater and steam valves to reposition automatically and also caused the loss of remote control of the affected valves from the control room. In addi-tion, the MFW pump turbines slowed to minimum speed, and the AFW pumps started. This sequence was the normally expected response to the loss of ICS transient; however, the operators did not realize that an ICS failure had occurred until 2 min after the initial failure. The Iimme-diate result was an RCS undercooling condition that resulted in the reactor tripping on high pressure. The reactor trip was followed by an overcooling condition that resulted in safety features actuation and excessive RCS cooldown.
The shift supervisor opened the pressurizer PORV to control RCS pres-sure. The atmospheric steam dump (ASD) and turbine bypass system (TBS) valves also opened to the halfway position on ICS failure. Rapid SG and RCS depressurization by means of the PORV, ASD, TBS, and AFW began. The low core decay heat exacerbated t'h-e* R.CS cool-down rate. The operators did not- find open switches S1 and S2 for 26 min despite checking the applicable cabinets several times. The operators were not immediately able to restore dc power within the ICS. As a result, nonlicensed operators were sent to isolate the affected steam and feedwater valves locally with handwheels. The operators failed to realize they could more quickly control the ASD and TBS valves from the remote shutdown panel or the ASD valves from their manual control station. The ASD valves and TBS valves were isolated within 9 min after the reactor trip. However, the operators experienced difficulty closing the ICS-controlled AFW flow control valves. One of the AEW flow control valves finally shut; however, the second ANW flow control valve was damaged and failed open.
Event Identifier: 312/85-025 E- 145
The associated AFW manual isolation valve was found to be stuck open.
Therefore, both AFW pumps continued to feed and overfill one steam generator. Because the plant has no main steam isolation valves, water began to overflow into the main steam lines. During the first 7 min of the incident, the excessive steam and feedwater flows resulted in a rapid RCS cooldown of >100*F. The pressurizer emptied, and a small bubble formed in the reactor vessel head. The RCS cooldown continued, and the RCS depressurized to '-1064 psig and then began to repres-surize. Vessel level was recovered by means of HPI. The safety fea-tures actuation system had actuated on low RCS pressure as designed, and this contributed to the rapid cooldown. About 26 min after the reactor trip, the operators restored power within the ICS by reclosing Si and S2 switches. The operators were then able to close the open AFW flow con-trol valve from the control room, which stopped the RCS cooldown, and started stabilizing the plant. The RCS had cooled down a total of 180*F in this 26-mmn period. The RCS repressurization. resulted in the RCS entering the B&W-designated pressurized thermal shock (PTS) region.
During changing of a valve lineup in the suction of the pump used to supply RCS makeup (HPI pump), the last suction valve to the pump was inadvertently shut. This resulted in the overheating and destruction of the pump. About 450 gal of contaminated water was spilled on the floor. This failure did not directly affect the incident because an additional HPI pump was available to supply RCS makeup. In addition, the spilled water did not result in any significant onsite or offsite radioactivity release or personnel dose.
Corrective Action The plant was taken to cold shutdown. 'The event was subsequently the subject of an NRC special investigation.
Plant/Event Data Systems Involved:
ICS, MFW, HFI, atmospheric dumps, and TBS Components and Failure Modes Involved:
120-V-dc power supply to ICS - failed during operation MFW pumps - experienced inadvertent runback during operation ASD and TBS valves - failed to close on demand AFW flow control valve and isolation. valve - failed to close on demand HPI pump - failed in operation because suction valve was inadvertently closed Event Identifier: 312/85-025 E- 146
Component Unavailability Duration: NA Plant Operating Mode: 1 (76% power)
Discovery Method: Operational event Reactor Age: 11.3 years Plant Type: PWR Comments-A complex event with safety train failures and an overcooling of the RCS MODELING CONSIDERATIONS AND DECISIONS Initiators Modeled and Initiator Nonrecovery Estimate Transient 1.0 Branches Impacted and Branch ,Nonrecovery Estimate HP I Base case one train failed Secondary-side 0.34 Valves 50% open because of ICS fail-release terminated ure; assumed potentially recoverable locally MFW 0.12 Assumed recoverable from control room, not a procedurally based response given the uncertainties imposed by the ICS failure PORV challenged 1 .0 Opened by operator Plant Models Utilized PWR plant Class D Event Identifier: 312/85 -025 E-147
CONDITIONAL CORE DAMAGE CALCULATIONS LER Number: 312/85-025 Event
Description:
Loss of Integrated Control System and Loss of Feedwater Event Date: 12/26/85 Plant- Rancho SeCo INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.OOOE+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CV TRANS 1.867E-04 Total 1.867E-04 CD TRANS 1.b601E-05 Total 1.60 1E-05 ATWS TRANS 3.OOOE-05 Total 3.OOOE-05 DOMINANT SEQUENCES End State: CV Conditional Probability: 1.766E-04 101 TRANS -RT -AFW- PORV.OR.SRV.CHALL -PORV.OR.SRV.RESEAT SS.RELEAS.TERM HPI End State: CD Conditional Probability: 1.342E-05 103 TRANS -RT -AFN PORV.OR.SRV.CHALL PORV.OR.SRV.RESEAT -HPI HPRI-HPI -SS.DEPRESS LPRI-HPI.HPR End State: ATWS Conditional Probability: 3.QOOE-O5 12B TRANS RT Event Identifier: 312/85-025 E-1 48
SEQUENCE CONDITIONAL PROBABILITIES Sequence End State Seq. Prob Non-Recott 101 TRANS -RT -AFW PORV.OR.SRV.CHALL -PORV.OR.SRY.RESEAT SS.RELE Cv 1.766E-04 I 1.76BE-01 AS.TERK HPI 102 TRANS -RT -AFW PORV.OR.SRV.CHALL PORV.OR.SRV.RESEAT -HP! HP CV 6.612E-06 2.964E-03 R/-HPI -SS.DEPRESS -LPR/-HPI.HPR 103 TRANS -RT -AFW PORV.OR.SRV.CHALL PORY.ORl.SRV.RESEAT -HP! HP ED 1.342E-05 I 2.964E-03 RI-HPI -SS.DEPRESS LPR/-HPI.HPR 104 TRANS -RT -AFW PORV.OR.SRV.CHALL PORV.OR.SRV.RESEAT -HPI HP CD 7.462E-07 2.964E-03 R/-HPI SS.DEPRESS 123 TRANS -RT AFN MFW -HPItF/B) HPR/-HPI -SS.DEPRESS COND/MFW CD 8.137E-07 6.26BE-04 126 TRANS -RT AFW NFW HPI(FIB) -SS.DEPRESS COND/MFW CD B.253E-07 5.030E-04 128 TRANS RT ATWS 3.0OOOE-05 I 1.200E-01 I dominant sequence for end state It 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:pwrdtree.cmp DATA: b~ranchpro.cmp No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail TRANS 1.030E-03 1.000E+00 LOOP 2.280E-05 3.400E-01 LOCA 4.170E-06 3.400E-01 RT 2.500E-04 1.200E-01 RT/LOOP 0.000E+00 1.O0OE+00 EMERB. POWER 2.8B50E-03 5.100QE-01 AFW 1.9 19E-03 2.700E-01 AFW/EMERB. POWER 5.OOOE-02 3.400E-01 IIFW 2.OOOE-0l > 1.O00E+00 3.400E-01 >1.200E-01 Branch Model: l.OF.1 Train I Cond Prob. 2.OOOE-01 ) l.OOOE+00 PORY. OR. SRV. CHALL 8.OOOE-02 > 1.000E+00 l.000E+00 Branch Model: l.OF.1 Train 1 Cond Prob: B.OOOE-02 > 1.OOOE+OO PORV. OR. SRV. RESEAT 1.00OOE-02 5.OOOE-02 PORV.OR. SRV.RESEAT/EMER6.POWER 1.OOOE-02 5. OOOE-02 Event Identifier: 312/85-025 E- 149
- 55. RELEAS. TERN 1.500E-02 > 1.000E+00 3.400E-01 Branch Model: 1.0F.1 Train I Land Prob: i.500E-02 > 1.000E+00 SS. RELEAS. TERM/-IIFW 1.500E-02 > 1.OO0E+00 3.400E-01 Branch Model: lA.0F.
Train I Cand Prob: 1.500E-02 ) l.000E+00 HP I 3.OOOE-04 > 1.000E-03 5.200E-01 Branch Model: 1.OF.3 Train I Land Prob: 1.000E-02 Train 2 Cand Prob: 1.OO0E-01 Train 3 Land Prab: 3.OO0E-0l > I.000E+00 HP] (F/B) 3.000'E-04 > 1.000E-03 5.200E-01 4.OOOE -02 Branch Model- L.OF.3+opr Train I Land Prab: 1.000E-02 Train 2 Cand Prab: 1.000OE-01 Train 3 Cond Prab: 3.OOOE-0l > 1.000E+00 HPRI-HPI 3.OOOE-03 5.600E-01 4.OOOE-02 SS. DEPRESS 3.600E-02 1.OOOE+00 COND/MFW 1.OO0E+00 3.400E-01 LPI/HPI 2.OOOE-03 3.400E-01 LPR/-HPI.HPR 6.700E-01 1.000OE+00 LPR/HPI 1.OOOE-03 1.OOOE+00 111 forced JD) HARRIS 10-07-1986 1327: 07 Event Identifier: 312/B5-025 E-150