ML20149K340
| ML20149K340 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1986-001-00 | |
| Download: ML20149K340 (5) | |
Text
PRECURSOR DESCRIPTION SHEET LER No.:
285/86-001 Event
Description:
Trip occurs, and automatic depressurization and turbine bypass sytem fails to open Date of Event:
July 2, 1986 Plant:
Ft. Calhoun EVENT DESCRIPTION Sequence At 0534 h, during normal operation while the reactor was at 100%
power, an instrument inverter trouble alarm was received in the control room.
Control room operators quickly diagnosed a failed instrument inverter feeding bus AI-40A.
They dispatched an equipment operator to the switchgear room to reenergize the bus manually by closing the breaker on a bypass transformer also feeding bus AI-40A.
The inverter failure placed the RPS in a half-trip condition because the RPS operates on a two-out-of-four logic and the failed inverter was one of four feeding the independent channels of the RPS.
About 10 s after the inverter failure, a reactor trip occurred when a second channel trip was received on the SG B low-level trip unit.
Several unusual transients were noted in the moments following the trip:
- 1. RCS pressure increased to -2400 psia for a short period of time.
This caused PORVs to be actuated.
- 2.
SG pressure increased to the set point of the secondary safety valves, causing them to be actuated.
- 3.
Overfeeding the SG resulted in abnormally high level and subsequent overcooling of the primary system.
As a result, RCS pressure decreased to a low of -1725 psia.
The overfeeding occurred because the main feed regulating valves failed to ramp down; the failure was due to loss of power to a relay when the inverter failed.
- 4.
Steam dump and bypass valves could not be opened because the in-verter power was lost to their controllers as well as to a relay that causes the dump valve to open.
- 5.
The operating charging pump stopped, and the two backup pumps could not be started because of loss of inverter power to the relay that controls the backup pump's operations.
Although the operating pump should not have stopped, for an unknown reason it did.
Within 1 min of the reactor trip, the equipment operator had reenergized the lost instrument bus, and control room operators were soon able to restore the plant to normal shutdown condition.
Event Identifier:
285/86-001 D-82
A diagnosis of the information revealed the following.
The deener-gized instrument bus AI-40A supplies power to electrohydraulic-control panel AI-50 with no alternate power.
A turbine first-stage pressure transmitter that sends a signal to the electrohydraulic-control load-control circuitry is powered from AI-50.
Loss of power caused a loss of signal to the load-control unit, resulting in the turbine control valves closing without a reactor trip.
This explains the high pressure seen in the primary system and the low SG level earlier in the transient.
Corrective Action Modification was made to the bus and inverter power transfer controls to provide backup power.
Plant/Event Data Systems Involved:
Atmospheric steam dump, turbine bypass, charging, Components and Failure Modes Involved:
Inverter -
failed in operation Automatic depressurization and turbine bypass demand Charging -
failed in operation and electrical system -
failed on Component Unavailability Duration:
Plant Operating Mode:
1 (100% power)
Discovery Method:
Operational event Reactor Age:
12.9 years Plant Type:
PWR Comments None MODELING CONSIDERATIONS AND DECISIONS Initiators Modeled and Initiator Nonrecovery Estimate Transient 1.0 No recovery Branches Impacted and Branch Nonrecovery Estimate SS depressuri-zation 0.34 Recoverable locally at the valves Plant Models Utilized PWR plant Class G Event Identifier:
285/86-001 D-83
CONDITIONAL CORE DAMABE PROBABILITY CALCULATIONS Event Identifiers 295/86-001 Event
Description:
Trip and ADS/TBS/ Fails to Open Event Dates 7/2/Sb Plant:
Fort Calhoun INITIATING EVENT NON-RECOVERABLE INITIATIN6 EVENT PROBABILITIES TRANS SEQUENCE CONDITIONAL PROBABILITY SUMS End State/initiator I.OE+00 Probability CV TRANS Total
- 1.
SE-Ob 1.3E-06 CD TRANS Total
- 4.
IE-05
- 4.
1E-05 ATWS TRANS Total 3.4E-05
- 3.
4E-05 DONINANT SEQUENCES End State:
CV Conditional Probability:
1.3E-06 101 TRANS -RT
-AFW PORV.OR.SRV.CHALL -PORV.OR.SRV.RESEAT SS.RELEAS.TERM HPI End State:
CD Conditional Probability:
4.OE-05 102 TRANS -RT -AFW PORV.OR.SRV.CHALL PORV.,R.SRV.RESEAT -HPI HPR/-HPI End State:
ATIS Conditional Probability:
3.4E-05 Event Identifier: 265/86-001 D-84
121 TRANS RT SEQUENCE CONDITIONAL PROBABILITIES Sequence End State Prob N Rec**
101 TRANS -RT -AFN PORV.OR.SRVCHALL -PORV.OR.SRV.RESEAT SS.RELE AS.TERM HPI 102 TRANS -RT
-AFW PORV.OR.SRV.CHALL PORV.OR.SRV.RESEAT -HPI HP R/-HP]
115 TRANS -RT AFW MFW -HPI(F/B)
HPR/-HPI -SS.DEPRESS -CONDJMFW 118 TRANS -RT AFW MFW HPI(F/B) -SS.DEPRESS -COND/MFW 121 TRANS RT CV CD 1.3E-06
- 2.9E-01 4.OE-05
- 5.OE-02 CV CV ATWS
- 1.
IE-07
- 1.
2E-07 3.4E-05 3.9E-02 3.2E-02 1.2E-01
- dominant sequence for end state
- non-recovery credit for edited case SEQUENCE MODEL:
BRANCH MODEL:
PROBABILITY FILE:
c:\\asp\\newmodel\\pwrqtree.cup c:\\asp\\newmode]\\calhoun.txt c:\\asp\\newmodel\\pwrb.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch TRANS LOOP LOCA RT RT/LOOP EMERS, POWER AFW AFW/EMERG.POWER MFW PORV.OR.SRV.CHALL Branch Model:
I.OF.1 Train I Cond Prob:
PORV.OR.SRV.RESEAT PORV.OR.SRV.RESEAT/EMERS.POWER SS.RELEAS.TERM SS.RELEAS. TERM/-MFW SS.DEPRESS Branch Model:
1.OF,1 Train I Cond Prob:
COND/MFW HP!
HP!(F/B)
Event Identifier: 285/86-001 System Non-Recov Opr Fail
- 4. BE-04 4.6E-06 2,4E-06 2.BE-04 O.OE+00 5.4E-04 3.8E-04 5.OE-02
- 2. OE-01 4.OE-02 > I.OE+O0 '4 4.OE-02 2.0E-02 2.0E-02 1.5E-02 i.5E-02 3.6E-02 > I.OE+00 3.bE-02 > Failed
- 1.
OE40C 3.0EE-04 3.0E-04 1.0E+00 3.9E-01 4.3E-01 1.2E-01 1.OE+00 S.OE-01 2.6E-O0 3.4E-01 3.4E-01 1.OE+00
- 5. OE-02 1.OE+O0 3.4E-01 3.4E-01 J.OE+00 > 3.4E-01 3.4E-01
- 8.
4E-01 B.4E-01 4.OE-02 D-85
PORV.OPEN HPR/-HPI CSR
- branch model file
- - forced Austin 09-11-1987 1225:00 Event Identifieri 285/86-001 1.0E-02
- 1.
- 2.
OE-03 1.OE+00 1.OE+00 3.E-01 D-86