ML20147A533
| ML20147A533 | |
| 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 1988-007-00 | |
| Download: ML20147A533 (6) | |
Text
B-25.3 Accident Sequence Precursor Program Event Analysis LER No:
346/88-007 Ri Event
Description:
Component cooling valves drift closed on loss of air Date of Event:
March 4,, 1988 Plant:
Davis Besse Summary During maintenance, it was discovered that a prolonged loss of instru-ment air would cause three service water valves to close.
This closure results in isolation of service water to the component cooling water heat exchangers, which faults the heat removal capability of this sys-tem.
This event has been modeled as a potential unavailability of COW during a long-term loss of instrument air.
The conditional probability of core damage associated with the event is estimated to be 1.6 x 10-6.
The relative significance of this event compared with other potential events at Davis Besse is shown below.
LER 346/88-007 Ri IE-8 1lE-7 I E-6 1E-5 I E-4 lE-3 IE-2 Trip L
EP Unavail Trip w/I AFW L(X)P (360 hrs)
Train Unavail AFW Unavail (360 hrs)
Event Description During troubleshooting, operations personnel isolated instrument air from service water air-operated valve SW1434 (service water heat exchanger temperature control valve) with the understanding that the valve would remain in its fail-safe full open position.
Approximately 1.5 h after the valve was isolated from instrument air, a control room alarm indicated that the valve was not fully open.
The valve drifted closed due to air leakage of its accumulator system.
Further investiga-tion revealed five other valves with similar problems.
The valves were the temperature control valves (SW1424 and SW1429) for the other two component cooling water heat exchangers, the temperature control valves
B-254 (CC1467 and CC1469) for the decay heat removal heat exchangers, and the component cooling nonessential header isolation valve CC 1495.
Upon discovery of the potential failure mode of valves CC1467 and CC1469, valve CC1469 was placed in the open position by engaging its manual operator.
There was, however, no corrective action immediately identified for the remaining valves.
Long-term corrective action was identified to modify the valves' actuator design and implement the modi-fications during the sixth refueling outage.
Event-Related Plant Design information The air-operated valves impacted by this event utilize two safety-grade accumulators as a backup pneumatic supply source.
One accumulator allows the valve to open on demand by providing pressurized air to the bottom of the valve's actuator piston, forcing the valve open as air is vented from above the piston.
Similarly, when closing the valve, the other accumulator provides pressurized air to the top of the valve's actuator piston, forcing the valve to close as air is vented from below the piston.
Valves SW1424, SW1429, and SW1434 are the service water heat exchanger temperature control valves to the respective E22-1, E22-2, and E22-3 component cooling water heat exchangers.
The component cooling water loops serviced by these heat exchangers provide cooling to many compo-nents, some of which are shown below:
(1) the high-pressure-injection-bearing oil coolers (2) the decay-heat-pump-bearing housing coolers (3) the emergency diesel generator jacket heat exchangers (4) the make-up pump gear and pump lube oil coolers (5) the seal return coolers (6) the letdown coolers (7) the control rod drive coolers If the three valves close, component cooling water will be able to cir-culate; however, heat removal capability for the system would be lost and would eventually render the above components inoperable if not corrected.
Valves CC1467 and CC1469 are the temperature control valves for the respective E27-1 and E27-2 decay heat removal heat exchangers.
Closure of these valves would render both trains of decay heat removal inoperable.
However, closure of the CCW heat exchanger valves results in the same impact.
B-255 ASP Modeling Assumptions and Approach This event has been modeled a long-term unavailability of instrument air.
Loss of instrument air results in a loss of feedwater due to unavailability of air to the feedwater control valves.
The frequency of loss of instrument air was assumed to be 0.01/yr (this is consistent with valves assumed in plant-specific PRAs).
The probability of not recovering instrument air prior to depletion of the individual valve accumulators at -1.5 h was assumed to be 0.34.
In addition,, operator action to restore service water to the heat exchangers [p(nonrecovery) 0.34] has been included in the analysis.
Analysis Results The core damage frequency estimated for the event is 1.6 x 10-6.
This value was developed from the probability of loss of instrument air over a 1-year period (p =0.01), failure to recover instrument air before 1.5 h (p = 0.34), failure to recover 0GW once the valves have closed on loss of instrument air (p =
0.34, included in transient calculation),
and p(core damage ILOFW and loss of HPI, feed and bleed, and HPR).
The dominant sequence (highlighted on the following event tree) involves loss of secondary-side cooling and failure of feed and bleed.
Note that this assessment may be conservative in that relaxed system success cri-teria resulting from 1.5 h of successful decay heat removal has not been factored into the analysis.
B-256 IPORV/
POWV PORV TRANS RT AFW MFW SRV SRV HPI HPR OE IICHAL RESEAT OPENI SEQ END NO STATE 11 12 OK OK CD CD OK OK OK 13 CD 14 C
1s 16 17 18 OK OK CD CD CD ATWS Dominant Core Damage Sequence f or LER 346/88-007 Ri
B3-257 CONDITIONAL CORE DAMIAGE PROBABILITY CALCULATIONS Event Identifier:
34 6/B8-007 Event
Description:
Component cooling valves drift close on loss of air Event Date:
03/04/88 Plant:
Davis-Besse INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator 3.4E01I Probability CD TRANS Total ATWS 4.6E-01 4.6E-04 1.1E-05 1.1E0-O TRANS Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence 17 TRANS -rt afw MEW
((P1(F/B) 18 TRANS rt
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence 1,7 TRANS -rt afw MFM NPI(F/B) 18 TRANS rE
- non-recovery credit for edited case SEQUENCE MODEL:
c: \\asp\\sealmod\\pwrbseal.cmp BRANCN MODEL:
c:\\asp\\sealmod\\davis.sll PROBABILITY FILE:
c: \\asp\\sealmod\\pwr bsll.pro No Recovery Limit BRANCB FREQUENCIES/PROBABILITIES Branch System TRANS 6.3E-04 > 1.0E+00 Branch Model:
INITOR Initiator Freq:
6.3E-04 > 1.00+00 End State CO ATNO End State CD ATMS Non-Recov 1.00+00 > 3.40-01 2.4E-01
- 4. 3E-01 1.2E-01 1.00+00 8.00-01 2.70-01 2.7E-01 3.4E-01 > l.CEi00 Prob
- 4. 60-04
- 1. 10-05 P rob, 4.60-04 1.10-05 N Roc**
9.20-02 4.10-02 N Nec-9.20-02 4.10E-02 Opr Fail loop loca rt rt/io~op emerq.power a Lw afw/emerg.power MFN B3ranch Model:
1.DF.l Train 1 Cond Prob:
Event Identifier: 346/88-007
- 1. 60-05
- 2. 4E0-06 2.80-04 0.00+00 2.80E-0 3 5.00E-03 5.0OE-03 2.00-01 > 1.00+/-00 2.00-01 > Unavailable
B-258 porv.or.srv. chall porv.or.srv. reseat porv.or.srv.reseat/emerg.power seal.boca ep. rec (si) ep. rec 801 Branch Model:
1.OF.2 Train I Coed Prob:
Train 2 Coed Prob:
HPT (F/B)
Branch Model:
l.OF.2-+opr Train 1 Coed ['rob:
Train 2 Cond Prob:
RPR/-OPI Branch Model:
1.OF.2+opr Train 1 Coed Prob:
Train 2 Coed Prob:
porv.open branch model file Sforced Minarick 11-14-1989 13:13:18 8. OE-02 I.0OE-02 1.01-02 o.01+00 o.01+00
- 4. 5E-01 1.01-03 > 1.01+00 1.OE-02 > Unavailable 1.01-01 > Unavailable 1.01-03 > 1.01-00 1.01-02 > Unavailable I.01-01 > Unavailable 1.5E-04 > 1.01+00 1.01-02 > Unavailable 1.5E-02 > Unavailable 1.0OE-02 1.01*00 1.1IE-02 1.01+00
- 1. 0E+00 1.01+00 1.01+00 8.4E-01 > 1.01+10 8.4E-01 > 1.01+00 1.01*00 1.01+00 1.0E-02 1.0E-03 4.0E-04 Event Identifier: 346/88-007