ML20149K480
| ML20149K480 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1982-009-00 | |
| Download: ML20149K480 (5) | |
Text
B.35-1 B.35 LER No. 339/82-009 Event
Description:
PORVs Inoperable due to Low Nitrogen Pressure Date of Event: March 8, 1982 Plant: North Anna 2 B.35.1 Summary On March 8, 1982, while cooling down for a refueling outage, the pressurizer PORVs were determined to be inoperable due to low nitrogen pressure. The increase in core damage probability (CDP), or importance, over the duration of the event is 4.3 x 10'. The base-case CDP over the duration of the event is 8.1 x 10"', resulting in an estimated conditional core damage probability (CCDP) of 8.5 x 10W.
B.35.2 Event Description On March 8, 1982, while the unit was in Mode 4 and cooling down to begin a refueling outage, the pressurizer power-operated relief valves (PORVs) were declared inoperable due to low nitrogen supply tank pressure.
The nitrogen tanks were refilled and the PORVs were restored to operability after about 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />. The PORVs were again declared inoperable due to inadequate nitrogen supply later in the same day, and the nitrogen tank was replenished again. A similar event occurred on March 10, 1982.
B.35.3 Additional Event-Related Information The licensee event report for this event indicates that the PORV nitrogen supply failures were caused by excessive use of nitrogen, excessive system leakage, and an inadequate makeup supply. The nitrogen supply is required to operate the PORVs when instrument air is unavailable.
B.52.4 Modeling Assumptions It was assumed that the PORV nitrogen system failures were latent during the prior operating cycle. Since the duration of the failures was not known, they were assumed to have existed for one-half of the annual operating cycle or 3,066 hours7.638889e-4 days <br />0.0183 hours <br />1.09127e-4 weeks <br />2.5113e-5 months <br />. It was assumed that the PORV nitrogen supply would have been required for PORV operation during postulated loss of offsite power (LOOP) events and that the PORVs would therefore not have been available for feed-and-bleed operations during LOOPs.
B.52.5 Analysis Results The increase in CDP, or importance, estimated for the event is 4.3 x 106. Adding this value to the nominal CDP in the unavailability period, 8.1 x 10', results in an estimated CCDP of 8.5 x 10'. The dominant LER No. 339/82-009
B.35-2 sequence, highlighted on the event tree shown in Figure B.35.1, involves a postulated transient, failure of auxiliary feedwater and main feedwater, and failure of feed and bleed.
LER No. 339/82-009
B.35-3
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Figure B.35.1 Dominant core damage sequence for LER 339/82-009 LER No. 339/82-009
B.35-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 339/82-009 Event
Description:
PORVs inoperable due to low nitrogen pressure Event Date: March 8. 1982 Plant: North Anna 2 UNAVAILABILITY. DURATION= 3066 NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 2.6E-02 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOOP 4.3E-06 Total 4.3E-06 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
216 loop -rt(loop) -ep afw -offsite.pwr.rec/-ep.and.afw FEED.BLEED CD 4.DE-06 2.4E-01 221 loop -rt(loop) -ep afw offsite.pwr.rec/-ep.and.afw FEED.BLEED CD 2.9E-07 2.4E-01
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
216 loop -rt(loop) -ep afw -offsite.pwr.rec/-ep.and.afw FEED.BLEED CD 4.OE-06 2.4E-01 221 loop -rt(loop) -ep afw offsite.pwr.rec/-ep.and.afw FEED.BLEED CD 2.9E-07 2,4E-01
- non-recovery credit for edited case Note: For unavailabilities, conditional probability values are differential values which reflect the added risk due to failures associated with an event. Parenthetical values indicate a reduction in risk compared to a similar period without the existing failures.
SEQUENCE MODEL: d:\asp\models\pwra8283.cmp BRANCH MODEL: d:\asp\models\nanna2.82 PROBABILITY FILE: d:\asp\models\pwr8283.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES LER No. 339/82-009
B.35-5 Branch System Non-Recov Opr Fail trans 8.OE-04 I.oE+O0 loop 1.6E-05 5.3E-01 l oca 2.4E-06 5.4E-01 sgtr 1.6E-06 I.OE+O0 rt 2.8E-04 I.OE-OI rt(loop) 0.OE+O0 1.OE+C0 Event Identifier: 339/82-009 afw 3.8BE -04 4.5E-01 afw/atws 4.3E- 03 I.OE+O0 afw/ep 5.OE- 02 3.4E-01 mfw 1.9E-01 3.4E-01 1.CE-03 porv.chall 4.OE- 02 I.OE+O0 porv .chal l/afw 1.DE+00 I.OE+O0 porv.chall/loop iDE-01 I.OE+O0 porv.chall/sbo 1.OE+00 1.0E+O0 porv. reseat 2.OE- 02 1.IE-02 porv. reseat/ep 2.OE- 02 ICOE+O0 srv.reseat(atws) iDE-0l 1.0E+O0 hpi 1.5E-03 8.9E-01 FEED. BLEED 2.DE-D2 > i.DE+OD
- 1.0E+O0 1.CE-02 Branch Model: 1.F.3+ser+opr Train 1 Cond Prob: 1.DE-02 Train 2 Cond Prob: iDE-0l Train 3 Cond Prob: 3.OE-01 Serial Component Prob: 2.OE-02 emrg. boration C.DE+00 1.0E+C0 1.0E-02 recov .sec.cool 2.OE-01 1.OE+C0 recov .sec.cool/offsite.pwr 3.4E-01 1.0E+C0 rcs.cooldown 3.OE-03 L.0E+O0 1.0E-03 rhr 2.2E-02 5.7E-02 1.0E-03 csr 7.5E-04 1.OE+O0 hpr 4.OE-03 1.0E+O0 ep 2.9E-03 8.9E-01 seal. loca 2.7E-01 1.OE+C0 offsite.pwr.rec/-ep.and.-afw 2.2E-01 1.OE+O0 offsite.pwr.rec/-ep.and.afw 6.7E-02 1.0E+C0 offsite.pwr.rec/seal loca 5.7E-01 1.0E+O0 offsite.pwr.rec/-seal .loca 7.OE-02 L.OE+O0 sg. iso. and. rcs. cool down 1.OE-02 I.OE-CI rcs.cool .below.rhr 3.OE-03 I.OE+O0 3.0E-03 prim. press. limited 8.8E-03 1.OE+O0
- branch model file
- forced LER No. 339/82-009