ML20156A222
| ML20156A222 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1983-063-00 | |
| Download: ML20156A222 (5) | |
Text
B.31-1 B.31 LER No. 327/83-063 Event
Description:
Failure of Power Operated Relief Valve Date of Event:
April 21, 1983 through September 12, 1983 Plant:
Sequoyah 1 B.31.1 Summary On April 21, 1983, one of two power operated relief valves (PORVs) failed to open when operators attempted to reseat it, apparently because of a failed solenoid. The same PORV was found to be leaking through following valve maintenance on September 12, 1983. Failure of the PORV resulted in unavailability of feed and bleed. The increase in core damage probability (CDP) over the duration of the event, or importance, is 1.9 x 10'-.
The base-case CDP over the duration of the event is 6.0-k 10, resulting in an estimated conditiional core probability of 7.9 x 10"'.
B.31.2 Event Description On April 21, 1983, with the unit at power, one of two PORVs failed to open when operators attempted to reseat it. The associated block valve was closed and power removed. Following maintenance during an outage on September 12, 1983, with the unit in Mode 3, the same PORV was found to be leaking through. The block valve was again closed and power removed. The most probable cause for the first event was determined to be a failed solenoid coil. The second event was attributed to the valve not fully closing. No root cause for this condition was provided. To correct the problem, it was decided to replace the PORV during the next refueling outage.
B.31.3 Additional Event-Related Information None.
B.31.4 Modeling Assumptions The NUREG-1 150 analysis of Sequoyah 1 (NUREG/CR-4550, Vol. 5, Rev.1, Part 1) indicates that both PORVs are required for successful feed and bleed. Since one PORV was presumably unavailable, the feed and bleed branch was modelled by setting the failure probability of the serial component, i.e. the PORVs, to
- 1. The probabilities for failure of the PORVs to reseat once challenged (branch models PORV.RESEAT and PORV.RESEAT/EP) were also revised to reflect this. The Sequoyah final safety analysis report (FSAR) specifies a maximum time between PORV tests of 18 months or between each refueling shutdown, whichever occurs sooner. The date of the last refueling outage prior to the April 23, 1983 valve failure was not provided.
In the absence of this information, the time the PORV was estimated to be unavailable was assumed to be half the maximum time between required PORV tests, i.e., nine months or 6570 hours0.076 days <br />1.825 hours <br />0.0109 weeks <br />0.0025 months <br />. Since this is greater than LER No. 327/83-063
B.31-2 one year of reactor operation, the unavailability time was taken to be 6,132 hours0.00153 days <br />0.0367 hours <br />2.18254e-4 weeks <br />5.0226e-5 months <br /> (one reactor year, assuming the unit was critical 70% of the time). Transient (TRANS), loss-of-offsite power (LOOP), loss-of-coolant accident (LOCA), and steam generator tube rupture (SGTR) were used as potential initiators in the unavailability analysis.
B.31.5 Analysis Results The increase in CDP, or importance, estimated for the event is 1.9 x 105'. Adding this value to the nominal CDP in the unavailability period, 6.0 x 105' (not shown on calculation sheet), results in an estimated conditional core damage probability of 7.9 x 10i The dominant sequence, highlighted on the event tree shown in Figure B.3 1. 1, involves a postulated transient, failure of auxiliary feedwater and main feedwater, and failure of feed and bleed.
LER No. 327/83-063
B.31-3 0 0 w
wcc C-cc)0 000 w (0 ow J ww w-Ji U-0 (n a-w 0I a.0 0
0 00 0 0 0
Figure B.3 1.1 Dominant core damage sequence for LER 327/83-063 LER No. 327/83-063
B.31-4 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
Event
Description:
Event Date:
Plant:
327/83-063 PORV fails to open during attempt to reseat 9/12/83 Sequoyah 1 UNAVAILABILITY.
DURATION= 6132 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS LOOP LOCA SGTR SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator CD TRANS LOOP LOCA SGTR Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER) 9.6E+00 5.3E-02
- 7.
9E- 03 1.OE-02 Probability 1.1E-05 7.3E-06 9.OE-08 0.OE+O0 1.9E-05 Sequence End State Prob 120 215 219 229 trans -rt afw mfw FEED.BLEED loop -rt(loop) -ep afw -offsite.pwr.rec/-ep.and.afw FEED.BLEED loop -rt(loop) -ep afw offsite.pwr.rec/-ep.and.afw FEED.BLEED loop -rt(loop) ep -afw/ep porv.chall/sbo PORV.RESEAT/EP CD CD CD CD 1.1E-05 7.BE-06 5.6E-07 C 1.3E-06 N Rec**
1.5E-01 2.4E-01 2.4E-01 4.7E-01
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob 120 215 219 229 trans -rt afw mfw FEED.BLEED loop -rt(loop) -ep afw -offsite.pwr.rec/-ep.and.afw FEED.BLEED loop -rt(loop) -ep afw offsite.pwr.rec/-ep.and.afw FEED.BLEED loop -rt(loop) ep -afw/ep porv.chall/sbo PORV.RESEAT/EP CD CD CD CD 1.1E-05 7.8E-06 5.6E-07 C 1.3E-06 )
N Rec**
1.5E-01 2.4E-01 2.4E-01 4.7E-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:
BRANCH MODEL:
PROBABILITY FILE:
c:\\asp\\1982-83\\pwrb8283.cmp c:\\asp\\1982-83\\sequoyl.82 c:\\asp\\1982-83\\pwr8283.pro LER No. 327/83-063
B.31-5 No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System trans loop l oca sgtr rt rt(loop) afw afw/atws afw/ep mfw porv.chall porv.chall/afw porv.chal l/loop porv.chall/sbo PORV. RESEAT Branch Model:
1.0F.1 Train I Cond Prob:
PORV. RESEAT/EP Branch Model:
1.OF.1 Train 1 Cond Prob:
srv. reseat(atws) hpi FEED. BLEED Branch Model:
1.DF.3+ser+opr Train 1 Cond Prob:
Train 2 Cond Prob:
Train 3 Cond Prob:
Serial Component Prob:
emrg.borat ion recov.sec.cool recov sec.cool/offsite.pwr rcs.cooldown rhr rhr.and.hpr hpr ep seal loca offsite.pwr.rec/-ep.and. -afw offsite.pwr.rec/-ep.and.afw offsite.pwr.rec/seaI loca offsite.pwr.rec/-seal. loca sg. iso.and. rcs.cooldown rcs.cool.below.rhr prim. press.limi ted 1.6E-03 1.6E-05 2.4E-06 1.6E-06 2.8E-04 O.OE+00 3.8E-04 4.3E-03 5.0E-02 2.OE-01 4.0E-02 1.0E+O0 I.0E-01 1.OE+O0 2.0E-02 > 1.0E-02 2.0E-02 > 1.OE-02 2.0E-02 > 1.0E-02 2.0E-02 > 1.OE-02 1.0E-01 1.DE-05 2.DE-02 > 1.0E+O0 1.0E-02 1.DE-01 1.0E-02 2.0E-02 > 1.0E+00 O.OE+00 2.0E-01 3.4E-01 3.0E-03 2.2E-02 1.0E-03 4.0E-03 2.9E-03 2.7E-01 2.2E-01 6.7E-02 5.7E-01 7.0E-02 1.0E-02 3.0E-03 8.8E-03 Non-Recov 1.0E+O0 5.3E-01 5.4E-01 1.0E+00 1.0E-01 I.OE+00 4.5E-01 1.0E+00 3.4E-01 3.4E-01 1.0E+00 1.0E+O0 1.0E+00 1.OE+00 1.1E-02 1.0E+00 1.0E+00 8.9E-01 1.0E+00 1.0E+00 1.0E+00 1.0E+00 1.OE+0D 5.7E-02 1.0E+00 1.0E+O0 8.9E-01 1.0E+00 1.0E+00 1.0E+0D 1.0E+O0 1.0E+00 1.0E-01 1.0E+00 1.0E+00 Opr Fail 1.OE-03 1.0E-02 1.0E-02 1.OE-03 1.0E-03 1.0E-03 1.0E-03 3.DE-03 branch model file
- forced LER No. 327/83-063