ML20135G732
| ML20135G732 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1990-011-00 | |
| Download: ML20135G732 (5) | |
Text
B-252 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.: 317/90-011 Event
Description:
Inoperable pressurizer PORV Date of Event: March 30, 1990 Plant: Calvert Cliffs 1 Summary One of two power-operated relief valves (PORVs) for Calvert Cliffs 1 was found to be inoperable for up to 19 months. This inoperability resulted in bleed and feed being unavailable for the same time period. The conditional core damage probability estimated for this event is 1.1 x 10-5. The relative significance of this event compared to other postulated events at Calvert Cliffs 1 is shown below.
LER 317/90-011 IE-7 1E-6 1EL5 1E41E-3 1E-2 Trip] L0WJ LonopiLj L360ohEP I T F 360 hAFW precursor cutoff Event Description On March 30, 1990, PORV ERV-402 was found to have been inoperable for up to 19 months. The thermal overload contact was found open, which indicated that an overcurrent condition had existed for a sufficient period of time to cause the circuit to open. No indication of this open contact existed in the control room. The thermal overload was the result of excessive current draw resulting from a stuck roller for the movable solenoid slug.
The same valve had been found in this condition in April of 1988. An inadvertent actuation of this valve occurred on June 26, 1988, and was the last time that this valve was known to have been operable.
B-253 Additional Event-Related Information The POR~s were originally installed to protect the pressurizer safety valves from lifting during plant operation. The POR~s (total of 2) are also relied on for emergency once-through core cooling (feed and bleed cooling) in the event of a total loss of all feedwater.
In the event that the steam generators become ineffective for heat removal, the RCS is vented to containment atmosphere via the POR~s, and makeup is provided via the high-pressure safety injection (HPSI) pumps. The loss of a single PORY would limit flow to the point that once-through cooling could probably not be maintained.
The PORVs are also used for low-temperature overpressure protection during cold shutdown. For this function, only one of two PORVs must be operable.
ASP Modeling Assumptions and Approach The event has been modeled as a 1-yr unavailability of bleed and feed. (One year is the longest unavailability normally modeled.)
Analysis Results The estimated conditional probability of severe core damage for this event over a 1-yr period is 1.1 X 1O-5. The dominant sequence involves loss of secondary-side cooling and failure of feed and bleed following a postulated trip. The dominant sequence to core damage is highlighted on the following event tree.
B-254 TRANS RT 1 AFW WW SRV SRV HPI HPR PORV CSR SEO END M W CHALL P R RESEAT P R OPEN NW STATE OK OK 11 Co 12 CD OK OK OK 13 CD 14 CD OK OK 19 CO 15 CD 16 CD 17 COD Is A7WS Dominant core damage sequence for LER 317/90-011
B-255 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 317/90-011 Event
Description:
Inoperable pressurizer PORV Event Date: 03/30/90 Plant: Calvert Cliffs 1 UNAVAILABILITY, DURATION- 6132 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRAN S 3. 1E+00 LOOP 5.3E-02 LOCA 6.3E-03 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRAN S 6.3E-06 LOOP 4.9E-06 LO0CA 4 .2E-08 Total 1 .1E-05 ATWS TRANS D.DE+00 LOO0P D.OE+00 LOCA 0.OE+D0 Total 0.OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
15 trans -rt afw mfw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 6.3E-06 S.SE-02 43 loop -rt/loop -emerg.power afw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 4.9E-06 1.4E-01
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
15 trans -rt afw mfw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 6.3E-06 8.8E-02 43 loop -rt/loop -emerg.power afw -hpi(f/b) -hpr/-hpi PORV.OPEN CD 4.9E-06 1.4E-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: c: \asp\1989\pwrgseal.cmp BRANCH MODEL: c:\aap\1989\calvertl.all PROBABILITY FILE: c: \asp\1989\pwr~bsil.pro Event Identifier: 317/90-011
B-256 No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 5 .OE-04 1. OE+00 loop 1.6E-05 5.3E-01 loca 2 .4E-06 4.3E-01 rt 2 .8E-04 1.2E-01 rt/loap 0. OE-400 1. OE+00 emerg .power 2.9E-03 B. CE-0l afw 3.8E-04 2. GE-0l afw/emerg .power 5.OE-02 3.4E-01 mfw 2 .OE-01 3.4E-01 porv.or.srv.chall 4.OE-02 1. CE+CC porv.or.srv. reseat 2 .OE-02 1.1E-C2 porv.or. srv. reseat/emerg .power 2 .OE-02 1.CE+00 seal. loca 4.6E-02 1.0E+00 ep.rec (si) 5.7E-01 1. CE+00 ep. rec 1. 4E-02 l.CE+CC hpi 3.OE-04 8. 4E-01 hpi (f/b) 3.OE-04 8.4E-01 1.OE-C2 PORV.OPEN 1.OE-02 > 1.OE+00 l.CE+CC 4.OE-04 Branch Model: l.OF.l+opr Train 1 Cond Prob: l.OE-02 > Failed hpr/-hpi 1 .5E-04 1.CE+CC cs r 2.OE-03 3. 4E-01
- branch model file
- forced Nina rick 08-06-1991 17:32:30 Event Identifier: 317/90-011