ML20135H302
| ML20135H302 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-017-00 | |
| Download: ML20135H302 (8) | |
Text
B-117 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.:-
Event
Description:
Date of Event:
Plant:
278/9 1-0 17, Control wiring for ADS/relief valves found damaged September 24, 1991 Peach Bottom 3 Summary Improperly installed insulation on the automatic depressurization system (ADS)/
safety relief valves (SRVs) resulted in damage to SRV control wiring. This condition existed throughout the refueling cycle. The high-pressure coolant injection (HPCI) system was also unavailable for periods of time during that interval. The conditional core damage probability estimated for this event is 3.3 x 10-4. The relative significance of this event compared to other postulated events at Peach Bottom 3 is shown below.
LER 278/91-017 IE-8 IE-7 1E-6 IE-5 1E-4 1-I 360 h precursor cutoff _j HPCI TLOFW RCIC
+ HPCJ Event Description Following shutdown for refueling in September. 199 1, three SRVs were removed for preventive maintenance. On examination, the control solenoid valve wiring insulation was found to be degraded on each SRV. The electrical cable insulation between the solenoid coil and its junction box was cracked and hardened on all three valves. A termination splice between the solenoid and the junction box was melted on two of the valves.
An investigation determined that the damage occurred because SRV insulation was improperly installed during the previous refueling-outage, which was completed in December 1989. The installed configuration left a significant portion of the piping
B-118 around the valve and adjacent to the solenoid uncovered. This allowed the solenoid and its wiring to experience temperatures in excess of 400'F. These high temperatures resulted in accelerated thermal aging -
qualified lifetimes were exceeded within 3 d after startup.
The solenoids for the eight remaining SRVs were operated and verified to function as expected under shutdown conditions. The valves were then removed and sent to a test facility to determine how well they could perform under accident conditions. Initial results of that testing indicated that some failures occurred, but final results were unavailable at the time that the LER and an associated Nuclear Regulatory Commission (NRC) inspection report were written.
The SRVs associated with the ADS system, in conjunction with low-pressure makeup sources, are intended to back up the HPCI system in maintaining vessel inventory during small-break loss-of-coolant accidents (LOCAs). The HPCI system was unavailable for an estimated total of -510 h during the 21-month period that the SRV control circuits were compromised (NRC Inspection Reports 50-277/91-33 and 50-278/91-33, dated December 23, 1991).
Additional Event-Related Information The SRVs at Peach Bottom are Target Rock two-stage, pilot-operated valves. As installed, high main steamline pressure will operate a pilot valve that will, in turn, operate the main valve in the unit to relieve steam to the suppression pool. An electrically operated solenoid valve can also be used to align compressed gas to open the main valve. This allows remote operation of the valve either by the operator or by the ADS.
ASP Modeling Assumptions and Approach Wiring for 8 of the 11I SRVs (including 4 of 5 dedicated to ADS) was shown to be still functional under normal conditions; the concern is that the valves might not have functioned under small-break LOCA conditions. As reactor core isolation cooling (RCIC) is assumed unable to provide sufficient makeup during a small-break LOCA in the Accident Sequence Precursor (ASP) models, HPCI provides the only reliable high-pressure source of makeup. Should HPCI be unavailable in this circumstance, the ADS system is required to rapidly depressurize the reactor vessel to permit makeup by low-pressure sources.
Small-break LOCAs may be classified into two categories: those involving relief valves that operate and fail to reseat correctly and those involving other reactor pressure boundary failures. Failures of the first type could be expected not to result in significant changes to the containment atmosphere. In accidents of this type, the
B-119 information available indicates that the ADS system would have worked.
Failures of the second type could result in release of steam to the containment atmosphere, and it is unclear whether the control circuits for the SRVs could have continued to function. For the purposes of this analysis, it was assumed that the SRVs would have failed in the event of a small-break LOCA other than those involving relief valve leakage.
The ASP Program estimates the small-break LOCA frequency at 3.3 x 10-6/h, with a nonrecovery probability of 0.5, and these values were used in this analysis. The ASP Program normally estimates a HPCI failure-on-demand probability of 0.029. In this case, however, data regarding HPCI unavailability during the event were available, which suggested that a higher value should be employed. As noted earlier, the HPCI system was unavailable for -510 h during the 21 months of operation with degraded SRVs. Assuming the plant was at power or in hot shutdown for 70% of the 21-month period results in an estimated HPCI unavailability of 0.047. (LER 278/91-017 reports an HPCI unavailability of -0.036 for the entire 21-month period.) The nonrecovery probability of 0.7 normally employed was used in this case.
This event was modeled as an unavailability of SRVs for the ADS function during a LOCA. HPCI was modeled as having a higher than usual failure probability. To estimate the relative significance of the event within a 1-yr observation period (the interval between precursor reports), a 1-yr unavailability period was utilized in the analysis (6132 h, assuming the plant was critical or at hot shutdown for 70% of the time).
Analysis Results The estimated core damage probability associated with this event is 3.3 x 10 -4. The dominant core-damage sequence, highlighted on the following event tree, involves a postulated LOCA with HPCI and ADS failures.
Additional information concerning this event is included in combined inspection reports 50-277/91-33 and 50-278/91-33 dated December 23, 1991.
B-120 LIOCA Rx IHPCI SRVa/
RHR RH RRS SHUT OR ADS U'CS LPC I (S
(RHR) IMODE I MODHEoR SEQ END NO STATE OK OK 71 CORE DAMAGE OK OK 72 CORE DAMAGE OK OK 73 CORE DAMAGE OK 74 CORE DAMAGE OK 75 CORE DAMAGE 76 CORE DAMAGE 77 CORE DAMAGE 96 ATWS Dominant core damage sequence for LER 278/91-017
B-121 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
278/91-017 Event
Description:
Control wiring for SRVs found damaged (TI LOCAs)
Event Date:
09/24/91 Plant:
Peach Bottom 3 UNAVAILABILITY, DURATION= 6132 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 3.4E+00 LOOP 2.3E-02 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANJS
- 0. OE+00 LOOP 0.OE+00 Total 0.OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
28 trans -rx.shutdown pcs/trans srv.chall/trans.-scram srv.close CD 1.5E-07 1.7E-01 fw/pcs.trans HPCI srv.ads 55 loop -emerg.power -rx.shutdown srv.chall/loop.-scram srv.close CD 1.3E-07 1.2E-01 (4PCI srv.ads 67 loop emerg.power -rx.shutdown/ep -ep.rec srv.chall/loop.-scram CD 9.7E-09 9.4E-02
-srv.close IIPCI rcic 69 loop emerg.power -rx.shutdown/ep -ep.rec srv.chall/loop.-scram CD 8.7E-09 1.3E-01 srv.close HPCI
~*non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
28 trans -rx.shutdown pcs/trans srv.chall/trans.-scram srv.close CD 1.5E-07 1.7E-01 fw/pcs.trans HPCI srv.ads 55 loop -emerg.power -rx.shutdown srv.chall/loop.-scram srv.close CD 1.3R-07 1.2E-01 HPCI srv.ads 67 loop emerg.power -rx.shutdown/ep -ep.rec srv.chall/loop.-scram CD 9.7E-09 9.4E-02
-srv.close HPCI rcic 69 loop emerg.power -rx.shutdown/ep -ep.rec srv.chall/loop.-scram CD 8.7E-09 1.3E-01 srv.close H4PCI Event Identifier: 278/91-017
B-122
- 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\\1989\\bwrcseal.cmp c: \\asp\\1989\\peach.sll c:\\asp\\1989\\bwr_csll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch trans loop loca rx. shutdown rx. shutdown/ep pcs/t rans srv.chall/trans.-scram srv.chall/loop.-scram srv -close emerg.power ep. rec fw/pcs.t rans fw/pcs. loca HPCI Branch Model:
l.OF.l Train 1 Cond Prob:
rcic crd srv.ads lpcs lpci (rhr) /lpcs rhr (sdc) rhr (sdc) /-lpci rhr (sdc) /lpci rhr (spcool) /rhr(sdc) rhr (spcool) /-lpci. rhr (sdc) rhr (spcool) /lpci.rhr(sdc) rhrsw
- branch model file
- forced Minarick 06-09-1992 0 9: 38:45 System 5.5E-04
- 1. 6E-05 3.3E-06
- 3. OE-05
- 3. 5E-04
- 1. 7E-01 1.OE+00 1.0E+00
- 3. 6E-02 1.4E-03 2.1E-01
- 4. 6E-01
- 1. OE+00 2.9E-02 > 4.7E-02 2.9E-02 > 4.7E-02
- 6. OE-02 l.OE-02 3.7E-03 3.OE-03 l.OE-03 2.1E-02 2.OE-02 1.OE+00 2.OE-03 2.OE-03 9.3E-02
- 2. OE-02 Non-Recov Opr Fail 1.OE+00 2.4E-01 5.OE-01 1.OE+00
- 1. OE+00
- 1. OE+00 1.OE+00 l.OE-400 l.OE+00 B.OE-01 l.OEi-00 3.4E-01 3.4E-01 7.OE-01 7.OE-0l l.OE+00
- 7. lE-01 3.4E-01 7.1E-01 3.4E-01 3.4E-01 l.0E+00 3.4E-01 3.4E-01 1.0E+00 3.4E-01 1.OE-02 1.OE-02 1.OE-03 l.OE-03 1.OE-03 2.OE-03 Event Identifier: 278/91-017
B-123 CONDITIONAL CORE DANAGE PROBABILITY CALCULATIONS Event Identifier:
278/91-017 Event
Description:
Control wiring for SRVs found damaged (non-RV LOCAs)
Event Date:
09/24/91 Plant:
Peach Bottom 3 UNAVAILABILITY, DURATION= 6132 NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOCA SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator 1.OE-02 Probability CD LOCA Total ATWS LOCA Total 3.3E-04 3.3E-04 0. OE+00 0.OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABI1LITY ORDER)
Sequence 77 loca -rx.shutdown HPCI SRV.ADS End State Prob N Rec**
CD 3.3E-04 3.5E-01
-t non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State 77 loca -rx.shutdown HPCI SRV.ADS CD non-recovery credit for edited case Note:
For unavailabilities, conditional probability values are differential values added risk due to failures associated with an event.
Parenthetical values indicate compared to a similar period without the existing failures.
SEQUENCE MODEL:
c:\\asp\\1989\\bwrcseal.cmp BRANCH MODEL:
c:\\asp\\1989\\peach.sll PROBABILITY FILE:
c:\\asp\\1989\\bwr-csll.pro P rob N Rec**
3.3E-04 3.5E-01 which reflect the a reduction in risk Opr Fail No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System 5.5E-04 Non-Recov l.OE+00 trans Event Identifier: 278/91-017
B-124 loop loca rx. shutdown rx. shutdown/ep pcs/t rans srv.chall/trans.-scram srv.chall/loop.-scram srv.close emerg.power ep. rec fw/pca.trans fw/pcs.loca I4PC I
Branch Model:
1.OF.1 Train 1 Cond Prob:
rcic crd SRV.ADS Branch Model:
1.OF.l+opr Train 1 Cond Prob:
lpcs lpci (rhr) /lpcs rhr (sdc) rhr (sdc) /-lpci rhrisdc) /lpci rhe (spcool) /rhr(sdc) rhr (spcool) /-lpci. rhr (adc rhr (spcool) /lpci.rhr(sad) rhrsw 1.6E-05 3.3E-06 3. OE-05 3.5E-04 1.7E-01 1.OE+00 1.02+00 3.G2-02 l.4E-03 2.1E-01
- 4. 6E-01 1.02+00 2.9E-02 > 4.7E-02 2.9E-02 > 4.7E-02 6.02-02 1.OE-02 3.7E-03
> 1.02+00 3.7E-03 > Failed 3.02-03 l.OE-03 2.1E-02 2.02-02 1.02+00 2.OE-03 2.02-03 9.3E-02 2.OE-02 2.4E-01 5.OE-01 1.02+00 1.02+00
- 1. OE+00 1.02-400 1.02+00 1.02+00 8.OE-01
- 1. OE+00 3.4E-01 3.4E-01 7.OE-01 7.OE-01 l.OE+00 7.1E-01 > 1.OE+OO 3.4E-01 7.1E-01 3.4E-01 3.4E-O1 1.02+00 3.42-01 3.42-01 1.02+00 3.42-01 1.02-02 1.02-02 1.OE-03 1.02-03 1.02-03 2.OE-03
- branch model file
- forced Minarick 06-09-1992 09:40:09 Event Identifier: 278/91-017