ML20156A257
| ML20156A257 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-011-01 | |
| Download: ML20156A257 (6) | |
Text
B-8 8 Accident Sequence Precursor Program Event Analysis LER No:
Event
Description:
Date of Event:
Plant:
280/88-011 RI Inoperable PORVs due to incorrectly torqued bolts and screws April 15, 1988 Surry Unit 1 Sunmmary During RCS depressurization, two power-operated relief valves failed to manually open on demand.
PORV failure was caused by improperly torqued bolts and
- screws, which allowed the actuator diaphragm to shift.
Unavailability of the PORVs results in unavailability of feed and bleed, which can provide core cooling in the event of loss of all feedwater.
The core damage estimate for this event is calculated at 1.5.
10-5.
The relative significance of this event compared to other potential events at Surry 1 is shown below.
1 E-8 1 E-7 1 E-6 LER 280/88-011 RI IE-5 1 E-4 1E-31E2 IFI I E-2 Trip Trip w/1 AFW Train Unavail 11 II EP Unavail LOOP (360 hrs)
AFW Unavail (360 hrs)
Event Description At 0505 on April 15, 1988, Surry 1 was in cold shutdown with the reactor coolant temperature at 130'F and pressure at 40 psig.
When the RCS tem-perature is below 350'F, Technical Specifications require that both PORVs be operational to provide relief capability to minimize pressure transients.
During routine RCS depressurization operations, power-operated relief valves PCV-1455C and PCV-1456 failed to manually open when operators tested the valves.
The operators unsuccessfully attempted to open both PORVs from their respective control room three-position selector switches by turning each switch from the AUTO to the
B-8 9 OPEN position.
Both valves were later opened by turning their switch from the CLOSE to the OPEN position.
Upon failure of the valves, both valves were declared inoperable and left open per plant Technical Speci-fication.
The cause of the PORV failures was a result of lack of procedural guid-ance regarding the correct torque values for the actuator diaphragm hold-down screws and bolts.
The improperly torqued screws and bolts allowed the actuator diaphragm to shift, resulting in intermittent PORV failure.
Event-Related Plant Design Information Two PORVs are provided on Surry 1.
These valves serve the following purposes:
PORVs are designed to lift prior to the safety valves, thereby preventing unnecessary safety valve challenges.
Each PORV is pro-vided with a series block valve, which can be closed to isolate the PORV if it fails open.
(Such a series valve is not allowed in a safety valve discharge path.)
PORV lift setpoints can be lowered to provide for overpressure protection during cold shutdown, when peak RCS pressure must be limited to values substantially below RCS design pressure.
PORVs, in conjunction with the safety injection system, provide feed and bleed capability that can be used to remove core decay heat in the event of unavailability of both main and auxiliary feedwater.
ASP Modeling Assumptions and Approach This event has been modeled as a loss of feed and bleed capability for the duration of one refueling cycle.
It is assumed that this problem existed at power and would not normally be detected except in a refuel-ing outage during PORV operational testing.
The plant was critical for approximately 11 990 h since the last refueling outage, which ended on July 17, 1986.
This is the plant vulnerability period assumed.
While operator action in the control room subsequently opened the valves (by placing the three-way switches first in the close position), the cause of the valve failures is believed to render such action unpre-dictable, and hence a nonrecovery likelihood of 1.0 for the valves has been conservatively assumed.
B-90 Analysis Results The conditional probability of core damage for this event is estimated to be 1.5 x 1i-5.
The dominant sequence for this event (highlighted on the following event tree) involves a postulated loss of offsite power occurring during the plant vulnerability period (p = 0.1),_a postulated failure of the auxiliary feedwater system (p =
9.9 x 1iO5), and PORV failure (which faults feed and bleed).
A sequence of almost the same significance involves a postulated LOFW with AFW failure and bleed and feed failure (again because of the faulted PORVs).
B-91 Ip A W I P VF P O R V" I
SA L
I P R GP.
OI R V LOOP RT/LOOP EP AF SRV SRV I
SEL EPC HI HR OE SR CHALL RESEAT LOCA (LONG)OPN SEO END NO STATE OK OK 41 CD 42 CD OK OK SE CD 43 GO 44 CD 45 GO OK 46 CD 47 GD 48 CD OK 49 CD 50 Go OK S1 CD 52 GO 53 GO OK 54 CD 55 CD 40 ATWS Dominant Core Damage Sequence for LER 280/88-011 Ri
B-92 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
Event
Description:
Event Date:
Plant:
280/88-011 Inoperable PORVs 04 /15/88 Surry 1 UNAVAILABILITY, DURATION-11990 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS LOOP LOCA SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator CD TRANS LOOP LOCA Total ATMO TRANS LOOP LOCA Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER) 4. 8E+00 1.0E-01 1.2E-02 Probability 6.2E-06
- 9. 1E-06 7.7E-08 1.5SE-O5 0.OE+00 0.
E+00 0.OE+00
- 0. OE+00 Sequence 43 loop -rt/loop -emerg.power afw -hpi(f/b) -hpr/-bpi PORV.OPEN 15 trans -rt afw rsfw -hpi(f/b) -hpr/-hpi PORV.OPEN End State Prob N Rec**
CD CD 9.1E-06 1.4E-Ol 6.2E-06 8.8E-02
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence 15 trans -rt afw rsfw -hpi(f/b).-hpr/-hpi PORV.OPEN 43 loop -rt/loop -emserg.power afw -hpi(f/b) -hpr/-hpi End State Prob N Rec-CD CD 6.2E-06 8.8E-02 9.1E-06 1.4E-01 PORV. OP EN
-* 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\\sealmod\\pwraseal.cmp c:\\asp\\sealmod\\surryl.s11 c:\\asp\\sealmod\\pwr_bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch trans loop 1 oca System 4.OE-04 1.6SE-OS 2.4E-06 Non -Recov 1.0 E-00
- 5. 3E-01
- 4. 3E-01 Opr Fail Event Identifier: 280/88-011
B-93 rt/ loop eme rg.power a fw afw/eme rg.power mfw porv.or.srv.chall porv.or.srv. reseat porv.or.srv.reseat/emerg.power seal.loca ep. rec (si) ep. rec hpi hpi (fib)
PORV. OPEN Branch Model:
l.OF.l+opr Train 1 Cond Prob:
hpr/-hpi car 2.8E-04
- 0. OE+00
- 2. 9E-03 3.8 E-04
- 5. OE-02
- 1. 9E-01 4.OE-02
- 2. OE-02 2.0E-02 2.7E-01 5.7E-01 7.0E-02 1.5E-03 1.5E-03 l.0E-02 > 1.00+00 1.00-02 > Failed 1.5E-04 9.3E-05 1.2E-01 1.00+00 8.00-01 2.60-01 3.40-01 3.4E0-01 1.00+00 1.1IE-02 1.00+00
- 1. 00+00 1.00+00 1.00+00 8.4E-01 8.4E0-01 1.00+00 1.00+00 1.00+00
- 1. 0E-02 4.0E-04 1.00-03
- branch model file
- forced Minarick 11-0 9-198 9 11:55:15 Event Identifier: 280/88-011