ML20140A211
| ML20140A211 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 05/19/2020 |
| From: | NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 280-1991-017 | |
| Download: ML20140A211 (5) | |
Text
B- 147 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No: 280/91-017 Event
Description:
Both emergency diesel generators for Unit 2 inoperable for 13 h Date of Event: July 15, 1991 Plant: Surry 2 Summary Both emergency diesel generators (EDGs) were inadvertently out of service at Surry 2 for 13 h.
EDG 3, the dual-unit swing diesel, had been unavailable since May 7, 1991, because of inadequate post-maintenance testing. EDG 2 was removed from service for 13 h on July 15, 1991.
The conditional probability of core damage estimated for this event is 2.9 x 10-6. The relative significance of the event compared to other postulated events at Surry 2 is shown below.
Event Description On August 9, 1991, with Unit 1 and Unit 2 at 100% power, it was discovered that EDG 3 had been inoperable since May 9, 1991. The discovery was made while investigating the cause for EDG 3 failing to achieve rated speed during a Unit 2 engineered safeguards features actuation on August 2, 1991. This safety injection/reactor trip occurred as a result of vital bus po wer oscillations on one channel and a failed steam generator (SG) pressure transmitter on another channel. During this event, EDG 3 achieved a speed of approximately 835 rpm, which is below the 870 rpm permissive needed to allow the output breaker to close. Therefore, operator action would have been required to bring EDG 3 up to speed should it have been necessary for the EDG to supply power to its emergency bus.
The failure of EDG 3 was due to previous maintenance that began on May 7, 1991, during which a
B- 148 replacement governor was installed. Adjustments were made to the governor to correct observed problems, but these adjustments rendered EDG 3 incapable of achieving rated speed when called upon to respond to a fast start signal. The required post-maintenance testing had not been performed on May 9, 1991, to verify proper response of EDG 3 to a fast start. Such testing would have detected the failure.
EDG 2 was inoperable (reason unknown) for approximately 13 h on July 15, 1991. Therefore, no EDGs were available for Unit 2 for 13 h on July 15, 1991. 'Since it was not known that EDG 3 was inoperable from May 9, 1991, EDGs 1 and 2 (the dedicated diesels for Units 1 and 2, respectively) were not tested daily, nor were the units placed in cold shutdown within the Technical Specification-required 7-d period.
Additional Event-Related Information The emergency power system for Surry consists of three EDGs for the two units. EDG 1 is dedicated to Unit 1, EDG 2 is dedicated to Unit 2, and EDG 3 is a "swing" diesel that serves as a backup for either Unit 1 or Unit 2. Each EDG has 100% capacity and is connected to independent 4.1 6-ky emergency buses. Each unit has two emergency buses, "H" and "J", and the "H" bus for each unit is connected to its exclusive EDG. The "J" bus of the affected unit would be supplied by EDG 3.
ASP Modeling Assumptions and Approach The event has been modeled as a potentially recoverable unavailability of emergency power for 13 h. A nonrecovery probability of 0.12 was utilized. This reflects the potential for recovery from the control room under burdened conditions following a blackout (LER 280/91-018 reports a similar incorrectly set governor for Unit 2, which was adjusted from the control room).
Analysis Results The conditional probability of subsequent core damage estimated for this event is 2.9 x 10-6. The dominant core damage sequence, highlighted on the following event tree, involves a postulated loss of offsite power (LOOP) with failure of emergency power, successful auxiliary feedwater (AFW) initiation, a reactor coolant pump seal loss-of-coolant accident (LOCA), and failure to recover AC power before core uncovery.
B-149 LOOE AFW PORV/
SRV 1 PRV/ SELOR E END L(/O OPCHALL I ]J SV RE E TEN LOCA (LNR P HPR PR S SEO NO STATE OK OK 57 CO 41 CO 42 CD OK OK 58 COD 43 CO 44 CO 45 CO OK 59 CO 46 CO 47 CO 48 CO OK 49 CO 50 CO OK so CO 51 CO 52 CO 53 CO OK 54 CO 55 CO 40 ATWS Dominant core damage sequence for LER 280/91-017
B-150 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 280/91-017 Event
Description:
Both ED~s for Unit 2 inoperable for 13 h Event Date: 05/09/91 Plant: Surry 2 UNAVAILABILITY, DURATION- 13 NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 1.1lE-04 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOOP 2 .9E-06 Total 2.9E-06 ATWS LOOP 0 .OE+00 Total 0 .OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
53 loop -rt/loop EMERG.POWER -afw/emerg.power -porv.or.srv.chall CD 1.9E-06 6.3E-02 seai.loca ep.rec(sl) 54 loop -rt/loop EMERG.POWER -afw/emerg.power -porv.or.srv.chall - CD 6.4E-07 6.3E-02 seal.loca ep.rec 55 loop -rt/loop EMERG.POWER afw/emerg.power CO 2.2E-07 2.2E-02 48 loop -rt/loop EMERG.POWER -afw/emerg.power porv.or.srv.chall - CD 7.8E-08 6.3E-02 porv.or.srv. reseat/emerg.power seal. loca ep.rec (sl)
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec-*
48 loop -rt/loop EMERG.PDWER -afw/emerg.power porv.or.srv.chall - CD 7.8E-08 6.3E-02 porv.or .srv.reseat/emerg.power seal. loca ep.rec (si) 53 loop -rt/loop EMERG.POWER -afw/erserg.power -porv.or.srv.chall CD 1.9E-06 6.3E-02 seal.loca ep.rec(si) 54 loop -rt/loop EMERG.POWER -afw/emerg.power -porv.or~srv.chall - CO 6.4E-07 6.3E-02 seal.loca ep.rec 55 loop -rtt'loop EMERG.POWER afw/emerg.power CD 2.2E-07 2.2E-02
- 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.
Event Identifier: 280/91-017
B-151 SEQUENCE MODEL: c: \asp\l989\pwraseal .cmp BRANCH MODEL: c: \asp\1989\surry2 .s11 PROBABILITY FILE: c: \asp\1989\pwr bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 8 .6E-05 1. OE+00 loop 1. 6E-05 5 .3E-01 loca 2. 4E-06 4.3E-01 rt 2.8E-04 1.2Z-O1 rt/ loop O.OE+OO 1. OE+OO EMERG.-POWER 2.9E-03 > 1.OE+00 e.OE-Ol > 1.2E-01 Branch Model: 1.OF.2 Train 1 Cond Prob: 5.OE-02 > Failed Train 2 Cond Prob: 5.7E-02 > Unavailable a fw 3.8E-04 2.6E-01 afw/emerg.power 5 .OE-02 3 .4E-01 mfw 1. 9E-01 3. 4E-01 pqrv.or.srv.chall 4 .OE-02 1.OE+00O porv.or.srv. reseat 2.OE-02 1.1lE-02 porv.or. srv. reseat /emerg.power 2 .OE-02 1.OE+00 seal. boca 2 .7E-01 1.OE+00 ep. rec (sl) 5 .7E-01 1.OE+00 ep. rec 7.OE-02 l.OE+00 hpi 1.5E-03 8 .4E-01 hpi (f/b) 1.5E-03 8. 4E-01 l.0E-02 porv. open 1.OE-02 1. 0E+00 4. 0E-04 hpr/-hpi 1.5E-04 l.OE+00 1.OE-03 csr 9.3E-05 1. OE+00
- branch model file
- forced tMinarick 06-08-1992 16:15:17 Event Identifier: 280/91-017