ML20156A255
| ML20156A255 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1989-012-00 | |
| Download: ML20156A255 (5) | |
Text
B-346 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No: 395/89-012 Event
Description:
Reactor trip causes trip of 3 other stations and an effective loss of offsite power Date of Event: July 11, 1989 Plant: Summer 1 Summary A loss of offsite power (LOOP) was initiated at Summer following a reactor trip that was caused by a main turbine trip from 100% power. The LOOP occurred when Summer tripped and caused three other generating stations to trip also. The loss of the four generating stations degraded the vital bus voltage to the point that the diesel generators started and sequenced on vital loads. The conditional core damage probability estimated for this event is 1.5 x 10-4. The relative significance of this event compared with other postulated events at Summer is shown below.
LER 395/89-012 1E-7 IE-6 1E-5 IE-4/ 1E-3 1E-2 Trip] L OF +
MTR AFW360 hAFW precursor cutoff Event Description On July 11, 1989, Summer was operating at 100% of rated power. An electrical technician accidently shorted the leads on a temperature converter in the "Generator Stator Cooling Water" cabinet. This blew an AC power fuse that, in turn, caused a loss of indication. Normally, if left unattended, this would run back the main generator to about 25% load; however, a turbine runback relay failed, and the generator remained above 80% load with a loss of stator water cooling indicated. Several minutes later the turbine automatically tripped. This initiated a reactor trip since power was greater than 50%.
When Summer tripped, it had an unusually high reactive load. This, plus the high grid
B-347 load and extremely sensitive settings on other plants' generator backup relays, caused three other generating stations to trip off also. This large perturbation on the grid caused a degraded grid voltage condition that lasted long enough to automatically start the diesel generators, shed ESF bus loads, and sequence on vital loads to the ESF buses supplied by the EDG. The diesel generators powered the ESF buses for 1 h and 35 min. At that time, normal power to the ESF buses was restored.
Additional Event-Related Information Summer's normal offsite source of power to the ESF 7200-V Bus A is from the Unit ESF transformers (two in parallel) supplied by the 115-ky switchyard. Alternate offsite power to this bus is from the 230-kY switchyard, stepped down through the emergency auxiliary transformer (EAT). In the event that all other sources of power are lost, emergency power to the bus is from EDG A. Summer's normal offsite source of power to the ESF 7200-V bus B is from the 230-kV switchyard, stepped down through the EAT. Alternate offsite power to this bus is from the Unit ESF transformers supplied by the 115-kY switchyard. In the event that all other sources of power are lost, emergency power to the bus is from EDG B. Note that the ESF buses A and B are supplied from offsite and never from the main generator. In this event, when the plant tripped and the other stations were lost, normal and alternate power supply to the buses was lost; however, the grid was still supplying power to the buses but at a reduced voltage. The diesel generators have an autostart signal when a reduced bus voltage is sensed for a fixed period of time; thus, when the timer ran out, as in this case, the diesels started and assumed their respective loads.
ASP Modeling Assumptions and Approach This event has been modeled as a grid-related loss of offsite power.
Analysis Results The conditional probability of severe core damage estimated for this event is 1.5 x 1-The dominant sequence associated with the event involves station blackout: failure to recover offsite power in the short term (p = 0.48), failure of emergency power (p = 2.3 x 10-3), a subsequent RCP seal LOCA (p = 0.21) prior to restoration of AC power, and failure to recover AC power (p = 0.45) prior to core uncovery due to the seal LOCA. This sequence is highlighted on the following event tree.
B3-348 ILOOPIRTILOOPI EP AFW I1SR V /
PORV SRV I SE PORVI SA I EPREC IHF4 (LONG) I HPR PORV IOPENI SEO NO END STATE IL CHALI. RESEAT *OC OK OK 41 CO 42 CO OK OK 43 CD (1) 44 CO 45 CO OK 46 CO 47 CO 48 CO OK 49 CO 50 CO OK 51 CO 52 CO 53 CO OK 54 CO 55 CO 40 ATWS (1) OK for Class D Dominant core damage sequence for LER 395/89-012
B-349 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 395/89-012 Event
Description:
Reactor trip causes effective loss of offsite power Event Date: 07/11/89 Plant: Summner 1 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 4.8E-01 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD LOOP 1. 5E-04 Total 1. 5E-04 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 9.8E-05 3.8E-01 SEAL.LOCA EP.REC(SL) CDý 54 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall - 2. 7E-05 3.8SE-01 SEAL.LOCA EP.REC CD 55 LOOP -rt/loop emerg.power afw/emerg.power 1.9E-05 1. 3E-01.
CD 48 LOOP -rt/loop emerg.power -afw/emerq.power porv.or.srv.chall - 3. 9E-0 6 3. 8E-01 porv.or.srv.reseat/ernerg.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.power -afw/emerg.power porv.or.srv.chall - CD 3.9E-0 6 3. BE-01 porv.or.srv.reseat/emerg.power SEAL.LOCA EP.REC(SL) 53 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall CD 9.8E-05 3.8E-01 SEAL.LOCA EP.REC(SL) 54 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall - CD 2. 7E-05 3. 8E-01 SEAL.LOCA EP.REC 55 LOOP -rt/loop emerg.power afw/emerg.power CD 1. 9E-05 1. 3E-01
- non-recovery credit for edited case SEQUENCE MODEL: c:\asp\1989\pwrbseal.cmp BRANCN MODEL: c:\asp\1989\sumnmer.sll PROBABILITY FILE: c:\asp\1989\pwr7bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Rec ov Opr Fail trans 5.9E-04 1.OE+00 LOOP l.GE-05 > 1.6E-05 5.3E-01 > 4.BE-Ol Branch Model: INITOR Event Identifier: 395/89-012
B-350 Initiator Freq: 1.6EE-05 loca 2.4E-06 4. 3E-01 rt 2.8BE-04 1.2E-01 rt/ loop 0.OE+00 1.00E+00 emerg. power 2. 9E-03 8. 0E-01 a fw 3.8BE-04 2. 6E-01 a fw/ eme rq -power 5. OE-02 3.4E-01 mfw 1. 0E+00 7. 00-02 porv.or.srv.chall 4. 0E-02 1. 00+00 porv.or.srv. reseat 3.0OE-02 1. 1E-02 porv.or.srv. reseat/emerg.power 3.0OE-02 1.OE+00 SEAL.-LOCA 2.7E-01 > 2.1E-0l 1. OE+00 Branch Model: l.OF.l Train 1 Cond Prob: 2.7E-01 > 2.1E-01 EP.REC (OL) 5.7E-01 > 4.5E-0l 1.00+00 Branch Model: l.OF.l Train 1 Cond Prob: 5.7E-0l > 4.5E-01 EP.REC 7.OE-02 > 3.3E-02 1. 00+00 Branch Model: l.OF.l Train 1 Cond Prob: 7.0E-02 > 3.3E-02 hpi 1.5E-03 8.4E-01 hpi (f/b) 1.5E-03 8.40-01 1 .00-02 hpr/-hpi 1.5E-04 1. 00+00 1.00-03 porv .open 1. 0E-02 1. 00+00 4 OE0-04
- branch model file
- forced Minarick 06-13-1990 12:20:17 Event Identifier: 395/89-012