ML20149K404
| ML20149K404 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-006-00 | |
| Download: ML20149K404 (5) | |
Text
B-117 Accident Sequence Precursor Program Event Analysis LER No: 309/88-006 Event
Description:
Loss of offsite power Date of Event: August 13, 1988 Plant: Maine Yankee Summnary A fault on an output transformer caused a momentary low-voltage condi-tion. This condition resulted in turbine and reactor trips and failure of automatic transfer of electrical power from the station service transformer to the reserve transformer. The emergency diesel generators started and loaded. One service water pump tripped when the DGs repowered the safety buses. A standby service water pump autostarted and supplied required loads. The core damage probability estimate for this event is 1.2 x 104 This event is considered significant from an ASP standpoint. The relative significance of this event compared with other potential events at Maine Yankee is shown below.
LER 309/88-006 1E-8 IE-7 1E-6 1E-5 1E-41E-1E3 IE-3 I E-2 LOOPI Tip EUnavail Trip w/1 AFW AFW Unavail (360 hrs)
Train Unavail (360 hrs)
Event Description While at 98% power, an internal phase to ground fault on the high side of the X-lA main output transformer propagated a voltage transient from the main generator output transformer, to the station service trans-former, and through the 345-ky grid back to the 115-ky reserve station transformer. This caused a simultaneous low-voltage condition on the station service and reserve station transformers. The coincident low-voltage condition prevented autotransfer of the source power to the reserve station transformer from the station service transformer. This caused a loss of offsite power to the unit. Both emergency diesel
B-118 generators started and loaded as designed. With no power to the reactor coolant pumps, the core was cooled by natural circulation. An intermittent failure of an overcurrent device in a circuit breaker caused a service water pump to trip after the generators repowered the vital buses. A standby service water pump autostarted on the trip of the primary pump and maintained service water flow. After the plant was stabilized, the reserve circuit breakers were manually closed, all buses were repowered from offsite sources, and a reactor coolant pump was started to reinstate forced circulation.
Event-Related Plant Design information Main generator output is provided to the Maine Yankee 345-ky distribu-tion switchyard via paralleled main transformers X-lA and X-lB. The switchyard initially supplies three other transmission systems, one of which is the Mason transmission line. The Mason line operates at 115-ky and provides two alternate transmission routes for Maine Yankee Reserve Station power. With the unavailability of both main and reserve power, emergency power was provided by two emergency diesel generators, DG-lA and DC-lB. Each of these generators provides power to their respective 4160-V emergency buses.
ASP Modeling Assumptions and Approach This event has been modeled as a grid-related loss of offsite power.
Since information regarding service water impacts was not readily avail-able in the FSAR and the standby service water pump started as designed, the potential impacts of a service water train, unavailability have not been modeled.
Analysis Results The conditional core damage probability for this event is 1.2 x 10-4.
The dominant sequence is a station blackout sequence involving failure to recover offsite power in the short term (p = 0.48), failure of emergency power (p = 2.3 x 10-3), subsequent RCP seal failure (p = 0.036), and failure to recover electric power prior to core uncovery (p = 0.44). This sequence is highlighted on the following event tree.
B-119 FI7 LOOP EP I AFW ISPRV' RVI
[JfrJCHALL I RTILOOPO ISORy SEAL IEPIEC POIRVIOP RESEAT LOCA (LONG)
HFI H PR 0 OEN SEC NO END STATE OK OK 41 CD 42 CD OK OK 43 Co 44 CD 45 Co OK 46 CD 47 COD 48 CD OK 49 CD 50 COD OK 51 COD 52 CO 53 CD OK 54 CD 55 CO 40 ATWS Dominant Core Damage Sequence for LER 309/88-006
B-120 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 309/88-006 Event
Description:
Momentary loss of offsite power Event Date: 08/13/88 Plant: Maine Yankee INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 4. 8E-01 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOOP 1.2E-04 Total 1. 2E-04 ATWS LOOP 0. 00+00 Total 0. OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State P rob N Rec**
54 LOOP -rt/loop emerg.power -a fw/eme rg. power -porv.or.srv.chall- CO 8. SE-OS 3. 8E-Ol SEAL.LOCA EP.REC 55 LOOP -ri/loop emerg.power afw/emerg.power CD 1 .9E0-O 1. 3E-01 53 LOOP -rt/loop emerg.power -afw/emerg.power -porv.or.srv.chall CO 1. 6E-05 3 . BE-01 SEAL.LOCA EP.REC(SL) 49 LOOP -rt/loop emerg.power -afw/emnerg. power porv.or.srv.chall- CD 3 . 3E-06 3. 8E-01 porv.or.srv.reseat/emerg.power -SEAL.LOCA EP.REC
-- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State P rob N Rec-*
49 LOOP -rt/loop emerg.power -afw/ernerg.power porv.or.srv.chall - CO 3.3E0-O 3. 8E-01 porv.or.srv.reseat/emerg.power -SEAL.LOCA EP.REC 53 LOOP -rt/ioop emerg.power -afw/ernerg.power -porv.or.srv.chall CO 1 .6E-05 3. 8E-01 SEAL.LOCA EP.REC(SL) 54 LOOP -rt/ioop emerg.power -afw/ernerg.power -porv.or.srv.chall - CO 8. OE-05 3. 8E-01 SEAL.LOCA EP.REC 55 LOOP -rt/ioop emerg.power afw/emerq.power CD 1. 9E-05 1.3E-01
--non-recovery credit for edited case SEQUENCE MODEL: a:\sealmod\pwrbseai.cmp BRANCH MODEL: a:\sealmod\mainyank.sll PROBABILITY FILE: a:\sealmod\pwr~bsll.pro Mo Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Rec ~ov Opr Fail trans 3.2E-04 1.OE+00 LOOP 2.OE-05 > 2.OE-05 5.BE-O1 -> 4 .8E-01 Branch Model: INITOR Event Identifier: 309/88-006
11-121 Initiator Freq: 2 .08-05 loca 2.4E-06 4 .3E-01 rt 2 .8E-04 1. 2E-01 rt/ loop 0. 08+00 1 .08+00 emerg. power 2 . 9E-03 8.0OE-01 afw 3. BE-04 2. 6E-01 afwlemerg .power 5. OE-02 3 .4E-01 mfw 1. 9E-01 3.48-01 porv .or .s rv .cha 11 4 .08-02 1.08+00 porv.or.srv. reseat 2.08E-02 1.1E-02 porv.or.srv.reseat/emerg.power 2.08E-02 1. 0 +00 SEAL.LOCA 5.58-02 > 3.68-02 1. 0 8+00 Branch Model: 1.08.1 Train 1 Cond Prob: 5.SE-02 > 3.68-02 EP.REC (SL) 6.5E-01 > 4.4E-01 1. 0 +00 Branch Model: 1.08.1 Train 1 Cond Prob: 6.5E-01 >4.4E-01 EP .REC 2.3E-01 > 8.08-02 1 .08+00 Branch Model: 1.0F.1 Train 1 Cond Prob: 2.3E-01 > 8.08-02 hpi 1.08E-03 8. 8E-01 hpi (fib) 1.08E-03 84 8E-01 1.08-02 hpr/-hpi 1 .58-04 1 .08+00 porv .open 0 .08+00 1.08E+00 0 .08*+00
- branch model file Sforced Minarick 11-14-1989 0 5:57 :3 3 Event Identifier: 309/88-006