ML20140A226
| ML20140A226 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 05/19/2020 |
| From: | NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-002-00 | |
| Download: ML20140A226 (6) | |
Text
B-7 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.: 029/91-002 Event
Description:
Loss of offsite power caused by lightning strike Date of Event: June 15, 1991 Plant: Yankee Rowe Summary Yankee Rowe lost offsite power for 24 min due to a lightning strike. All three emergency diesel generators (EDGs) operated as designed. As a result of the lightning, surge protection fuses from the normal DC supplies. blew on both vital power supply inverters.
Both inverters transferred to their alternate (EDG-backed) AC sources. However, in the event of failure of the EDGs, 120-VAC instrument. p~wer would have been lost.
The conditional probability of core damage associated with this event is 6.1 x 10-4 The relative significance of the event compared to other postulated events at Yankee Rowe is shown below:
Event Description Yankee Rowe experienced a lightning strike that caused a total loss of offsite AC power on June 15, 1991. The lightning strike (1) destroyed the phase A lightning arrestor on station service transformer (SST) 3, which is connected to the Cabot (Y-177) 115-ky transmission line, and (2) caused a flashover of an insulator on phase A of the Harriman (Z-126) 115-ky transmission line disconnect switch. Offsite AC power was lost for 24 min.,
An automatic reactor scram and turbine trip occurred as a result of the loss of offsite
B-8 power. All three EDGs operated as designed. EDGs 1 and 3 started automatically in response to the deenergization of both offsite transmission lines. EDG 2 was manually started by operators in anticipation of securing the main generator in accordance with plant procedures. Lightning also caused blown surge protection fuses in the normal DC input supplies to both vital buses. Upon deenergization, both vital bus inverters automatically transferred to and were energized by their backup sources (ED~s 1 and 3).
The nonessential uninterruptible power supply (NEUPS) failed to automatically transfer to its backup source after the lightning strike. Consequently, various plant communication systems experienced failures. The plant commercial phone system failed to operate after the NEUPS was reenergized due to lightning-induced failures of two critical circuit packs.
While attempting to realign the emergency buses to offsite power, an inadvertent safety injection (SI) actuation signal was initiated. No actual injection occurred due to adequate main coolant system pressure.
An unusual event was declared due to the loss of offsite power and a fire emergency caused by the smoldering lightning arrestor on SST 3. The unusual event was elevated to an alert based on the continued inoperability of communication systems, the deenergized NEUPS, and the existence of degraded plant equipment.
Additional Event-Related Information Yankee Rowe has two independent sources of offsite power, the 115-ky Z-126 transmission line from the Harriman station, and the 115-ky Y-177 transmission line from the Cabot station. Normal operation is with both of these lines in service. Z-126 feeds the SST 2 and Y-177 feeds the SST 3. The SST 1 is connected to the outdoor section of the generator bus.
The station service system consists of three 2400-V buses, each supplied from an SST.
Each 2400-V bus, in turn, supplies a 480-V station service switchgear bus. Three emergency 480-V buses (vital bus 1, vital bus 2, and transformer A bus) are fed independently from the 480-V station service buses or, upon loss of AC power, from each of the three independent EDGs. Backup DC power consists of three 125-VDC station batteries and three associated battery chargers.
ASP Modeling Assumptions and Approach The event has been modeled as a plant-centered loss of offsite power (LOOP).
Probabilities for LOOP nonrecovery (short term) and failure to recover AC power prior to battery depletion were revised to reflect values associated with a plant-centered LOOP
,B-9 (see ORNL/NRC/LTR-89/1 1, Revised LOOP Recovery and PWR Seal LOCA Models, August 1989). Both trains of 120-VAC vital power were assumed to be unavailable if all three EDGs failed to start or run (only the backup source to the vital buses was available since the DC fuses supplying power to both inverters failed open after the lightning strike). Unavailability of both instrument buses was assumed to proceed to core damage, since steam generator (SG) and reactor coolant system (RCS) parameters would be unavailable for monitoring and control.
Analysis Results The conditional probability of core damage estimated for this event is 6.1 x 10 -4. The dominant core damage sequence, highlighted on the following event tree, involves a station blackout. In the event that RCS and SG parameters can be successfully monitored without 120-VAC power to allow decay heat removal, the conditional core damage probability estimated for the event is 4.3 x 10-4
B-10 LO TLO POR V / I PO RV / E L I EP M IP R SEO END EP IAFW ISRV SRV HPI HPR OPEN NO STATE I ICHALL RESEAT LOCA (LONG) OPEN OK OK 41 C 42 CD OK OK 43 CO 44 CD 45 CD 46 CD 40 ATWS Dominant core damage sequence for LER 029/91-002
B-li CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 029/91-002 Event
Description:
LOOP and degraded instrument power caused by lightning Event Date: 06/15/91 Plant: 029/91-002 INITIATING EVENT NON-RECOVERAB3LE INITIATING EVENT PROBABILITIES LOOP 5 O0E-01 SEQUENCE CONDITIONAL PROBABI1LITY SUNS End State/Initiator Probability CD LOOP 6.*1E-04 Total 6. 1E-04 ATWS LOOP 0 .OE+00 Total 0 .OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State P rob N Rec**
46 LOOP -rt/loop emerg.power CD 2 .2E-04 4 .OE-0l 45 LOOP -rt/loop -emnerg.power afw/emerg.power hpi(f/b) CD 2 .OE-04 1.4E-01 43 LOOP -rt/loop -emerg.power afw/emerg.power -hpi(f/b) -hpr/-hpi CD 1 .7E-04 1.7E-01 porv.open 44 LOOP -rt/loop -emerg.power afw/emerg.power -hpi(f/b) hpr/-hpi CD 1.7E-05 1.7E-01
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
43 LOOP -rt/loop -ereerg.power afw/emerg.power -hpi(f/b) -hpr/-hpi CD 1.7E-04 1.7E-01 p0 rv. open 44 LOOP -rt/loop -emerg.power afw/emerg.power -bpi(f/b) hpr/-hpi CD 1.7E-05 1. 7E-01 45 LOOP -rt/loop -emerg.power afw/emerg.power hpi(f/b) CD 2.OE-04 1.4E-01 46 LOOP -rt/loop emerg.power CD 2.2E-04 4. OZ-01
- non-recovery credit for edited case SEQUENCE MODEL: c:\asp\1989\02991002.cmp BRANCH MODEL: c:\asp\1989\yrowe.sll PROBAB3ILITY FILE: c:\asp\1989\pwr~bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Event Identifier: 029/91-002
B-12 Branch System Non-Recov Opr Fail trans 2.5E-04 1.02+00 LOOP 1.6E-05 > 1.6E-05 5.3E-01 > 5.02-01 Branch Model: INITOR Initiator Freq: 1. 6E-05 loca 2.4E-06 4.32-01 rt 2.8E-04 1.2E-01 rt/loop 0.02+00 1.02+00 ernerg.power 5. 4E-04 8.02-01 a fw 1.3E-03 2. 6E-01 afw/emerg.power 1.02-01 3.42-0 1 mfw 2.02-01 3.42-0 1 porv.or. srv.chall 4.OE-02 1.02+00 porv.or. srv.reseat 2.OE-02 1.1E-02 porv.or. srv. reseat/emerg.power 2.OE-02 1.02+00 seal. loca 0. OE+00 l.OE+00 ep. rec 4sl) 0. 02+00 1.02+00 EP.REC 1.7E-01 > 1.1E-01 l.OE+00 Branch Model: 1.OF.l Train 1 Cond Prob: 1.7E-01 > 1.1E-01 hpi 3.02-0 4 8.42-01 hpi (f/b) 2.4 -03 8 .4E-01 1.02-02 hpr/-hpi 1.5E-05 1.02+00 1.02-03 porv.open 1.02-02 1.02+00 4.02-04
- branch model file
- forced Minarick 06-11-1992 20:07:53 Event Identifier: 029/91-002