ML20135G727
| ML20135G727 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1990-018-00 | |
| Download: ML20135G727 (5) | |
Text
B-370 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.: 456/90-018 Event
Description:
SSPS train "B" trips reactor, AFW pump "A" trips Date of Event: September 29, 1990 Plant: Braidwood 1 Summary A spurious solid state protection system (SSPS) train "B" actuation caused a reactor trip, containment isolation, and safety injection. Following the reactor trip, one of the two AFW pumps tripped on low suction pressure. The conditional core damage probability estimated for this event is 2.1 x 10-5. The relative significance of this event compared to other postulated events at Braidwood is shown below.
LER 456/90-018 L iL IE-7 IE-6 1E-5I IE-4 1E-3 1E-2 I I 360h Trpi360h ItEJnvi I ~AFW unavail LOOPuavi precursor cutoff -J LO0FW +1 M TR AFW Event Description A SSPS train "B" actuation resulted in a "B" train safety injection (SI), containment isolation actions, and turbine and reactor trips. The running feedwater pumps did not trip and their motor-operated discharge valves did not close automatically, as was expected.
Investigation revealed that the failures of the feedwater system to perform as expected were due to the very short duration of the spurious actuation signal. The actuation signal was not maintained for sufficient time to cause operation of nonlatching auxiliary relays in the circuitry, which initiates feedwater pump trip and closure of the feedwater pump discharge valves.
The SI was reset and a cooldown was begun to place the plant in cold shutdown. About 7 h after trip, trouble-shooting of the SSPS resulted in an inadvertent SI and steamline isolation. The steamline isolation interrupted cooling via the steam dump valves, and
B-371 reactor coolant system (RCS) temperature began to increase. At about the same time, IlA auxiliary feedwater (AFW) pump tripped on low suction pressure and l B was started.
The operator then opened three power-operated relief valves (PORVs) to limit the RCS temperature increase. The steaniline isolation was reset, and the steam dump valves were returned to service. RCS temperature increased from about 340*F to 352 0 F before the increase was stemmed. This represented an inadvertent entry into mode 3 conditions.
An attempt was made to restart AFW pump IlA, but it tripped again. The operator closed the PORVs, experiencing some difficulty in closing one. The plant then proceeded to cool down to mode 5 in an uneventful fashion.
The failure of 1lA AFW pump was attributed to oscillation of AFW flow control valves while controlling at low flow rates. A modification was made to the flow control valves to eliminate the condition.
The spurious SSPS train "B" actuations were attributed by the utility to internal problems with the SSPS processing circuitry. While trouble-shooting did not positively identify a defective component, the most likely cause was believed to be in Safeguards Output Card A516. This card, along with one other output card, six logic cards, and a power supply, was replaced.
Additional Event-Related Information There are two AFW pumps at Braidwood. The 1A AFW pump is motor-driven and the 1B AFW pump is driven by a diesel engine. Either pump can provide sufficient flow to the steam generators.
ASP Modeling Assumptions and Approach This event has been modeled as a trip with one train of AFW inoperable. The potential inoperability of the SSPS, beyond the spurious actuation, has not been addressed.
Analysis Results The conditional probability of severe core damage estimated for this event is 2.1 x 1i-5.
The dominant sequence to core damage, highlighted on the following event tree, involves failure of AFW, failure to recover main feedwater (p--0.07), and failure of feed and bleed.
B-372 LTRAN T IS IAAW IICHAL AFITM I F PORV/
SRV PORV/
SRV RESEAT HPI HPR OR OE OPEN I SEO NO END STATE OK OK 11 CD 12 CD OK OK OK 13 CD 14 CD OK OK 15 CD (1) 16 CD 17 CD 18 ATWS (1)OK for Class D Dominant core damage sequence for LER 456M90-018
B-373 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 456/90-018 Event
Description:
Reactor trip due to SSPS trip plus AFW pump trip Event Date: 09/29/90 Plant: Braidwood I INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1 .DE+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 2 .1E-05 Total 2 .1ED05 ATWS TRANS 3.4E-05 Total 3.4E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
17 trans -rt AFW mfw hpilf/b) CD 1.OE-D5 1.5E-02 15 trans -rt AFW mfw -hpi(f/b) -hpr/-hpi porv.open CD 9.5E-06 1.8E-02 16 trans -rt AFW mfw -hpi (f/b) hpr/-hpi CD 1.1E-06 1.8E-02 18 trans rt ATWS 3.4E-05 1.2E-D1
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
15 trans -rt AFW mfw -hpi~f/b) -hpr/-hpi porv.open CD 9.5E-06 1.BE-02 16 trans -rt AFW mfw -hpiff/b) hpr/-hpi CD 1.1E-D6 l.BE-02 17 trans -rt AFW mfw hpi(f/b) CD l.OE-D5 1.5E-02 18 trans rt ATWS 3.4E-05 1.2E-01
- non-recovery credit for edited case SEQUENCE MODEL: c: \asp\1989\pwrbseal.cmp BRANCH MODEL: c:\asp\1989\braid.sll PROBAJ3ILITY FILE: c:\asp\1989\pwrbsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail Event Identifier: 456/90-018
B-374 trans 4.3E-04 1. 05+00 loop 1.6E-05 5.3E-01 loca 2.4E-06 4.3E-01 rt 2 .8E-04 1.2E5-01 rt/loop O.OE+OO 1. OE+00 eme ra *power 2.9E-03 B.05-01 AFW 1.3E-03 > 5.OE-02 2. 6E-01 Branch Model: l.OF.2+ser Train 1 Cond Prob: 2.OE-02 > Failed Train 2 Cond Prob: 5.OE-02 Serial Component Prob: 2 .8E-04 afw/emerg.power 5.OE-02 3.45-01 mfw 1.OE+OO 7.05-02 porv.or.srv.chall 4.OE-02 1. OE+00 porv.or. srv. reseat 2 .OE-02 1. IE-02 porv.or. srv. reseat/emerg .power 2 .OE-02 1. OE+00 seal. loca 2.7 E-O1 1.05+00 ep.rec(sl) 5.8E-01 1. OE+00 ep. rec 1.8E-02 1.OE+00 hpi l.OE-03 B.4-01 hpi (f/b) l.OE-03 6. 4E-01 1.05-02 hpr/-hpi 1.5E-04 1.05+00 1.0OE-03 porv.open 1 .OE-02 1. 05+00 4.OE-04 branch model file
- forced Minarick 08-06-1991 17:32:51 Event Identifier: 456/90-018