ML20156A237
| ML20156A237 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-027-00 | |
| Download: ML20156A237 (6) | |
Text
B-177 Accident Sequence Precursor Program Event Analysis LER No:
Event
Description:
Date of Event:
Plant:
328/88-027 Ri Reactor trip following loss of main feedwater with turbine-driven AFW pump unavailable June 6, 1988 Sequoyah Unit 2 Summnary A reactor trip occurred because of a loss of main feedwater caused by closure of a feedwater control valve during testing as a result of a missing diode in a blocking circuit.
The turbine-driven auxiliary feed-water pump was also unavailable.
The conditional probability of severe core damage estimated for this event is 4.5.
10-6.
The relative significance of this event compared with other potential events at Sequoyah 2 is shown below.
1 E-8 LER 328/88-027 RI]
I II I E-4 I E-3 IE-2 Trip AEW Unavail IL EP Unavail (360 hrs)
(360 hrs)
LOOP Trip w/1 AFW Train Unavail Event Description At approximately 14:15 EDT with unit 2 at 98% power, a reactor trip occurred due to a steam/feedwater flow mismatch coincident with a low steam generator level in loop 4. The steam/flow mismatch and low steam generator level were both caused by the closure of the loop 4 feedwater flow control valve (2-FCV-3-103).
The valve closed during the perform-ance of surveillance instruction 618, "Engineered Safety Features Actua-tion System Block Tests."
A missing diode in the blocking circuit caused the flow control valve to close.
During the recovery from the trip, several variations from the normal trip recovery occurred.
These variations were:
B-178 (1) Flow control valves 2-FCV-1-139 (MSR-C2) and 2-FCV-1-153 (MSR-B1) would not close while trying to isolate steam to the moisture separator reheaters.
The torque switch settings for these valves were too low to close against the high pressure steam.
(2) A condenser vacuum breaker was erroneously opened by an assistant unit operator and resulted in a loss of condenser vacuum and disabling of the steam dump valves.
High noise levels in the area contributed to the miscommunication that caused the operator to open the vacuum breaker.
The intent was to open the extraction low-point drain valves using the handswitch near the condenser vacuum breaker.
(3) Because the main control room operator was unaware of the vacuum breaker being opened locally, two vacuum pumps were turned off in an att empt to stabilize condenser vacuum.
(4) The assistant unit operator noted that the condenser vacuum breaker was open from the local handswitch and immediately closed the vacuum breaker.
The vacuum breaker was open for approximately 2 min. Condenser vacuum restabilized, and the " B" vacuum pump was restarted.
(5) Unit operators noted that the number 3 steam generator level was rising and reverted to manual control.
The turbine-driven aux-iliary feedwater control valve (2-LCV-3-172) was not closing com-pletely on demand, and the turbine-driven auxiliary feedwater pump was continuing to fill steam generator number 3. The valve was not closing properly because of a stem maladjustment.
Manual control of auxiliary feedwater was undertaken, and the turbine-driven aux-iliary feedwater pump was shut off.
(6) The power-operated relief valve for steam generator number 2 opened prematurely at approximately 1005 psig instead of 1040 psig.
This valve opened prematurely because of an out-of-calibration condition on the pressure switch.
None of the above variances from normal responses prevented the opera-tors from recovering safely from the reactor trip.
Changes to the FSAR have made it unnecessary to perform surveillance instruction 618; there-fore, this surveillance test will no longer be performed.
Event-Related Design Information There are two turbine-driven, variable-speed main feedwater pumps that are capable of delivering feedwater to the four steam generators.
The feedwater system normally operates at full load with three hotwell, three demineralized condensate, three condensate heater, and two main feedwater pumps in service.
B-179 The auxiliary feedwater system consists of two motor-driven pumps and one turbine-driven pump.
Each of the motor-driven pumps serves two steam generators; the turbine-driven pump serves all four.
ASP Modeling Assumptions and Approach The event has been modeled as a loss of main feedwater and failure of the turbine-driven feedwater pump in the auxiliary feedwater system.
Analysis Results The conditional probability of severe core damage estimated for this event is 4.5 x 10-6.
Dominant sequences leading to core damage for this event involve the mintiatinu transient and successful
6.0 x 10- ),
failure to recover main feedwater (p =0.34), and failure of feed and bleed (p =
0.02, including failure to initiate).
The dominant sequence associated with the event is highlighted on the following event tree.
B-180 TRN IT F
F PORV/ I RV/
HII P
M AW S
RV ISRV HI OE CHAL I PRESEATE SEQ END NO STATE OK OK S
CD 12 CD OK OK OK 13 CD 14 cD OK OK 15 CD 16 CD 17 CD 18 ATWS Dominant Core Damage Sequence for LER 328/88-027 Ri
B3-i18 CONDITIONAL CORE DANAGE PROBABILITY CALCULATIONS Event Identifier:
32 8/88-027 Event
Description:
Trip from steam/feedwater flow mismatch and low SO level Event Date:
06/06/88 Plant:
Sequoyah 2 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS
- 1. 0E+00 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probability CD TRANS 4.5E-06 Total 4.5Ef-06 ATWS TRANS
- 3. 4E-05 Total 3.4E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
17 trans -rt AEW NEW hpi(f/b)
CD 2.2E-06 7.4E-02 15 trans -rt AEW NEW -hpi(f/b) -hpr/-hpi porv.open CD 2.1E-06 8.8E-02 18 trans rt ATWS 3.4E-05 1.2E-01
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Ret" 15 trans -rt AEW NEW -hpi(f/b) -hpr/-hpi porv.open CD 2.TE-06 8.8E-02 17 trans -rt AEW NEW hpi(f/b)
CD 2.2E-06 7.4E-02 18 trans rt ATWS 3.4E-05 1.2E-01 non-recovery credit for edited case SEQUENCE NODEL:
a:\\sealmod\\pwrbseal.cmp BRANCB MODEL:
a:\\sealmod\\sequoyah.sl1 PROBABILITY FILE:
a:\\sealmod\\pwr~bsll.pro No Recovery Limit BRANCB EREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 7.7E-04 1.01+00 loop 1.6E-05 5.3E-01 iota 2.4E-06 4.3E-01 rt 2.8E-04 1.2E-01 rt/loop S.OE-i00 1.01*00 emerg.power 2.9E-03 8.01-01 AEW 3.8E-04 > 2.3E-03 2.6E-01 Branch Model:
1.OE.3+ser Train 1 Cond Prob:
2.01-02 Train 2 Cond ProD:
1.01-01 Train 3 Cond ProD:
5.01-02 > Failed Event Identifier: 328/88-027
3-182 Serial Component Prob:
AEW/EMER. POWER Branch Model:
l.OF.l Train 1 Cond Prod:
MEW Branch Model:
l.OF.l Train 1 Cond Prod:
5orv.or.srv.chall porv.o~r.srv. reseat porv.or.srv. reseat/emerg.power seal.boca ep. ree (si) ep. rec hp i hpi (f/b) hpr/-hpl porv. open
- 2. 8E-04 SAJE-02 > l.OE-+00 5.OE-02 > Failed l.OE+00 > l.OE+00 1.OE+00 > Failed 4.OE-02 2.OE-02 2. OE-02
- 2. 7E-01 5.7E-01
- 0. OE+00 1.OE-03 l.OE-03
- 1. 5E-04
- 1. SE-02
- 3. 4E-01 7.OE-SO > 3.4E-01 1.SE+0S
- 1. lE-02
- 1. SEi-S
- 1. SEi-O 1.5 Li-S
- 1. SE+SS 8.4 E-O1
- 8. E-O1 1.0K-tOO 1.OSEi-S
- .SE-02 1.OE-03 4.OE-54
- branch model file Sforced Minarick 11-12-1989 15:45:08 Event Identifier: 328/88-027