ML20149K477
| ML20149K477 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 05/28/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-002-00 | |
| Download: ML20149K477 (5) | |
Text
B-190 Accident Sequence Precursor Program Event Analysis LER No: 338/88-002 Event
Description:
Plant trip with failure of one AFW train Date of Event: January 8, 1988 Plant: North Anna 1 Summnary While at 100% power, all three circulating water pumps tripped off for an unknown reason. In anticipation of a loss of condenser vacuum, the reactor was manually tripped. Forty minutes after the trip, the one auxiliary feedwater pump tripped when a stroke limiter/regulator valve failed. The conditional probability of core damage for this event is calculated at 1.4 x 10-5. The relative significance of this event com-pared with other potential events at North Anna 1 is shown below.
LER 338/88-002 1IE-8 I1E-7 I1E-6 IE- 1E-4 IE-3 I E-2 LOO L EP Unavail Trip LOP (360 hrs)
Trip w/1 AFW AE navail Train Unavail (360 hrs)
Event Description At 0438 h the plant was manually tripped due to a spurious failure of the circulating water system. All three operating circulating water pumps tripped off for an unknown reason. The reactor was tripped in anticipation of a loss of condenser vacuum. Prior to the reactor trip, the circulating water pumps were frequently stopped to clear fish from the intake screens and to replace the traveling screen shear pins. All emergency systems responded as required except the turbine-driven AFW train, which first started and then tripped off after 40 min when a stroke limiter/regulator valve failed due to a blown plug. It is believed that the blown plug may have possibly struck the trip valve linkage or allowed the steam to impinge on the trip valve linkage, caus-ing the valve to shut. The rapid increase in condenser pressure caused seven of the low-pressure turbine rupture discs to blow out.
B-191 Corrective action was taken to modify the turbine-driven AFW pump by removing the stroke limiter/regulator valve. The unit 2 turbine-driven AFW pump will be similarly modified. One of the circulating water train "A" solenoid valves was replaced after it did not open the associated trip valve.
Event-Related Plant Design information The circulating water system protection circuitry is designed with vari-ous interlocks to protect the circulating water intake structure from pressure transients. Two of the interlocks related to the event are (1) when all the CW pumps are tripped, the condenser waterbox vacuum breakers open automatically; and (2) when one of two, condenser waterbox vacuum breakers opens on at least two of four waterboxes, all the remaining CW pumps are tripped. After extensive testing, it was not clear which interlock may have initiated the trip of all the CW pumps.
ASP Modeling Assumptions and Approach This event has been modeled as an effective loss of feedwater trip (cir-culating water pump failure will cause a loss of feedwater) and failure of the turbine-driven AFW pump.
Analysis Results The conditional core damage probability estimated for the event is 1.4 . 10-5. The two dominant sequences involve loss of AFW following the loss of feedwater (p = 6.0 x 10-4) and failure of feed and bleed (p = 0.02, including failure to initiate). The dominant sequence asso-ciated with the event is highlighted on the following event tree.
B-.192 TRANS RT AFW MFiN ISRV PORV/
CHALL PORV/
SRV RESEAT HPI HPR PORV OPEN CSR SEO NO END STATE OK OK 20 CD 11 12 OK OK OK 21 CD 13 CD 14 OK OK 22 CD 15 CD 16 CD 17 CD 18 ATWS Dominant Core Damage Sequence for LER 338/88-002
B-193 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 338/88-002 Event
Description:
Plant trip with failure of one AFW pump Event Date: 01/08/88 Plant: North Anna 1 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.OE+/-00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 1. 4E-05 Total 1. 4E-05 ATWS TRANS 3. 4E-05 Total 3.4E-055 SEQUENCE CONDITIONAL PRO13ABILITIES (PROBABILITY ORDER)
Sequence End State P rob N Rec--
17 trans -rt AFW MFW hpi(f/b) CO 6. 7E-06 2 . 2E-01 15 trans -rt AFW MFW -hpl(f/b) -hpr/-hpi porv.open CO 6.1E-06 2 .81-01 16 trans -rt AFW MFW -hpi(f/b) hpr/-hpi CO 6. 7E-07 2 .6E-01 22 trans -rt AFM MFW -hpi(f/b) -hpr/-hpi -porv.open csr CO 6. 3E-07 2.6SE-01 18 trans rt AIMS 3.4E-05 1.21-01
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State P rob N Rec'-
22 trans -rt AFW MFW -hpi(f/b) -hpr/-hpi -pr rv.open csr CD 8 .3E-07 2.81E-01 15 trans -rt AFW MFW -hpi(f/b) -hpr/-hpi pr rv.open CD 6. 1E-0 6 2.61E-01 18 trans -rt AEW MFW -hpi~f/b) hpr/-hpi CD 6.7E-07 2.61-01 17 trans -rt AEM MEW hpi(f/b) CD 6.7E-06 2 . 2E-01 18 trans rt AIMS 3. 4Z-05 1. 2E-01
-- non-recovery credit for edited case SEQUENCE MODEL: c:\asp\sealmod\pwraseal.cmp BRANCH MODEL: c:\asp\sealmod\northanl.sil PROBABILITY FILE: c:\asp\sealmod\pwr_bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 2.4E-04 1 .01*00 loop 1.6E-05 5.31-01 100 a 2 .4E-0 6 4. 31-01 rt 2.81-04 1.2E-01 rt/loop 0.06+00 1 .01+00 emerg.power 2.9E-03 8.01-01 AEW 3.8E-04 > 2.3E,-03 2.6E-01 Event Identifier: 338/88-002
B-194 Branch M~odel: 1.CF.3+ser Train 1 Cond Prob: 2 .0E-02 Train 2 Cond Prob: 1.00E-01 Train 3 Cond Prob: 5.00-02 > Failed Serial Component Prob: 2.8E-04 AFW/EMERG .P01400 5.00-02 > 1.00+00 3.4E-01 Branch M~odel: l.OF.l Train 1 Cond Prob: 5.00-02 > Failed MFI4 1.9E-01 > 1.00+00 3.4Z-01 > 1.01>00 Branch M~odel: 1.0F.1 Train 1 Cond Prob: 1.9E-01 > Failed porv.or .srv.chall 4 . OE-02 1 .00+00 porv.or.srv. reseat 3.00E-02 1.10E-02 porv.or.srv.reseat/emerg.power 3.0 E-02 1.00+00 seal .loca 2 . 7E-01 1 .00+00 ep. rec Isi) 5. 7E-01 1.00+00 ep. rec 7.0E-02 1.00+00 hpi 1 .5E-03 8 .40E-01 hpl (f/b) 1. 5E0-03 8.40E-01 1.00-02 porv .open 1.00-02 1.00+00 4.00-04 hpr/-hpl 1.50-04 1.00+00 1.00E-03 csr 9. 3E-05 1.00+00
- branch model file
- forced tMinarick 11-13-1989 13:47:34 Event Identifier: 338/88-002