ML20135G979
| ML20135G979 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 423-1990-011 | |
| Download: ML20135G979 (5) | |
Text
B-330 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.: 423/90-0il Event
Description:
Reactor trip and one containment spray recirculation pump unavailable Date of Event: March 30, 1990 Plant: Millstone 3 Summary The reactor was manually tripped in anticipation of a loss of main condenser vacuum while the automatic screen wash system was unavailable because of maintenance. One containment recirculation pump was out of service at the time. The conditional core damage probability estimated for this event is 1.1 X 10-6. The relative significance of this event compared to other postulated events at Millstone 3 is shown below.
Event Description In early March 1990 the automatic circulating water inlet screen wash system was taken out of service for repairs. At this time of the year, Long Island Sound undergoes a significant increase in its seaweed loading. On March 30, 1990, Millstone 3 was at 100% power when the plant initiated manual screen washing to clean the seaweed from the screens on the intake structure, since not all the repairs were completed on the automatic system. The manual cleaning dumped the seaweed back into the Sound away from the plant; however, a tide change brought the seaweed back into the circulating water intake. Operations began decreasing power in an attempt to get below the power level where a turbine trip causes a scram, which was anticipated if the seaweed clogged the intake structure and caused a loss of vacuum in the main condenser. Since two circulating water pumps tripped on intake screen high differential pressure before power
B-331 got below this point, the operators manually scrammed the reactor from 80% power.
One containment recirculation pump was out of service for a high oil level problem in its lower motor bearing.
Response to the trip was complicated when thermal relief valves for two feedwater heaters separated at the point where the relief valve piping connected to the heaters. The turbine building was evacuated for -30 min following the piping failures until it could be determined that no personnel hazard existed.
Additional Event-Related Information The containment recirculation system is comprised of two identical subsystems each containing two 3950-gpm capacity pumps and two recirculating coolers. The two subsystems share two 360'F containment spray headers. The containment recirculation system provides for decay heat removal following a loss-of-coolant accident (LOCA).
Operability of two of four pumps is required for system success.
ASP Modeling Assumptions and Approach This event has been modeled as a reactor scram and loss of feedwater due to loss of condenser vacuum with one containment recirculation pump out of service.
Analysis Results The conditional probability of severe core damage estimated for this event is 1.1 X 10-6.
The dominant sequence to core damage, highlighted on the following event tree, involves failure of secondary-side cooling and failure of feed and bleed.
B-332 SEO END TRNS I RTISRV AF MFW PORVI PORV/I SRV H-PI IHPR IPR OPEN S NO STATE CHALL IRESEATI I OK OK 20 CD 11 CD 12 Co OK OK OK 21 CD 13 CD 14 CD OK OK 22 CD 16 CD 16 a, 17 a, 18 ATWS Dominant core damage sequence for LER 423/90-011
B-333 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 423/90-011 Event
Description:
Reactor trip and one CSR pump unavailable.
Event Date: 03/30/90 Plant: Millstone 3 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.DE+0O SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 1 .1E-06 Total 1 .1E-06 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 hpijf/b) CD 3.6E-07 7 .4E-02 15 trans -rt afw MFH -hpi(f/b) -hpr/-hpi porv.open CD 3.5E-07 8.8E-02 22 trans -rt afw MPH -hpi(f/b) -hpr/-hpi -porv.open CSR CD 2 .7E-07 S.SE-02 20 trans -rt -afw porv.or.srv.chall porv.or.srv.reseat -hpi -hpr/ CD 7 .2E-08 1. lE-02
-hpi CSR 16 trans -rt afw MPH -hpi (f/b) hpr/-hpi CD 3.8E-08 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 Rec**
20 trans -rt -afw porv.or.srv.chall porv.or.srv.reseat -hpi -hpr/ CD 7.2E-08 1.lE-02
-hpi CSR 22 trans -rt afw MPH -hpi(f/b) -hpr/-hpi -porv.open CSR CD 2 .7E-07 S.SE-02 15 trans -rt afw MFH -hpi(f/b) -hpr/-hpi porv.open CD 3.5E-07 B.BE-02 16 trans -rt afw MPH -hpi(f/b) hpr/-hpi CD 3.8E-08 B.8BE-02 17 trans -rt afw MPH hpi(f/b) CD 3. 6E-07 7 .4E-02 is trans rt ATWS 3.4E-05 1 .2E-01
- non-recovery credit for edited case SEQUENCE MODEL: c :\asp\1989\pwraseal .cmp BRANCN MODEL: c: \asp\1989\inillstn3. all PROBABILITY FILE: c :\asp\l989\pwrbsll.pro Event Identifier: 423/90-011
B-334 No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Racov Opr Fail trans 4.6E-04 1.0E+00 loop~ 1 .8E-05 3.3E-01 loca 2 .4E-06 4. 3E-01 rt. 2 .8E-04 1.2E-01 rt /loop 0 .OE+00 1.0E+00 emerg.power 2.9E-03 B. OE-01 afw 3.8E-04 2.6E-01 afw/emerg .power 5.OE-02 3.4E-01 MFW 2.OE-01 > l.0E+00 3.4E-01 Branch Model: l.OF.1 Train 1 Cond Prob: 2.OE-0l > Unavailable porv.or.srv.chall 4 .OE-02 1. OE+00 porv.or.srv. reseat 2 .OE-02 1.1E-02 porv.or.srv. reseat/emerg.power 2 .OE-02 l.0E+00 seal .loca 3.5E-01 l.OE+00 ep. rec (si) 7 .6E-01 1.OE+00 ep. rec 1 .5E-01 1.0E+00 hpi 1.0E-03 8.4E-01 hpi (f/b) l.OE-03 S. 4E-01 1.OE-02 porv.open 1.OE-02 1.OE+00 4. OE-04 hpr/-hpi 1.59-04 1.OE+00 1.OE-03 CSR 9.3E-05 > 7.2E-03 1. OE+00 Branch Model: 2.OFi4 Train 1 Cond Prob: 1.OE-02 > Failed Train 2 Cond Prob: 3.OE-02 Train 3 Cond Prob: l.OE-0l Train 4 Cond Prob: 3 .0E-01
- branch model file
- forced Mina rick 08-06-1991 17:30:22 Event Identifier: 423/90-011