ML20147A480
| ML20147A480 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter Office of Nuclear Regulatory Research |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1988-008 | |
| Download: ML20147A480 (5) | |
Text
3-36 5 Accident Sequence Precursor Program Event Analysis LER No:
Event
Description:
Date of Event:
Plant:
45 5/88-008 Loss of feedwater and one train of auxiliary feed-water tripped July 14, 1988 Byron Unit 2 Summnary An operator inadvertently caused a loss of feedwater while operating at 95% power.
During the recovery from the reactor trip, one train of auxiliary feedwater tripped and was restored approximately 4 min later. 105The core damage probability estimated for the event is 4.0 x l0.
The relative significance of this event compared with other potential events at Byron 2 is shown below.
1 E-8 I
E-7 LER 455/88-008 IE-6 IE-5 I E-4 1
E-3 I
E-2 Tnip I Trip w/I ARM Train Unavail IL EP Unavail LOOP (360 hrs)
AFRM Unavail (360 hrs)
Event Description The feeder breaker from the unit auxiliary transformer to nonsafety-related bus 243 had been operating at elevated temperatures, and it was decided to rack the breaker out for a visual inspection.
Therefore, the feeder breaker from the system auxiliary transformer was closed and the feeder breaker from the unit auxiliary transformer was opened.
While attempting to place the opened feeder breaker in "pull-to-lock," the operator inadvertently operated the handswitch for the feeder breaker from the system auxiliary transformer, which de-energized bus 243.
This resulted in loss of several secondary-side pumps, including the 2B and 2C main feedwater pumps.
Steam generator levels decreased rapidly, and an automatic reactor trip occurred.
A main feedwater isolation occurred
B-366 as expected shortly after the trip and auxiliary feedwater pumps 2A and 2B started automatically at 0114; however, the 2B AFW pump tripped on a false overspeed signal caused by a loose terminal grounding connec-tion.
The 2B AFW pump was reset and was automatically started at 0118 and operated normally for the duration of the event.
The abnormally high temperature of the feeder breaker for bus 243 was caused by failure of a heater control circuit used to keep breaker cubicles warm during cold weather.
ASP Modeling Assumptions and Approach The event was modeled as a loss of feedwater and one train of auxiliary feedwater unavailable.
Analysis Results The conditional probability of severe core damage estimated for this event is 4.0 x 10-5.
The dominant sequences to core damage involve failure of auxiliary feedwater (p =5.2 x 10-3),
failure to recover main feedwater (p = 0.34), and failure of feed and bleed capability (p =
0.02, including operator failure).
The dominant sequence for this event is highlighted on the following event tree.
3-367 1PORV/ PORV/
TRANS RT AFW MW SRy SRV HPI HPR IPORV CHAL RESEAT OPEN SEO END NO STATE 11 12 OK OK CD CO OK OK OK 13 CD 14 CD OK OK 15 CD 16 CD 17 00 18 ATWS Dominant Core Damage Sequence f or LER 455/88-008
B-368 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
455/88-008 Event
Description:
Loss of feedwater and one train of AFW tripped Event Date:
07/14/88 Plant:
Byron 2 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS 1.11+00 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 4.01-05 Total 4.01-05 ATWS TRANS
- 3. 4E-05 Total 3.4E-05 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
CD 1.9E-0S 7.4E-02 15 trans -rt AFW MEW -hpi(f/b) -hpr/-hpi porv.open CD 1.8E-05 8.8E-02 16 trans -rt AFW MEW -hpi(f/b) hpr/-hpi CD 2.OE-06 8.8E-02 18 trans rt AIMS 3.41-05 1.2E-01
-non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
15 trans -rt AFW MEW -hpi(f/b) -hpr/-hpi porv.open CD 1.8E-05 8.8E-02 16 trans -rt AFW MEW -hpi(f/b) hpr/-hpi CD 2.OE-06 8.8E-02 17 trans -rt AFW MEW hpi(flb)
CD 1.9E-05 7.4E-02 18 trans rt ATWS 3.4E-05 1.2E-01
-~ non-recovery credit for edited case SEQUENCE MODEL:
a:\\sealmod\\pwrbseal.cmp BPANCH MODEL:
a:\\sealmod\\byron.sll PROBABILITY FILE:
a:\\sealmod\\pwr~bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opt Fail trans 5.3E-04 1.01-00 loop 1.6E-05 5.3E-I1 loca 2.41-06
- 4. 31-El rt 2.81-04 1.21-01 rt/loop 0.01+00 1.01+00 cmerg.power 2.9E-03 8.01-01 AFW
- 1. 3E-03 > 2.01-12 2.61-01 Branch Model:
1.OF.2.-ser Train 1 Cond Prob:
2.01-02 Event Identifier: 455/88-008
B3-369 Train 2 Coed 2mob:
Serial Component Prmb:
AtM! EMERBC. POWER Branch Model:
l.OF.1 Train 1 Cond 2mob:
MEW Branch Model:
l.OF.1 Train 1 Cond 2mob:
porv.or.srv.chall porv.or.srv. reseat porv.or.srv.rescat/e~merg.power seal.looa ep. rec (si) ep. rec hpl hpi (f/b) hpr/-hpi porv.open 5.05-02 > Failed 2.8E-04 5.05-02 > 1.05+00 5.05-02 > Failed 1.054-00 > 1.05+00 1.05+00 > Failed 4.OE-02 2.0E-02 2.OE-02 2.7E-01 5.8E-01 1.8E-02 l.OE-03
- 1. OE-03 1.5E-04 1.OE-02 3.4 E-01 7.OE-02 > 3.4E-01 1.OS-tOO
- 1. 15-02 1.OE+00
- 1. 0E+00
- 1. 05+00
- 1. 05+00 8.4 E-01 8.4ES-01
- 1. OE+00
- 1. OE+00 10 OE-02 1.OE-0 3 4.OE-04 branch model file forced minarick 11-12-1989 22 :2 1 :32 Event Identifier: 455/88-008