ML20147A477
| ML20147A477 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 05/26/2020 |
| From: | Christopher Hunter Office of Nuclear Regulatory Research |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1989-009-00 | |
| Download: ML20147A477 (6) | |
Text
B-143 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No: 324/89-009 Event
Description:
Reactor manual scram causes a loss of offsite power Date of Event: June 17,1989 Plant: Brunswick 2 Summary A loss of offsite power (LOOP) occurred following a manual scram from 76% power at Brunswick 2. The LOOP was preceded by the loss of the station auxiliary transformer (SAT) when a technician shorted its neutral grounding transformer primary winding while troubleshooting a ground alarm on the SAT. The high-pressure coolant injection (HPCI) system, the reactor core isolation cooling (RCIC) system, and the safety relief valves (SRVs) were used to control reactor pressure and level following the LOOP.
When attempting to place the unit in cold shutdown, it was discovered that one of two trains of LPCI/RHR was inoperable because of a stuck-closed injection valve.
The conditional probability of core damage associated with this event is estimated to be 3.6 x 10O5. The relative significance of this event compared with other potential events at Brunswick 2 is shown below.
LER 324/89 -009
] 1 -
IE-7 1E-6 1E-5 I L-E3I E31-L JfLOI L360ItEP 360Oh HPCI LJF +
+ RCIC HPCI precursor cutoff Event Description Brunswick 2 was at 76% power on June 17, 1989, when the control room received annunciation of a ground on the SAT. Since the SAT is maintained and repaired by the Wilmington Area Transmission Maintenance Unit (TM), which is not part of the Brunswick staff, the load dispatcher was contacted to send a repair crew out to the site to
B-144 repair the SAT. While the repair crew was troubleshooting the problems with the SAT, the operators and the plant General Manager were discussing the consequences of the potential loss of the SAT. A previous plant modification had shifted the power source for the recirculation pumps to the SAT, so that a loss of the SAT would cause a dual recirculation pump trip. This dual pump trip, in turn, would lead to a plant scram because an NRC Bulletin (1IE 88-07, which concerned thermal-hydraulic instability problems for BWRs) required BWRs to immediately shut down if they found themselves operating at relatively high power with no forced recirculation. It was decided to reduce power below the region of concern stipulated in IE 88-07. However, power had been reduced to only 76%, when a technician from TM shorted out the transformer, causing a dual recirculation pump trip. Upon receipt of the pump trip, the operator scrammed the reactor in accordance with procedures, thus separating the main generator from the grid and tripping off the unit auxiliary transformer (UAT) and causing a LOOP.
While attempting to place the unit in cold shutdown 2 d later, the outboard RHR injection valve 2-El 1-FO17A was discovered stuck closed. The valve disk had separated from the stem, a result of incorrect installation of two locking pins when the valve was rebuilt in 1986. Because of the nature of the valve failure mode, the valve appeared open based on control room indication and valve stem position.
Additional Event-Related Information The normal offsite power supply to the Unit 2 safeguards buses is provided through the startup auxiliary transformer. The alternate nonemnergency source is from the unit auxiliary transformer, which is energized by the main generator.
When the SAT was lost, the unit initially remained on line, and the main generator supplied house loads through the UAT. By procedure, the SAT was the only permissible power source for the recirculation pumps, and the loss of the SAT de-energized them.
Given the unavailability of the recirculation pumps, procedures then required that the unit be scrammed. This removed the remaining source of nonemnergency power to Unit 2, and a LOOP resulted. The emergency diesels started and carried the essential electrical loads as designed.
ASP Modeling Assumptions and Approach This event has been modeled as a plant-centered loss of offsite power with one unavailable train of LPCI/RHR.
B-145 Analysis Results The conditional probability of severe core damage for this event is 3.6 x 10-5. The dominant sequence involves failure to recover offsite power in the short term, failure of emergency power, and failure to recover AC power prior to battery depletion. This sequence is highlighted on the following event tree.
B-146 tX EP lSHUT DOMY REG tcm)D CHAt
~VC a ORW IWCS m UVI ADS CS m (SDO MOE (SI' MODE) alR SEQ 60 END)
STATE OK OR OK OK OK 142 ccR DAMAGE OK 445 c AIO OK 45 COREDAMAGE
.47 C-R DAM-AG
___________________ 48 ccmI DAMAGE OK OR 46 CaORDAMAGES OK OK OK si R DAMAGE840 OK 52 cc 4a OK 54 COREDAMAGE OK 57 COREDAMAGE OK OK 56 caE DAMAGE Ox COGR AMG 64 CORED6AMAE 62 Oa PUO 6 c DAMAGE 65 COREDAMAGE as as 66 COREDM OKc~DA6G Os
- 11 cTOZDAMAGE Ld Dominant core damage sequence for LER 324189-009
B- 147 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 324/89-009 Event
Description:
Scram results in LOOP and LPCI degraded Event Date: 06/17/89 Plant: Brunswick 2 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 3.OE-0l SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOOP 3. 6E-05 Total 3. 6E-05 ATWS Loop 9.OE-06 Total 9.OE-06 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec-*
83 LOOP emerg.power -rx.shutdown/ep EP.REC CD 2.5SE-OS 2 . 4E-01 40 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scram -srv.close CD 7. 9E-06 3 . 4E-02
-hpci RHR(SDC) rhr(spcool) /rhrisdc) 55 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scram srv.close CD 2.8E-06 1.5SE-01 hpci srv.ads 98 LOOP -einerg.power rx.shutdown ATWS 9.OE-06 3.OE-01
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec-*
40 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scram -srv.close CD 7.9E-06 3. 4E-02
-hpci RHR(SDC) rhr(spcool) /rhr(sdc) 55 LOOP -emerg.power -rx.shutdown srv.chall/loop.-scram srv.close CD 2.8E-06 1.5SE-01 hpci srv.ads 98 LOOP -einerg.power rx.shutdown ATWS 9.OE-O6 3. OE-Ol 83 LOOP emerg.power -rx.shutdown/ep EP.REC CD 2.5SE-05 2. 4E-01
- non-recovery credit for edited case SEQUENCE MODEL: c:\asp\1989\bwrcseal.cmp BRANCN MODEL: c:\asp\1989\brunswck.sll PROBABILITY FILE: c:\asp\1989\bwr-csll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Rec :ov Opr Fail trans 2.3E-04 l.OE+0C LOOP 1.6E-05 > 1.6E-05 3.6E-01 > 3.OE-O1 Branch Model: INITOR Initiator Freq: 1. 6E-05 loca 3. 3E-06 5. OE-01 Event Identifier: 324/89-009
B-148 rx -shutdown 3.06-05 1.0 6+00 rx. shuidown/ep 3. 5E-04 1.06+00 pcs/trans 1.7E-01 1.06E+00 srv.chall/trans .-scram 1. OE+OO 1.06+00 srv.chall/loop.-scram 1.06+00 1.06+00 srv.close 3. 6E-02 1. 0 +00 emerg. power 2.1E-03 8. OE-01 EP.REC 1.6E-01 > 4.9E-02 1 .06+00 Branch Model: 1.OF.1 Train 1 Cond Prob: 1.6E-01 > 4.9E-02 fw/pc . trans 4.6E-01 3.46-01 fw/pcs .loca 1. OE+00 3.46-01 hpci 2. 9E-02 7.06-01 rcic 6. OE-02 7.06E-01 c rd 1.06-02 1.06+00 1.06-02 srv.ads 3.7E-03 7.16-01 1.06-02 lpcs 3.06-03 3.4 -01 LPCI (RHP) /LPCS 1.06-03 > 1.06-01 7.16-01 Branch Model: l.OF.2 Train 1 Cond Prob: 1.06-02 > Failed Train 2 Cond Prob: 1.06-01 Rim (SDC) 2.1E-02 > 1.2E-01 3.46-01 1 .OE-03 Branch Model: l.OF.2+ser+opr Train I Cond Prob: 3.06-03 > Failed Train 2 Cond Prob: 3.06-01 > l.OE-01 Serial Component Prob: 2.06-02 rhr (sdc) /-lpci 2.06-02 3. 46-01 1.06E-03 rhr (sdd /lpci 1.06+00 1.06+00 1.06-03 rhrlspcool) /rhr(sdc) 2.06-03 3.46-01 rhr (spcool) f-lpci .rhr (sdd 2.O0 -03 3.4 -01 rhr(spcool) /lpci.rhr(sdc) 9. 3E-02 1 .06+00 rhrsw 2.06-02 3. 4 -01 2.06-03
- branch model file
- forced Minarick 0 6-14-1990 16: 59:18 Event Identifier: 324/89-009