ML20135G974
| ML20135G974 | |
| Person / Time | |
|---|---|
| Site: | McGuire  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1991-001-00 | |
| Download: ML20135G974 (6) | |
Text
B-373 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.: 369/91-001 Event
Description:
Switchyard breaker test results in loss of offsite power Date of Event: February 11, 1991 Plant: McGuire 1 Summary Errors and equipment failures during installation of new switchyard relay protection resulted in the opening of all switchyard breakers connecting Unit 1 to the grid. Reactor and turbine trips followed, and both diesel generators (DGs) started and loaded. An excessive cooldown rate resulted in safety injection (SI) actuation and main steam isolation valve (MSIV) closure. Subsequently, reactor coolant pressure increased, and two pressurizer power-operated relief valves (PORVs) actuated. Containment pressure increased to 0.76 psig. Offsite power was restored, and operators began unloading the DGs after about 1.25 h. The conditional core damage probability estimated for the event is 2.6 x 10-4. The relative significance of this event compared to other postulated events at McGuire 1 is shown below.
LER 369/89-001 1E-7 IE-6 1E-5 1E-4] IE-3 1E-2 Trip] LOFW +J [I~ LLoop I MTR AFW 16 F L- precursor cutoff Event Description Prior to the event, Unit 1 was operating at 100% power while relay protection modifications for the switchyard autotransformer were in progress. At McGuire, the autotransformer serves to crosstie the Unit 1 230-ky switchyard to the Unit 2 525-ky switchyard. A sudden pressure fault detection relay had been added to the autotransformer, and testing was in progress to verify that operation of the relay would properly isolate the autotransformer. The autotransformer feeder breakers' trip coil circuits were blocked to prevent their actuation, and a simulated fault pressure signal was introduced. Additional protective relaying that had not been blocked detected the
B-374 simulated sudden pressure relay operation and the failure of the autotransformer breakers to open. This actuated a breaker failure scheme that cleared both main buses in the 525-kV and 230-ky switchyards. Each McGuire unit remained connected to the grid through two transmission lines, which connect directly to the main transformer outputs, bypassing the switchyard buses.
The output from the Unit 1 generator was directed to the Craighead and Mecklenburg transmission lines. The increased current in the Craighead line was detected by relay protection as an overcurrent condition. This, in conjunction with a failed distance relay for the Craighead line, resulted in opening of the Craighead line feeder breaker. The entire output of the unit was then directed to the Mecklenburg line, which was sized for only one-half of the unit's output. The feeder breaker for this line tripped on overcurrent, and all offsite power was lost to Unit 1.
When all connections to the grid were lost, the generator output frequency rose, increasing reactor coolant pump (RCP) speed and flow.' The resulting increase in reactor power initiated'a high flux rate reactor trip, which was followed by a turbine trip. DGs IlA and IlB started and automatically picked up their emergency loads.
The loss of power to nonsafety-related valves prevented operators from isolating certain main steam loads, and excessive cooling of the reactor coolant system (RCS) resulted.
Low steamline pressure initiated a SI and automatic isol 'ation of the MSIVs. RCS temperature and pressure then began rising until limited by operation of two PORVs (a third valve was unavailable because of maintenance). Containment pressure began rising, ultimately reaching 0.76 psig.
The SI signal was reset, and power was restored to the switchyard. About 75 min into the event, operators began removing loads from the DGs and restoring the plant to a normal alignment.
Additional Event-Related Information The utility reported that 4 min into the event, the condenser "was in full load rejection mode." It was also reported that an inability to isolate steamline drains and other main steam valves contributed to excessive steam demand and cooldown.
At McGuire, air is removed from the condenser by steam-jet air ejectors. This system might not be impacted by a loss-of-offsite power (LOOP). Condenser cooling is provided by the condenser circulating water system (CCW). The CCW system is crosstied between Units 1 and 2 via an 84-in, line. As power to Unit 2 was apparently maintained during the event, some CCW may have been available to the Unit 1 condenser. These features may have contributed to the excessive cooldown of the, main steam system.
B-375 ASP Modeling Assumptions and Approach This event was modeled as a plant-centered loss of offsite power with pressurizer PORVs demanded. Probabilities for LOOP non-recovery (short term) and failure to recover AC power prior to battery depletion were revised to reflect values associated with a plant-centered LOOP (see ORNL/NRCILTR-89/1 1, Revised LOOP Recovery and PWR Seal LOCA Models, August 1989). The two operable PORVs were assumed to be adequate for feed and bleed.
Analysis Results The estimated core damage probability associated with this event is 2.6 x 10-4 The dominant core damage, sequence, shown on the following event tree, involves a postulated loss of emergency power following the LOOP, and failure to recover AC power prior to battery depletion. If the unavailable PORY is assumed to fail feed and bleed, the resultant core damage probability is estimated to be 2.9 x 10-4.
B-376 LOPE I I 1-OP E ICHALL PORV/ PORVI I RESEAT ELIEFCPORV LOCA (LONG) I IOPEN SEO NO END STATE OK OK 41 CO 42 CD OK OK 43 CD (1) 44 CD 45 CD OK 46 CO 47 CO 48 CO OK 49 CO 50 CO OK 51 CO 52 CD 53 CO OK 54 CO 55 CO 40 ATWS (1)0OK for Class D Dominant core damage sequence for LER 369/9 1-001
B-377 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 369/91-001 Event
Description:
Switchyard breaker test results in a LOOP Event Date: 02/11/91 Plant: McGuire 1 INITIATING EVENT NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP 3.OE-01 SEQUENCE CONDITIONAL PROBABILITY SUNS End state/Initiator Probability CD LOOP 2.6E-04 Total 2.6E-04 ATWS LOOP 0 . DE+00 Total 0.05+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State P rob N Rec-*
49 LOOP -rt/loop emerg.power -afw/ernerg.power PORV.OR.SRV.CN4ALL - CO 2.3E-04 2. 4E-01 porv.or.srv. reseat/erserg.power -seal. loca EP.REC 50 LOOP -rt/loop emerg.power -afw/exeerg.power PORV.OR.SRV.CHALL CD 2.OE-05 2.4E-01 porv.or.srv. reseat/erserg.power 55 LOOP -rt/loop emerrj.power afw/einerg.power CD 1 .2E-05 8.2E-02
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
49 LOOP -rt/loop emerg.power -afw/ernerg.power PORV.OR.SRV.CHALL - CD 2.3E-04 2.4E-01 porv.or.srv.reseat/emerg.power -seal.loca EP.REC 50 LOOP -rt/loop emerg.power -afw/emerg.power PORV.OR.SRV.CHALL CD 2.0E-05 2 .4E-01 porv.or.srv.reseat/emerg .power 55 LOOP -ri/loop emerg.power afw/emerg.power CD 1.2E-05 8.2E-02
-* non-recovery credit for edited case SEQUENCE MODEL: c:\asp\1989\pwrbseal.cmp BRANCH MODEL: c:\asp\1989\mcguire.sll PROBAB3ILITY FILE: c:\asp\1989\pwr;bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Event Identifier: 369/91-001
B-378 Branch System Non-Recov Opr Fail trans 4.3E-04 1.02+00 LOOP 1.6E-05 > 1.6E-05 3.62-01 > 3.02-01 Branch Model: INITOR Initiator Freq: 1.6E-05 loca 2.4E-06 4.32-01 rt 2. 8Z-04 1.2E-01 rt/loop 0 .02+00 1.02+00 emerg.power 2 . 9E-03 8.02-01 a fw 3 .8E-04 2.62-01 afwlemerg .power 5.02-02 3.42-01 mfw 1.02+00 7 .02E-02 1.02-03 PORV.OR. SRV.CHALL 4.02-02 > 1.02+00 1.02+00 Branch Model: l.OF.l Train 1 Cond Prob: 4.02-02 > 1.02+00 porv.or. srv.reseat 3.02-02 1.1E-02 porv.or. srv. reseat /emerg.power 3. 02-02 1. 02+00 seal.boca 0. 02+00 1.02+00 ep. rec (s1) 0.02+00 1. 02+00 EP.REC 4.52-01 > 3.5E-01 1.02+00 Branch Model: 1.OF.l Train 1 Cond Prob: 4.52-01 > 3.52-01 hpi 1.02-03 6.42-01 hpi (f/b) 2.22-03 8.42-01 1.02-02 hpr/-hpl 1.5E-04 1.02+00 1.02-03 porv. open 1 .02-02 1.02+00 4.02-04
- branch model file
- forced Minarick 03-11-1992 12:32:35 Event Identifier: 369/91-001