ML20135H304
| ML20135H304 | |
| Person / Time | |
|---|---|
| Site: | Perry  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 440-1991-009 | |
| Download: ML20135H304 (6) | |
Text
B-467 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER.No.:
Event
Description:
Date of Event:
Plant:
440191-009 Two EDGs inoperable March 14, 1991 Perry Summary Perry was operating at 100% power on March 14, 199 1, when the Division 2 emergency diesel generator (EDG) failed a surveillance test. Subsequently, the Division 1 EDG also failed its surveillance test. It took 11 h and 55 min to restore one EDG to operable status.
It was later determined that one EDG had been inoperable for over 28 d, and the other EDG was potentially unavailable for 15 d. The conditional core damage probability est.imated for this event (assuming -both EDGs were unavailable for 15 d) is 5.3 x 10-4 The relative significance of this event compared to other postulated events at Perry is shown below.
-LER 440/91-009 1E71I IE-IE-E31-L 360 hHPCS Trip LOFW +
precursor cutoff -'
HPCS Event Description Perry was operating at 100% of rated power at 0915 hours0.0106 days <br />0.254 hours <br />0.00151 weeks <br />3.481575e-4 months <br /> on March 14, 199 1, when the Division 2 EDG failed its monthly surveillance test. The EDG reached rated speed but was unable to generate any output voltage because the field contactor failed to close.
Subsequent investigations revealed that excessive play in the pivot point of the latch mechanism allowed the latch arm to twist sideways and weld the contacts in the trip position of the KI close coil of the field contactor. It was concluded that this could only have occurred during shutdown of the EDG during the last surveillance test performed February 14, 1991. Thus, the EDG had been out of service for the entire 28-d period between tests.
B-468 When the Division 2 EDG failed its surveillance test, plant Technical Specifications required that the Division 1 EDG be demonstrated operable. During this demonstration run, the Division 1 EDG started and came up to rated speed, but the operators were unable either remotely or locally to synchronize the generator to the grid. The Division 1 EDG was also declared inoperable. In this situation, the plant Technical Specifications require either one EDG to be returned to operability within 2 h or for shutdown to begin.
In spite of this, the plant remained at full power throughout the -12 h time required to repair the Division 2 EDG.
The plant decided to repair the Division 2 EDG first, since the failed component was known. Field contactor repair parts for the Division 2 EDG were not available from warehouse stock and were instead obtained from the Division 1 EDG. The Division 2 EDG was repaired, tested, and placed back in service at 0220 hours0.00255 days <br />0.0611 hours <br />3.637566e-4 weeks <br />8.371e-5 months <br /> on March 15, 1991.
At this time, investigation and troubleshooting was begun on Division 1 EDG. The utility concluded that the lower limit switch on the motor-operated potentiometer was malfunctioning although no conclusive cause was ever identified. If this was the case, then the EDG would be expected to function in the event of a loss of offsite power (LOOP). However, after inspection, cleaning, exercising, and post-maintenance testing, the problems that prevented the governor speed control from functioning during the surveillance test could not be recreated. The utility believed that the conditions causing the event were corrected during the maintenance activities. Division 1 EDG was placed back in service at 2305 hours0.0267 days <br />0.64 hours <br />0.00381 weeks <br />8.770525e-4 months <br /> on March 15, 1991. Both EDGs were inoperable at the same time for repairs for 11 h and 55 min If the cause of the Division 1 EDG failure would have prevented its operation following a LOOP, then both EDGs would have been inoperable for an expected 15 d.
Additional Event-Related Information Offsite power is available to the Perry 1 345-ky switchyard from five 345-ky transmission circuits. Engineered safety features (ESF) loads are assigned to three independent load groups designated Divisions 1, 2, and 3. Divisions 1 and 2 are redundant, while Division 3 supplies power for the high-pressure core spray (HPCS) system. Each division consists of a 4. 16-ky switchgear bus and assembly, diesel generator standby power supply, motor control centers, batteries, and battery chargers.
The preferred power supply for the class 1E buses is the startup transformer. Alternate power is available through a manual normally-open disconnect from the unit 2 startup transformer. Emergency loads for Divisions 1 and 2 are supplied from EDGs rated at 7,000 kW.
ASP Modeling Assumptions and Approach Division 2 EDG was inoperable for more than 28 d, and Division 1 EDG was unable to synchronize to the grid when required. Since the cause for the latter unavailability was
B-469 never positively identified, nor was it demonstrated that the EDG would have assumed loads following a postulated LOOP, it was assumed that the Division 1 EDG was unavailable for one half its surve 'illance period of 15 d. Therefore, this event has been modeled as a postulated LOOP with both the EDGs unavailable and nonrecoverable for 15 d.
Analysis Results The conditional probability of subsequent core damage estimated for two EDGs unavailable for 15 d is 5.3 x 10O4. A sensitivity analysis was performed to ascertain the effect of out of service time on the conditional probability. If it is assumed that the two EDGs were only unavailable for the 11.9 h repair time, the conditional probability of core damage decreases by more than an order of magnitude to 1.7 x 1i-5.
The dominant core damage sequence, highlighted on the following event tree, involves a postulated LOOP, a nonrecoverable emergency power system failure, and failure to restore AC power prior to battery depletion.
B-470 Rn LOOP RHR pa 666 RHRSW E~SHUT REG OR 650~
Q 6
46W LC (SDC Sp OR SEQ END 00404C 'L,,,,
Cý PS ADS M4ODE) hDOE, OTHER NO STATE 40 DOE DM4AGE 41 0066 WMAGE CK OK ODE DMG OK DOR EDAMAGE 44 R DEAMAGE OK 45 ORE DAMAGE 46 COEDMG 44 COE0AMAG 4a COE DAMAGE 44 GOE DAMAGE GE COR EDAMAGE DOE DMG 51 ~
EDAMAGE GE 53 COE DAMA 54 ODEDAMAO 5
CORE DAMAGE GE DOR
~ EAMAGE O
EDAMAGE 66 REDAMAG so OOE DAMAGE 64 R
EDAMAGE 85 DOE DAMAGE 66 OO6E DAMAGE OE DAMAGE OK GE so OOE DAMAGE 67 OOE DAMAGE a2 EDAMAGE GOR~E DAMAGE C
AT.AMG Dominant core damage sequence for LER 440/91-009
B-471 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
440/91-009 Event
Description:
Two emergency diesel generators inoperable Event Date:
03/14/91 Plant:
Perry 1 UNAVAILABILITY, DURATION-360 NON-RECOVERAB3LE INITIATING EVENT PROBABILITIES LOOP SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator 3.1lE-03 Probability CD LOOP Total ATWS 5.3E-04 5.3E-04 0.OE+00 0.OE+00 LOOP Total SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence 83 loop EMERG.POWER -rx.shutdown/ep ep.rec
-* non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
End State Prob N Rec**
CD 5.3E-04 5.3E-01 Sequence End State 83 loop EMERG.POWER -rx.shutdown/ep ep.rec CD
- non-recovery credit for edited case Note:
For unavallabilities, conditional probability values are differential values added risk due to failures associated with an event.
Parenthetical values indicate compared to a similar period without the existing failures.
SEQUENCE MODEL:
c: \\asp\\1989\\bwrcseal.cmp BRANCH MODEL:
c:\\asp\\1989\\perry.sll PROBAB3ILITY FILE:
c:\\asp\\1989\\bwr_csll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES P rob N Rec**
5.3E-04 5.3E-01 which reflect the a reduction in risk Opr Fail Branch trans System 7.7E-04 Non-Recov l.OE+00 Event Identifier: 440/91-009
B-472 loop loca rx -shutdown rx. shutdown/ep pcs/trans srv.challftrans.-scram srv.chall/loop.-scram srv.close EMERG.POWER Branch Model:
1.OF.2 Train 1 Cond Prob:
Train 2 Cond Prob:
ep. rec fw/pcs.trans fw/pc. loca hpci rcic crd srv.ads lpCS lpci (rhr) /lpcs rhr (sdc) rhr (sdc) /-lpci rhr (ado)/lpci rhr (spcool)/rhr (sdc) rhr(spcool) /-lpci.rhr(sdc) rhr(spcool) /lpci.rhr(adc) rhrsw 1.6E-05 3.3E-06 3.OE-05 3.5E-04 1.7E-01.
l.OE+OO 1.OE+OO 6.3E-02 2.9E-03 > l.OE+OO 5.OE-02 >Failed 5.7E-02 > Failed 1.7E-01 4.6E-01 l.OE+OO 2.OE-02 6.OE-02 l.OE-02 3.7E-03 2.OE-02 6.OE-04 2.3E-02 2.09-02 1.OE+OO 2.OE-03 2.OE-03 9.3E-02 2.OE-02 5.3E-01 5.OE-Ol 1.OE+OO l.OE+OO l.OE+OO l.OE+OO l.OE+0O 1.aE+oo S.OE-Ol
> l.OE+OO 1.OE+00
- 3. E-OI.
3.4E-01
- 3. 4E-01 7.OE-Ol 1.OE+0O 7.1E-Ol 3.4E-01 7.1E-01 3.4E-Ol 3.4E-Ol
- 1. OE +00 3.4E-01 3.4E-01 l.OE+00 3.4E-01 l.OE-02 l.OE-02 l.OE-03 l.OE-03 l.OE-03 2.OE-03
- branch model file
- forced Minarick 03-14-1992 16:23:45 Event Identifier: 440/91-009