ML20135G751
| ML20135G751 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 285-1990-020 | |
| Download: ML20135G751 (5) | |
Text
B-161 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No.: 285/90-020 Event
Description:
EDG failure, similar condition with other EDG Date of Event: September 13, 1990 Plant: Fort Calhoun Summary High ambient air temperatures resulted in failure of an emergency diesel generator (EDG) during a performance test. Electronic components of the generator's voltage regulator were located in an unvented cabinet. After 2 h of operation, these components overheated and failed. The other EDG was not subjected to a similar test but was identically configured. The conditional probability of core damage associated with this event is 6.5 x 10-4 The relative significance of the event compared to other postulated events at Fort Calhoun is shown below.
Event Description Approximately 2 h into a test run of EDG 1, generator output current increased from a normal operating level of 385 amp to 575 amp. Operators attempted to reduce the generator output but were unsuccessful. When they opened the generator output breaker, the generator output voltage dropped to 2120 V-AC, about one-half of normal. Operators then shut down the EDG and declared it inoperable.
An investigation into the failure determined that components in the generator exciter circuit had overheated and failed. These components were located in an unvented cabinet in the room with the EDG. Ambient air temperature was elevated and temperatures as
B- 162 high as 1401F were measured on the surface of the exciter cabinet. This was judged sufficiently high to cause semiconductor components of the exciter system to fail. An identical design problem was identified on the other EDG.
Additional Event-Related Information Two diesel generators at Fort Calhoun are intended to provide AC power to vital equipment in the event of a loss of offsite power (LOOP). In the event of a LOOP and coincident failure of the EDGs, two batteries can provide instrumentation and control power for a period of up to 8 h. There is one turbine-driven auxiliary feedwater pump, which can provide water to the steam generators to remove decay heat during this time.
ASP Modeling Assumptions and Approach The event has been modeled as a potential LOOP with both EDGs unavailable after 2 h.
A frequency of 0.012/reactor year was assumed for a LOOP with a duration greater than 2 h (see ORNL/NRC/LTR-89/1 1, Revised Loop Recovery and Seal LOCA Models, August 1989). A period of vulnerability of 0.2 yr was assumed, since it is believed that high ambient temperatures were a necessary contributor to the EDG failure.
The contributions of core damage probability of seal failure sequences were neglected in this analysis as they are accounted for in another analysis. LER 285/90-025 details a finding that, as a result of design deficiencies, a loss of instrument air would result in a failure of the component cooling water (CCW) system. As CCW cools the charging pumps, they are aiso considered to be rendered inoperable by a loss of instrument air.
Failure of both CCW and charging would place the reactor coolant pump (RCP) seals at risk. These failures could be expected during any LOOP, and the associated risks related to seal failures are considered in the analysis for 285/90-025.
Analysis Results The probability of a LOOP greater than 2 h in duration is about 2 x 10-6 /h. The probability of such a LOOP over a 0.2-yr period (-1226 critical h) is therefore 0.0024.
With the EDGs assumed unavailable, recovery requires restoration of offsite power in the long term (nonrecovery probability, p = 0.256). The conditional core damage probability estimated for this event is 6.4 x 10-4 The dominant core damage sequence is highlighted on the following event tree. This sequence involves a LOOP with a duration greater than 2 h, emergency power system failure, and failure to recover AC power prior to battery depletion.
B- 163 ILOOP RT/LOOPI EP IAMW SRV SRV ISEAL EP REC IHPI HPR OPEN CSR SEC END NO STATE II CHALL ReSEAT LOCA I(LONG) I I IOE OK OK 41 CD 42 CO OK OK 50 CD 43 CD 44 CD 45 CD OK 46 CD 47 CD 48 CO OK 40 CO 50 CO OK 5i CO 52 CO 53 CO OK 54 CO 55 CO 40 ATWS Dominant core damage sequence for LER 285/90-020
B-164 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 285/90-020 Event
Description:
ED~s unavailable under high amb~ient temperatures Event Date: 09/13/90 Plant: Fort Calhoun UNAVAILABILITY, DURATION- 6132 NON-RECOVERABLE INITIATING EVENT PROBABILITIES LOOP2 .4E-03 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOOP 6.5E-04 Total 6.5E-04 ATMS LOOP 0.OE+00 Total0.OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
54 LOOP -rt/loop EMERG.POWER -afw/emerg.power -porv.or.srv.chall - CD 5.9E-04 1.2E-01 SEAL.LOCA EP.REC 55 LOOP -fl/loop EMERG.POMER afw/emerg.power CD 3.9E-05 4.1E-02 49 LOOP -fl/loop EMERO.POWER -afw/emerg.power porv.or.srv.chall - CD 2.4E-05 l.2E-01 porv.or.srv.reseat/emerg.power -SEAL.LOCA EP.REC
- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
49 LOOP -fl/loop EMERG.PUMER -afw/emerg.power porv.or.srv.chall - CO 2.4E-05 1.2E-01 porv.or.srv.reseat/emerg.power -SEAL.LOCA EP.REC 54 LOOP -fl/loop EMERG.POMER -afw/emerg.power -porv.or.srv.chall - CO 5.9E-04 1.2E-0l SEAL.LOCA EP.REC 55 LOOP -fl/loop EMERG.POWER afw/emerg.power CO 3.9E-05 4.lE-02
-* non-recovery credit for edited case Note: For unavailabilities, conditional probability values are differential values which reflect the added risk due to failures associated with an event. Parenthetical values indicate a reduction in risk compared to a similar period without the existing failures.
SEQUENCE MODEL: c:\asp\19B9\pwrgseal.cmp BRANCH MODEL: c:\asp\1989\calhoun.sll PROBABILITY FILE: c:\asp\19B9\pwr~bsll.pro Event Identifier: 285/90-020
B- 165 No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 6.OE-OS 1. OE+OO LOOP 1.6E-05 > 3.3E-06 5.3E-01 > 1.2E-01 Branch Model: INITOR Initiator Freq: 1.6E-05 > 3.3E-06 loca 2 .4E-06 4.3E-01 rt 2 .8E-04 1.2E-01 rt /loop O.OE+OO 1. OE+OO EMERG.POWER 2.9E-03 > l.OE+OO 8.OE-Ol > 1.OE-i-O Branch Model% l.OF.2 Train 1 Cond Prob: 5.OE-02 > Failed Train 2 Cond Prob: 5.7E-02 > Failed afw 2. 3E-03 2. 6E-Ol afw/emerg.power 5.OE-02 3. 4E-01 mfw 2 .OE-Ol 3.4E-01 porv.or.srv.chall 4. OE-02 1. OE+OO porv.or .srv. reseat 2 .OE-02 l.lE-02 porv.or.srv. reseat/emerg.power 2 .OE-02 1.OE+OO SEAL.LOCA 4.6E-02 > O.OE+OO
- l.OE+0O Branch Model: l.OF.1 Train I Cond Prob: 1 .4E-02 hpi 1 .OE-03 8.4E-01 hpi (f/b) 1 .OE-03 8. 4E-01 1. OE-02 porv.open l.OE-02 1. OE+OO 4. OE-04 hpr/-hpi 1 .5E-04 1. OE+OO car 2 .OE-03 3. 4E-01
- branch model file
- forced Minarick 08-06-1991 17:32:47 Event Identifier: 285/90-020