ML20156A234
| ML20156A234 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 06/04/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1989-033-00 | |
| Download: ML20156A234 (9) | |
Text
B-166 ACCIDENT SEQUENCE PRECURSOR PROGRAM EVENT ANALYSIS LER No:
Event
Description:
Date:
Plant:
327/89-033 RWST level and other instrument transmitters inoperable December 16, 1989 Sequoyah 1 and 2 Summary During a cold snap, all four level transmitters that indicate refueling water storage tank (RSW*T) level on Unit 1 froze and failed to indicate correctly. This would have prevented automatic switchover from the RWST to the containment sump in the event of a LOCA or feed and bleed. Main steam pressure and feedwater flow indicators also froze and failed.
Similar difficulties were experienced on Unit 2. The conditional probability of core damage estimated for this event is 4.8 x 10-6 on Unit 1 and 3.6 x 10-6 for Unit 2. The relative significance of this event compared with other postulated events at Sequoyah is shown below.
Event Description On December 16, 1989, Sequoyah 1 was at 100% power when, one after the other, all four refueling water storage tank level transmitters (LT 63-50, -51, -52, -53) froze and failed high. These failures were attributed to the unusually low temperature experienced (8'F), as well as to other causes, including the deenergization of heat tracing on the instruments. This condition existed for approximately 39 h on Unit 1. A similar condition (three of four transmitters inoperable) existed on Unit 2 for approximately 25 h.
B-167 The RWST level transmitter heat tracing was de-energized around September 1989, in response to concerns that the heat trace thermostats were not known to be class lE rated but were capable of impairing the performance of class lE equipment. Calculations performed at the time indicated that the level transmitters would function acceptably at low temperatures. The heat tracing was originally installed in response to 1980-8 1 incidents in which the RWST sensing lines also froze.
Main steam pressure transmitter PT-lI-9A froze and failed high on Unit 1. This pressure transmitter indicates when main steam pressure is low, providing input to flow transmitter FT7-1-1 OA. Low main steam pressure, coincident with high steam line flow, initiates steam line isolation and safety injection. The instrumentation remained inoperable for about 37 h.
Feedwater flow transmitter FT-3-90A froze and failed high on Unit 2. This transmitter provides indication to the steam flow/feed flow mismatch reactor trip circuitry. Steam flow/feed flow mismatch coincident with low steam generator level will initiate a reactor trip. At the time it failed, the flow transmitter's signal was also being used for steam generator level control. When it failed high, flow to the loop 3 steam generator was reduced, requiring prompt operator intervention to prevent a steam generator low level reactor trip.
The transmitter was apparently inoperable for about 33 h.
The instrumentation failures were attributed to the temperature, to improper insulation, and to greater than expected cooling by ventilation air flow.
Additional Event-Related Information During a LOCA, the residual heat removal (RHR) system automatically switches from borated water injection to recirculation when the RWST level transmitters indicate a low level (coincident with high level in the containment sump). The RWST level transmitters failed "high," indicating the presence of a high level in the tank. Had a LOCA occurred during the time that the transmitters were inoperable, or if bleed and feed had been required for trip mitigation, the automatic switchover of suction supply to the RHR pumps from RWST to sump would not have occurred. The switchover could have been accomplished manually, however, if done before the RHR and HPI pumps were damaged.
ASP Modeling Assumption and Approach The event has been modeled as a 38.75-h unavailability of high-pressure recirculation on Unit 1. The likelihood of not recovering from the unavailability was assumed to be 0. 12,
B-168 indicative of a nonroutine and nonproceduralized in-control room action.
For Unit 2, the high potential for reactor trip when FT-3-90A failed high was addressed by modifying the transient frequency used in the calculation such that the probability of trip over the 25-h unavailability of RWST indicaton was 0.3. Other assumptions for Unit 2 were the same as for Unit 1.
Analysis Results The conditional core damage frequency estimated for the event on Unit 1 is 4.8 x 10-6.
For Unit 2, the estimated core damage probability is 3.6 x 10-6. For both events, the dominant sequence involves a postulated LOCA with successful high-pressure injection and failure of high-pressure recirculation due to failure to correct switch from the RWST to the containment sump. This sequence is highlighted on the following event tree.
B-169 FiOCA RT IAFW IMFW HPI IHPR P01W I[I_ _
OPEN SEO END NO STATE OK 71 CD 72 00 OK 73 CD 74 OD OK 75 GO0(1) 76 CD 77 CD 78 ATWS (1)0OK for Class D Dominant core damage sequence for LER 327/89-033 (Sequoyah 1 calculation)
B-170 LOA RT AFW MFW HPI HPR ýPRV SEO END
________________________OPEN NO STATE OK 71 CD 72 00 OK 73 00 74 CD OK 75 CD (1) 76 CD 77 C0 78 ATWS (1)0OK for Class D Dominant core damage sequence for LER 327189-033 (Sequoyah 2 calculation)
B3-171 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
327/89-033 Event
Description:
RWST level and other instrument transmitters inoperable Event Date:
12/16/89 Plant:
Sequoyah 1 UNAVAILABILITY, DURATION= 38.75 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS
- 4. OE-05 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD TRANS 5.5E-08 LOOP 1.5E-08 LOCA 4.7E-06 Total 4.8E-06 ATWS TRANS
- 0. OE+00 LOOP
- 0. OE+00 LOCA
- 0. OE-I-C Total
- 0. OE+00 SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec*t 71 loca -rr -afw -hpi NPR/-NPI CD 4.7E-06 5.2E-02 non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
71 loca -rt -afw -hpi NPR/-NPI CD 4.7E-06 5.2E-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\\1989\\pwrbseal.cmp BRANCH MODEL:
c:\\asp\\1989\\sequoyah.s11 PROBABILITY FILE:
c:\\asp\\1989\\pwr7bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Non-Recov Opr Fail trans 7.7E-04 l.0E+00 loop 1.6E-05 5.3E-01 loca 2.4E-06 4.3E-01 rt 2.8E-04 1.2E-01 rt/loop 0.OE+00 1.05+00 Event Identifier: 327/89-033
B -172 erserg. power afw afwfemerq.power sifw porv.or.srv.chall porv.or.srv. reseat porv.or.srv.reseat/emerg.power seal. loca ep.rec (si) ep. rec hpi hpi (f/b)
HPR/-HPI Branch Model:
1.OF.2+opr Train 1 Cond Prob:
Train 2 Cond Prob:
porv.open
- 2. 9E-03 3.8E-04 5.OE-02
- 1. OE+OO 4.OE-02
- 2. OE-02
- 2. OE-02
- 2. 7E-01 5.7E-01 7.OE-02 1.OE-03 1.OE-03 1.5E-04 > l.OE-s-O *
- 1. OE-02 1.5SE-02
- 1. OE-02
- 8. OE-O1 2.6SE-01 3.4 E-01
- 7. OE-02
- 1. OE+OO 1.1E-02
- 1. OE+OO
- 1. OE+OO
- 1. OE-4-O
- 1. OE+OO
- 8. 4E-01 8.4 E-01 l.OE+OO > 1.2E-O1
- 1. OE+OO 1.OE-02 1.OE-03 > O.OE+OO 4.OE-04
- branch model file
- forced Minarick 06-14-1990 11:28:20 Event Identifier: 327/89-033
B-173 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier:
327/89-033 Event
Description:
Potential trip with unavailable RMST Indication Event Date:
12/16/89 Plant:
Sequoyah 2 UNAVAILABILITY, DURATION= 25 NON-RECOVERABLE INITIATING EVENT PROBABILITIES TRANS LOOP LOCA SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator
- 3. OE-01 2.2E-04
- 3. 6E-06 8.1E-06
- 0. OE+00
Sequence End State 71 loca -ft -afw -hpi HPR/-HPI CD 11 TRANS -rt -afw porv.or.srv.chall porv.or.srv.reseat -hpi NPR/
CD
-HPI 16 TRANS -rt afw mfw -hpi(f/b)
HPR/-NPI CD 18 TRANS rt ATWS
-- non-recovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State 11 TRANS -rt -afw porv.or.srv.chall porv.or.srv.reseat -hpi HPR/
CO
-NP I 16 TRANS -rt afw mfw -hpi(f/b)
NPR/-HPI CD 18 TRANS rt ATWS 71 loca -rt -afw -hpi NPR/-HPI CD non-recovery credit for edited case Note:
For unavailabilities, 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\\pwrbseal.cmp BRANCH MODEL:
c:\\asp\\1989\\sequoyah.sil PROBABILITY FILE:
c:\\asp\\1989\\pwr bsll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Prob
- 3. IE-06 2.-7E-07 2.2E-07 8.1E-06 Prob 2. 7E-07 2.2E-07 8.1lE-06 3.1E-06 N Rec**
5.2E-02
- 1. 3E-03 2.2E-03 1.2E-01 N Rec t t 1.36-03
- 2. 2E-03
- 1. 2E-01 5.2E-02 which reflect the a reduction in risk Event Identifier: 327/89-033
B -174 Branch System Non-Recov Opr Fail TRANS Branch Model:
rNITOR Initiator Freq:
loop 1 oca tt rt/loop emerg. power a fw afw/emerg.power infw Porv.or.srv~chall Porv.or.sri, reseat porv.or.srv reseat/emerg power seal. loca ep.rec(sl) ep. rec hpi hpi (f/b)
HPR/-HPI Branch Model:
l.OF.2+opr Train 1 Cond Prob:
Train 2 Cond Prob:
porv.open
- branch model file
- forced 7.7E-04 > 1.2E-02 *
- 7. 7E-04
- 1. 6E-05 2.4E-06 2.8E-04
- 0. OEfOO
- 2. 9E-03
- 3. 8E-04 5.0OE-02 I.0E+00 4.0E-02
- 2. 0E-02 2.05-02
- 2. 7E-01
- 5. 7E-01 7.OE-02 1.OE-03 2.05-03 1.5E-04 > 1.0E÷00
- 1.05-02 1.5E-02 1.05-02
- 1. OE+00
- 5. 3E-01
- 4. 3E-01 I.2E-01 I. 05÷00 8 OE-01
- 2. 65-01 3 4E-01 7.0E-02 1.05+00 1.1E-02
- 1. 5E+00 I. 5E+00 1.05+00
- 1. OE+00 8.4 E-01
- 8. 4E-01 l.0E+/-00 > 1.2E-01
- 1. 05+00
- 11.
OE-02 1.OE-03 > 0.05÷00 4.0E-04 Minarick 06-14-1990 11:25:09 Event Identifier: 327/89-033