ML20135G720
| ML20135G720 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 05/14/2020 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Littlejohn J (301) 415-0428 | |
| References | |
| LER 1993-003-00 | |
| Download: ML20135G720 (8) | |
Text
A.5-1 A. 5 LER No. 3 13/93-003 Event
Description:
Both Trains of Recirculation Inoperable for 14 h Date of Event: September 30, 1993 Plant: Arkansas Nuclear One, Unit 1 A. 5.1 Summary On September 30, 1993, an engineering evaluation was completed at Arkansas Nuclear One, Unit 1, which indicated that the B decay heat removal/low-pressure injection (DHRILPI) pump might have been incapable of performing its recirculation mode function following a loss-of-coolant accident (LOCA). This condition existed from May 24, 1993, while that plant was at power, until the plant shutdown on September 9, 1993.
In addition, the A DHRILPI pump was also inoperable for 14 h during this time period for routine maintenance and surveillance. The estimated conditional core damage probability for this event is 5. 1 x 100.
The relative significance of this event compared to other postulated events at Arkansas Nuclear One, Unit 1, is shown in Fig. A. 5. 1.
LER 313/93-003 1E-7 1E-6 I E-5 1E-4 1E-3 I E-2 TRI L360 h AF Loo360 hEP Precursor Cutoff LOFW & 1 MTR AFW Fig. A. 5. 1 Relative event significance of LER 313/93-003 compared with other potential events at Arkansas Nuclear One, Unit 1 A. 5.2 Event Description On September 9, 1993, at 0432 hours0.005 days <br />0.12 hours <br />7.142857e-4 weeks <br />1.64376e-4 months <br />, a routine plant shutdown was in progress to begin a refueling outage at Arkansas Nuclear One, Unit 1. With the reactor coolant system (RCS) temperature at 180 0F, the B DHR/LPI pump was placed into service. At 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br />, the outboard motor bearing for the B DHRILPI pump alarmed on high temperature, and the pump was secured. The A DHRILPI pump remained in service.
Following verification of the operation of the oil slinger and testing of the pump oil, the B pump was restarted at 1224 hours0.0142 days <br />0.34 hours <br />0.00202 weeks <br />4.65732e-4 months <br />. The outboard motor bearing temperature again increased, and the pump was secured and declared inoperable at 1430 hours0.0166 days <br />0.397 hours <br />0.00236 weeks <br />5.44115e-4 months <br />.
Troubleshooting efforts indicated that there was no bearing damage. However, the pump and the motor were not properly coupled. The coupling hub on the pump shaft was installed -0. 316 in. too far toward the motor.
This condition caused the motor to be pushed off its magnetic center in the outward direction. Because the pump thrust bearing is on the opposite side of the pump from the motor, thermal expansion of the pump shaft while pumping hot fluids would push the shaft coupling farther in the outboard direction, creating increased thrust loading on the outboard motor bearing.
LER No. 3 13/93-003
A.5-2 The outboard motor bearing temperature response during past surveillance tests was reviewed. Although the tests are terminated before the bearings reach stabilization temperature, the stabilization temperature can be determined from the strip chart data. This review indicated that higher than normal (but acceptable) bearing temperatures were observed after May 24, 1993. A review of maintenance records indicated that the B DHR/LPI pump coupling was greased during a system mini-outage on May 24, 1993.
The B DHRILPI pump was considered capable of performing its LPI function throughout this period.
However, from May 24, 1993, until the plant shutdown on September 9, 1993, the B pump was considered inoperable in the recirculation mode while pumping hot water from the reactor building sump. The A DHR/LPI pump was also inoperable for 14 h during this time period for routine maintenance and surveillance activities.
A. 5.3 Additional Event-Related Information The DHR/LPI pumps are used in three modes. The first is the DHR mode that is used during plant shutdowns, heatups, and outages. In this mode the pumps take suction from one of the RCS hot legs. After passing through the pumps and DHR coolers, cooled water is returned to the RCS cold legs. The B pump failed while operating in this mode. The second is the LPI mode; in this mode, the pumps take suction from the refueling water storage tank (RWST) and discharge it to the RCS loops. This LPI mode is automatically initiated following a safety injection signal. The third is the recirculation mode that is used following the depletion of the RWST. The pump suction is aligned to the containment sump. After passing through the pumps, the water is cooled in the DHR coolers. The discharge can then be aligned directly to the RCS, or if RCS pressure is above the DHR/LPI pump shutoff head, to the suction of the high-pressure injection (HPI) pumps for "piggyback" operation.
During the LPI mode, when the pumps take suction on the cooler water in the RWST, the licensee indicates' that the B DHR/LPI pump will operate. However, when the warmer water from the containment sump passes through the pumps during the recirculation mode, the increased heat addition from the warmer water may cause the pump to fail due to the improper coupling of the motor.
A. 5.4 Modeling Assumptions Two cases were run. In the first case, both the A and B DHRILPI pumps were assumed inoperable in the recirculation mode for 14 h. This calculation was performed because both trains of the system (high- and low-pressure recirculation) were inoperable. The B pump would have initially operated but would have subsequently failed as described above. The A pump was out-of-service for routine maintenance and surveillance testing. It was assumed that the A pump could potentially be recovered during the injection phase of a postulated LOCA event, making it available for the recirculation mode. A nonrecovery factor of 0.34 was assumed (NUREG/CR-4674, Vol.17, Sect. A. 1.3, Recovery Class R2, failure appeared recoverable in the required period at the failed equipment, and the equipment was accessible; recovery from the control room did not appear possible).
The second case addresses the long-term unavailability of the B DHRILPI pump. This calculation was performed because the long-term unavailability of a single train of recirculation significantly impacts the conditional core damage probability for the event. The pump was assumed to be inoperable in the recirculation mode for 3048 h (from May 5, 1993, to September 9, 1993). The A pump was considered operable during this period with nominal failure rates and nonrecovery values applied.
LER No. 313/93-003
A.5-3 A. 5.5 Analysis Results The estimate of the conditional core damage probability for this event is 5.1 x 10-5. This consists of.a contribution of 4.9 x 10-6 for case I (both trains inoperable for 14 h) and 4.6 x 10-5 for case 2 (train B inoperable for 3048 h). The dominant core damage sequence for both cases, shown in Fig. A. 5.2, involves a postulated LOCA, successful reactor trip, auxiliary feedwater, and HPI, followed by failure of high-pressure recirculation.
LER No. 3 13/93-003
A.5-4 LOCA RT AFW MFW HPI HPR PORV OPEN SEQ END NO STATE OK 71 CD 72 CD OK 73 CD 74 CD OK 75 OK 76 CD 77 CD 78 ATWS Fig. A.5.2 Dominant core damage sequence for LER 313/93-003 LER No. 313/93-003
A.5-5 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 313/93-003 Event
Description:
Both trains of recirc inoperable Event Date: 09/30/93 Case: Case 1 - Both trains inoperable for 14 hours1.62037e-4 days <br />0.00389 hours <br />2.314815e-5 weeks <br />5.327e-6 months <br />, A train recoverable Plant: AND - Unit 1 UNAVAILABILITY, DURATION= 14 NONRECOVERABLE INITIATING EVENT PROBABILITIES LOCA 1.4E-05 SEQUENCE CONDITIONAL PROBABILITY SUNS End State/Initiator Probabi lity CD LOCA 4.9E -06 Total 4.9E-06 ATWS LOCA O.OE+OO Total O.OE+OO SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
71 Loca -rt -afw -hpi NPR/-HPI CD 4.9E-06 1.5E-01
- nonrecovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
71 Loca -rt -afw -hpi HPR/-HPI CD 4.9E-06 1.5E-01
- nonrecovery credit for edited case Note: For unavailabitities, 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 peried without the existing failures.
SEQUENCE MODEL: s:\asp\prog\models\pwrdseaL .cmp BRANCH MODEL: s:\asp\prog\models\anol .sil PROBABILITY FILE: s:\asp\prog\models\pwr-bsil .pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Nonrecov Opr Fail trans 1.4E-04 1.OE+0O loop 1.6E-05 3.6E-01 loca 2.4E-06 4.3E-01 rt 2.8E-04 1.2E-01 rt/Loop O.OE+OO 1.OE+00 LER No. 313/93-003
A.5-6 emerg power 2.9E-03 8.OE -01 afw 2.3E-03 2.6E-01 afwlenierg .power 5.OE-02 3.4E-01 mfw 2.OE-01 3. E-01 porv.or.srv.chaL I 8.OE- 02 1.OE+00 porv.or.srv. reseat 1.OE-02 1.1E-02 porv. or. srv. reseat/elnerg.power I OE-02 1.OE+00 seat. Loca 4.OE-02 1.OE+00 ep.rec(sl) 5.9E-01 1.OE+00 ep. rec 1.5E-01 1.0E+00 hpi 3.OE-04 8. E-01 hpi( f/b) 3.OE-04 8.4E-01 .1O0E-02 HPR/-HPI 1.5E-04 > 1.OE+00 1.OE+00 > 3.4E-01 1.0E-03 Branch Model: 1.OF.2+opr Train 1 Cond Prob: 1.OE-02 > Failed Train 2 Cond Prob: 1.5E-02 > Failed
- branch model file
- forced LER No. 3 13/93-003
A.5-7 CONDITIONAL CORE DAMAGE PROBABILITY CALCULATIONS Event Identifier: 313/93-003 Event
Description:
Both trains of recirc inoperable Event Date: 09/30/93 Case: Case 2 - Long term mnop of train B, Nominal values for A train Plant: ANO - Unit 1 UNAVAILABILITY, DURATION= 3048 NONRECOVERABLE INITIATING EVENT PROBABILITIES LOCA 3.1 E-03 SEQUENCE CONDITIONAL PROBABILITY SUMS End State/Initiator Probability CD LOCA 4.6E-05 Total 4.6E-05 ATWS LOCA O.OE+OO Total 0.OE+OO SEQUENCE CONDITIONAL PROBABILITIES (PROBABILITY ORDER)
Sequence End State Prob N Rec**
71 loca -rt -afw -hpi HPR/-HPI CD 4.6E-05 4.3E-01
- nonrecovery credit for edited case SEQUENCE CONDITIONAL PROBABILITIES (SEQUENCE ORDER)
Sequence End State Prob N Rec**
71 Loca -rt -afw -hpi HPR/-HPI CD 4.6E-05 4.3E-01
- nonrecovery 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: s:\asp\prog\models\pwrdseaL .cmp BRANCH MODEL: s:\asp\prog\modeLs\anol .sil PROBABILITY FILE: s:\asp\prog\models\pwr~bs~ll.pro No Recovery Limit BRANCH FREQUENCIES/PROBABILITIES Branch System Nonrecov Opr Fail trans 1.4E-04 1.DE+00 Loop 1.6E-05 3.6E-01 toca 2.4E-06 4.3E-01 rt 2.8E-04 1.2E-01 rt/Loop O.OE+OO 1.OE+00 LER No. 313/93-003
A.5-8 emerg .power 2.9E-03 8.0E -01 afw 2.3E-03 2.6E-01 afwlernerg .power 5.0E-02 3.4E-01 mfw 2.OE-01 3.4E-01 porv.or.srv.chal I 8.0E-02 1 O0E+00 porv.or.srv. reseat 1.OE-02 1.1E-02 porv.or.srv. reseat/emerg.power 1.OE-02 1.0OE+00 seal. Loca 4.OE-02 1.OE+00 ep.rec(sL) 5.9E-01 1.0OE+00 ep. rec 1.5E-01 1.0OE+00 hpi 3.OE-04 8.4E-01 hpi(f/b) 3.0E-04 8.4E-01 1 O0E-02 HPR/- HP I 1.5E-04 > 1.5E-02 1 O0E+00 1.OE-03 Branch Model: l.OF.2+opr Train 1 Cond Prob: 1.0E-02 > Failed Train 2 Cond Prob: 1.5E-02
- branch model file
- forced LER No. 313/93-003