ML042960260

Page Starts with Risk Significance of the Event...

ML042960260
Person / Time
Site:	Palo Verde
Issue date:	10/08/2004
From:	- No Known Affiliation
To:	Office of Nuclear Reactor Regulation
References
FOIA/PA-2004-0307
Download: ML042960260 (2)

Text

David Loveless - Risk AIT Reportwwdd Inputttto Page Pane 113 David Loveless Risk Input to AlT Report.wpd Risk Significance of the Event The initial risk assessment for Unit 2 resulted in a conditional core damage probability (CCDP) of 6.5 x 10 4. The initial CCDP for Units 1 and 3 was estimated as 3.2 x 1 04 per unit.

Subsequently, the team, assisted by Office of Nuclear Regulatory Research personnel, completed a detailed risk assessment for the event. This analysis used the Standardized Plant Analysis Risk (SPAR) Model for Palo Verde 1, 2, & 3, Revision 3.03, to estimate the risk. The analyst assumed that 95 percent of loss of offsite power events, similar to the June 14th event, would be recovered within 2-1/2 hours. The resulting CCDPs were 3 x 10 5, 4 x 104, and 3 x 10-for Units 1, 2, and 3, respectively.

The team gathered information concerning the failed emergency diesel generator and charging pump in Unit 2. Other equipment problems including turbine-driven auxiliary feedwater pump drains, power-operated relief valves problems, and i breaker issues were assessed. In addition, the team evaluaod theW qhilitv of th, licen Wee. .ovfry offsite power, the probability that power could [ ?rc . .. ..... vial buses from the gas turbine generators had it been needed, and the c ability of vital and nonvital batteries to continue to provide control power, had a station black ut occurred.

The team made the following assumptions critical to the analysis:

• The Unit 2 Emergency Diesel Generator A failed and could not have been recovered prior to postulated core damage.

• A Unit 2 licensed operator misaligned the suction path to Charging Pump E causing the pump to trip on low suction pressure. The pump could not have been recovered prior to postulated core damage because the pump was air bound.

• The required mission times, during this specific event, for the emergency diesel generators and the turbine-driven auxiliary feedwater pump were 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.

• Recovery of vital power to the first vital bus was possible within one hour following a postulated station blackout.

• The probability that operators failed to restore offsite power within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> was 4 x 10.2 as determined using the SPAR-H method. The nominal action failure rate of 0.001 was modified because the available time was barely adequate to accomplish the breaker alignments necessary, the operator stress level would have been high, and the actions required were of moderate complexity.

• The probability that operators failed to restore offsite power prior to the core becoming uncovered during a reactor coolant pump seal LOCA was estimated as 4 x 104. The same performance shaping factors were used as for the 1-hour recovery with the exception of the time available. The team determined that the time available was nominal, because there would be some extra time, above what is minimally required, to execute the recovery action.

• The failure probability for recovery of offsite power prior to battery depletion during a station blackout was estimated as 4 x 103. The same performance shaping factors were used as for the seal LOCA recovery.

David Loveless - HiK Input to Al I Keport.wpa_

David Loveless t-IIsk Input 10 Al I HeDort.wpd

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• The team concluded that the failures of 13.8 kV feeder breakers in Units 1 and 3 would have increased the complexity in recovering offsite power for these units. However, the potential contribution of common cause failure probabilities would not greatly impact the nonrecovery probabilities described previously for Unit 2.

• The Palo Verde gas turbine generators used for station blackout could be started and loaded within one hour of blackout initiation. One gas turbine generator can provide power to switchyard components and supply one Unit 1vital 4160 volt bus. Both generators can provide one vital bus on Units 1 and 2 or Units 1 and 3, but not Units 2 and 3.

To account for the offsite power circumstances on June 14, 2004, the team modified the SPAR to include updated loss of offsite power recovery curves based on actual grid conditions and estimated probabilities of'human actions failing. Additionally, modeling of the Palo Verde gas turbine generators was improved to better represent their contribution in providing power to vital buses if needed. The team determined that this modified SPAR was an appropriate tool to assess the risk of this event.

The team set the likelihood of a loss of offsite power to 1.0, and the likelihood of all other initiating events were set to the house event FALSE, indicating the assumption that it is unlikely that two initiating events would occur at the same time. The failure to start and failure to run basic events for both Emergency Diesel Generator A and Charging Pump E were set to the house event TRUE, permitting calculation of the probability that similar components would fail from common cause. The SPAR model was quantified following the modifications, and the mean of the best estimate CCDPs were obtained through Monte Carlo simulation of the event.