Traditionally, probabilistic risk assessments (PRAs) of severe accidents at nuclear power plants have explored accidents initiated during full-power operation. However, in 1989 the U.S. Nuclear Regulatory Commission (NRC) initiated an extensive program to examine carefully the potential risks during low-power and shutdown operations. The program included two parallel projects, one at Sandia National Laboratories studying a boiling water reactor (Grand Gulf), and the other at Brookhaven National Laboratory studying a pressurized water reactor (Surry Unit 1). Both the Sandia and Brookhaven projects have examined only accidents initiated by internal plant faults-so-called "internal initiators". This project, which has explored the likelihood of seismic-initiated core damage accidents during refueling outage conditions, is complementary to the internal-initiator analyses at Brookhaven and Sandia. This report covers the seismic analysis at Grand Gulf, while a companion report documents the Surry seismic analysis. All of the many systems modeling assumptions, component non-seismic failure rates, and human error rates that were used in the internal-initiator study at Grand Gulf have been adopted here, so that the results of the study can be as comparable as possible. Both the Sandia study and this study examine only one shutdown plant operating state (POS) at Grand Gulf, namely POS 5 representing cold shutdown during a refueling outage. This analysis has been limited to work analogous to a level-1 seismic PRA, in which estimates have been developed for the core-damage frequency from seismic events during POS 5. The methodology is almost identical to that used for full-power seismic PRAs, as widely practiced in the nuclear industry. The results of the analysis are that the core-damage frequency for earthquake-initiated accidents during refueling outages in POS 5 is found to be quite low in absolute terms, less than 10"r/year. The core-damage frequency is also low relative to the frequency during POS 5 for internal initiators, as analyzed in the companion study by Sandia.
Traditionally, probabilistic risk assessments (PRAs) of severe accidents at nuclear power plants have explored accidents initiated during full-power operation. However, in 1989 the U.S. Nuclear Regulatory Commission (NRC) initiated an extensive program to examine carefully the potential risks during low-power and shutdown operations. The program included two parallel projects, one at Brookhaven National Laboratory studying a pressurized water reactor (Surry Unit 1) and the other at Sandia National Laboratories studying a boiling water reactor (Grand Gulf). Both the Brookhaven and Sandia projects have examined only accidents initiated by internal plant faults-so-called "internal initiators". This project, which has explored the likelihood of seismic-initiated core damage accidents during refueling shutdown conditions, is complementary to the internal-initiator analyses at Brookhaven and Sandia. This report covers the seismic analysis at Surry Unit 1, while a companion report documents the Grand Gulf seismic analysis. All of the many systems modeling assumptions, component non-seismic failure rates, and human error rates that were used in the internal-initiator study at Surry have been adopted here,so that the resultsof the two studiescan be as comparable as possible.Both the Brookhaven study and thisstudy examine only two shutdown plantoperatingstates (POSs) during refuelingoutages at Surry,calledPOS 6 and POS 10,which representmid-loop operationbefore and afterrefueling, respectively. This analysishas been limitedto work analogousto a levcl-IseismicPRA, in which estimateshave been developed for the coredamage frequency from seismiceventsduring POSs 6 and 10. The methodology isalmost identical tothatusedforfull-powerseismicPRAs, aswidciypracticed inthenuclearindustry. The results of the analysisare that the core-damage frequency for earthquake-initiated accidentsduring refueling outagesin POS 6 and POS I0 isfound tobe low in absoluteterms, lessthan 10"6/year.The core-damage frequenciesare alsolow relativeto the frequencies during POS 6 and POS I0 for internalinitiators, as analyzed in the companion study by Brookhaven.
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