Containment event analysis for postulated severe accidents: Surry power station, Unit 1: Volume 1: Draft report for comment

A study has been performed as part of the Severe Accident Risk Reduction Program (SARRP) to investigate the response of a particular pressurized water reactor with a subatmospheric containment (Surry Unit 1) to postulated severe accidents. A detailed containment event tree for the Surry plant has been devised to describe the various possible accident pathways that can lead to radioactive releases from containment. Data and analyses from a large number of NRC and industry-sponsored programs have been reviewed and used as a basis for quantifying the event tree, i.e., determining the likelihood of each pathway for a variety of accident sequence initiators. A generalized containment event tree code, called EVNTRE, has been developed to facilitate the quantification. The uncertainty in the results has been examined by performing the quantification three times, using a different set of input each time to represent the variation of opinion in the reactor safety community. In the so-called ''central'' estimate, the likelihood of early containment failure (occurring before or at the time of reactor vessel breach) was found to be very low for most accident sequence initiators. However, uncertainties surrounding the issues of direct containment pressure capacity could lead to much higher early failure likelihoods. This work supports NRC's assessment of severe accident risks to be published in NUREG-1150