United States experience in probabilistic risk assessment for a graphite moderated, channel reactor

The United States operated a number of graphite moderated, water cooled, pressure tube (or channel type) reactors at the Hanford Site in Washington State. The last of these reactors to operate was the Hanford N Reactor which was designed to produce steam for the generation of electricity for civilian energy use as well as special nuclear material. In 1982, a review team recommended that a probabilistic risk assessment (PRA) be conducted for the reactor. The basis for this recommendation was that the reactor represented a unique facility and therefore the safety insights gained from operations at other facilities were not directly transferable to it. Furthermore, the reactor was going through a series of upgrades and it was felt that the selection of competing improvements would benefit from a risk perspective to help judge the relative value of these upgrades. The initial Level 1 PRA was initiated just prior to the time of the Chernobyl accident in 1986. After the accident, a limited scope PRA was also conducted. The limited scope PRA was completed in July 1987. The original Level 1 PRA was brought to completion in August 1988 and was followed by complete Level 3 analyses including an evaluation of external initiating events. This paper will focus on the Level 1 effort. The objectives of the Level 1 PRA for N Reactor were: identification of the plant features that are the most important contributors to fuel-damage frequency; development of plant logic models for use in analyzing safety and reliability issues; identification of dominant accident sequences and their frequencies for input into the Level 2/3 PRA and external events analyses; and establishment of a Hanford team with demonstrable expertise in performing risk and reliability analyses