The evolution of computer codes for the analysis of severe accidents in light water reactors, WASH-1400 to present

This paper describes the evolution of our understanding of the potential consequences of severe accidents in light water based on research and model development undertaken by the Nuclear Regulatory Commission through its contractors and by the U.S. nuclear industry. Major milestones were activities undertaken by Brookhaven National Laboratory in support of the WASH-740 study, based on a very limited experimental database, by AEC contractors in support of the Reactor Safety Study (WASH-1400), which was the first major attempt to quantify the risk to the public from severe accidents, to the follow-on NUREG-1150 risk study and most recently to the insights provided by the NRC's SOARCA program. The Three Mile Island Unit-2 accident, which shortly followed after the release of WASH-1400, provided impetus to the regulatory authority and the regulated industry to perform research to better understand the potential frequency and consequences of severe accidents. This paper describes the development of the tools used to predict accident consequences based on research performed by the NRC (and its contractors) and by the nuclear industry (including the Electric Power Research Institute). Over time, these tools have matured, have been validated against research results, and have gained a level of configuration management enabling them to be used in demonstrating safety adequacy for the licensing and regulatory oversight of these reactors. This paper has been limited in its scope to severe accident analysis development activities in the U.S.