U.S. Nuclear Regulatory Commission

The modeling approached used in the State-of-the-Art Reactor Consequence Analysis project to characterize the release of radionuclides to the environment accompanying a postulated severe (core damage) accident is based on plant-specific applications of the MELCOR computer code. MELCOR is a state-of-the-art computational model developed by Sandia National Laboratories for the U.S. Nuclear Regulatory Commission. Due to large uncertainties in many aspects of severe accident behavior, MELCOR provides the code user a wide spectrum of options for modeling uncertain physical phenomena and characterizing plant response to beyond design basis accident conditions. Choices made by the code user to select among these options or, when necessary, develop new or enhanced modeling features are, therefore, an important aspect of the overall modeling approach. This document describes the specific manner in which MELCOR modeling capabilities were used to represent important, but uncertain, aspects of severe accident behavior. This description includes choices made among alternate modeling options offered through code input, changes to selected input parameters from those offered as 'default ' values, and in some case