In the summer of 1974, the Theoretical Division of the Los Alamos Scientific Laboratory began several research programs in the area of reactor safety for the United States Nuclear Regulatory Commission. Research efforts were started in the Liquid Metal Fast Breeder (LMFBR) and the Light Water Reactor (LWR) safety programs. The character of the Theoretical Division was to develop computer codes for the safety analysis of these reactor systems. The question of whether or not, during the course of a hypothetical accident sequence in an LMFBR, the core will subside to a coolable configuration without secondary critical bursts has never been resolved. To aid the study of this question, a computer program called SIMMER (S/sub N/, Implicit, Multified, Multicomponent, Eulerian Recriticality) was to be developed to predict the dynamics of extreme hypothetical accident sequences during which extended core motion is expected. This time-dependent computer code called for combining an advanced multidimensional, multiphase fluid dynamic methodology with multidimensional neutron transport theory and improved equation-of-state technology. In the LWR program, the research emphasis was to push forward in two areas: (1) the development of advanced multiphase fluid dynamic methods and computer programs for performing basic research and analyzing areas in thermal hydraulics important to the safety of water reactors, and (2) the development of an advanced ''best estimate'' systems code called TRAC (Transient Reactor Analysis Code) for analyzing loss-of-coolant accidents and anticipated-transients-without-scram in light water reactors.
