[en] Physical models, and numerical methods are presented for SIMMER-II, a computer program to predict the neutronic and fluid dynamic behavior of an LMFBR during a hypothetical core disruptive accident. Either the time-dependent multigroup neutron diffusion or neutron transport equation is solved by the quasistatic method to predict the reactor neutronic behavior during the accident. Cross sections depend on temperature and background cross sections. Three fields, a structure field, a liquid field, and a vapor field, are modeled to predict the fluid dynamic behavior of the reactor. Each field consists of density components, to follow the material motion, and energy components, to predict the material temperatures. For typical accident calculations, the materials are fertile fuel, fissile fuel, stainless steel, sodium, control material, and fission gas. Heat, mass and momentum transfer are calculated among the three fields and their components