Abstract: 3-D Monte-Carlo method was employed for analyzing Liquid Metal Fast Breeder Reactor (LMFBR) sodium two-phase flow in fuel bundle under hypothetical loss of coolant accident (LOCA) conditions. Based on molecular kinetic theory, a microscopic model was developed for determining the evaporation and condensation rates of sodium in a reactor bundle. Three dimensional Monte-Carlo methodology was applied to simulate the molecular trajectories, collision rates among vapor molecules and between molecules and surfaces of pins as well as bundle canning wall. Re-wetting of dried out regions of the cladding surfaces was simulated with a dynamic liquid film model. The source of molecules re-entering into the Monte-Carlo calculation of molecular dynamics due to the evaporation of sodium from the condensed liquid film was computed. The study also reported the numerical simulation of an out-of pile sodium boiling experiment made at Karlsruhe Research Center (Forschungszentrum Karlsruhe) of Germany.