The aim of this work is to study the behavior of the temporal evolution of the distributions of the hydrogen concentration and temperatures profiles in the core of the boiling water reactor (BWR) during a severe accident. The core of the BWR is considered as an effective medium where the fuel assembly in the core consists of arrays of in-line fuel rod. The governing effective-medium equations for momentum, heat and mass transfer in terms of effective transport coefficients that capture the essential features of the in-line rods assembly were computed using CFD method. The dependence of the evolution of the dimensionless hydrogen concentration and temperatures profiles with Peclet and Biot numbers show the ability of the effective medium for accurately describing the transport phenomena for BWR during severe accident. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights' reserved.