Application of the subcell analysis method for thermohydraulic calculations of boiling water cooled reactor rod fuel assemblies

On the basis of the results of experimental investigation of steam content distribution over the cross section of adjacent cells of boiling water cooled reactor fuel assembly simulator the dependence of an intercell flow enthalpy on the two-phase flow regime is found. The physical model of mass transfer process inside the fuel assemblies is suggested, approxim ted equations are derived. They permit to determine the beginning of intercell flow enrichment with a steam phase as well as the enthalpy of these flows depending on mass velocity, coolant steam content and fuel element power density. Physical and analytical model of the developed surface boiling regime beginning is elaborated. The model does not contain empirical coefficients, while the results obtained using it correlate well with experimental data. Calculational analysis of the studied experimental regimes of the boiling coolant distribution over the cross section of heated models of rod fuel assemblies is performed. The method for taking into account the effect of unheated surface on the boundaries of two-phase flow regime, the application of which gives a satisfactory agreement between experimental and calculational data, is proposed