A porous media method three-dimensional numerical investigation of two-phase flow thermal-hydraulics within advanced nuclear fuel rod bundles

Abstract

The modem design optimisation of the nuclear fuel assemblies for the Boiling Water Reactors posed a need for the simulation and analyses of strong multidimensional thermal-hydraulic processes in order to investigate and prove the effectiveness of the nuclear fuel assembly cooling, as well as to determine steady-state and transient operational conditions which leads to the Critical Heat Flux (CHF) conditions. Numerical simulation and analyses of the CHF conditions and nuclear fuel rod bundle steady-state and transient thermal-hydraulics is performed with the three-dimensional two-fluid model and by the application of the porous media concept for the coolant two-phase flow within bundle complex geometry. The ability of the applied methodology to predict multidimensional two-phase flow thermal-hydraulics is an improvement in regard to the commonly used one-dimensional subchannel analyses methods. In regard to the full CFD calculation of two-phase flow, with the implementation of the boundary conditions at the walls of all fuel rods in the bundle, here proposed methodology requires much less computational effort with the acceptable reliability of obtained results for the engineering applications. The obtained results show the influence of the bundle design characteristics, such as positions of the part-length rods, on the two-phase mixture cross flow, steam void distribution, and spatial positions of dry-out onsets.