Development of a Numerical Analysis Method for Cross-sectional Void Fraction Distribution in Subchannels of Nuclear Fuel Assemblies

Detailed information about the void fraction distribution in fuel assemblies is increasingly important with the development of high burn-up fuels. A numerical method has been developed for the steady cross-sectional void fraction profile in fuel assemblies using a marching method in the axial direction, considering cross-flows due to lift forces, void diffusion and momentum balance. Uniform pressure in a cross section was assumed under the dominant vertical flow and the secondary lateral flow condition in each subchannel. The merit of this simplified method is its high-performance computation using many BFC meshes for expression of complex void fraction and velocity distributions inside the subchannels. The calculated results were compared with the observed void distributions obtained with X-ray computed tomography in the NUPEC tests of full-scale advanced BWR fuels. The comparison showed the capability of this method for predictions of overall void fraction distributions inside the subchannels. This method will provide a good tool for void fraction profile prediction in high burn-up fuels, while future studies for reliable correlations of lift forces are required over a wide range of flow conditions.