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 asse mblies is increasingly important with the development of high burn-up fuels. A numerical method has been developed for the steady cross-sect ional void fraction prose in fuel assemblies using a marching method i n the axial direction, considering cross-flows due to lift forces, voi d diffusion and momentum balance. Uniform pressure in a cross section was assumed under the dominant vertical how and the secondary lateral how condition in each subchannel. The merit of this simplified method is its high-performance computation using many BFC meshes for expressi on of complex void fraction and velocity distributions inside the subc hannels. The calculated results were compared with the observed void d istributions obtained with X-ray computed tomography in the NUPEC test s of full-scale advanced BWR fuels. The comparison showed the capabili ty of this method for predictions of overall void fraction distributio ns inside the subchannels. This method will provide a good tool for vo id fraction profile prediction in high burn-up fuels, while future stu dies for reliable correlations of lift forces are required over a wide range of now conditions.