Investigation of Thermal and Epithermal Neutron Flux Distributions in Universal Nuclear Icebreaker Fuel Assemblies

The results of experimental investigations of the spatial and energy distributions of the thermal and epithermal neutrons in a cassette fuel assembly, similar in design to that developed for the RITM-200 reactor core in the universal nuclear icebreaker, are presented. The measurements were performed using activation detectors with the isotopes 63Cu, 55Mn, and 197Au. It is shown that the structural nonuniformities of the fuel assembly (central water cavity, reactivity compensation system absorbing rod, strongly screened rods with consumable absorber) create significant thermal neutron flux nonuniformity over the cross-section of a fuel assembly. The flux distribution of epithermal neutrons with energy >0.4 eV is almost uniform. The experimental data are supported by calculations.