Concept of sodium fast power reactor with intracassette heterogeneity

The use of a sodium fast reactor with intracassette (assembly) heterogeneity permits a core breeding ratio of greater than 1, stabilization of the output from fuel assemblies, elimination of channel ducts, and increase in fuel replacement interval. Uniform distribution of burning fuel elements and parent substance fuel elements in the assemblies permit utilization of migration effects of absorbed gamma rays in parent substances owing to uniform output distribution among the fuel elements; increase in reliability of fuel element tubes owing to reduction of linear output from oxide elements near the end of operation; etc. The safety level can be increased by: a constant reactivity realized by stabilized components; improvement in the characteristics of heat flow in the core; optimization of the structural material ratio, core height, etc.; elimination of channel ducts; decrease in swelling of structural material due to decreased neutron irradiation; decrease in PCI rate; etc. An increase in the fuel replacement interval by one year or longer can lead to an increased ulitization rate. A uniform distribution of neutrons eliminates the necessity of rearrangement and recharging of assemblies. Improved neutron coupling and reduced neutron leak permit control of output distribution and allow a uniform distribution to be achieved by a smaller number of light control rods. (Nogami, K.)