Design of JMTR high-performance fuel element

For test and research reactors, the core conversion to low-enriched uranium fuel is required from the viewpoint of non-proliferation of nuclear weapon material. Improvements of core performance are also required in order to r espond to recent advanced utilization needs. In order to meet both requirem ents, a high-performance fuel element of high uranium density with Cd wires as burnable absorbers was adopted for JMTR core conversion to low-enriched uranium fuel. From the result of examination of an adaptability of a few group constants generated by a conventional transport-theory calculation with an isotropic scattering approximation to a few group diffusion-theory core calculation f or design of the JMTR high-performance fuel element, it was clear that the depletion of Cd wires was not able to be predicted accurately using group c onstants generated by the conventional method. Therefore, a new generation method of a few group constants in consideration of an incident neutron spe ctrum at Cd wire was developed. As the result, the most suitable high-perfo rmance fuel element for JMTR was designed successfully, and that allowed ex tension of operation duration without refueling to almost twice as long and offer of irradiation field with constant neutron flux.