Conceptual study of subcritical assembly with neutron generator for reactor physics experiments and neutron utilization

In Japan, critical assemblies are regulated under the "Act on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactions" which provides guidelines for the construction and the operation of nuclear facilities: the core configurations of nuclear experiments are constrained accordingly. In addition, the nuclear facilities are required to comply with strict administrations if the nuclear facilities have more than the certain amount of nuclear fuels. On the other hand, subcritical assemblies are not subjected to the nuclear reactor regulation because the subcritical assemblies are not classified as nuclear reactors. We propose the subcritical assembly with a pulsed accelerator neutron source, which uses nuclear fuels below the regulation amount to ease maintenance and to widen experimental capabilities. Conceptual study of such system was carried out to calculate the deviation between reactivity difference in critical state and that in subcritical state by using the MCNP 6.2 code and the JENDL 4 library. Neutron source multiplication method and pulsed neutron source method were performed. The deviation can be estimated around 10 % by setting detectors in appropriate positions. Additionally, neutron radiography was studied as a simple application. The geometrical shapes of sample rods were seen clearly.