Conceptual design study of light water subcritical assembly

Abstract

Due to high level of inherent safety, subcritical assemblies are widely used for research and training purposes. This paper presents the neutronic design of a typical research subcritical assembly with domestic fuel rods. This assembly consists of fuel sub-assemblies with 4 × 4 square arrays of fuel pins. Two types of fuel pins, metal uranium with Al cladding and uranium dioxide with Zr–Nb 1% cladding, are studied. It is cooled and moderated using ordinary water and is surrounded by Beryllium as reflector. According to sub-criticality of the assembly, a 5-curie Am–Be source was located in the center of the assembly to sustain chain reactions.The simulations were implemented by MCNPX code and neutronic parameters were calculated accordingly. Optimal fuel rod pitch and fuel rod radius were investigated and effect of different reflectors on the effective multiplication factor (keff) were studied. Then thermal, epithermal, fast and total average flux were calculated. Considering the importance of subcritical assembly feedbacks, moderator and fuel temperature coefficients of reactivity were obtained. Since the effective delayed neutron fraction and neutron generation time play an important role in the reactor kinetics, they were also calculated for this subcritical assembly.Results of the simulation show that by arranging the fuel sub-assemblies in a 9 × 9 array, multiplication factor of about 0.8 for metal U and about 0.7 for UO2 is achieved for subcritical assembly. The optimum thickness of Beryllium reflectors was found to be 26 cm for metal U and 31 cm for UO2. Results also show that the subcritical assembly with metal U fuel has higher average total flux but the subcritical assembly with UO2 fuel has larger reactivity coefficient. However, the results also prove that the subcritical assembly with both fuels is inherently safe due to negative fuel and moderator temperature coefficients of reactivity.