Investigation on mitigating neutron streaming effect for the water-loop facility used in the MIT reactor

Highlights: •MCNP5 is adopted for the investigation of neutron streaming effect. •Manual adjustment of neutron importance is applied to the refined mesh discretization. •Neutron streaming is quantitatively identified in the Water-loop facility at the MITR. •Stepped thimble design could effectively mitigate the neutron streaming phenomenon. -- Abstract: The Massachusetts Institute of Technology Reactor (MITR) is a 6 MW tank-type reactor with a compact core design. It is moderated and cooled by light-water and has a heavy-water reflector. The MITR is a unique test bed for a wide range of nuclear material and fuel research. In one of the recent fuel cycles, detectors found elevated neutron levels on the reactor top lid during the operation with the Water-loop facility installed. A preliminary investigation theorized that the neutrons were streaming out via a thin CO₂ gas gap, which is used as a thermal isolation space for the in-core experiment. The current study employs neutron transport simulations to investigate the physical interpretation of the observed phenomenon. The state-of-the-art general purpose Monte Carlo code MCNP5 is adopted as the computational tool. The calculation results identify the typical characteristics of the streaming effect by quantifying the fraction of upward-streaming neutrons. Efforts have also been made to explore feasible engineering options for mitigating this neutron streaming from the Water-loop facility. It is concluded that a borated guide tube is not sufficient to reduce the neutron level on the reactor top lid; however, a stepped thimble design is effective in reducing the neutron source contribution.