Boise State University recently procured Thermo Fisher Scientific P385 neutron generators to test novel sensors and materials for next generation nuclear reactors.
In order to safely operate the neutron generators without exposing personnel to excess radiation, the Nuclear Regulatory Commission (NRC) requires a dosimetry study before installation. This study sought to design shielding that was not prohibitively large or expensive and which reduces dosage rates to allow 100 annual hours of operation within the NRC dose limits.
Monte-Carlo N-Particle Transport code (MCNP) was used to model the generator and surrounding room. A variety of materials and geometries were investigated to determine the most efficient neutron attenuator as well as gamma ray shields.
Borated polyethylene and graphite were found to be the most effective shielding while an exterior lining of lead proved necessary to decrease gamma radiation. In order to reduce cost and volume of shielding around the generator, an additional ‘labyrinth’ of conventional bricks was modeled within the room. The size of the room was also adjusted to determine the minimum space required for safe operation. Shielding materials and dimensions were optimized to meet the NRC’s safe dose requirements
