Design of Miniature Neutron Source Reactor with LEU Core

MNSR with HEU core is mainly used for the neutron activation analysis(NAA), some short-live isotopes production and as training and teaching tool. In order to enlarge the usage of MNSR, such as, medical treatment, physics experiment and prompt gamma neutron activation analysis (PGNAA), the thermal and epithermal neutron beams on the horizontal direction are designed on the both sides of the reactor core, which will be used for boron neutron capture therapy (BNCT) and neutron radiography. Improved MNSR with thermal power 30kW is an under moderated reactor of pool-tank type, UO₂ with enrichment of 13.0% as fuel, light water as coolant and moderator, and metal beryllium as reflector. There are totally 346 fuel pins and 4 support pins in the core arranged in 10 concentric circles. The diameter of the core is 231 cm and its height is 258 cm. There is only 1 control rod located in the center which can fulfill the operation of startup, power operation and shutdown. The excess reactivity is 3.5~4mk, and the reactivity temperature coefficient is minus, so it is safe and has good inherent safety. MCNP code is employed to perform the calculations of the frame design of the thermal neutron beam and the epithermal neutron beam(0.4eV~10keV), the epithermal neutron beam is mainly composed of neutron moderation layer, thermal neutron absorption layer, gamma ray shielding layer, neutron collimator parts. The moderator materials used probably for the design of the epithermal neutron beam, such as, water, graphite, aluminum, Al₂O₃, Fluental and other material, are selected according to neutron absorption cross section and scattering cross section of the different nuclides, Fluental with thickness of 50cm is selected as the final moderator material according to the calculating values of the epithermal neutron flux density, gamma and fast neutron contamination at the exit of the epithermal neutron beam for the different moderator materials. For thermal neutron beam facility, the graphite moderator is adopted as moderator. The bismuth, lead and poly-pb are selected as gamma shielding materials, Poly-boron and poly-lithium are as the neutron shielding. (author)