Neutron flux distribution in the irradiation channels of Am-Be neutron source irradiation facility

Monte Carlo (MCNP-5) simulations of the neutron fluxes were performed to determine the radial and axial neutron fluxes of the two irradiation sites of the 20 Ci ²⁴¹Am-Be neutron irradiation facility at NNRI. The geometry of the ²⁴¹Am-Be source as well as the irradiator design, constituted one cylindrical neutron source at the center of a cylindrical barrel with water as moderator. In the far and the near irradiation sites that were 13.1 cm and 4.2 cm, respectively, from the source, the average thermal, epithermal and fast neutron fluxes axially increase exponentially from the bottom and peak at the center of the source 3.0 cm from the bottom of the source and decrease to a very low value at the end of the tube. The percentage of the average thermal flux increases as the distance from the source increases, while the percentages of the epithermal and fast fluxes decrease as the distance from source increases. In the far and near irradiation sites the average radial thermal neutron flux decreases at the rates of 307.02 n cm^-2 s^-1 and 961.54 n cm^-2 s^-1 per cm along the diameter, respectively. The average radial, epithermal and fast neutron fluxes were fairly uniform along the diameter in the two irradiation sites.