Design and modeling of a fast neutron beam at a research reactor

A fast neutron irradiation beam has been designed and modeled in MCNP6, (Monte Carlo N-Particle) for use in the beam port 4 facility at the TRIGA Mark-II reactor at The University of Texas at Austin. This facility will be built to provide neutrons with energies resembling the Watt fission neutron spectrum by reducing thermal and epithermal neutron flux. A filter uses elements of lead, borated polyethylene, naturally enriched boron, 96% enriched boron-10 powder, and boron nitride. Sensitivity studies of the filter design were performed to produce an economic and effective facility. To filter the beam, natural boron, enriched boron, and boron nitride were used; to collimate the beam, elements of borated polyethylene and natural boron were employed. The filter is modular to allow for the order and contents of each filter element to be reproducible and exchangeable. This flexibility allows the system to improve over time should additional funding become available or other functionality be desired. The filter as modeled will produce a 10 cm diameter beam of filtered neutrons with a maximum flux of 3.75 × 10⁶ n cm⁻² s⁻¹ at a reactor power of 950 kW. The resulting neutron flux resembles the pure ²³⁵U Watt fission spectrum with a slight elevation in the epithermal neutron flux.