Design, construction, and characterization of a fast neutron beam port facility at the University of Texas at Austin TRIGA reactor

Barron, Daniel Caldwell
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A fast neutron irradiation facility has been designed, modeled, and constructed in the beam port 4 facility at The University of Texas at Austin’s TRIGA Mark-II Reactor. This facility targets the Watt-fission neutron spectrum in a controlled environment by reducing the present thermal and epithermal flux while preserving the fast neutron flux. The present facility will open new avenues in nuclear non-proliferation for fast-fission yields in addition to measuring radionuclide migration. The filter system was designed using MCNP and Solidworks and consists of a lead plug to stop gamma-rays, filter elements of natural boron and 96% enriched B¹⁰, collimation elements of borated polyethylene and natural boron, and an exit filter of boron nitride. A beam stop was constructed to reduce the ambient dose rate using borated paraffin wax, polyethylene, cadmium, and lead. Sensitivity studies were performed to configure an economic facility by optimizing the amounts and configurations of materials used in the filter. The filter is modular to allow for rearrangement of elements and the ability to change the materials used as needed should higher efficiencies be desired or a higher total flux. Initial results indicate the facility produces a 10 cm diameter beam with an integrated flux of 6.63x10⁵ n/cm²/s at a reactor power of 950 kW and resembles the Watt-fission spectrum well with a slightly elevated epithermal neutron flux. The fast neutron flux above 0.1 MeV constitutes 98.77% of the total flux and the thermal neutron flux only 0.0014% of the total flux. STAYSL PNNL was used to unfold the neutron spectrum from 9 measurable reactions in 5 flux foils. Results suggest that the fast neutron flux is higher than anticipated in all STAYSL runs although the total flux is lower than anticipated.