Computational design and optimization of a neutron imaging beamline using Monte Carlo and deterministic SN radiation transport for the Utah TRIGA reactor

This paper describes efforts to establish a neutron imaging radiography capability in an available beam port at the University of Utah 90-kW TRIGA Reactor (UUTR) from computational analysis of the proposed beamline. We employ methods in 3-D Monte Carlo and SN deterministic radiation transport methods to characterize the neutron beamline before its construction. Incident beam currents established using results from 3-D SN and Monte Carlo full core models were employed as a starting point for the source term in a full-scale MCNP6 computational model of the neutron beam port system to predict the spatial and energy-dependent neutron flux expected to exit the beam pipe onto the imaging operations floor. Our analysis reveals a full-power total neutron flux of 2.5 x 107 +/- 1.4 x 105n/cm2/s, where 92% of this flux streams through the surface normal within angle tally bin of 0 degrees < theta < 2 degrees . This neutron flux is above the threshold of industrially relevant neutron flux magnitudes necessary to probe non-destructive imaging in high-Z materials and opens up possibilities for tomographic imaging. Further analysis regarding the functionality and optimal design of the beam port utilizes both Monte Carlo and deterministic 3-D radiation transport methods to characterize principal neutron beam line attributes such as the L/D ratio, theoretical cadmium ratios, neutron-to-photon ratio, effective beam diameter estimates, and predicted neutron flux to irradiated foil samples. These supplemental models are developed in efforts to solidify consistency between Monte Carlo and deterministic methods, as subsequent development of a radiation beamstop will employ a hybrid fashion of the two for variance reduction purposes. In addition, primary and secondary radiation dose rates are evaluated to assess parameters for safe installation, guide future irradiation tests, and design of a beam stop for optimal shielding.