Utilizing the Oak Ridge electron linear accelerator facility for benchmarking neutron thermalization in moderators

The Oak Ridge Electron Linear Accelerator facility (ORELA) was investigated as a neutron source for validating neutron thermalization in moderators/reflectors of interest to Generation IV and space reactor designs. ORELA is a powerful accelerator-based neutron source located in the Physics Division of Oak Ridge National Laboratory. It produces intense nanosecond bursts of neutrons with energies from 10 ^-3 to 10 ⁸ eV, a maximum source strength of ∼5×10 ¹⁸ n/s (at the target), and a maximum frequency of 1000 Hz. In this work, Monte Carlo simulations (MCNP-4C2) were performed to design a pulsed neutron slowing-down-time experiment to benchmark temperature dependent neutron thermalization in reactor grade graphite. In our model, we assumed that the graphite assembly would be located at the end of ORELA's shortest beam tube, which is 10 m long. The effects of the size of the graphite assembly, the geometry of the heating elements, the neutron detector type and arrangement, and the temperature of the graphite were investigated. The results of this work simulate and discuss the response (time dependent reaction rate) of a Pu-239 detector (placed outside the pile) under various experimental conditions. Consequently, this work demonstrates the utility of a facility such as ORELA in performing cross-section validation experiments, and the value of using the slowing-down-time concept as the basis for such experiments. Future work will apply the results of these simulations to benchmark the fundamental data and the approach that are utilized to determine the scattering laws used to generate the thermal neutron scattering cross sections.