  Monte Carlo-transport codes are designed to simulate the complex neutron
transport physics associated with nuclear systems. These codes are tasked with
simulating phenomena such as temperature effects on cross-sections,
thermo-physical effects, reaction rates, and kinematics. It is not
computationally possible to simulate the physics of a system exactly. However,
many of the approximations made by modern simulation codes have been well
validated. This article investigates an impactful simulation error caused by an
approximation made in many Monte Carlo-transport codes. The approximation that
target-at-rest is valid for neutrons at energies 400 times that of the thermal
energy of the target particle is found to be inaccurate in certain scenarios.
This paper identifies such cases, notably TRISO [1] fuel and instances where
fuel infiltrates the pores of graphite in Molten Salt Reactors. The breakdown
of this approximation occurs particularly when there exists a small length
scale between fuel, a material with absorption resonances, and moderator, a
scattering material. When threshold values are too small, resonance escape
probabilities can deviate by as much as 1% per resonance, forming a baseline
defect. Furthermore, two distinct anomalies were observed upon temperature
variation, directly attributed to the transition between target-at-rest and
target-in-motion physics. Equations provided in this study offer predictions
for the temperature ranges within which these anomalies occur, based on system
temperature and threshold value. The recommendations put forth in this paper
advocate for incorporating the threshold value as a user-defined variable in
transport Monte Carlo codes employing this approximation. Additionally, users
are advised to conduct convergence studies to ensure that the chosen threshold
value is sufficiently high to mitigate the influence of baseline defects and
anomalies.
