Validation of the two-region Rossi-alpha model for reflected assemblies

摘要

Nuclear criticality safety, nonproliferation and safeguards, emergency response, and stockpile stewardship utilize estimates of the k_(eff) multiplication factor. The value of k_(eff) cannot be directly measured, but it can be inferred from the prompt neutron period. One modality of measuring the prompt neutron period is the Rossi-alpha method, which is predicated on fitting a histogram of detection time differences due to the nonrandom temporal distribution of same-fission-chain neutrons. Recent works have developed the motivation and theory to expand traditional one-region point kinetic models resulting in one-exponential histogram fits to two-region models that result in two-exponential fits. This paper validates the new two-region model using organic scintillator measurements of copper-reflected weapons-grade plutonium (0.83 ≤ k_(eff) ≤ 0.94) and high-density-polyethylene-reflected highly enriched uranium (0.73 ≤ k_(eff) ≤ 0.95). Furthermore, the results show that more thermal systems have shorter prompt neutron periods in the core region due to increased induced-fission probabilities for moderated neutrons. A new parameter introduced by the two-region model is also shown to be correlated to the amount of reflection, and may be used to infer assembly properties such as type and amount of reflector.