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

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Nuclear criticality safety, nonproliferation and safeguards, emergency response, and stockpile stewardship utilize estimates of the keff multiplication factor. The value of keff 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≤keff≤0.94) and high-density-polyethylene-reflected highly enriched uranium (0.73≤keff≤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.