On the Feynman-alpha method for reflected fissile assemblies

Hua, Michael Y.; Hutchinson, Jesson D.; McKenzie, George E.; Clarke, Shaun D.; Pozzi, Sara A.

doi:10.1016/j.anucene.2020.108082

On the Feynman-alpha method for reflected fissile assemblies

Journal Article · Fri Feb 12 00:00:00 EST 2021 · Annals of Nuclear Energy (Oxford)

DOI:https://doi.org/10.1016/j.anucene.2020.108082· OSTI ID:1783280

Hua, Michael Y. ^[1]; Hutchinson, Jesson D. ^[2]; McKenzie, George E. ^[2]; Clarke, Shaun D. ^[3]; Pozzi, Sara A. ^[3]

Univ. of Michigan, Ann Arbor, MI (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States); University of Michigan
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Univ. of Michigan, Ann Arbor, MI (United States)

The Feynman-alpha method is a neutron noise technique that is used to estimate the prompt neutron period of fissile assemblies. The method and quantity are of widespread interest including in applications such as nuclear criticality safety, safeguards and nonproliferation, and stockpile stewardship; the prompt neutron period may also be used to infer the k_eff multiplication factor. The Feynman-alpha method is predicated on time-correlated neutron detections that deviate from a Poisson random variable due to multiplication. Traditionally, such measurements are diagnosed with one-region point kinetics, but two-region models are required when the fissile assembly is reflected. This paper presents a derivation of the two-region point kinetics Feynman-alpha equations based on a double integration of the Rossi-alpha equations, develops novel propagation of measurement uncertainty, and validates the theory. Validation is achieved with organic scintillator measurements of weapons-grade plutonium reflected by various amounts of copper to achieve k_eff values of 0.83–0.94 and prompt periods of 5–75 ns. The results demonstrate that Feynman-alpha measurements should use the two-region model instead of the one-region model. The simplified one-region model deviates from the validated two-region models by as much as 10% in the estimate of the prompt neutron period, and the two-region model reduces to the one-region model for small amounts of reflector. The Feynman-alpha estimates of the prompt neutron period are compared to those of the Rossi-alpha approach. The comparative results demonstrate that the Feynman-alpha method is more precise than the Rossi-alpha method and more accurate for k_eff < 0.92, whereas the Rossi-alpha method is generally more accurate for higher multiplications. Here, the uncertainty propagation developed in this work should be used for all Feynman-alpha measurements and will therein improve fitting accuracy and appropriate precision estimates.

View Accepted Manuscript (DOE)

Research Organization:: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States); Univ. of Michigan, Ann Arbor, MI (United States)

Sponsoring Organization:: National Science Foundation (NSF); National Science Foundation Graduate Research Fellowship; USDOE National Nuclear Security Administration (NNSA); USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)

Grant/Contract Number:: 89233218CNA000001; NA0002534; NA0003920

OSTI ID:: 1783280

Alternate ID(s):: OSTI ID: 1874916
OSTI ID: 1905334

Report Number(s):: LA-UR-20-28930

Journal Information:: Annals of Nuclear Energy (Oxford), Journal Name: Annals of Nuclear Energy (Oxford) Vol. 155; ISSN 0306-4549

Publisher:: ElsevierCopyright Statement

Country of Publication:: United States

Language:: English

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