Parametric Study of ²³⁹Pu and ²³⁵U Mass Equivalency in Spherical Geometries

Technical Report · Thu Jul 20 04:00:00 EDT 2023

DOI:https://doi.org/10.2172/1993192· OSTI ID:1993192

Mahlen, Caden Lee ^[1]; Zhang, Ning ^[1]

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1. Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

In this technical document, parametric studies were performed to determine if a bounding mass equivalency can be used to simplify multi-material models in MCNP6 for criticality evaluations. The studies explored the simplification of MCNP6 models by substituting materials, particularly ²³⁹Pu for ²³⁵U. The initial hypothesis for the material simplification is based on the subcritical mass limits from the ANSI/ANS- 8.1 standard which states that for water reflected metal spheres the limits are 20.1 kg ²³⁵U and 5.0 kg ²³⁹Pu. Using those masses, it was hypothesized that a 20.1 kg sphere of ²³⁵U and a 5.0 kg sphere of ²³⁹Pu could be considered to have the same multiplication. Given that equivalency, 4kg ²³⁵U could be modeled by 1kg ²³⁹Pu (i.e., a mass ratio of 4:1). The parametric studies compare the materials ²³⁹Pu and ²³⁵U independently and in multi-material spherical models. Critical mass, critical thickness, critical mass ratio and critical thickness ratio values are presented as functions of inner radius. keff values are presented as a function of inner radius at different mass ratios and total masses. Results indicate that the conservative bounding mass equivalency is one. That is, if substituting ²³⁹Pu for ²³⁵U it should be done on a gram for gram basis.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• DOE Contract Number: 89233218CNA000001
• OSTI ID: 1993192
• Report Number(s): LA-UR--23-28520
• Country of Publication: United States
• Language: English

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