Evaluation of the Sum-of-Fractions Methodology for Water and Polyethylene Moderated Systems

Conference · Sun Oct 01 00:00:00 EDT 2023

OSTI ID:2340140

Zipperer, Travis J. ^[1]; Prichard, Andrew W. ^[2]; Greene, Travis M. ^[3]; Marshall, William J. ^[3]; Lang, Alex ^[3]

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1. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
2. TradeWind Services, LLC, Richland, WA (United States)
3. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sum-of-Fractions is a method intended to assure a subcritical margin for aqueous solutions and slurries of fissionable isotopes. The method indicates that a system is subcritical if the sum of the ratios of the mass of each isotope in a mixture to its individual minimum subcritical mass limit is less than or equal to one. The basis of the Sum-of Fractions has historically been derived from allowances given in ANSI/ANS-8.15-1981. However, the allowance was removed in ANSI/ANS-8.15-2014 due to a lack of technical basis. A methodology was developed to assess the validity of using the Sum-of-Fractions for water or polyethylene moderated systems for the following nuclides: ²³²U, ²³³U, ²³⁴U, ²³⁵U, ²³⁷Np, ²³⁶Pu, ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²⁴²Pu, ²⁴¹Am, ²⁴²mAm, ²⁴³Am, ²⁴²Cm, ²⁴³Cm, ²⁴⁴Cm, ²⁴⁵Cm, ²⁴⁶Cm, ²⁴⁷Cm, ²⁴⁹Cf, and ²⁵¹Cf. The methodology uses available benchmark data for mixtures of ²³³U, ²³⁵U, and ²³⁹Pu to establish the calculational margin, and a mass limit reduction to establish the margin of subcriticality. Water or polyethylene moderated and reflected mixtures containing the nuclides are evaluated with SCALE 6.2.4. Including the calculational margin, subcritical mass limits for each nuclide were computed for optimally water or polyethylene moderated and fully reflected systems. These masses were used to create nuclide mixtures in which the sum of the mass to subcritical mass limit ratios is one. The various nuclide mixtures were modeled over a range of moderation and demonstrate the k_eff does not exceed the calculational margin. For additional assurance of subcriticality, a significant mass reduction is applied to each computed minimum critical mass of the nuclides without adequate benchmark data consistent with the method in ANSI/ANS-8.15-2014.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States); TradeWind Services, LLC, Richland, WA (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 2340140
• Report Number(s): PNNL-SA--185366
• Country of Publication: United States
• Language: English

Related Subjects

07 ISOTOPE AND RADIATION SOURCES
Actinide
Analytical Methods
Critical Mass
Criticality Safety
Moderation
Nuclear Criticality Safety Program (NCSP)
Sum-of-Fractions