Utility of Monte Carlo Modelling for Holdup Measurements.

Conference · Fri Jun 10 00:00:00 EDT 2005

OSTI ID:977994

Belian, Anthony P. ^[1]; Russo, Phyllis A ^[1]; Weier, Dennis Ray ^[1]

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

Non-destructive assay (NDA) measurements performed to locate and quantify holdup in the Oak Ridge K25 enrichment cascade used neutron totals counting and low-resolution gamma-ray spectroscopy. This facility housed the gaseous diffusion process for enrichment of uranium, in the form of UF₆ gas, from {approx} 20% to 93%. Inventory of ²³⁵U inventory in K-25 is all holdups. These buildings have been slated for decontamination and decommissioning. The NDA measurements establish the inventory quantities and will be used to assure criticality safety and meet criteria for waste analysis and transportation. The tendency to err on the side of conservatism for the sake of criticality safety in specifying total NDA uncertainty argues, in the interests of safety and costs, for obtaining the best possible value of uncertainty at the conservative confidence level for each item of process equipment. Variable deposit distribution is a complex systematic effect (i.e., determined by multiple independent variables) on the portable NDA results for very large and bulk converters that contributes greatly to total uncertainty for holdup in converters measured by gamma or neutron NDA methods. Because the magnitudes of complex systematic effects are difficult to estimate, computational tools are important for evaluating those that are large. Motivated by very large discrepancies between gamma and neutron measurements of high-mass converters with gamma results tending to dominate, the Monte Carlo code MCNP has been used to determine the systematic effects of deposit distribution on gamma and neutron results for ²³⁵U holdup mass in converters. This paper details the numerical methodology used to evaluate large systematic effects unique to each measurement type, validates the methodology by comparison with measurements, and discusses how modeling tools can supplement the calibration of instruments used for holdup measurements by providing realistic values at well-defined confidence levels for dominating systematic effects.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)
• OSTI ID: 977994
• Report Number(s): LA-UR-05-5102; TRN: US1004178
• Resource Relation: Conference: Institute of Nuclear Materials Management 46. Annual Meeting, Phoenix, AZ (United States), 10-14 Jul 2005
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
97 MATHEMATICS AND COMPUTING
CALIBRATION
CRITICALITY
DECOMMISSIONING
DISTRIBUTION
GASEOUS DIFFUSION PROCESS
NEUTRONS
NUCLEAR MATERIALS MANAGEMENT
SAFETY
SIMULATION
SPECTROSCOPY
URANIUM
WASTES
Nuclear Criticality Safety Program (NCSP)
Non-destructive assay (NDA) Measurements
Monte Carlo code MCNP