ENVIRONMENTAL RADIATION DOSE ASSESSMENT AND VALIDATION FOR URANIUM HEXAFLUORIDE STORAGE MANAGEMENT
This research entailed development and application of an efficient indirect Monte Carlo simulation methodology for performing environmental radiation dose assessment for storage of a large number (thousands) of uranium hexafluoride (UF6) cylinders in an outdoor storage area. The primary objective was to substantially improve the Monte Carlo computational efficiency by an order of magnitude or better as compared to the previous time-consuming methodology using the direct, standard simulation approach. The collateral objective was to support onsite storage of UF6 cylinders at uranium enrichment facilities, addressing dose impacts on storage capacity, footprint requirements, and cylinder placement in terms of spacing, array and stacking. The research was carried out in four sequential phases: Phase 1 — UF6 radiation source term development; Phase 2 — single UF6 cylinder dose evaluation for parameter selection; Phase 3 — multiple UF6 cylinder dose assessment methodology development and application; and Phase 4 — validation of computational efficiency and dose simulations. Phases 1 and 2 were a prerequisite for Phase 3 to provide the necessary data and input for use in Phase 3. The results of the single and multiple cylinder dose calculations from Phases 2 and 3 were validated in Phase 4 including quantification of computational efficiency. The analytical tools for the research consisted of the ORIGEN-S module in the SCALE 6.1.2 code package for source term calculations, the MAVRIC module in SCALE 6.1.2 for source importance data generation, and MCNP5 (v1.60) for radiation dose calculations. The indirect Monte Carlo simulation process developed from Phase 3 was applied to a case study involving different storage configurations to investigate the dose trend and sensitivity to storage capacity, and dose impacts to storage management. The case study produced the results of the neutron and photon dose rates from multiple UF6 cylinders in various storage arrays and stacks for impact analysis and demonstration of regulatory compliance at environmental locations such as the site boundary. The computational efficiency of the indirect Monte Carlo dose simulations was quantified in terms of figure of merit, confirming the accomplishment of the objective for substantial improvement over the direct, standard approach. In addition, comparison of the dose rate results for single and multiple cylinders from this research against the field measurements and other calculations available validated the environmental dose assessment with respect to the degree of accuracy and conservatism. As a result of this research, future related work was identified and recommended for further development and enhancement.
UF6, Radiation Dose, 48Y Cylinder, Storage, Environment, Validation
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