An Analysis of External Radiation Dose Fields in an Underground Facility at the Potential HLW Geologic Repository at Yucca Mountain, Nevada

Abstract

Radioactive waste-handling operations at a potential high-level radioactive waste (HLW) geologic repository at Yucca Mountain, Nevada, would involve receiving, processing, aging, and emplacing HLW equivalent to 70 000 t of heavy metal. This paper presents results of an independent analysis of the external radiation dose fields for various designs at a potential repository underground facility, performed at the Center for Nuclear Waste Regulatory Analyses. In order to analyze dose fields, radiation source terms have been developed for commercial spent nuclear fuel (SNF) assemblies. Full-scale three-dimensional models of waste packages and underground repository drifts were used in the Monte Carlo radiation transport simulations. Radiation dose rates along the drifts have been evaluated for multiple waste packages for SNF-emitted photons, neutrons, and ⁶⁰Co photons. Analysis shows that SNF photons contribute >68%, ⁶⁰Co photons contribute <30%, and neutrons contribute <2% to the total dose rates. The contribution of photons scattered off the drift walls is <10% of the total dose rates. The drift elbow significantly blocks direct radiation shine and reduces scattered dose rates at the junction of turnout and main access drifts where workers could be present.