Performance Assessment for Depleted Uranium Disposal in a Near-Surface Disposal Facility

Abstract

The U.S. Nuclear Regulatory Commission (NRC) staff has conducted a technical analysis to assess the potential impacts of disposal of large quantities of depleted uranium in a nearsurface disposal facility. The nature of the radiological hazards associated with depleted uranium presents challenges to the estimation of long-term effects from its disposal – namely that its radiological hazard gradually increases over time due to the in-growth of decay products. In addition, these decay products include a daughter in gaseous form (Rn-222), which has significantly different mobility in the environment than the parent radionuclides. NRC staff developed a screening performance assessment model of a reference low-level radioactive waste (LLRW) disposal facility to evaluate the risk and uncertainties associated with the disposal of depleted uranium as low-level waste. The model was constructed with the dynamic simulation software package GoldSim®, a Monte Carlo simulation software solution for dynamically modeling complex systems. The depleted uranium source is modeled as releasing to a backfill assumed to surround the depleted uranium in the disposal cells. Radionuclides released to the backfill are vertically transported via advection through unsaturated zone cells to an underlying aquifer, where they are transported to a receptor well. Radon that emanates from radium present in the depleted uranium is modeled as diffusing through an engineered cap into the interior of a residence placed over the disposal area or to the external environment. The model evaluates the radiological risk to future residents and intruders (acute or chronic exposures) near or on the land overlying the disposal facility. Calculations were performed probabilistically to represent the impact of variability and uncertainty on the results. Key variables evaluated included: disposal configurations, performance periods, institutional control periods, wasteforms, site conditions, pathways, and scenarios. The impact of these variables on projected radiological risk can be significant. For example, estimated risks are very sensitive to the performance period, and estimated disposal facility performance is strongly dependent on site specific hydrologic and geochemical conditions. In addition, radon fluxes to the environment are very sensitive to the long-term moisture state of the system and to the disposal depth.