Simulation of the radon flux attenuation in uranium tailings piles

Abstract

Tailings wastes are generated during the milling of certain ores to extract uranium and thorium. In the recent past uranium mill tailings consisted of fine-grained sand and silt materials, usually disposed in large piles in an open air area. Radium is probably the most hazardous constituent of uranium tailings. It produces radon, a radioactive gas which can easily spread into the environment. Airborne radon decays into a series of short half-life products that are hazardous if inhaled. Tailings also emit gamma radiation which can increase the incidence of cancer and genetic risks. Post closure and site rehabilitation involves, among other situations, controlling and estimating radon release from the surface of the tailings pile. Generally the primary cleanup method consists of enclosing the tailings with compacted clay or native soil to prevent the release of radon and then covering this layer with rocks and vegetation. This implies a cover design and placement which will give long term stability and control to acceptable levels of radon emission and gamma radiation, preventing also erosion and water infiltration into the tailings. An algorithm based on the theoretical approach of diffusion was developed to estimate radon attenuation originated by a cover system placed over the tailings pile and subsequently the resulting concentration in the breathing atmosphere. The one dimensional steady-state radon diffusion equation was applied to a porous and multiphase system to estimate the radon flux from the tailings to the surface. The thickness of a cover that limits the radon flux to a stipulated value was performed for a particular contaminated site. The efficiency of the cover attenuation was evaluated from the comparison with the resulting radon concentration in the absence of any cover system.