Radon control by multilayer earth barriers. 1. Modeling of moisture and density effects on radon diffusion from uranium mill tailings

A new approach is proposed and used to analyze radon emissions from uranium mill tailings. The approach has been developed based upon the analysis of partially saturated water flow effects on diffusion of radon through the cover and tailings materials. Such an analysis provides the moisture distribution with depth and time in the profile, hence, the gas filled pore space available for radon diffusion is known as a function of both depth and time. Through using the diffusion coefficient for radon gas in air, appropriately reduced to account for the porous material at the particular gas filled pore space, the bulk diffusion coefficient, which in the past could only be obtained experimentally, is provided directly. Solution for the radon concentration in the engineered tailings-cover profile is then possible so the radon emission rate at the ground surface can be calculated directly. Results for evaluation of tailings and multilayered cover materials are presented using the new approach. Some of the engineered covers analyzed by the method were installed in a field trial at Grand Junction, Colorado, last summer. Initial measurements were made on the installed cover and generally good agreement is found between calculated and field measured emission rates. Continuing measurementsmore » are being made for longer term comparison with calculated results and to verify the effectiveness of thinner covers with use of engineered moisture retention for radon emissions control.« less