Source Term Evaluation Model for Uranium Tailings Ponds

A multicompartmental source term model, which is designed to assess the radionuclide leaching and transport in a saturated or unsaturated porous medium such as a uranium tailings pond (or near-surface radioactive waste disposal facility) is developed. The subsurface environment is assumed to be composed of a series of compartments. In the case of a uranium tailings pond, these compartments represent the natural evolution of several layers of uranium mill wastes due to their disposal into a tailings pond over a few decades. The bottommost layer forms at the beginning of the disposal, and the topmost layer develops at the flag end of the disposal. The model incorporates the ingrowths of progeny from the parent radionuclide. Upon entry of radionuclides into a layer, they mix, sorb, decay, and are eventually removed by the downward movement of water, ultimately reaching the aquifer below the bottommost layer. Each compartment may have its own unique properties. The primary outputs of the model are the radioactivity and concentrations in different layers. The quantity of interest is the radionuclide release from the bottommost layer, which is the top surface of an aquifer. For verification purpose, the model is applied for the leaching of ${}^{241}\mathrm{Pu}$ and its daughter ${}^{241}\mathrm{Am}$ from a near-surface radioactive waste disposal facility, and the results are compared with those generated by FOLAT, a widely used software. Good agreement of results is observed between both the models. Application of the model to a uranium tailings pond shows that ${}^{222}\mathrm{Rn}$ is released to the aquifer at the highest rate, followed by ${}^{210}\mathrm{Bi}$, ${}^{210}\mathrm{Pb}$, ${}^{210}\mathrm{Po}$, and ${}^{226}\mathrm{Ra}$. The radioactive release rate of ${}^{238}\mathrm{U}$, which is the parent radionuclide, is about 500 times lower than that of ${}^{222}\mathrm{Rn}$. Sensitivity of radioactivity release to different parameters such as infiltration velocity, residual moisture content, and distribution coefficient is evaluated. Radioactivity release increases with the increase in infiltration velocity; however, release persists longer for lower infiltration velocity. Radioactivity release is higher for higher residual moisture content and lower distribution coefficients. The main use of the model is for estimating the source input data for groundwater contaminant transport models.