Abstract
NexGen Energy Ltd (NexGen) proposes to develop a new uranium mining and milling operation in northern Saskatchewan, Canada. NexGen adopted a proactive tailings management strategy that will effectively manage the acid rock drainage (ARD) and metal and radionuclide leaching (M&RL) risks associated with the mill tailings. The tailings management strategy includes the underground disposal of cemented uranium paste tailings and process wastes in mined-out stopes and a purposely built underground tailings management facility (UGTMF). Predictive source terms of the tailings and process wastes stored in the stopes and UGTMF were developed to support an environmental impact statement and meet Canadian Nuclear Safety Commission regulatory requirements. The paper presents the source term derivation approach and methods. Conceptualization of mass transport processes and fit-for-purpose design assumptions were used to reduce the necessary parameters and variables to those that can be measured using standard and modified kinetic tests. Input data for the source terms were derived from a multi-year geochemical characterization program with empirical measurements of key mass transport processes. Geochemical speciation modeling was used to evaluate the potential effect of secondary mineral and ion exchange controls. Simulated porewater qualities and elemental diffusivity values were provided as input to the groundwater solute transport model. Integrated environmental effects assessment of mass loading from the indicates acceptable effects on downstream water systems over a modeling period of 10,000 years. Combined with other benefits such as long-term stability and limited requirement for post-closure management, the study indicates the effective long-term management of ARD/M&RL risks associated with the uranium mill tailings.