COMPUTATIONAL MODELLING OF CRITICAL PROCESSES IN THE RELEASE AND TRANSPORT OF RADIONUCLIDES IN THE NEAR-FIELD

A system analysis of the performance of a geologic repository generally involves the simulation of a connected series of processes, beginning with radionuclide release from the waste form and ending with a calculation of dose to an exposed individual. Although the limitations of such analyses have been discussed in detail [1, 2], this begs of question of whether there is not a better approach. An alternative may be to break the analysis into “parts” that are focused on specific barriers.

The barrier most amenable to analysis is the waste form and its interactions with the near-field. Radionuclide release will be critically sensitive to variations in the temperature, the radiation field, redox conditions, pH, pCO₂, surface area to solution volume and the presence and type of near-field materials. Among the important processes are: 1.) the kinetics of waste form corrosion; 2.) the mechanisms of waste form corrosion; 3.) the formation of secondary, alteration phases; 4.) the sorption/reduction reactions at the surfaces of near-field materials; 5.) the formation and mobility of colloids; 6.) microbial interactions with radionuclides and materials in the near-field. Each of these processes are complicated and are expected to vary in importance with changing conditions over time. One approach may be to consider the radionuclide inventory as it changes with time and to identify the critical processes within each time frame that hold the promise or potential for reducing the mobility of specific radionuclides. Such an approach may reduce the complexity of safety assessments.

We use atomistic simulations to assess specific processes in the near field, such as the mechanism and kinetics of UO₂ oxidation/corrosion in dry and wet conditions [3], Np incorporation into U⁶⁺ alteration phases such as studtite [4] and boltwoodite, and uranyl, neptunyl, and plutonyl adsorption to iron oxide phases.

[1] Ewing et al. (1999) Risk Analysis 19, 933-958.

[2] Ewing (2006) In Uncertainty Underground – Yucca Mountain and the Nations’s High-Level Nuclear Waste. MIT Press, 71-83.

[3] Skomurski et al. (2008) Corrosion of UO₂ and ThO₂: A quantum-mechanical investigation. Journal of Nuclear Materials, 375, 290-310.

[4] Shuller et al. (2010), Quantum-mechanical evaluation of Np-incorporation into studtite, American Mineralogist, 95, 1151-1160.