Multiscale diffusion model of NpO2+ through compacted bentonite in saline solution

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Approximately 16% and 60% of electricity generation are covered by nuclear energy in Canada and Ontario, respectively. The safe management of used nuclear fuel is essential to protect our health and environment, and critical to increasing the public confidence of Canadians in the use of nuclear energy. The result of safety assessment for used nuclear fuel disposal is sensitive to the values of sorption and diffusion coefficients of important radionuclides (RI) such as Np-237. Since Np-237 is an alpha-emitter with a long half-life (2.14 million years) and diffuses slowly under reducing conditions, but diffuses moderately under oxidizing conditions, Np is one of key RI which dominate the long-term risk in the safety assessment of used nuclear fuel.    Saline solution is a typical feature of porewaters in deep rocks in Southern Ontario (e.g. ionic strength of saline is up to about 6 mol/kgw). However, little data on sorption and diffusion of RI in compacted bentonite and host rocks in saline solution is available. The diffusion of RI in compacted bentonite is generally discussed by the sorption distribution coefficient (Kd) obtained by batch test. However, there is frequently a large discrepancy between the actual diffusivity of RI and their diffusivity predicted by Kd value. We do not completely understand why this discrepancy takes place and do not have a diffusion model which is applicable in compacted bentonite.    Our research program will study the diffusion of Np(V) and HTO (tritiated water) in compacted bentonite as a function of porewater salinity, develop a diffusion model in compacted bentonite, and elucidate the mechanisms on this discrepancy.    The diffusion behaviours of Np(V) and HTO through compacted bentonite will be measured by a through-diffusion method. A diffusion model will be developed by combining with the sorption model developed in the current NSERC Discovery Grant research (2014-2019) and by considering electrostatic double layer theory, water movement by molecular dynamic simulation (MDS), surface charge density by density functional theory (DFT), and electro viscose effect. By discussing the validity of measured diffusion coefficient values and by comparing those with MDS results, the diffusion model will be validated and the reasons for the discrepancy will be studied.     Six Master, two Ph.D. and five undergraduate students will participate in this research. One Postdoctoral fellow will also participate in this research, with expertise in material sciences. Training of HQP will be accomplished through the research-based development of their skills in unsealed actinide handling, diffusion theory, MDS and DFT, and of their knowledge of radiochemistry and interface chemistry.     With uncertainties and incomplete public deliberations on the nuclear energy development, the results of this research will advance the methods of safety assessment and enhance the training of individuals in radioactive waste management.