Sorption of Np(V) and Np(IV) on bentonite in brine groundwater

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In Canada, New Brunswick, Ontario, Saskatchewan, and even Alberta (heretofore a “non-nuclear” province) have considered adding nuclear capacity to their energy mix. However, for any nuclear energy policy taken by the government and utilities, a solution to the problem of used nuclear fuel management and disposal is essential for public acceptance of the policy. According to the research of the Nuclear Waste Management Organization (NWMO) in Canada, currently both sedimentary and crystalline rocks are considered as potential hosts for a deep geological repository. One of the unique characteristics of groundwaters, where many municipalities are expressing their interest to host the used nuclear fuel disposal repository, is brine (total dissolved solids > 100 g/L). In the safety assessment of used nuclear fuel disposal, neptunium (Np)-237 will dominate the long-term radiological risk, and the quantitative understanding of sorption of Np-237 on the engineered barrier (bentonite) is a key factor to improve the reliability of safety assessment. However, the sorption of Np (Np(V) in aerobic condition and Np(IV) in anaerobic condition) on bentonite in brine has not been studied at all. We will (1) investigate the sorption behaviour of Np(V) and Np(IV) on bentonite in synthesized brine, (2) formalize the sorption reactions and (3) evaluate the equilibrium constants of the sorption reactions. The equilibrium constants and the methodology to evaluate the equilibrium constants in brine are essential to establish a technologically, physically, chemically and mathematically robust safety assessment methodology which is the long-term goal of my research program. First, the sorption of Np(V) and Np(IV) on the MX-80, which is a promising candidate material of bentonite used as engineered barrier in Canada, will be measured at varying pH, ionic strength, Na+/Ca2+ ratio at constant ionic strength and Np concentration by batch sorption method. By integrating constant capacitance surface complexation model, Density Functional Theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) measurement, the sorption reactions of Np(V) and Np(IV) on the MX-80 will be identified and formalized. The equilibrium constants of the sorption reactions will be determined and the constant at zero ionic strength will be evaluated by extrapolation using Pitzer model. Two to three full time Master’s and Ph.D. students and one undergraduate summer student will participate in this proposed research every year. Training of HQP will be accomplished through the research-based development of their skills on unsealed actinide handling, surface complexation model, DFT and EXAFS, and of their knowledge on actinide chemistry, radiochemistry and interface chemistry. The achievements of this research are critical to nuclear industries in Canada, especially the NWMO, Ontario Power Generation and the regulation authorities. Our proposed research will establish a cutting-edge research and education core in actinide chemistry and safety assessment research on used nuclear fuel management at McMaster University. This research will contribute to establishing the research methodology on sorption of redox sensitive radionuclides in brine. Furthermore, since the equilibrium constants of sorption reactions are essential parameters in safety assessment, this research will contribute to the safety of used nuclear fuel management in Canada and other countries where the disposal in brine is considering. These achievements will improve the reliability of safety assessment of radioactive waste disposal, and therefore, will contribute to the safety of the sustainable use of nuclear energy and radioisotopes in Canada and throughout the world.