A bench-top type ultracentrifuge to secure the reliability of safety assessment of used nuclear fuel disposal

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In this NSERC RTI Grant, a bench-top type ultracentrifuge is required. This is a critical tool to measure the values of sorption distribution coefficient (Kd) of elements which dominate the long-term risk of used nuclear fuel disposal.*** Since the migration of elements in the repository, the geosphere and the biosphere is assessed by their Kd values, the ability to reliably measure these values is critical in the safety assessment of disposal. Furthermore, the sensitivity analysis on the safety assessment by the Nuclear Waste Management Organization (NWMO), the Japan Atomic Energy Agency and other institutes shows that the uncertainty of the Kd value significantly and directly affects the result of safety assessment. *** Five municipalities in Ontario are under review for the site selection of used nuclear fuel repository by the NWMO. Three are the crystalline rock media and two are the sedimentary rock media. The unique feature of groundwater in these areas is the high ionic strength (saline and brine). Even when the rocks and the bentonite (a candidate material for engineered barrier in Canada) contact saline and brine, small particles (about 2.5 nm to several tens nm) are released into the solution. One of the most critical factors to measure the reliable Kd values is how completely we can separate the liquid from the solid including the small particles. The ultracentrifugation is the only way to separate such small particles from liquid phase, and is a global standard method to measure the reliable Kd values. Incomplete separation of small particles leads to large uncertainties of Kd values.*** The NWMO plans to select one municipality to host the repository in 2023, and to submit the Environmental Assessment and Licence application to the Federal Government in 2028. This means that the reliable Kd values have to be measured as soon as possible, and are essential to protect our health and environment and to improve our confidence in the used nuclear fuel disposal in Canada.*** The bench-top type ultracentrifuge requested in this grant will be used in the research funded by the NWMO. Because of the bench-top type, it can be installed in a glove box in which the atmosphere is strictly controlled to achieve the experimental conditions equivalent to deep geological media in Ontario. The Kd values of Se, Tc and Np on Canadian rocks in saline and brine under the reducing and oxidizing conditions will be measured, and will be compiled in the NWMO Sorption Database. Furthermore, the NWMO lists 19 elements of interest which are important in the safety assessment, and only few works on their sorption in saline and brine under the reducing conditions have been conducted. McMaster University is the only institute in Canada where the sorption behaviours of the important elements can be studied, and so the opportunity to use the ultracentrifuge will certainly be expanded in future works including NSERC Discovery Grant applied for now.