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

在加拿大和安大略，核能分别占发电量的16%和60%。安全管理使用过的核燃料对保护我们的健康和环境至关重要，对提高加拿大公众对使用核能的信心至关重要。废核燃料处置的安全评价结果对Np-237等重要放射性核素的吸附系数和扩散系数十分敏感。由于Np-237是具有长半衰期（214万年）的α发射体，并且在还原条件下缓慢扩散，但在氧化条件下适度扩散，因此Np是在用过的核燃料的安全评估中主导长期风险的关键RI之一。 盐水溶液是安大略南部深层岩石中孔隙水的典型特征（例如，盐水的离子强度高达约6 mol/kgw）。然而，很少有数据的吸附和扩散的RI在压实膨润土和寄主岩石在盐溶液中。用静态吸附试验得到的吸附分配系数（Kd）讨论了RI在压实膨润土中的扩散。然而，经常有一个很大的差异之间的实际扩散系数的RI和它们的扩散系数预测的Kd值。我们不完全理解为什么会发生这种差异，也没有适用于压实膨润土的扩散模型。 我们的研究计划将研究Np（V）和HTO（氚化水）在压实膨润土中的扩散作为孔隙水矿化度的函数，建立压实膨润土中的扩散模型，并阐明这种差异的机制。 用穿透扩散法测定了Np（V）和HTO在压实膨润土中的扩散行为。扩散模型将结合当前NSERC发现资助研究（2014-2019）中开发的吸附模型，并考虑静电双电层理论，分子动力学模拟（MDS）的水运动，密度泛函理论（DFT）的表面电荷密度和电粘效应。通过讨论实测扩散系数值的有效性，并将其与MDS结果进行比较，验证扩散模型的有效性，并研究产生差异的原因。 六个硕士两个博士五名本科生将参与这项研究。一名博士后研究员也将参与这项研究，具有材料科学方面的专业知识。对HQP的培训将通过以研究为基础发展他们在未密封的锕系元素处理、扩散理论、MDS和DFT方面的技能以及他们在放射化学和界面化学方面的知识来完成。 由于核能发展的不确定性和公众审议的不完整性，这项研究的结果将促进安全评估的方法，并加强对放射性废物管理人员的培训。

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.