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

在加拿大和安大略省，核能发电分别占总发电量的16%和60%。安全管理使用过的核燃料对于保护我们的健康和环境至关重要，对于提高加拿大人对使用核能的信心也至关重要。乏燃料处置安全性评价结果对重要放射性核素（如Np-237）的吸附和扩散系数值敏感。由于Np-237是一种半衰期长（214万年）的α -排放物，在还原条件下扩散缓慢，而在氧化条件下扩散适中，因此在乏燃料安全评价中，Np是主导长期风险的关键RI之一。***盐溶液是南安大略深部岩石孔隙水的典型特征（如盐离子强度可达6 mol/kgw左右）。然而，关于RI在盐溶液中压实膨润土和寄主岩石中的吸附和扩散的数据很少。RI在压实膨润土中的扩散一般用批量试验所得的吸附分配系数Kd来讨论。然而，RI的实际扩散系数与Kd值预测的扩散系数之间往往存在较大差异。我们不完全理解为什么会发生这种差异，也没有一个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随孔隙水盐度的变化，建立压实膨润土中的扩散模型，并阐明这种差异的机理。***采用透扩散法测量Np(V)和HTO在压实膨润土中的扩散行为。扩散模型将结合目前NSERC发现资助研究（2014-2019）中开发的吸附模型，并考虑静电双层理论、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胶效应。通过讨论测量的扩散系数值的有效性，并将其与MDS结果进行比较，验证扩散模型并研究差异的原因。***硕士6人，博士2人，本科生5人。一名具有材料科学专长的博士后也将参与这项研究。HQP的培训将通过他们在非密封锕系元素处理、扩散理论、MDS和DFT方面的技能以及他们的放射化学和界面化学知识的研究发展来完成。***由于核能发展的不确定性和公众讨论的不充分，本研究的结果将促进安全评估方法和加强放射性废物管理方面的个人培训。

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.