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

在加拿大，新不伦瑞克省、安大略省、萨斯喀彻温省，甚至阿尔伯塔省（迄今为止是一个“非核”省份）都考虑在其能源结构中增加核电容量。然而，对于政府和公用事业公司采取的任何核能政策，解决乏燃料管理和处置问题是公众接受政策的关键。根据加拿大核废料管理组织（NWMO）的研究，目前沉积岩和结晶岩都被认为是深层地质储存库的潜在宿主。地下水的独特特征之一是盐水（总溶解固体含量100克/升），许多城市都表示有兴趣在地下水中建立乏燃料处置库。在乏燃料处置安全性评价中，镎(Np)-237将主导长期辐射风险，定量了解工程屏障（膨润土）对镎-237的吸附是提高安全性评价可靠性的关键因素。然而，目前还没有研究过膨润土对Np(好氧条件下的Np（V)和厌氧条件下的Np(IV)）的吸附。我们将(1)研究Np(V)和Np（IV）在合成盐水中对膨润土的吸附行为，(2)形式化吸附反应，(3)评估吸附反应的平衡常数。平衡常数和评估盐水平衡常数的方法对于建立技术、物理、化学和数学上可靠的安全评估方法至关重要，这是我研究计划的长期目标。首先，采用批吸附法测定了不同pH值、离子强度、恒定离子强度和Np浓度下Na+/Ca2+比对Np(V)和Np（IV）在MX-80上的吸附。MX-80是加拿大用作工程屏障的有前途的膨润土候选材料。通过集成恒电容表面络合模型、密度泛函理论（DFT）计算和扩展x射线吸收精细结构（EXAFS）测量，将识别Np(V)和Np（IV）在MX-80上的吸附反应并形式化。利用Pitzer模型外推，确定了吸附反应的平衡常数，并计算了离子强度为零时的常数。每年将有2 - 3名全日制硕士、博士研究生和1名本科生暑期生参与本拟研究。对HQP的培训将通过以研究为基础的发展他们在非密封锕系元素处理、表面络合模型、DFT和EXAFS方面的技能，以及他们在锕系元素化学、放射化学和界面化学方面的知识来完成。这项研究的成果对加拿大的核工业至关重要，特别是对NWMO、安大略发电公司和监管当局。我们提出的研究将在麦克马斯特大学建立锕系化学和乏燃料管理安全评估研究的前沿研究和教育核心。本研究将有助于建立卤水中氧化还原敏感放射性核素吸附的研究方法。此外，由于吸附反应的平衡常数是安全性评估的重要参数，因此本研究将有助于加拿大和其他考虑在盐水中处置的国家的乏燃料管理的安全性。这些成就将提高放射性废物处置安全评估的可靠性，因此将有助于加拿大和全世界可持续使用核能和放射性同位素的安全。

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.