Computational studies of chemistry underpinning the Enhanced Actinide Removal Plant at Sellafield

支持塞拉菲尔德强化锕系元素去除工厂的化学计算研究

• 批准号: 2657365
• 金额: --
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2657365
• 关键词: Computational; studies; chemistry; underpinning; Enhanced

The Enhanced Actinide Removal Plant (EARP), located on the Sellafield site, is one of the UK's most crucial radioactive effluent treatment plants. The EARP removes actinides and selected fission products from reprocessing effluents by association with a ferric iron (oxyhydr)oxide floc, which is precipitated from acidic effluent streams by the addition of NaOH. Historically, the EARP has treated radioactive effluents from, for example, the Magnox reprocessing plant (in which ferrous sulfamate is added as a reductant to separate Pu from U (reducing Pu(VI) to Pu(IV) while leaving U as U(VI)) but, as the Sellafield site transitions from its current reprocessing operations to post-operational clean-out and accelerated decommissioning activities, the effluent compositions that the EARP receives will change in character. Hence, detailed understanding of the iron (oxyhydr)oxide formation processes occurring in the EARP, and how these species interact with radioactive elements, will underpin not only optimisation of current plant efficiency, but will allow better prediction of changes in efficiency as effluent composition varies. While important progress has recently been made,1, 2 many details of the processes by which iron (oxyhydr)oxides form, and actinides are removed, remain unclear.The three most common Fe(III) (oxyhydr)oxide phases are ferrihydrite, hematite and goethite. Ferrihydrite is thermodynamically metastable, and is typically the first phase to precipitate from acidic ferric solutions; it is poorly structured and nanocrystalline, and is believed to be the dominant phase which forms as the EARP pH rises. In 2016, the co supervisors showed that Fe13 Keggin clusters form at very low pH and begin to aggregate above pH 1,2 and hence are implicated in the formation of ferrihydrite from base hydrolysis of very acidic Fe(III) solutions.Very recently, the co supervisors reported an EXAFS study of plutonium sorption during ferrihydrite nanoparticle formation,1 concluding that Pu(IV) strongly adsorbs via a tetradentate inner-sphere complex during the formation of ferrihydrite as the pH increases, while noting that "the exact nature of the Pu(IV) tetranuclear complex on the ferrihydrite surface is unclear without additional information. However, one possibility is bonding to the "square" window of the Fe13 Keggin unit". They also find that, while precipitation to form PuO2 is not the dominant Pu(IV) sequestration pathway, there is evidence that PuO2 is a minor product.This PhD project will study computationally, using molecular quantum chemical techniques based on density functional theory, the mechanisms by which Fe based clusters form, and how those clusters bind actinides, in conditions relevant to the EARP. The student will begin by examining the process of Fe13 Keggin formation from monomeric Feaq3+, via olation and oxolation reactions which create multinuclear intermediate species. With this in hand, work will proceed to study the interactions of these multinuclear Fe clusters, including the Fe13 Keggin, with actinides. Initial focus will be on Pu(IV), to link with the previously obtained experimental data. We will establish the most likely candidate for the experimentally observed inner-sphere, tetradentate species, and the mechanism(s) by which it forms. Pathways to the PuO2 minor product will also be explored. Once the Pu(IV) work is complete, the interactions of mono and multinuclear Fe clusters with other actinides of relevance to the EARP will be studied; key targets are U(VI), Np(IV), Np(V) and Am(III). Regular meetings of the student with all of the supervisory team will ensure continual close linking of the calculations with previous and ongoing experimental work, and provide valuable perspective and context to the direction of the computational research.References1. KF Smith et al., ACS Earth and Space Chemistry, 2019, 3, 2437-24422. JS Weatherill et al., Environ. Sci. Technol., 2016, 50, 9333-9342

位于塞拉菲尔德的强化锕系元素去除厂（EARP）是英国最重要的放射性污水处理厂之一。EARP通过与三价铁（羟基）氧化物絮凝物结合，从后处理流出物中去除锕系元素和选定的裂变产物，该絮凝物通过加入NaOH从酸性流出物流中沉淀出来。从历史上看，EARP处理了来自Magnox后处理厂等的放射性废水（其中添加氨基磺酸亚铁作为还原剂，以将钚与铀分离（将钚（六）还原为钚（四），同时将铀作为铀（六）留下），但随着塞拉菲尔德场址从其目前的后处理作业过渡到运行后清除和加速退役活动，EARP接收的流出物组合物的性质将改变。因此，详细了解EARP中发生的铁（氢氧化物）氧化物形成过程，以及这些物质如何与放射性元素相互作用，不仅将支持当前工厂效率的优化，而且将允许更好地预测效率变化，因为流出物成分不同。虽然最近已经取得了重要进展，1，2铁（羟基）氧化物的形成和锕系元素被去除的过程的许多细节仍然不清楚。三种最常见的铁（III）（羟基）氧化物相是水铁矿，赤铁矿和针铁矿。水铁矿是结晶亚稳的，并且通常是从酸性三价铁溶液中沉淀的第一相;其结构不良并且是纳米晶体，并且被认为是随着EARP pH升高而形成的主要相。在2016年，共同监督人发现，Fe 13 Keggin簇在非常低的pH值下形成，并且开始在pH 1，2以上聚集，因此与非常酸性的Fe（III）溶液的碱水解形成水铁矿有关。最近，共同监督人报告了水铁矿纳米颗粒形成过程中钚吸附的EXAFS研究，1的结论是，随着pH值的增加，在水铁矿形成过程中，Pu（IV）通过四齿内球络合物强烈吸附，同时指出“在没有额外信息的情况下，水铁矿表面上Pu（IV）四核络合物的确切性质尚不清楚。然而，一种可能性是键合到Fe 13 Keggin单元的“正方形”窗口。他们还发现，虽然沉淀形成PuO 2不是主要的Pu（IV）螯合途径，有证据表明PuO 2是一个次要的products.This博士项目将研究计算，使用基于密度泛函理论的分子量子化学技术，Fe基簇形成的机制，以及这些簇如何结合锕系元素，在相关的条件下的EARP。学生将开始通过检查Fe 13 Keggin从单体Feaq 3+形成的过程，通过olation和oxolation反应，产生多核中间体物种。有了这个在手，工作将继续研究这些多核铁团簇，包括Fe 13 Keggin，与锕系元素的相互作用。最初的重点将放在Pu（IV）上，以与先前获得的实验数据相联系。我们将建立实验观察到的内球，四齿物种，以及它的形成机制（S）的最可能的候选人。还将探索PuO 2次要产品的途径。一旦Pu（IV）的工作完成，单核和多核Fe团簇与其他与EARP相关的锕系元素的相互作用将被研究;关键目标是U（VI），Np（IV），Np（V）和Am（III）。学生与所有监督团队的定期会议将确保计算与以前和正在进行的实验工作的持续密切联系，并为计算研究的方向提供有价值的观点和背景。KF Smith等人，ACS地球与空间化学，2019，3，2437-24422。JS Weatherill等人，Environ. Sci.技术人员：2016，50，9333-9342