Actinide uptake by carbonates: modelling of molecular processes in radioactive waste systems

锕系元素是核燃料循环所有阶段产生的放射性废物的主要成分，从矿山废物和加工副产品到高放射性的使用过的核燃料。虽然铀（U）占主导地位，但放射性废物也可能含有镎（Np）、钚（Pu）、镅（Am）和铥（Cm）。特别是，U、Np和Pu的几种形式是长寿命的，可以以不同的化学状态存在，其中一些在环境中是非常移动的。在这种情况下，溶液中的锕系元素与岩石和矿物表面的相互作用将是阻止它们进入生物圈的主要机制，因此了解锕系元素如何与表面相互作用以及表面本身如何表现非常重要。一种关键的矿物类型是方解石（碳酸钙），它通常以石灰石的形式存在，也可以在地下废物储存库中形成，这是水泥与预先存在的岩石相互作用的结果。碳酸钙的另一种形式，文石，也很重要。事实上，方解石和文石中天然存在的锕系元素可用于确定岩石和矿物的年龄，并揭示过去的环境条件，从锕系元素分布到二氧化碳浓度。因此，锕系元素在碳酸盐环境中的行为在环境研究的许多领域中具有重要意义。锕系元素的实验非常困难，因为这些元素都是放射性的，有些元素，如钚和Am，是极其危险的。因此，如果我们能够通过使用计算机模型来理解和预测锕系元素的行为，那将是非常有帮助的。然而，如果我们要做出可信的预测，这些模型必须基于在分子尺度上对锕系元素-方解石反应的详细描述。因此，本项目的第一个目标是对方解石中的关键过程以及与方解石表面的反应有一个很好的理解。第二个目标是利用这种理解来定义通过锕系和不同碳酸盐的趋势的原则。我们和其他几个人已经做了锕系元素与方解石反应的实验，我们与德国核废料研究所INE Karlsruhe进行了良好的合作，该研究所拥有出色的实验工作设施。这些实验的结果可用作开发和测试计算机模型的起点。我们将详细描述锕系元素及其环境，然后利用这些结果开发更简单的模型，这些模型可用于研究锕系元素-矿物相互作用所需的大型复杂模拟。我们将与国家统计研究所密切合作，包括交换数据和研究人员，这将使我们能够获得最新的实验研究，并使我们能够开展协调一致的理论和实验工作方案。

The actinide elements are major components of the radioactive wastes produced in all stages of the nuclear fuel cycle, from mine wastes and byproducts of processing to highly radioactive used nuclear fuel. Whilst uranium (U) is dominant, radioactive wastes may also contains neptunium (Np), plutonium (Pu), americium (Am) and curium (Cm). In particular, several forms of U, Np and Pu are long-lived and can exist in different chemical states, some of which are very mobile in the environment. In these circumstances, the interactions of actinides in solution with the surfaces of rocks and minerals will be the principal mechanism by which they will be prevented from entering the biosphere so it is important to understand how actinides interact with surfaces, and also how the surfaces themselves behave. One key mineral type is calcite (calcium carbonate), which commonly found as limestone, and which can also form in, for example, underground waste repositories as a result of the interactions of cement with the pre-existing rock. Another form of calcium carbonate, aragonite, is also important. Indeed, naturally occurring actinides in calcite and aragonite can be used to find the ages of rocks and minerals, and to reveal past environmental conditions, from actinide distributions to carbon dioxide concentrations. The behaviour of actinides in carbonate environments is thus of great importance in many areas of environmental research. Experiments with actinides are very difficult because these elements are all radioactive and some, such as Pu and Am, are extremely hazardous. It would therefore be very helpful if we could understand and predict actinide behaviour by using computer models. However, if we are to make credible predictions, these models have to be based on a detailed description of the actinide-calcite reaction at the molecular scale. The first aim of this project is therefore to develop a good understanding of the key processes involved in incorporation into calcite and reactions with calcite surfaces. The second aim is to use this understanding to define the principles that govern trends both through the actinide series and across different carbonates. We, and several others, have done experiments on actinide reactions with calcite and we have a good collaboration with INE Karlsruhe, a German nuclear waste research institute, which has excellent facilities for experimental work. The results of these experiments can be used as the starting point for both developing and testing computer models. We will devise detailed descriptions of the actinides and their environments and then use these results to develop simpler models that can be used in the large, complex simulations needed to investigate actinide-mineral interactions. We will collaborate closely with INE, including exchanges of data and research staff, which will give us access to the latest experimental studies and allow us to carry out a coordinated programme of theoretical and experimental work.