Compositional and Structural Evolution of Plutonium Dioxide: Underpinning Future Decisions

二氧化钚是一种非常有活力的材料。放射性衰变会破坏晶格，还会在材料中形成其他元素。氦是一种惰性气体，可能被局域化或被困在晶格中，也可能被释放。铀同位素(由钚-238、239、240衰变形成)和Am-241(由钚-241衰变形成)在二氧化钚晶格内一个原子一个原子地形成。英国有140吨二氧化钚形式的分离钚，这是世界上最大的民用储备。在过去的半个世纪里，它已经被分离出来，在最终使用之前，需要在未来储存几十年。目前，政府打算将这些材料中的大部分制成核反应堆燃料(“混合氧化物燃料”)，还有一小部分不能制成燃料的材料作为废物处置，尽管政策变化可能会导致更多的材料被指定为废物。无论钚的最终命运如何，这种材料都需要被加工成适合其最终用途的形式，而它在储存过程中的演变将影响它是否适合加工。因此，我们需要能够预测二氧化钚在储存过程中将如何变化，这样我们才能知道它是否适合最终使用。这个项目的目的是了解二氧化钚是如何变化的，这样我们就可以做出这些预测。我们将用二氧化钚进行实验测量，以确定辐射损伤、氦形成和衰变产物形成在长达几十年的时间尺度上对材料的影响。将使用一系列模型样品和从英国库存中提取的材料来探索二氧化钚的演变。腐烂产品的性能将使用储存的材料来确定。我们将使用同步加速器技术(X射线吸收光谱、衍射和层析成像)、电子显微镜和比表面积测量来表征材料。这些实验的结果将被用于开发二氧化钚演化的计算模型。由于衰变产物逐个原子形成，而衰变过程影响材料的电子结构，我们需要在单个原子和原子的小聚集体的尺度上对所有这些过程进行建模，但由于我们感兴趣的性质在晶格尺度上是明显的，我们还需要了解原子尺度效应是如何传递到更大的尺度上的，我们还将开发我们可以在更大尺度上使用的模型。

Plutonium dioxide is a very dynamic material. Radioactive decay damages the lattice and also forms other elements in the material. Helium, an inert gas, may be localised or trapped in the lattice, or maybe released. Uranium isotopes (formed from the decay of plutonium-238, 239, 240) and americium-241 (formed from decay of plutonium-241) are formed atom-by-atom within the plutonium dioxide lattice. The UK has 140 tonnes of separated plutonium in the form of plutonium dioxide, the World's largest civil stockpile. This has been separated over the last half century and will need to be stored for several decades into the future before its end use. Currently, Government intends most of this material to be made into nuclear reactor fuel ('mixed oxide fuel'), with a small proportion, which cannot be made into fuel, being disposed of as waste, although policy changes could lead to more of it being designated as waste. Whatever the final fate of the plutonium, the material will need to be processed into a suitable form for its end use, and its evolution while it is being stored will affect its suitability for processing. We therefore need to be able to predict how plutonium dioxide will change in storage, so we know whether it will be suitable for its final use. The purpose of this project is to understand how plutonium dioxide changes so we can make these predictions. We will make experimental measurements with plutonium dioxide to define the effects of radiation damage, helium formation and decay product formation on the material over timescales up to several decades. The evolution of plutonium dioxide will be explored using both a series of model samples and materials drawn from the UK stockpile. Behaviour of decay products will be determined using the stockpile materials. We will use synchrotron techniques (X-ray absorption spectroscopy, diffraction and tomography), electron microscopy and specific surface area measurements to characterise the materials. The results of these experiments will be used to develop computational models of plutonium dioxide evolution. Because decay products form atom-by-atom, and decay processes affect the electronic structure of the material, we need to model all these processes at the scale of individual atoms and small aggregates of atoms, but because the properties we are interested in are manifest at the lattice scale, we also need to understand how the atomic-scale effects carry across to this larger scale, and we will also develop models which we can use at this larger scale.