Caesium Mobility and Phase Separation Processes in Borosilicate Glasses

当核燃料棒从反应堆中取出时，它们除了含有二氧化铀（最初是235U浓缩的）外，还含有包括钚在内的多种元素的混合物，这些元素是由反应堆中发生的核反应产生的。在英国和法国，这些棒被“再加工”以提取钚，钚可以再次用作核燃料，而铀，现在在235U中耗尽，可以与钚混合制成所谓的混合氧化物或MOx燃料。这种提取被称为PUREX过程，包括将棒溶解在高酸性溶液中，并用有机溶剂提取钚和铀。在反应堆过程中“分裂”的原子（称为裂变产物）被留在高酸性和放射性的溶液中。这种分离极大地减少了需要处理的核废料的体积。提取钚和铀后剩下的高酸性和放射性液体被干燥成氧化物粉末，与主要由氧化硼和二氧化硅制成的基玻璃混合，加热到1100摄氏度左右，就变成了玻璃。对于这种材料的质量（长期稳定性）来说，所有的裂变产物都充分地融入玻璃中是很重要的。此外，生产玻璃所需的条件-高温和强辐射场-使得该工艺本身在操作温度，熔化设备寿命和生产最高质量的废玻璃（即避免高溶解度结晶沉淀）之间的平衡方面具有挑战性。本课题拟利用高温核磁共振（NMR）技术监测两种问题元素铯（Cs）和钼（Mo）在硼硅酸盐熔体中的溶解以及冷却过程中Cs2MoO4及相关化合物的析出。核磁共振对于这些材料来说是一种非常有吸引力的技术，因为它们的组成非常复杂。利用核磁共振，可以独立于材料的其余部分观察到特定的元素，因此在确定哪些元素参与了特定的过程时，复杂的成分不是一个问题。通过观察硼和硅的行为，这种高温核磁共振方法已经成功地用于观察熔融硅酸盐和硼硅酸盐。在这里，我们采用这些技术来观察铯和钼。结果应该是了解Cs和Mo在简化熔体中的溶解度，以及由法国核运营商（commissarial’energie Atomique， CEA）和英国核运营商（Nexia Solutions， BNFL集团）提供的真正的废玻璃（非放射性版本）。

When nuclear fuel rods are removed from a reactor they contain, in addition to uranium dioxide (originally 235U enriched), a mixture of elements, including plutonium, that have been generated by the nuclear reactions that occur in the reactor. In the United Kingdom and in France these rods are 're-processed' to extract out plutonium, which may be used again as a nuclear fuel, and the uranium, now depleted in 235U, which can be mixed with the plutonium to make what is called mixed oxide or MOx fuel. This extraction is called the PUREX process and involves dissolving the rods in a highly acidic solution and extracting the plutonum and uranium with organic solvents. The atoms that have 'split' (called fission products) during the reactor process are left in the highly acidic and radioactive solution.This separation dramatically reduces the volume of nuclear waste material that needs disposal. The highly acidic and radioactive liquor remaining after the extraction of the plutonium and uranium is dried to an oxide powder and mixed with a base glass, made primarily from boron oxide and silicon dioxide, and heated to around 1100 degrees centigrade and turned into a glass. It is important for the quality (long term stability) of this material that all the fission products are adequately incorporated into the glass. Furthermore, the conditions required to produce the glass - high temperatures and intense radiation fields - make the process itself challenging with respect to the balance between temperature of operation, lifetime of melting equipment and the production of the highest quality waste glass i.e., avoidance of high solubility crystalline precipitates.This proposal intends to use high temperature nuclear magnetic resonance (NMR) spectroscopy to monitor the dissolution of two problematic elements, caesium (Cs) and molybdenum (Mo) into the borosilicate melt and the precipitation of Cs2MoO4 and related compounds during cooling. NMR is a very attractive technique for these materials because they are so complex in terms of composition. With NMR, specific elements can be observed independently of the rest of the material and so complex compositions are not an issue in determining which elements are involved in particular processes. This high temperature NMR method has been used successfully to look at molten silicates and borosilicates in the past by observation of boron and silicon behaviour. Here we are adapting these techiques to observe caesium and molybdenum. The outcome should be an understanding of the solubility of Cs and Mo into simplified melts and the real waste glasses (non-radioactive versions) provided by the French nuclear operators (Commissariat a l'Energie Atomique, CEA) and the UK nuclear operators (Nexia Solutions, BNFL group).