Behaviour of UK Specific Spent Fuels Under Conditions Relevant to Geological Disposal.

传统上，英国对其核燃料棒进行再加工，提取并再利用从反应堆中取出后剩余的约95%的铀和约1%的钚。经过政府机构的审查，越来越清楚的是，一些英国核燃料现在不会再进行再加工，而需要直接处置在一个合适的地质储存库中，这是处置英国核废料的最佳选择。由于以前的核燃料后处理政策，在英国对这种直接处置方案进行的实验研究很少。然而，国际上对来自压水反应堆（PWR）或沸水反应堆（BWR）的燃料进行了大量的研究。本提案中涉及的核燃料是英国独有的，来自先进气冷反应堆（AGR）。与国际上使用的更常见的水慢化反应堆相比，它们在反应堆冷却剂（二氧化碳）和慢化剂（石墨）方面有所不同，而且在工作温度（AGR为825℃，而压水堆或沸水堆为~300℃）方面也有所不同。与国际上研究的乏燃料相比，这些差异造成了这些燃料在储存库中的行为方式的不确定性。这项建议的目的是将来自该国领先的研究型大学和核研究大学以及国家核实验室和核退役管理局的研究人员聚集在一起，建立研究能力，调查这些不同燃料特性对燃料行为的影响，并将其与国际上的发现进行比较。其目的是利用以英国具体研究为基础的国际研究来了解储存库中使用过的AGR燃料的长期行为，从而为其处置的安全评估做出贡献。为此，该项目将雇用5名研究人员和6名博士生来创建模拟核燃料，该核燃料由特殊制备的二氧化铀（贫铀）组成，具有与AGR燃料相同的微观结构特征，以及AGR乏核燃料的额外化学成分，这些化学成分是由铀和钚原子在反应堆中裂变（分裂）产生的。这将通过深入研究英国这些特征微观结构和化学成分的数据来确定，这些数据是通过对过去高放射性乏燃料的艰难检查确定的。该团队将预测燃料的化学成分在1000到10万年之间，这被认为是跨越处置罐的寿命，因为它们受到腐蚀的影响。然后，他们将确定模拟燃料在不同水成分下的溶解速率。这将是一些有代表性的天然地下水成分和由燃料棒和外部容器的钢包层溶解控制的水成分所预期的。最终，这些燃料的表面会形成铀矿物，它们可能会含有钚和镎。这些相的稳定性将决定这些元素释放回环境的速度，它们将被测试对未来很长一段时间内仍然存在的辐射的损害的抵抗力。该小组将在一个国际咨询委员会的指导下，在配备现代分析方法的国际设施进行实验，这些方法可用于处理放射性样品。这将有助于在国际上建立该小组，并建立英国的放射性实验研究能力，以便它可以支持英国核废料处置库的许可。

The UK has traditionally reprocessed its nuclear fuel rods to extract and re-use the ~95% uranium and ~ 1% plutonium that remains once they are removed from the reactor. It is becoming clear after reviews by government bodies that some UK nuclear fuels will not now be re-processed and will need to be directly disposed of in a suitable geological repository, recommended as the best option for disposing of the UK's nuclear wastes.Because of the previous nuclear fuel re-processing policy, very little experimental research has been carried out in the UK on this direct disposal option. However, a large amount has been carried out internationally on fuels that come from pressurised water (PWR) or boiling water reactors (BWR). The nuclear fuels concerned in this proposal are unique to the UK and come from Advanced Gas-cooled Reactors (AGR). These differ in the reactor coolant (carbon dioxide) and the moderator (graphite) from the more common water-moderated reactors used internationally and also in the operating temperature (825 C for AGR compared with ~300 C for a PWR or BWR). These differences create uncertainties in the way that these fuels would behave in a repository when compared with the spent fuels studied internationally.The aim of this proposal is to bring together researchers from the country's leading research universities and nuclear research universities and both the National Nuclear Laboratory and the Nuclear Decommissioning Authority to build research capacity to investigate the effect of these different fuel characteristics on the fuel behaviour and compare it with that found internationally. The aim is to use the international research underpinned by UK specific research to understand the long term behaviour of spent AGR fuel in repository so contributing to the safety assessment for its disposal.To do this the project will employ 5 investigators and 6 PhD students to create simulated nuclear fuels that consist of specially prepared uranium dioxide (depleted) with the same microstructural characteristics expected for AGR fuels and the additional chemical components of AGR spent nuclear fuel that result from uranium and plutonium atoms that have fissioned (split) during its time in the reactor. This will be determined by a deep search of UK data on these characteristic microstructures and chemical compositions determined from difficult examinations of the highly radioactive spent fuel in the past.The team will predict the chemical composition of the fuel for ages between 1,000 and 100,000 years which is thought to span the lifetime of the disposal canisters as they are affected by corrosion. They will then determine the rates of dissolution of the simulated fuels under different water compositions. These would be those expected for some representative natural ground water compositions and to water compositions governed by the dissolution of the steel cladding of the fuel rods and the external containers.Eventually, uranium minerals will form on the surface of these fuels and they will probably incorporate plutonium and neptunium. The stability of these phases will determine the rate of release of these elements back into the environment and they will be tested for resistance to damage by the radiation that remains at long times into the future.The group will be guided by an international advisory board and undertake experiments at international facilities equipped for modern analytical methods enabled for work with radioactive samples. This will help establish the group internationally and build the UK capacity for this radiological experimental research so it can support the licensing of a nuclear waste repository in the UK.