An investigation of corrosion and leaching of carbide fuels in a GDF setting

碳化铀（UC）被认为是一种来自英国民用核反应堆试验计划的外来燃料材料。这种材料已在苏格兰北方的Dounreay等地点用作原型高密度燃料。核退役管理局（NDA）拥有一份辐照和非辐照碳化铀库存，作为其遗产内的废弃奇异燃料材料的一部分，因此承担其安全管理和最终处置的责任（由NDA的子公司放射性废物管理（RWM）负责）。碳化铀被认为是一种反应性和潜在的自燃材料，其反应性与铀金属相当。地质处置是国际公认的最安全的高活性放射性废物的长期解决方案，英国的外来乏燃料（如碳化物乏燃料）打算以这种方式管理。目前的博士项目将调查辐照和未辐照碳化物乏燃料在类似于地质处置设施（GDF）的条件下（一）关闭前和（二）关闭后的腐蚀和浸出行为。英国的乏燃料库存大部分是陶瓷二氧化铀燃料。氧化物提供了一种稳定的基质，当与地下水接触时，预计会显示出很高的化学稳定性，除了在颗粒边界和燃料的可接触部分快速释放放射性核素（即时释放分数）外，容器失效后放射性核素的释放速率（基质溶解速率）将很低。外来燃料和金属燃料的处置路线尚未完全确定。然而，预计碳化物和金属燃料在接触地下水时会相对较快地腐蚀。本研究的目的是更好地支持预期的行为碳化物乏燃料处置后，在GDF.Experimental方法：利用由国家核实验室（NNL），斯普林菲尔德实验室提供的原始碳化铀燃料材料，目前的学生将使用尖端材料分析技术，以提供一个纳米到微米到毫米级的碳化物腐蚀行为的观察。技术将包括X射线断层扫描（XRT），高速原子力显微镜，二次离子质谱，高分辨率电子显微镜和X射线衍射。这些技术在布里斯托的IAC都是常规使用和可用的。为了补充材料分析，沥滤研究将利用溶液分析技术，如ICP-MS和ICP-OES，以确定不同GDF类似地下水溶液（好氧和缺氧）中的铀浓度。此外，该项目还将利用萨里大学独有的TRLFS仪器，作为TRANSCEND项目的一部分，该仪器正在开发用于水溶液锕系元素分析。该项目将设置一系列封闭池实验，使用密封的充水玻璃外壳将小型碳化铀“棒”样品固定在固定位置。这些特殊的电池将允许使用TRLFS定期测量不断变化的水化学，并使用X射线断层扫描仪测量腐蚀进程。这些分析技术将能够详细研究腐蚀和沥滤行为，但不会破坏实验系统。由碳化物（13.66 g/cc）腐蚀形成的氧化物（10.97 g/cc）的较低密度意味着在不同条件（温度、溶解的O2和水化学）下的氧化速率可以通过使用XRT测量氧化物的演变厚度来确定。

Uranium carbide (UC) is considered an exotic fuel material which has arisen from the UK's civil nuclear reactor test programme. This material has served as a prototype high density fuel at sites like Dounreay in northern Scotland.The Nuclear Decommissioning Authority (NDA) has an inventory of irradiated and non-irradiated uranium carbide as part of its legacy waste exotic fuel materials within its estate and therefore carries the liability for its safe management and ultimately its disposal (for which Radioactive Waste Management (RWM), a subsidiary of the NDA are responsible). Uranium carbide is considered a reactive and potentially pyrophoric material with a reactivity comparable to uranium metal. Geological disposal is internationally recognised as the safest long-term solution for higher activity radioactive wastes and the UK's exotic spent fuels (such as carbide spent fuel) are intended to be managed in this way. The current PhD project will investigate corrosion and leaching behaviour of irradiated and unirradiated carbide spent fuel under conditions analogous to a Geological Disposal Facility (GDF) both (i) pre-closure and (ii) post-closure. Most of the UK's spent fuel inventory is in the form of ceramic uranium dioxide fuels. Oxide provides a stable matrix that is expected to display high chemical stability when contacted by groundwater and, apart from the rapid release of radionuclides at the grain boundaries and in accessible parts of the fuel (the instant release fraction), the rate of release of radionuclides after container failure (the matrix dissolution rate) will be low. Disposal routes for exotic and metallic fuels are yet to be fully determined. It is however expected that carbide and metallic fuels will corrode relatively quickly when accessed by groundwater. This study aim to better underpin the expected behaviour of carbide spent fuel following disposal in a GDF.Experimental Approach:Utilising virgin Uranium carbide fuel material provided by the National Nuclear Laboratory (NNL), Springfields laboratory, the current studentship will use cutting edge materials analysis techniques to provide a nano to micro to millimetre scale observation of carbide corrosion behaviour. Techniques will include X-ray tomography (XRT), high-speed atomic force microscopy, secondary ion mass spectrometry, high-resolution electron microscopy and X-ray diffraction. The techniques are all routinely used and available at the IAC in Bristol. To compliment the materials analysis, leaching studies will utilise solution analysis techniques such as ICP-MS and ICP-OES to determine evolving U concentrations in different GDF-analogous groundwater solutions (oxic and anoxic). In addition, the project will also utilise the unique TRLFS instrument available at the University of Surrey which is being developed for aqueous actinides analysis as part of the main TRANSCEND project.The project will setup a series of enclosed cells experiments, using sealed, water-filled glass housings to hold small uranium carbide 'stick' samples in a fixed position. These special cells will permit periodic measurement of the evolving water chemistry using TRLFS and also the corrosion progression using X-ray tomography. These analysis techniques will enable a detailed study of corrosion and leaching behaviour but without disrupting the experimental system. The lower density of the oxide (10.97 g/cc) which forms from corrosion of the carbide (13.66 g/cc) means that rates of oxidation under different conditions (temperature, dissolved O2 and water chemistry) can be determined by measuring the evolving thickness of the oxide using XRT.