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

碳化铀（UC）被认为是一种奇异的燃料材料，起源于英国的民用核反应堆试验计划。这种材料在苏格兰北部的唐尼等地被用作高密度燃料的原型。核退役管理局（NDA）有一份辐照和未辐照碳化铀的清单，作为其遗产废物外来燃料材料的一部分，因此对其安全管理和最终处置负有责任（NDA的子公司放射性废物管理公司（RWM）负责）。碳化铀被认为是一种活性物质，具有与铀金属相当的反应性。地质处置是国际公认的最安全的高放射性废物的长期解决方案，英国的外来乏燃料（如硬质合金乏燃料）打算以这种方式进行管理。目前的博士项目将在类似地质处置设施（GDF）的条件下(i)关闭前和（ii）关闭后，研究辐照和未辐照碳化物乏燃料的腐蚀和浸出行为。英国大部分乏燃料库存都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，预计将显示出很高的化学稳定性，而且，除了在晶界和燃料可接近部分的放射性核素迅速释放（即时释放部分）外，容器失效后放射性核素的释放率（基质溶解率）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地下水中会腐蚀得相对较快。本研究旨在更好地支持在GDF处理后的硬质合金乏燃料的预期行为。实验方法：利用美国国家核实验室（NNL） Springfields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的材料分析技术，对碳化铀的腐蚀行为进行纳米到微米到毫米尺度的观察。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。在布里斯托尔的IAC，这些技术都是常规使用的。为了补充材料分析，浸出研究将利用ICP-MS和ICP-OES等溶液分析技术来确定不同gdf模拟地下水溶液（含氧和无氧）中不断变化的U浓度。此外，该项目还将利用萨里大学提供的独特的TRLFS仪器，作为TRANSCEND主要项目的一部分，该仪器正在开发用于含水锕系元素分析。该项目将设置一系列封闭的细胞实验，使用密封的，充满水的玻璃外壳将小的碳化铀“棒”样品保存在固定位置。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，也可以使用x射线断层扫描测量腐蚀过程。这些分析技术将使腐蚀和浸出行为的详细研究，而不会破坏实验系统。由碳化物（13.66 g/cc）腐蚀形成的氧化物（10.97 g/cc）密度较低，这意味着在不同条件下（温度、溶解的O2和水化学）的氧化速率可以通过使用XRT测量氧化物的厚度来确定。

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.