CHaracterisation, Imaging and MaPping of fuel debris for safe retrieval (CHIMP)

这项研究是英国和日本联合开展的，旨在支持福岛第一核电站（NPP）正在进行的清理工作。2011年3月11日，海啸吞没了福岛第一核电站，导致冷却剂丢失事故和1 - 3号沸水反应堆部分熔毁。熔毁后，立即向高温反应堆堆芯注入海水进行紧急冷却，但这还不够，温度上升到2000摄氏度以上，导致UO2核燃料颗粒熔化，并与蒸汽氧化的锆燃料包壳发生反应。由此产生的材料是高度危险的，包括UO2燃料、包层中的锆和其他熔化的反应堆部件的混合物，如燃料压力容器中的混凝土和钢。这些材料被称为燃料碎片和堆芯，具有高放射性，因为它们也含有裂变产物和燃料裂变产生的少量锕系元素（例如钚）。自事故发生以来，水一直不断地注入反应堆，成功地冷却了燃料碎片和堆芯。在这种所谓的“稳定冷停堆条件”下，对冷却水流出物的分析显示含有钚，这表明燃料碎片正在溶解。这促使了从反应堆中回收燃料碎片和堆芯的重大努力，但在制定任何回收计划和选择安全进行回收操作的技术之前，有必要了解：1)燃料碎片和堆芯的化学和物理性质；2)燃料碎片和堆芯在反应堆内驻留的地方。由于在“正常”的核退役操作中找不到这种材料，而且由于极端的辐射场，不可能进入反应堆取样或定位其位置，因此需要其他解决方案。在这个研究项目中，我们将使用掺有“冷”裂变产物类似物（如稳定同位素）的UO2，与锆包层和其他反应堆部件材料（如混凝土和钢）一起熔化，制造替代燃料碎片和堆芯材料。与真实的燃料碎片和堆芯相比，这些在实验室处理是安全的。通过分析这些材料，这将是迄今为止创造的最真实的燃料碎片和堆芯模拟物，我们将建立对其特性的理解，这将支持退役方法的选择（例如，哪种类型的机器人和切割工具）以及如何处理材料，一旦它们从反应堆中取出。这些材料还将用于验证我们的日本项目合作伙伴目前正在开发的两项重要技术，从反应堆中回收燃料碎片和堆芯。第一个是现场远程3D成像技术，能够在不直接接触的情况下识别反应堆内任何给定的燃料碎片或堆芯中的放射性元素类型。我们建议在福岛第一核电站的项目中测试这种方法。第二个是统计映射模型，该模型将使用模拟燃料碎片和堆芯材料的特征输入，结合福岛第一核电站燃料碎片释放的伽马辐射数据，绘制出反应堆内燃料碎片和堆芯的预测地图。该模型、我们的材料以及英国和日本在该项目中的合作伙伴之间的合作，将有力地支持将于2021年开始的安全燃料碎片回收作业的规划和实施。

This research is a joint UK and Japan effort to support ongoing clean-up operations at the Fukushima Daiichi Nuclear Power Plant (NPP). On March 11 2011, the tsunami that engulfed the Fukushima Daiichi NPP resulted in a loss of coolant accident and the partial meltdown of boiling water reactor Units 1 - 3. Immediately after the meltdown, seawater was injected into the high-temperature reactor cores for emergency cooling, however this was not sufficient and temperatures rose in excess of 2000C, causing pellets of UO2 nuclear fuel to melt and react with steam-oxidised zircaloy fuel cladding. The resulting material is highly hazardous and comprises a mixture of UO2 fuel, zirconium from the cladding and other melted reactor components, such as concrete and steel from the fuel pressure containment vessel. These materials are known as fuel debris and corium and are highly radioactive since they also incorporate fission products and minor actinides (e.g. plutonium) from the fission of the fuel. Since the accident, water has been continuously injected to the reactor, which has successfully cooled the fuel debris and corium. In this so-called "stable cold shutdown condition", analysis of coolant water effluent has been shown to contain plutonium, which indicates that the fuel debris is being dissolved. This has prompted significant efforts to retrieve the fuel debris and corium from the reactors, but before any plans for retrieval can be made, and technologies selected to safely undertake the retrieval operations, it is necessary to understand: 1) the chemical and physical properties of the fuel debris and corium; and 2) where the fuel debris and corium reside within the reactor.Because this material is not found in 'normal' nuclear decommissioning operations, and because it is not possible to enter the reactor to take samples, or to locate its position due to the extreme radiation field, other solutions are required. In this research project, we will make surrogate fuel debris and corium materials using UO2 doped with "cold" fission product analogues (e.g. stable isotopes), melted together with zirconium cladding and other reactor component materials (e.g. concrete and steel). In comparison with real fuel debris and corium, these will be safe to handle in the laboratory. By analysis of these materials, which will be the most realistic fuel debris and corium simulants created to date, we will build an understanding of their properties, which will support the choice of decommissioning methodologies (e.g. which type of robot and cutting tool) and how to handle the materials once they have been retrieved from the reactor. These materials will also be used to validate two important techniques currently under development by our Japanese project partners to retrieve fuel debris and corium from the reactors. The first is an on-site remote 3D imaging technique, capable of identifying the type of radioactive elements in any given piece of fuel debris or corium within the reactor, without direct contact. It is proposed to test this method during our project at the Fukushima Daiichi NPP. The second is a statistical mapping model, which will use input from the characterisation of our simulant fuel debris and corium materials, combined with data from the gamma radiation emission from the fuel debris within the Fukushima Daiichi NPP, to make a predictive map of where the fuel debris and corium reside within the reactor. This model, our materials and the collaboration between UK and Japan partners in this project, will strongly support the planning and implementation of safe fuel debris retrieval operations, which are due to begin in 2021.