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

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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.