Characterisation of severely damaged nuclear fuel materials for treatment and disposal

The severe nuclear reactor accidents that have happened across the world resulted in severe consequences, including the release of radionuclides and the generation of large amounts of highly radioactive nuclear fuel-containing materials. To reduce the radioactive hazard to public and environment and to inform and guide the management of severely damaged nuclear fuels, investigation of a limited number of samples collected from affected nuclear reactors has been conducted. However, analysis was restricted by their high radioactivity and heterogenous properties. To gain a deeper insight into the physical and chemical properties, simulant materials with low radioactivity have been synthesised in laboratory and characterised by synchrotron X-ray microanalysis. This thesis presents a series of studies on the simulation and characterisation of radioactive nuclear materials, including Advanced Gas-cooled Reactor (AGR) spent nuclear fuel, Chernobyl lava-like fuel containing material (LFCM) and Fukushima molten corium concrete interaction (MCCI) products, and the development of a software, containing the fundamental functions of X-ray microanalysis and a new approach for generation of 2D speciation maps. The main results are summarised below:
A simulant AGR spent nuclear fuel (SIMFuel) was manufactured by Hot Isostatic Pressing (HIP), with retention of volatile fission products. Characterisation of HIP AGR SIMFuels demonstrated fission product partitioning, phase assemblage, microstructure and porosity in good agreement with real spent nuclear fuels. Cs was found at the can-material interface, similar to its location in actual fuels, where it is distributed in the gap between fuel pellets and cladding.
Micro-focus synchrotron X-ray analysis was applied to investigate several representative regions of the heterogeneous simulant LFCMs and Fukushima MCCI products. Combining the analyses from micron-resolved chemical probes (X-ray fluorescence, µ-XRF and X-ray Absorption Spectroscopy, µ-XAS) and diffraction analysis (µ-XRD), the crystalline phase assemblage was established and the oxidation state and local coordination of uranium was determined in both crystalline and amorphous fractions of the material, giving novel insight to nuclear accident material chemistry. The data obtained were interpreted to provide information on the chronology of crystallisation of phases after the accident, which was in good agreement with the limited information available on real LFCM samples. The crystalline phase assemblage revealed the presence of the expected minerals such as a range of U-Zr-O containing crystallites in both LFCMs and Fukushima MCCI products. In addition, crystalline silicate phases including CaSiO3, SiO2-cristobalite and (Ce,Nd)2Si2O7 were detected. Micro X-ray absorption spectroscopy analysis determined that the majority of U was present in tetravalent species, whereas Ce was observed to be trivalent, concurrent with the highly reducing conditions of synthesis.
A new approach to construct 2D speciation maps, based on synthetic Chernobyl LFCMs, was proposed and applied to obtain information on the spatial variation and gradients of the oxidation state of U. The maps were calibrated to the normalised absorption of U L3 XANES spectra of relevant reference compounds, modelled using a combination of arctangent and pseudo-Voigt functions (to represent the photoelectric absorption and multiple scattering contributions). The result showed that calibration of speciation maps can be improved by determination of the normalised X-ray absorption at specific excitation energies selected to maximise oxidation state contrast.
To assist in the wider application of the methods developed in this thesis, the XAiSL software was developed. It allows synchrotron µ-focus XRD, XRF and XAS data to be imported, processed for analysis and plotted with minimal user manipulation through a user friendly graphical user interface (GUI). Combining these analyses allows the construction of two-dimensional maps of crystalline phases and oxidation states.
The work performed in this thesis has demonstrated that µ-focus X-ray analysis of very small fractions of material can yield rich chemical information, which can be applied to real particles of nuclear-melt down materials to aid decommissioning operations. The different distribution of phases assemblages and chemical properties of simulant materials can guide the ongoing decommissioning work, especially in terms of phase identification, degradation prediction and retrieval of real materials within an accident affected nuclear reactor. For instance, the proper cutting and collection technology for sample removal can be investigated based on these simulant materials with low radioactivity and cost. In the future, these materials can even be used as a model in the research and development of wasteform for disposal.