Within this PhD project, the goal is to separate out the phenomenon of recrystallisation and provide a scientific description. All efforts to ‘simulate’ the effect of fission on the fuel matrix have important limitations, so to achieve a representative study, it is necessary to use irradiation. Within this PhD, the ultimate goal is to irradiate small disks of U7Mo in the BR2 reactor at representative temperatures and up to appropriate fission densities. Possibilities for such an irradiation have been verified, but the candidate will need to work out the details of the experiment. Collaborations with PNNL, INL and ANL in the USA are possible for this. By such an experiment, a systematic matrix of materials irradiated at different temperatures and up to different fission densities will be generated and the irradiated disks can be examined with a variety of techniques, particularly microhardness measurements, SEM-TEM for microstructure work, laser flash (collaboration with ITU) for thermal conductivity, X-ray diffraction for lattice parameter, etc. This will allow a deeper understanding of the mechanisms underlying the recrystallisation, help in defining thresholds for recrystallisation in terms of fission density, temperature and the influence of the microstructure on the recrystallisation effect. In the end, this data will provide input for mechanistic modeling (not within the scope of this work) of fuel behaviour.
Preparative work can be accomplished by the irradiation of U7Mo disks with ion beams. Although this only partially simulates the situation in pile and limits the effect to a thin layer on the surface of only a few µm thick, it is the closest possible simulation and a very useful way to provide important first indications. Furthermore, there are important advantages since the samples do not get activated and temperature control is easier. Collaborations with the University of Munich or the Argonne National Lab in Chicago are immediately available for the ion irradiations, but possibilities also exist in France (GANIL).
The candidate will be involved in the preparation of the UMo discs needed for the irradiations (ion and neutron), the preparation and design of the in-pile experiment (based on existing technology), will be directly involved in the ion irradiation experiments and post-irradiation examinations of the ion-irradiated specimens. As part of an ongoing irradiation and PIE program that will enter the hot cells in 2015, the candidate will have the opportunity to study irradiated U7Mo fuel at different levels of recrystallisation. Although in that case different parameters are harder to separate, it will provide a good basis to maintain the technological relevance of the scientific approach with the disc irradiation. It can also provide backup materials in case the irradiation project suffers delays, which is always a risk in the nuclear field. Immediate opportunities for publication exist as the data is technologically and scientifically highly relevant.