Fission gas release mechanism between 600 °C and 800°C during thermal transient on irradiated fuel

The subject of this thesis concerns the evaluation of the source term (fission gas release) in incidental situations of type LOCA (Loss of Coolant Accident) of nuclear fuels. During such thermal transients, the fission gas release is characterized by successive bursts: the first one occurring between 600 and 800 C, and a second one at about 1100 C. This work is about the first burst release. It appear that this one come from the centre of the fuel pellet. The aim of this thesis is to study the mechanisms responsible for the fission gas release between 600 C and 800 C. To this purpose, we collected more than 200 results of thermal treatments carried out using the experimental platform MERARG. The analysis of this database reveals two important results: under the experimental conditions of MERARG, no significant fragmentation of the fuel was observed at temperatures below 1000 C; the amount of fission gas release between 600 C and 800 C appears to increase with fuel storage time. The first result suggests the fragmentation of the fuel is not a necessary condition for the release of fission gas in this temperature range. Other mechanisms may then be responsible for this gas release. During its storage, the fuel undergoes α particle self-irradiation. We demonstrate that the kinetics of fission gas release between 600 C and 800 C is simultaneous with the kinetics of the annealing of self-irradiation defects at this same temperature. The mechanism involves an autocatalytic process leading to a kinetic of fast germination-growth of gas nano-clusters. This model perfectly explains the experimental results in the database. To confirm this mechanism, a study on surrogate materials demonstrates that, in UO₂ pellets sintered and implanted by Xe, irradiations in the electronic regime actually promote the release of implanted Xe at those temperatures. The re-dissolution of the fission gas bubbles by this kind of irradiation is consistent with observations in other contexts. Those conclusions allow to extend the mechanism for release to irradiated fuel. During the storage of the fuel, α self-irradiation promotes the re-dissolution of the trapped gas. The consequences of this effect are particularly important in the region close to the grain boundaries, where the concentration of defects is also larger. The irradiation mechanism increases the fraction of fission gas available for release, depleting the amount of gas initially trapped in bubbles. The gas in solution can effectively be carried by crystal defects, largely available in the irradiated fuel and whose migration at 600-800 C induces the mobility of the fission gas. When they reach an outlet path, the gas can be released from the pellet in a way consistent with the model of autocatalytic germination-growth we developed to explain the macroscopic results of the database. In conclusion, this work supports the hypothesis of a mechanism of fission gas release in the range 600-800 C via a mechanism involving the migration and annealing of defects without significant fragmentation of the fuel. (author)