Innovative production of nuclear fuel by microwave internal gelation: Heat transfer model of falling droplets

Based on closed packed microspheres produced by internal gelation, the Sphere-pac concept provides a safer and cleaner process and better remote production capability compared to the traditional production of nuclear fuels. Whereas classical reactor fuel rods are fed with UO2 pellets, this advanced concept is suggested for minor actinide bearing fuels. This paper focuses on the thermal analysis of the microspheres during their preparation by microwave-assisted internal gelation (MIG). Internal gelation is a sol-gel process initiated by a temperature shift within aqueous droplets. Alternatively this critical temperature increase takes place by contact heat transfer from a hot silicon-oil bath. Microwave heating of the droplets during their free fall provides a simpler and cleaner process, where the experimental parameters have to be carefully optimized. A matlab model is developed to study the thermal behaviour of a droplet during processing inside a microwave resonant cavity. This finite elements model takes into account the most relevant parameters (size of droplet, thermal properties of the solution, heat and mass transfer …) and calculates the temperature profile inside the droplet. The model is validated with experimental data from literature. The influence of each parameter on the temperature profile is investigated and the optimal microwave power can then be determined and applied to the production unit.