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.

Standardisation des outils de calcul pour les ADS et leur application à différents scénarios de transmutation des déchets

Marco Cometto

The management of radioactive wastes from the nuclear fuel cycle has become an important issue in the development of future, more sustainable nuclear energy systems. Partitioning and transmutation (P&T) of actinides and some long-lived fission products could reduce the mass and radiotoxicity of highlevel wastes and possibly ease repository licensing requirements. Influenced by political and technological developments, an increasing number of countries employing nuclear power have become interested in P&T technology which involves the development of new types of critical and/or sub-critical reactors and very efficient fuel cycle strategies. The present thesis is closely connected to two P&T related projects of the OECD Nuclear Energy Agency in Paris, a numerical benchmark exercise for an accelerator-driven minor actinides burner and a calculational system study on the role of accelerator-driven systems (ADS) and advanced fast reactors (FR) in advanced nuclear fuel cycles. The author co-coordinated the benchmark exercise and performed the comparative analysis of the different "fuel cycle schemes" investigated in the system study. The benchmark exercise contributed to a greater understanding of the physical phenomena characteristic of the transmutation of actinides in a closed fuel cycle, and allowed to test and improve the tools used for modeling ADSs and corresponding advanced fuel cycles. This work thus effectively led to the development of an efficient and more complete calculational scheme, which has permitted an in-depth and reliable analysis of different transmutation scenarios to be carried out. To quantitatively assess the advantages and drawbacks of transmutation, we considered a limited number of "base" transmutation scenarios which constitute an envelope for various possible transmutation strategies employing fast spectrum systems. We then evaluated, for an equilibrium situation, the main aspects which characterise the different transmutation strategies, such as the nuclear park composition, the mass and toxicity of the waste, the need for natural uranium, the in-pile and out-pile mass inventories and, finally, the fuel cycle requirements. Thus, this study has allowed for the reliable comparison of performances of the three main options in the nuclear waste management, viz. open cycle, recycling of plutonium alone and recycling of all the actinides, as well as an evaluation of the advantages and disadvantages of ADSs in comparison to advanced critical reactors in the management of minor actinides or transuranics. The last part of the dissertation is dedicated to a phase-out study for two schemes employing either ADSs or critical reactors. It has been shown thereby that advanced systems, optimised for an equilibrium fuel composition, can indeed be used to reduce the inventory of transuranics. A comparison of ADS and FR performances has been made in this context, and the relative benefits of transmutation have been quantified in each case taking into consideration, more realistically, the residual mass inventory after the shutdown of all nuclear installations.

EPFL2003

Standardisation des outils de calcul pour les ADS et leur application à différents scénarios de transmutation des déchets

Marco Cometto

EPFL2003