Home > Publications database > Dissolution Behaviour of Innovative Inert Matrix Fuels for Recycling of Minor Actinides |
Book/Dissertation / PhD Thesis | FZJ-2017-05787 |
2017
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-238-2
Please use a persistent id in citations: http://hdl.handle.net/2128/15153 urn:nbn:de:0001-2017081734
Abstract: During the peaceful use of nuclear energy high level wastes, which contain long-lived radionuclides (plutonium, minor actinides) with high radiotoxicity, are generated worldwide. About 10,000 t of spent fuel are unloaded from commercial reactors each year. Most countries including Germany favour the direct disposal of spent fuel in deep geological formations. Other countries prefer the reprocessing of the spent fuel to recycle uranium and plutonium. In Europe commercial reprocessing is currently performed in La Hague and Sellafield. A future alternative would be provided by closing the fuel cycle in the context of the partitioning and transmutation strategy (P&T). This method separates and converts the long-lived radionuclides into stable or short-lived nuclides via neutron induced reactions in dedicated facilities. The P&T strategy has potential to significantly reduce the radiotoxicity and the volume of the radioactive waste; however it cannot obviate the need of a final repository. The transmutation of minor actinides can be performed in different reactor types, including accelerator driven systems, which consist of a subcritical reactor core and an external accelerator. The accelerator driven system (ADS) fuel consists of the fissile material (AnO$_{2}$) which is spread in an inert matrix to improve the thermal properties of the fuel. Within this work two different fuels containing actinide oxides as fissile material and ceramic magnesium oxide (CerCer) or metallic molybdenum (CerMet) as matrix material are under investigation. The dissolution and separation issues for inert matrix fuels (IMF) have not yet been investigated coherently. It is of crucial importance to take into account the behaviour of the matrix elements in the dissolution and separation processes and to check their compatibility with future waste management requirements. The dissolution and the subsequent hydro or pyrometallurgical treatment of the fuel are crucial steps in the reprocessing process. A complete dissolution of the actinide oxide and the matrix material, or a selective dissolution, where one of the components remains undissolved, can be considered. To investigate there processability of molybdenum and magnesia based inert matrix fuels reference samples containing variable amounts of CeO$_{2}$, which serves as surrogate for plutonium dioxide, have been prepared based on a comprehensive compactibility and sinterability investigation. The pellets were thoroughly characterized by means of density measurements, micro hardness measurements, scanning electron microscopy (SEM) investigation, and X-ray diffraction(XRD). The dissolution rate was studied in macroscopic experiments as a function of acid concentration and temperature. Magnesium oxide is soluble even under mild conditions. The dissolution rates of MgO at different acid concentrations are rather similar, whereas the dissolution rate is strongly dependent on the temperature. Additionally, the MgO dissolution process was investigated following a microscopic approach. Detailed SEM investigations show a heterogeneous reactivity of the MgO pellet’s surface. A model was developed to describe the evolution of the pellet surface area and a surface normalized dissolution rate was calculated. The activation energies of MgO dissolution in nitric acid have been calculated from the Arrhenius plot for different acid concentrations and indicate a surface controlled dissolution mechanism. During the dissolution of MgO/CeO$_{2}$ pellets the MgO dissolves completely, while the bulk of CeO$_{2}$ remains undissolved, allowing a separation of the actinides and the matrix during the dissolution process.
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