Dissolution Behaviour of Innovative Inert Matrix Fuels for Recycling of Minor Actinides

Autor: Mühr-Ebert, Elena Laura
Jazyk: angličtina
Rok vydání: 2017
Předmět:
DOI: 10.18154/rwth-2017-06658
Popis: RWTH Aachen University, Diss., 2017; Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Schriften des Forschungszentrums Jülich, Reihe Energie & Umwelt, 378, 1 Online-Ressource (xii, 164 Seiten) : Illustrationen, Diagramme(2017). = RWTH Aachen University, Diss., 2017
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 (AnO2) 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 the reprocessability of molybdenum and magnesia based inert matrix fuels reference samples containing variable amounts of CeO2, 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/CeO2 pellets the MgO dissolves completely, while the bulk of CeO2 remains undissolved, allowing a separation of the actinides and the matrix during the dissolution process. The dissolution of molybdenum is more complex due to its specific aqueous redox chemistry (Mo(-II) – Mo(VI)). Molybdenum metal is soluble in nitric acid. However, it is oxidized to MoO3, which then precipitates. The dissolution rate strongly depends on the acid concentration. Higher acid concentrations or temperatures result in faster dissolution but also in more precipitation. The addition of ferric nitrate to the acid improves the dissolution properties and prevents precipitations at low acid concentrations. However, the addition of a great amount of Fe3+ to the solution might influence the extraction process. To get closer to the real system, unirradiated PuO2/Mo pellets were dissolved in collaboration with NRG and the Karlsruher Institut für Technologie, Institut für Nukleare Entsorgung. The presence of Fe(III) increases the solubility of Mo as described above, while the solubility of the actinides is decreased. In 1 and 3 mol/L HNO3 without the addition of iron about 2 and 3% of the plutonium dissolved respectively, while less than 0.1% of the Pu dissolved in the presence of iron.Dissolution in nitric acid is the first step in the head end of the reprocessing of spent fuel. Therefore, detailed knowledge of the speciation of molybdenum in nitric acid medium is crucial on the one hand to understand this dissolution process and on the other hand as a basis for the design of a tailored extraction process. The solution species of Mo in strongly acidic media have been characterized and quantified comprehensively. For this purpose electrospray ionization mass spectrometry (ESI-MS), which can probe the stoichiometry and relative abundances of solution species, was applied in collaboration with the Leibniz Universität Hannover, Institut für Radioökologie und Strahlenschutz. The method delivers unique insights into the solution speciation of molybdenum as a function of acid- and Fe(III)-concentration. The solution species Mo forms in the presence of iron were investigated in order to understand the effect of Fe(III) on the dissolution. The ESI-MS reveals the formation of mixed Mo-Fe species which explains the increased solubility. Moreover, a method to analyse solutions of molybdenum with natural isotopic composition with a commercially available ESI/MS/MS instrument (QTrap) was developed in collaboration with the Forschungszentrum Jülich, Zentralinstitut für Engineering, Elektronik und Analytik 3. The commercial instrument permits routine measurements due to significantly reduced measurement times. As an alternative to the very complex dissolution of Mo based IMF the separation of the matrix material from the fuel by thermal treatment was considered. This exploits that molybdenum is oxidized in air at temperatures from 400 °C and the resulting MoO3 sublimes at 800 °C. It is expected that the volatile components of the fuel are not deposited together with the molybdenum and the remaining components of the fuel do not evaporate at these tem-peratures. Molybdenum was quantitatively evaporated and recovered as MoO3, confirmed by XRD. In the case of thermal treatment of Mo/CeO2 mixtures a solid state reaction of MoO3 and CeO2 occurs at 674 °C. Depending on the temperature and Mo-Ce ratio CeMo5O8, Ce8Mo12O49, and Ce2Mo4O15 form. In the case of thermal treatment of Mo/PuO2 mixtures at equivalent conditions a mixed oxide PuMo2O8 is expected to form, which will affect the evaporation in a way similar to the MoO3 and CeO2 solid state reaction. However, the bulk of the Mo should be evaporated and the mixed oxide PuMo2O8 can be easily dissolved in nitric acid. The thermal treatment of the fuel is a promising method for the separation of the bulk molybdenum to simplify the dissolution and minimize the effect on the liquid-liquid extraction.
Published by Forschungszentrum Jülich GmbH, Zentralbibliothek, Jülich
Databáze: OpenAIRE
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