Das Verhalten der Urancarbide und -oxicarbide in endlagerrelevanten aquatischen Phasen

The reactors of the next generation proposed as part of the Gen IV Initiative also include a reactor type that has already been successfully tested in Germany, i.e. the graphite-moderated high-temperature reactor (HTR). Apart from the high operating temperature, this reactor type also enables the use of a large number of nuclear fuels of different compositions. As part of the EU RAPHAEL project ($\mathbf{ReA}$ctor for $\mathbf{P}$rocess heat, $\mathbf{H}$ydrogen $\mathbf{A}$nd $\mathbf{EL}$ectricity generation), studies were initiated on the advantages and disadvantages of uranium and uranium/thorium mixed oxides as potential fuels. These studies were extended to include the topic of carbide-based fuel compositions. In addition, consideration was also given to various accident scenarios of long-term relevance with respect to the disposal of these nuclear fuels. One of the possible scenarios concerns an ingress of water into the repository and the associated corrosion of the containers as well as a possible release of radioactivity. It is therefore important that the behaviour of the potential HTR fuel compositions should be investigated and understood in the repository-relevant aquatic phases. In the present work, analytical procedures such as X-ray diffraction, scanning electron microscopy, mass spectrometry, $\gamma$-spectrometry and gas chromatography were applied for this purpose. The results show that, depending on the composition of the aquatic phases and the experimental conditions, the carbide-based uranium compounds displayed different uranium dissolution behaviour. For example, a complete conversion of UC$_{2}$ into UO$_{2}$ was determined with the MgCl$_{2}$-rich brine 2 solution at 90 °C. This conversion delayed the uranium dissolution compared with the experiments involving clay pore water. However, the presence of iron significantly increased uranium dissolution in the brine 2 solution in comparison to that in clay pore water. These initial results can make an important contribution towards the development of future reactors, particularly with respect to safety analysis