Bildung von sekundären Phasen bei tiefengeologischer Endlagerung von Forschungsreaktor-Brennelementen - Struktur- und Phasenanalyse

For the assessment of a confident und sustainable final disposal of high level radioactive waste –fuel elements of german research reactors also account for such waste – in suitable, deep geological facilities, processes of the alteration of the disposed of waste and therefore the formation of the corrosion products, i. e. secondary phases must be well understood considering an accident scenario of a potential water inflow. In order to obtain secondary phases non-irradiated research reactor fuel elements (FR-BE) consisting of UAl$_{x}$-Al were subjected to magnesium chloride rich brine (brine 2, salt repository) and to clay pore solution, respectively and furthermore of the type U$_{3}$Si$_{2}$-Al were solely subjected to magnesium chloride rich brine. Considering environmental aspects of final repositories the test conditions of the corrosion experiments were adjusted in a way that the temperature was kept constant at 90 °C and a reducing anaerobic environment was ensured. As major objective of this research secondary phases, obtained from the autoclave experiments after appropriate processing and grain size separation have been identified and quantified. Powder X-ray diffraction (PXRD) and the application of Rietveld refinement methods allowed the identification of the corrosion products and a quantitative assessment of crystalline and amorphous contents. Scanning and transmission electron micoscopy were additionally applied as a complementary method for the characterisation of the secondary phases. The qualitative phase analysis of the preprocessed secondary phases of the systems UAl$_{x}$-Al and U$_{3}$Si$_{2}$-Al in brine 2 shows many similarities. Lesukite – an aluminium chloro hydrate – was observed for the first time considering the given experimental conditions. Further on different layered structures of the LDH type, iron oxyhydroxide and possibly iron chlorides, uncorroded residues of nuclear fuel and elementary iron were identified as well. Depending on preceding preparation procedures of the secondary phases the phase mixture resulted in different constituents. By preparation in water the stable phases obtained were aluminium hydroxides and not lesukite. Crystalline phases containing silicon were solely observed in the system U$_{3}$Si$_{2}$-Al in brine 2 being prepared with water. Contrary to these observations the system UAl$_{x}$-Al in Mont Terri solution (clay pore water) qualitatively exhibits distinct different phase contents. Goethite, gypsum, hemi hydrate (bassanite), and accessory hematite were the only crystalline phases observed. Additionally aluminium and UAl4 as residues of nuclear fuel were also found. [...