Synthese und Identifizierung von substituierten Mg-Al-Cl-Doppelhydroxidverbindungen mit Schwerpunkt IR-Spektroskopie

On the basis of the political decision that there should be no reprocessing of spent research reactor fuel elements (RR-FE) in Germany, direct disposal in deep geological formations is under discussion. The RR-FE are classified as heat-generating high-level radioactive waste and their disposal is aspired preferential in salt formations. A possible accident scenario for long-term safety analysis in a salt repository is a water ingress, whereby highly concentrated salt brines will arise. Leaching experiments with irradiated and unirradiated metallic UAl$_{x}$-Al- and uranium-silicide RR-FE in repository-relevant solutions, such as clay pore water (Mont-Terri-Type) or MgCl$_{2}$-rich salt brine (brine 2) were therefore undertaken at the institute of energy research (department safety research and reactor technology, IEF-6) of the Research Centre Jülich (Curtius et al., 2006). Under the test conditions the RR-FE corroded completely within one year and the contained radionuclides were mobilized primary. But most of the radionuclides were immobilized again by the secondary phases formation (corrosion products) showing that these secondary phases act as a near-field barrier against radionuclide migration (Brücher et al., 2001). A Mg-Al-Cl layered double hydroxide (LDH) was identified as a crystalline component of the secondary phases arisen from the non irradiated RR-FE in brine 2 (Mazeina et al., 2003). Structurally, the LDHs are composed of brucite-like layers, in which some of the divalent cations are replaced isomorphously by trivalent cations (Reichle, 1986). The resultant positive charge excess is compensated by anions in the interlayer. Water is also present in the interlayer. LDHs have attracted attention recently because of their especial layered structures and their large anion exchange capacities of up to 3 meq/g (Miyata, 1983). Due to the large anion exchange capacities they are used e.g. in order to separate heavy metals. LDHs can be synthesized readily by coprecipitation methods (Miyata, 1975; Cavani et al., 1991). In a repository it is assumed that during water ingress beside the pure Mg-Al-Cl LDH different solid solution-LDH-compounds might be found, because of the fuel element container being additional present beside the RR-FE. In this study the incorporation of cadmium (contained in control rods), iron and manganese (FE container materials) into the structure of a Mg-Al-Cl LDH was therefore investigated. Synthesis of several LDHs by a coprecipitation method was performed. In these LDHs some molar amounts of the magnesium cations were replaced successfully by other divalent cations (Cd$^{2+}$, Fe$^{2+}$ or Mn$^{2+}$). Even cations in the size of Cd$^{2+}$ (Cd$^[2+}$: 95 pm, Mg$^{2+}$: 72 pm, Shannon, 1976) can be incorporated. In order to determine, if these substituted LDHs have better retention properties for radionuclides, the sorption behavior of selenium (selenium is a long-living fission product and [...