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The monazite-cheralite solid solutions LnPO4-Ca0.5Th0.5PO4 with Ln = La, Gd were prepared via a co-precipitation route, showcasing an optimised, scalable synthesis procedure for a possible waste form accommodating high level liquid waste streams. A distortion of the cheralite structure with respect to the monazite structure was observed throughout both solid solutions as evidenced by a deviation of the lattice parameters from the linear behaviour known from other monazite solid solutions. Using a high temperature flux method, cheralite single crystals were grown for the first time for in-depth structural investigations. Both thorium and calcium were found to deviate from the central position of the LnO9 polyhedron, supporting previous neutron diffraction investigations of identical cheralite samples.

Zirconia-based ceramics are promising host matrices for the immobilization of radionuclides in high-level waste streams due to their high radiation resistance and chemical stability. This study explores coprecipitation and different solid-state synthesis techniques to produce phase-pure zirconia-based ceramics with varying cerium and neodymium co-doping. Varying the dopant concentration enabled the synthesis of zirconates with monoclinic, cubic defect fluorite, and cubic pyrochlore structures. Powder X-ray diffraction was used for phase identification. In the case of coprecipitation, all synthesized compositions were predominantly phase-pure. Solid-state synthesis techniques included manual mixing of metal oxide powders with mortar and pestle, mechanical mixing in a ball mill, and magnetic mixing in a slurry. All solid-state mixing methods produced heterogeneous ceramics, featuring multiple phases, with manual mixing yielding the most phase-pure product. Extending the grinding time, re-sintering of the solid phases, and an increased Nd content were found to enhance the phase purity.

Ce-doped yttria-stabilized zirconia (YSZ) and pure YSZ phases were subjected to irradiation with 14 MeV Au ions. The irradiation studies were performed to simulate long-term structural and microstructural damage due to self-irradiation in YSZ phases hosting alpha-active radioactive species. It was found that both the Ce-doped YSZ and YSZ phases were rather tolerant to irradiation at high ion fluences and the bulk crystallinity was well preserved. Nevertheless, local microstrain increased in all the studied compounds after the irradiation, with the Ce-doped phases being less affected than pure YSZ. Doping with cerium ions increased the microstructural stability of YSZ phases through a possible reduction in the mobility of oxygen atoms, which limits the formation of structural defects. Doping of YSZ with tetravalent actinide elements is expected to have a similar effect. Thus, YSZ phases are promising for the safe long-term storage of radioactive elements. Using synchrotron radiation diffraction, measurements of the thin irradiated layers of the Ce-YSZ and YSZ samples were performed in grazing incidence (GI) mode. A corresponding module for measurements in GI mode was developed at ROBL and relevant technical details of sample alignment and data collection are also presented.

Zirconia is largely studied because of its ability to incorporate lanthanides and actinides; this has been one of the strategies adopted to try to immobilize ions from High-Level Radioactive Waste (HLW). Furthermore, cerium is often used as a surrogate for plutonium due to its analogous chemical properties1. In this study, zirconia samples with different Ce concentrations were syntehsized via the co-precipitation route. A small amount of Eu as co-dopants was added during synthesis to enable luminescence spectroscopic analyses of the solids phases2. Six zirconia compositions doped with 15 to 80 mol%Ce were prepared. The phase composition was evaluated by site selective luminescence spectroscopy (ss-TRLFS) and Synchrotron Powder X-Ray Diffraction. The latter method showed, that more than one phase coexists in all samples with concentrations lower than 80 mol% Ce. The tetragonal phase is predominant until 30 mol% Ce-doping, and above this concentration, a metastable phase (t’’) starts becomes dominant. The phase transformation can also be deduced from the coordination environment of europium, showing a small band at 583 nm, characteristic for the monoclinic phase, and another one at around 581.1 nm for Eu incorporated in the tetragonal phase. The systematic shift of this signal to shorter wavelengths points toward a lengthening of the Eu-O bonds with increasing Ce concentration, as the unit cell becomes systematically larger. The excitation peaks are clearly broader for the samples with predominantly t’’-ZrO2, which points toward a less ordered environement. This is in agreement with the hypothesized displacement of oxygen atoms from ideal (cubic) crystallographic positions, in this metastable (transition) phase.

This contribution provides an overview of a current research network funded by the German Federal Ministry of Education and Research (BMBF), entitled “Fundamental investigations of actinide immobilization by incorporation into solid phases relevant for final disposal” – AcE. The AcE project aims at understanding the incorporation and immobilization of actinides (An) in crystalline, repository-relevant solid phases, such as zirconia (ZrO2) and UO2, but also in zircon (ZrSiO4), pyrochlores (Ln2Zr2O7) and orthophosphates of the monazite type (LnPO4), which may find use as host matrices for the immobilization and safe disposal of high-level waste streams. Recent studies by the AcE-project consortium, addressing the structure, properties, and the radiation tolerance of monazites and Zr(IV)-based solid phases containing actinides or their surrogates from the lanthanide series will be presented. Material synthesis strategies in the AcE project have aimed at generating single-phase solid solutions in the form of polycrystalline powders, dense ceramics, and single crystals. Structural studies using powder X-ray diffraction at ambient conditions, but also at high temperatures and pressures have been complemented with a wide range of microscopic and spectroscopic techniques to address differences between the host- and dopant environments in the solid matrices at ambient and extreme conditions. The radiation tolerance of the synthetic solid phases have been investigated by combining external heavy-ion irradiation of inactive Ln-doped materials and in situ self-irradiation of 241Am-doped Zr(IV)-phases with monoclinic, cubic defect fluorite and pyrochlore structures. The latter experiments have been conducted in joint efforts with the Joint Research Center in Karlsruhe within the ActUsLab programme.

Zirconium dioxide is a corrosion product of the Zircaloy cladding, which houses the nuclear fuel pellets. It can form solid solutions with uranium as well as fission and activation products. Moreover, ZrO2 and other zirconium bearing crystalline solid phases, such as pyrochlores, are being investigated as potential host matrices for the immobilization of radionuclides present in high-level waste streams. Zirconate pyrochlores are characterized by high chemical durability and radiation resistance. However, upon irradiation, some zirconate pyrochlores undergo a phase transition to the defect fluorite crystal structure, or become amorphous, which could hamper a continued immobilization of radionuclides in the solid matrix. In the current study, the influence of different synthesis methods on the phase purity of Ce/Nd-co-doped zirconates was investigated. Furthermore, phase transformations occurring in zirconate phases as a result of different U/Y-dopant concentrations were studied. Ce/Nd-co-doped zirconates and U/Y-co-doped zirconates were obtained via coprecipitation. In addition, identical zirconate compositions were synthesized using three different solid-state methods involving manual mixing with a mortar and pestle, mechanical mixing with a ball mill or magnetic mixing in a slurry. PXRD measurements of all solids were done for crystal structure analysis. For the Ce/Nd-co-doped zirconates synthesized via coprecipitation, rather phase-pure monoclinic, cubic defect fluorite and cubic pyrochlore structures could be obtained. The samples synthesized via solid-state methods were found to contain multiple phases due to insufficient mixing of the educts. Manual mixing led to the most phase-pure ceramics and was therefore chosen for further investigation. It was shown that re-sintering the ceramics as well as longer grinding time resulted in enhanced phase purity. Furthermore, an increase of the Nd-content also correlates with an improved phase purity. PXRD data of the U-d

Marine biodiversity loss is accelerating, leading to the elevated extinction risks of many species, including sharks and rays. To mitigate these losses, information on their distribution and community composition is needed. Monitoring these (often) mobile species is challenging, especially in remote, highly dynamic and turbid coastal areas. Here, we use an environmental DNA (eDNA) approach to: (1) establish the presence and distribution of elasmobranch species, (2) compare this to a conventional fisheries-dependent approach, and (3) determine the influence of season, area-based protection and habitat on elasmobranch community composition in the highly dynamic Bijagós Archipelago in Guinea-Bissau (West Africa). We collected 127 seawater samples and detected elasmobranch DNA in 58 (45.7%) of these samples, confirming the presence of 13 different elasmobranch species (2 sharks, 11 rays), including seven threatened species. Eight of the species detected by the eDNA-approach were also recorded in a fisheries observer program, which recorded another eight species not detected by the eDNA approach. The most commonly occurring species, based on the number of eDNA sampling locations were the pearl whipray (Fontitrygon margaritella), smalltooth stingray (Hypanus rudis), scalloped hammerhead shark (Sphyrna lewini), and the blackchin guitarfish (Glaucostegus cemiculus). Species composition and richness differed significantly before (January-March) and after the rainy season (November-December). Furthermore, we showed that community composition and species richness did not differ between protected (MPA) and non-protected areas of the archipelago. Thus, we confirm that eDNA approaches are a valuable and non-invasive tool to study threatened shark and ray species in data-deficient and dynamic coastal areas, especially when combined with conventional monitoring methods such as fisheries-dependent information.