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Crystalline materials such as monazite have been considered for the storage of radionuclides due to their apparent radiation stability. Understanding their structural chemical response to radiation damage is a key component of determining their suitability for this application. Herein, high resolution structural studies were performed on the monazite solid solution La1-CexPO4 (x = 0.25, 0.5, 0.75, 1) in order to understand the role of structural chemistry on irradiation stability. Ceramic samples were irradiated with 14MeV Au ions with 1014 ions/cm2 and 1015 ions/cm2 to simulate the recoil of daughter nuclei from the alpha decay of actinides. The extent of radiation damage was analysed in detail using scanning electron microscopy (SEM), Raman spectroscopy, grazing incidence X-ray diffraction (GI-XRD) and high-energy-resolution fluorescence detection extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy. SEM and Raman revealed extensive structural damage as well as the importance of grain boundary regions, which appear to impede the propagation of defects. Both radiation-induced amorphisation and recrystallisation were studied by grazing incidence X-ray diffraction, highlighting the exceptional ability of monazite to remain crystalline at high fluences throughout the solid solution. Both, diffraction and HERFD-EXAFS experiments show that while atomic disorder is increased in irradiated samples compared to pristine ceramics, the short-range order was found to be largely preserved, facilitating recrystallisation. Crucially, this is consistent for all investigated samples, despite the chemical disorder introduced in the solid solution.

To mimic radiation damage by recoiling nuclei following alpha-decay, ceramics and single crystals of LaPO4 monazite doped with Eu(III) were irradiated with 14 MeV Au5+ ions at three different fluences. The crystallinity, local coordination environments, and topography of the samples were probed using numerous methods including grazing-incidence X-ray diffraction (GIXRD), vertical scanning interferometry (VSI), scanning electron microscopy (SEM), Raman, and luminescence spectroscopy. GIXRD data collected from the irradiated regions of the ceramics revealed fluence dependent amorphization. A similar level of amorphization was detected for samples irradiated with 5×1013 ions/cm2 (fluence, F1) and 1×1014 ions/cm2 (F2), while a slightly lower contribution to the scattering signal from the amorphous part was obtained for the sample irradiated with the highest fluence of 1×1015 ions/cm2 (F3). VSI showed clear swelling of entire grains at the highest ion fluence, while more localized damage to grain boundaries was detected for ceramic samples irradiated at the lowest fluence. Single crystal specimens showed no pronounced topography changes following irradiation. SEM backscattered electron images revealed that the ceramic irradiated at the highest fluence exhibited topological features indicative of grain surface melting or softening and displacement of grains. Finally, Raman and luminescence data showed a different degree of disorder in polycrystalline vs. single crystal samples. While changes to PO4 stretching and bending vibrations could be observed in the ceramics, these changes were more subtle or not present in the single crystals. The opposite was observed when probing the local Ln-O environment using Eu(III) luminescence, where the larger changes in terms of an elongation of the Eu-O (or La-O) bond and an increasing relative disorder with increasing fluence were observed only for the single crystals. The dissimilar trends observed in irradiated single crystals and ce

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.

The incorporation of tetravalent cerium into the monazite structure via Ca(II)-coupled substitution was investigated using a solid state and a co-precipitation route. Based on powder XRD measurements and elemental mappings an optimised synthesis procedure was developed that averts the formation of secondary phases and allows the stabilisation of tetravalent cerium with a ratio of up to 0.21 Ce(IV)/Ce(III). In-situ HERFD-XANES measurements at the Ce L3 edge at up to 800 °C were performed to study the cerium oxidation state during the phase transformation from rhabdophane to monazite and the sintering process, revealing an unexpected non-linear behaviour as well as a charge-directing effect of the lanthanum cation.

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.

Cr-doped UO2 is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO2 matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO2 single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U+4 in the fluorite lattice as Cr+3 with formation of additional oxygen vacancies. Bulk material measurements reveal the complexity of Cr states, where metallic Cr (Cr0) and oxide related Cr+2 and Cr+32O3 were identified and attributed to grain boundary species and precipitates, with concurrent (Cr+3xU+41-x)O2-0.5x lattice matrix incorporation. The deconvolution of chemical states via crystal vs. powder measurements enables the understanding of discrepancies in literature whilst providing valuable direction for safe continued use of Cr-doped UO2 fuels for nuclear energy generation.

Experimental and fitted EPR data of Cr-UO2 single crystal grains, EXAFS data Cr-UO2 single crystal grain and powder with Cr redox standards, XANES data Cr-UO2 single crystal grain and powder with Cr redox standards

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. The main objectives of the AcE project are (i) the development of synthesis strategies for An(IV)-doped solid phases, (ii) understanding their associated structural and physical properties using combined modelling and experimental approaches and (iii) determining their performance after irradiation with particular regard to an assessment of their long-term stability, dissolution behavior, and suitability for An matrix incorporation. Recent results obtained for ZrO2, the main corrosion product of the Zircaloy cladding material surrounding nuclear fuel rods, will also be discussed. ZrO2 is monoclinic phase (P2(1)/c) at ambient conditions, and transforms into tetragonal (P4(2)/nmc) and cubic phases (Fm3 ̅m) at high temperatures of around 1200 °C and 2370 °C, respectively. However, particle size effects, the incorporation of foreign ions such as the actinides, as well as high radiation fields are known to also influence the stability fields of the polymorphs. A short overview of experimental studies conducted by the AcE partners, addressing both the ZrO2 bulk structure, irradiation-induced changes, as well as the An environment during and after such structural transformations, will be given.