For several decades, ceramic matrices have been studied as potential materials for the specific immobilization of long-lived radionuclides such as iodine, cesium and actinides. Several of them have been considered and optimized on the basis of the existence of natural analogues. The first step in the search of a radwaste ceramic concerns the phase's ability to immobilize the targeted radionuclides and their daughter products. With this in mind, systematic studies are being carried out to analyze the various substitutions required for effective long-term immobilization. While the initial studies reported in literature involved dry chemistry routes, more recent studies involving wet processes have been developed. The latter make it possible to improve synthesis conditions, particularly in terms of homogeneity, radionuclide incorporation rate and reactivity of the initial powders. This can induce improved sinterability. Several phosphate, silicate and oxide ceramics have been recently prepared in this way, using sol-gel, direct precipitation or hydrothermal processes.

Since radionuclides must be immobilized as monoliths, optimized precursors must be densified by melting or, more frequently, sintering. To achieve such a densification, the powders are generally shaped by uniaxial pressing at room temperature, then the pellets are subjected to a high-temperature calcination step. Several microstructural parameters must then be considered, such as grain size (thus grain boundaries density), absence of secondary phases that could alter the chemical durability of the final ceramics, or presence of pore networks that could contribute to accelerated leaching rate.

Chemical durability during leaching or weathering tests under long-term storage conditions is one of the most important properties for validating the use of a radwaste matrix. This study must usually include a kinetic component, allowing the determination of multiparametric dissolution law. This is obtained by independently varying several parameters such as solution pH, temperature and concentrations of active species at the solid/liquid interface. Under these conditions, qualification tests such as MCC1 do not provide sufficient knowledge of the system to allow long-term extrapolation of the long-term behavior of the ceramic material. This kinetic study must be complemented by a thermodynamic approach aimed at describing saturation phenomena. This includes the identification of potential secondary phases likely to act retroactively on the long-term behavior of ceramics. Coupling structural and chemical characterization of these phases with speciation calculations in solution enables us to assess their solubility product and thus their capacity to delay radionuclide migration. As the confined elements are radioactive, irradiation phenomena play an important role both during the elaboration and sintering stages and during leaching tests, notably through radiolysis phenomena in solution and at the solid/solution interface.

This presentation will focus on several examples to illustrate the various steps involved in qualifying a specific radwaste matrix. We will also discuss the pitfalls that can lead to poor knowledge of the system under study, and impact extrapolation of the long-term behavior of the ceramic prepared.

The idea of immobilizing radionuclides in crystalline host materials was put forward 70 years ago. Since then, continuous research has been conducted on a wide variety of crystalline materials that are considered as possible host matrices. However, many challenges remain, owing, e. g., to the complex chemistry of nuclear waste streams and the exceptionally high requirements regarding physical and chemical long-term stability.

Monazite (LnPO_4, Ln = La-Gd) has long been considered as one of the most promising crystalline host materials for long-term storage of radionuclides, especially actinides. The main reasons for this are its chemical flexibility, its excellent aqueous durability and its low recrystallization temperature, which allows for rapid self-healing of radiation induced damages. It has been shown that monazites can accommodate large amounts of trivalent actinides within their crystal structure. However, the incorporation of tetravalent dopants via coupled substitution with divalent cations has proven challenging, even though natural monazite is known to contain significant amounts of Th and U (combined up to 27 wt-%).

To facilitate assessments with respect to selection criteria such as chemical flexibility, radiation resistance and aqueous durability, efforts are made to identify inactive surrogate-models. The use of cerium as a surrogate for tetravalent actinides will be discussed for monazite-type phases based on the solid solution La_1-x(Ca,Ce)_xPO₄ which was extensively studied using powder and single crystal XRD, electron imaging techniques including EPMA, SEM and TEM as well as spectroscopic measurements including Raman, TRLFS, EXAFS and in-situ XAS experiments. Based on these findings the synthesis of active monazites containing up to 50 % Th⁴⁺ was successfully performed as shown by PXRD measurements.

The poster will focus on irradiation studies of monazite-type ceramic pellets from the solid solution La_1‑xCe_xPO₄. Monazite is known for its remarkable ability to recover from radiation damage by a combination of low recrystallisation temperatures (~570 K) and low activation energies for thermal annealing (<3 eV) as well as irradiation-induced recrystallisation which was observed both from external irradiation and self-irradiation.

While various studies have been published investigating the effects of radiation on the monazite structure, the impact of disorder introduced by solid solutions has not yet been studied extensively. For this reason, various compositions of the aforementioned solid solution were irradiated with Au ions at two different fluences and subsequently analysed with SEM, gracing incidence XRD and Raman spectroscopy in order to gain a better understanding of their radiation stability and recrystallisation properties.

The safe disposal of nuclear waste is one of the intergenerational issues which needs to be solved. A potential route to effectively immobilize radionuclides could be realized by their incorporation into crystalline solid phases in future radioactive waste repositories. In particular, the immobilization of specific waste streams containing minor actinides (Np, Am, Cm) or plutonium in crystalline solid phases may be advantageous compared to glass matrices, which may be less resistant to leaching and disintegration [1-3]. Due to their radiation stability and chemical and structural flexibility, monazite-type compounds are considered suitable matrix materials [4]. To better understand structural changes due to radiation damage, synthetic monazite single crystals with different chemical compositions (La, Nd, Pm, Sm)PO₄ were synthesized by a high-temperature (flux method). Irradiation experiments were performed at the UNILAC beamline of GSI Helmholtz-Centre Darmstadt using 1.7 GeV Au ions and fluences of up to 1e13 ions/cm². The irradiated single crystals were characterized by Raman spectroscopy, secondary electron microscopy and single crystal X-ray diffraction. The irradiation of monazite with 1.7 GeV Au ions results in an embrittlement of the crystals and the formation of a glassy surface layer of about ~48 μm thickness, which correlates well with the projected range of ~44 µm according to SRIM-2013 calculations [5]. The irradiation results in a significant broadening of the Raman modes and further changes in the lattice dynamics. X-ray diffraction experiments revealed the amorphization of the surface layer.

The presentation gives an overview of the structural changes of La monazite single crystals under swift heavy ion irradiation at ion fluences of 5e11 ions/cm² ,1e12 ions/cm² and 2e12 ions/cm². After irradiation, cross sections of the single crystals were prepared and additionally polished with an Ar ion mill to investigate the surface damage along the path of the fast heavy ions using optical light microscopy and Raman spectroscopy. The methods used show a strong surface damage within the projection range of the gold ions due to color change, increase of the FWHM of the Raman band and decrease of crystallinity.

[1] Donald et al. (1997) J. Mater. Sci. 32; [2] Ewing (1999) PNAS, 96; [3] Lumpkin et al. (2006) Elements, 2; [4] Schlenz et al. (2013) Z. Kristallogr. Cryst. Mater. 228; [5] Ziegler et al. (2010) Nucl. Instrum. Methods Phys. Res. B 268

The authors acknowledge the BMBF for financial support in the project No. 02NUK060.

Monazites are rare earth phosphates that are potential host matrices for the immobilization of actinides in high-level radioactive waste streams. This is due to their ability to incorporate various cations through different substitution mechanisms as well as their radiation resistance as observed in natural monazite mineral samples. In this study, LnPO₄ monazite ceramics and single crystals doped with 500 ppm Eu^III as a luminescent probe were irradiated with heavy ions to simulate the recoil of daughter products that occurs during alpha decay of the actinides. More specifically, irradiation experiments were conducted either with 14 MeV Au ions at fluences ranging from 5×10¹³ – 1×10¹⁵ ions/cm² or with swift 1.7 GeV Au ions at fluences of 5×10¹¹ – 2×10¹² ions/cm².

Irradiated monazite ceramics were analyzed with electron microscopy (SEM), vertical scanning interferometry (VSI), grazing incidence diffraction (GID), Raman spectroscopy, and luminescence spectroscopy to probe long- and short-range order of the monazite microstructure.

SEM micrographs and VSI data show clear damage of the irradiated regions of the ceramics, in the form of swollen grains and enlarged grain boundaries. GID images and powder patterns reveal diffuse scattering and amorphous contributions in irradiated samples. Solid solution compositions show larger damage than corresponding monazite endmembers, while polycrystalline and single crystal samples show a similar level of amorphization. In the local coordination environments, Raman spectra of irradiated samples display a shoulder on the ν₁ peak, indicating disruption in the vibrational modes of the phosphate tetrahedra. Luminescence data illustrate ion-irradiation-induced changes in the local LnO₉ polyhedral environment in the monazites. Integrated excitation spectra show a difference in the intensity and position of the excitation peak with irradiation. Especially single crystal data show a systematic decrease of the local site symmetry of the Eu³⁺ cation, and a general broadening of emission spectra, indicative for reduced local order following amorphization.

When placed underground for long-term storage (~10^6 years), phases containing radioactive species are exposed to elevated temperature, T, and pressure, P. Corresponding values can reach ~400˚C and 0.4 GPa for repositories situated 15 km underground. Moreover, formation of He bubbles as a result of undergoing α-decay generates regions where P can reach 10 GPa. Temperature and pressure are powerful thermodynamic parameters that can influence structural and physical properties of materials. These factors, therefore, have to be considered when evaluating performance of nuclear phases to be placed in underground repositories for eternity. Corresponding experimental simulations of accelerated aging can be achieved by subjecting candidate materials to extreme conditions. This would allow to conclude on their long-term stability under harsh conditions. We illustrate this approach on Zr-based ceramic materials as hosts for actinide elements.

Studied phases were Y-stabilized ZrO₂ (YSZ), ZrSiO₄ and GdZr₂O₇ doped with tetravalent ions. Zr-based materials can incorporate substantial amount of actinide elements, as was shown for instance of YSZ. In addition, corresponding tetragonal and cubic YSZ modifications exhibit excellent phase stabilities at elevated T. While HP induced phase transformation in tetragonal YSZ, concentration of the incorporated Th ions remained constant up to at least 12 GPa. Contrary, application of HP induced discharge of incorporated Th atoms in ThxZr1-xSiO₄ system. Nevertheless, stable compounds in this system have been identified and corresponding formations regions were found to be strongly influenced by synthetic conditions. While GdZr₂O₇-based phases were found to be stable at lower P, complete amorphization was observed at P > 40 GPa. Corresponding behavior will be illustrated on synchrotron radiation diffraction experiments with in situ application of extreme T and P. We propose that studies under extreme conditions to be included in a standard protocol for evaluation of materials to be placed for long-tern in underground nuclear waste storage.