Several materials have or are currently being investigated for nuclear waste sequestration applications, including crystalline ceramic oxides, glasses, and glass–ceramic composites. Rare-earth phosphates have been investigated extensively for this application owing to the range of structures that the hydrous or anhydrous versions can adopt as well as the fact that naturally occurring rare-earth phosphates have been found to contain U or Th. The purpose of this talk is to discuss (generally) the properties that must be considered when identifying nuclear wasteform materials and (more specifically) the structure and properties of rare-earth phosphates. This talk will cover the synthesis of anhydrous and hydrous rare-earth phosphates and the effect of single- and dual ion irradiation on the structure of rare-earth phosphates.

Due to various properties of interest, several phosphate-based ceramics have been considered for long as potential matrices for the long-term conditioning of radionuclides including actinides, in a deep geological storage. Among these ceramics, monazite-derived phases (Ln,An^III)PO₄ have been widely studied due to their high structural flexibility and remarkable chemical durability.

However, while the direct incorporation of An(III) has been already reported using both wet and dry synthesis methods, the incorporation of An(IV) is limited to only few dry studies due to problems associated to wet chemistry precipitation of initial precursors. Nevertheless, wet synthesis methods can lead to significant improvements in terms of chemical homogeneity and sintering capability, which can enhance the chemical durability of materials during leaching tests.

This work was devoted to the elaboration of synthetic monazites doped with tetravalent actinides (An = U, Th) by coupled substitution in order to prevent the formation of cationic vacancies. This mechanism consists in substituting two Ln(III) cations by one An(IV) and one M(II) cations.

The incorporation of An(IV) in the monazite structure was first performed using rhabdophane as starting precursor of Nd_1-2xCa_xAn_xPO₄ (x ≤ 0.1). The protocol based on hydrothermal conditions was optimised by varying the Ca/An stoichiometric ratio in order to prepare single phase samples. The thermal conversion of rhabdophane into monazite-cheralite was investigated under inert atmosphere and in air. It was followed by direct sintering in air for various holding times and heating temperatures. This one-step process allowed the direct preparation of dense pellets of monazite-cheralite. Sintering maps were further built in order to master the final microstructure of the ceramics. Finally, dissolution experiments were performed for various temperatures and leaching media to establish multiparametric dissolution rate laws. Preliminary leaching experiments revealed chemical durability as high as those obtained for undoped monazites.

The United Kingdom holds a substantial inventory of PuO₂, forecast to reach approximately 140 teHM (tonnes equivalent heavy metal) upon completion of reprocessing. This material presents a unique decommissioning prospect for which there is a need to develop a robust management strategy. Prompt immobilisation and disposal within a geological disposal facility (GDF) is a promising route towards ultimate disposition, yet in order to safely underpin the safety case for the geological disposal of Pu, it is necessary to understand the long term evolution of candidate wasteform materials in simulated repository environments. Moreover, there is a need to develop suitable wasteform materials capable of co-accommodating Pu, prescribed quantities of neutron poisoning species, trace processing impurities and transition metal cations capable of providing charge balance for non-stoichiometric compositions. Several baseline wasteform formulations derived from zirconolite, pyrochlore and fluorite-type matrices have been proposed on the basis of high chemical durability, radiation stability and moderate ease of processing. Herein, this talk will provide an overview in recent advances in the formulation refinement and fundamental characterisation of candidate wasteform materials for UK Pu. This includes detailed scoping trials aiming to characterise the incorporation of a representative U, Th and Ce surrogate fraction within zirconolite and pyrochlore phases, fabricated by conventional sintering (CPS), hot isostatic pressing (HIP) and reactive spark plasma sintering (RSPS).

Zirconolite (CaZrTi₂O₇) ceramics are proposed as a promising host-matrix for the immobilization of plutonium stockpile. In the literature, zirconolite ceramics are generally synthesized by conventional solid-state methods. Solid-state synthesis of Zirconolite ceramics require higher temeperatures and also leads to the formation of minor quantities of a chemically less durable perovskite (CaTiO₃) ceramic as a secondary phase. In order to prevent the formation of perovskite phase and to reduce the synthesis temperature, a low-temperature synthetic method such as the sol-gel method has been used in this study to synthesize a phase pure zirconolite ceramic. The following zirconolite compositions has been synthesized via sol-gel methods at lower synthesis temperatures: CaZrTi₂O₇, Ca_1-xCe_xZrTi_2-xFe_xO₇, and Ca_1-xCe_xGd_xZrTi_2-2xFe_2xO₇. The as-synthesized ceramics were characterized using powder XRD, SEM-EDS, and TEM. In order to study the effect of radiation on the structure of zirconolite ceramics, in-situ ion irradiation of synthesized ceramics was performed and the structural response was monitored using in-situ TEM.

Plutonium (Pu) wastes generated in the nuclear fuel cycle have significant radiotoxicity and require long-term immobilization. Zirconolite is one of the key mineral phases in the original titanate-based Synroc formulation and has long been considered a radiation tolerant and chemically durable host phase for Pu wastes. However, the Pu-bearing wasteform and the metallic canister used for containment of the waste material during hot isostatic pressing (HIPing), can potentially interact. This interaction may produce phases that are different to the bulk wasteform material or possibly with different chemical composition to that of the bulk, due to the potential for elemental diffusion across the canister-wasteform interface. In the present work, Pu-bearing zirconolite-rich full ceramic wasteforms were produced via HIPing in research-scale HIP canisters. Two different canisters, made from stainless steel (SS) and nickel (Ni), were used for this study to compare the effects of canister material on the wasteform-canister interaction. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDS) was utilized to perform a detailed investigation of the elemental compositions of the phases formed over the canister-wasteform interaction zone. These were then compared with phase compositions from regions near the center of the HIP canister. The wasteform sample HIPed in the SS canister showed ~100-120 µm of interaction zone dominated by high temperature Cr diffusion from SS to the wasteform with the Cr predominantly incorporated into the durable zirconolite phase. The wasteform sample HIPed in the Ni HIP canister showed almost no interaction zone with only minor diffusion of Ni from the canister into the wasteform near the interface. Though the HIP canister-wasteform interaction extends to ~120 µm when using a SS HIP canister, this translates to ∼0.2–0.3 vol% for an industrial-scale HIPed wasteform.