Monazite-type ceramics as nuclear waste form: crystal structure, microstructure and properties

Abstract In the last decades, various single- and polyphase ceramic materials have been proposed as potential waste forms for the immobilisation of specific nuclear waste streams, such as separated plutonium from civilian or military sources unsuitable for further use, or separated minor actinides. Among them, phosphate-based materials with monazite structure emerged as promising candidates for ceramic nuclear waste forms due to their specific physico-chemical properties including high structural flexibility allowing for significant waste loadings, high chemical durability, and high resilience against radiation damage. This work focuses on synthesis and fabrication of monazite-type ceramics for the immobilisation of Pu and minor actinides, and their properties. Key points addressed comprise the formation of monazite and rhabdophane solid solutions by structural incorporation of actinides (Cm3+, Pu3+) or their non-radioactive surrogates (Eu3+, Gd3+), studied by combined X-ray diffraction, spectroscopic and computational techniques; the comparison of various synthesis methods, leading to monazite precursors with different crystal structure and morphology; the effect of precursor morphology on the microstructural evolution of monazite-type ceramics during sintering; as well as correlations between microstructure, texture effects and mechanical properties. Besides the mechanical properties (elastic moduli, microhardness and fracture toughness), thermal properties (thermal expansion, and heat capacity) of monazite solid solutions were studied in detail. Particular attention is paid to the properties that govern the long-term stability of the waste form under conditions relevant to geological disposal. The enthalpies of formation and mixing of single phase monazite La1-xLnxPO4 (Ln = Eu and Gd) solid solution series, determined in this work using high temperature oxide melt solution calorimetry, provide key input parameters for calculation of stability and solubility of the monazite solid solutions as a function of temperature, thus contributing to the long-term stability prediction of monazite nuclear waste forms. Additionally, the dissolution kinetics of LaPO4 of the monazite structure-type was systematically studied in dynamic dissolution experiments in acidic media at different temperatures to gain a deeper insight into the mechanism governing the dissolution of monazite waste forms. The potential formation of La-rhabdophane as secondary phase in the dissolution experiments was explored here in the temperature range of interest by thermodynamic modelling. In conclusion, this work provides a perspective on the recent advancements in the understanding of the properties of monazite ceramics in respect to their application as nuclear waste forms.