Abstract
The paper consists of two main parts: a microscopic and spectroscopic investigation of the single crystal of 17-year-old 238Pu-doped Eu-monazite, and a theoretical calculation of the properties of several structural types of orthophosphates. It is shown that actinide-doped monazite is prone to the formation of mechanically weak, poorly crystalline crust, presumably consisting of rhabdophane. Its formation is likely promoted by the formation of peroxides and, potentially, acidic compounds, due to the radiolysis of atmospheric moisture. The calculations of mixing the enthalpies and Gibbs energies of binary solid solutions of Pu and rare earth element (REE) phosphates that were performed for the principal structural types—monazite, xenotime, rhabdophane—show that, in the case of light REEs, the plutonium admixture is preferentially redistributed into the rhabdophane. This process strongly affects the behavior of actinides, leached from a monazite-based waste form. The applications of these results for the development of actinide waste forms are discussed. The current data on the behavior of real actinide-doped monazite suggest that this type of ceramic waste form is not very resistant, even in relatively short time periods.
Highlights
The modeling of the long-term behavior of crystalline forms for actinides immobilization greatly benefits from investigation of the natural minerals comprising radioactive elements, such as uranium and thorium
We present new experimental and computational results relevant for the understanding of the long-term behavior of monazite-based waste forms loaded with several atomic percentages of plutonium
It has been shown that conclusions about the high performance of monazite waste forms based on the studies of simulant materials, that is, those with rare earth elements only, are too optimistic
Summary
The modeling of the long-term behavior of crystalline forms for actinides immobilization greatly benefits from investigation of the natural minerals comprising radioactive elements, such as uranium and thorium. Ideally, a rare earth element phosphate (REEPO4 ) with monoclinic structure—occupies a special place among actinide-containing minerals with remarkable chemical stability, as natural monazite is almost never present in metamict, that is, an amorphized state. Only very few works report diffuse X-ray diffraction patterns of natural Th-rich monazite [1]. The samples studied in [1] were likely huttonite–monazite solid solutions, which are less radiation-resistant than pure monazite. The radiation damage in natural monazite is largely confined to small domains, eventually enriched in impurities [2,3]. A low critical temperature of recovery from radiation damage in monazite [4]
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