Detailed X-ray diffraction analysis of Ce1-xNdxO2-(x/2) as a surrogate for substoichiometric americium oxide

Abstract

European Space Agency (ESA) radioisotope power systems will use 241 Am as their heat source. The chemical form of the americium oxide has yet to be decided but an option that may be investigated in future research are certain Ia-3 AmO2-(x/2) phases. In a previous investigation, Ia-3 (C-type) Ce1-xNdxO2-(x/2) with x values between 0.5 and 0.7 were proposed as candidate surrogates (Watkinson et al., 2017). A continuous oxalate precipitation and calcination route for fabricating such oxides was presented and the means to target an x value was described (Watkinson et al., 2017). An initial estimate of the lattice parameter was provided in the previous study by fitting Gaussians to X-ray diffraction data peaks. Previous quantitative X-ray fluorescence analysis suggested an x-value of 0.61 had been achieved. Characterising the crystal structure and the oxygen-to-metal ratio, which corresponds to x, of an oxide powder will be essential data for future sintering studies where changes in these parameters will be investigated. In this study, a Rietveld refinement investigation is presented, which has enabled the lattice parameter of the material to be better constrained and estimated more precisely. The result is consistent with the literature; a lattice parameter-x value relationship in the literature was used to estimate the x value using this improved estimate and it confirmed an x value of 0.6 has been made. This investigation has thus further validated the previously presented synthesis method and characterised the crystal structure of the material more precisely in preparation for future sintering trials.

Investigation of Rare Earth-Containing Double Phosphates of the Type A3Ln(PO4)2 (Ln = Y, La, Pr, Nd, and Sm–Lu) as Potential Nuclear Waste Forms

“…11−13 The exploration of potential waste form materials for the minor actinides utilizes stable surrogate elements that are considered chemically close matches, typically Nd 3+ and Sm 3+ due to their similar oxidation states, Shannon ionic radii, and coordination number preferences. 14 The potential of phosphate matrices for HLW actinide immobilization has been recognized for decades. 15−24 Naturally occurring minerals with phosphate-based structures have been widely studied for their chemical and mechanical stability, particularly monazite (LnPO 4 , P2 1 /n, Ln = Y and La−Dy) because it is the most abundant natural lanthanide phosphate and demonstrates outstanding structural stability over hundreds of millions of years of geochemical events.…”

“…High-level nuclear waste (HLW) contains numerous elements that will persist for tens to hundreds of thousands of years, including most of the transuranic (TRU) elements, which will require highly stable and robust frameworks in which to sequester them. − Compared to uranium and thorium, there is a pronounced dearth of known extended structures for the minor actinides, including americium and curium. Developing materials that can potentially function as effective waste forms for the minor actinides is important as they are sufficiently long-lived to pose significant hazards for millennia. − The exploration of potential waste form materials for the minor actinides utilizes stable surrogate elements that are considered chemically close matches, typically Nd 3+ and Sm 3+ due to their similar oxidation states, Shannon ionic radii, and coordination number preferences …”