Incorporation of U(IV) in monazite-cheralite ceramics under oxidizing and inert atmospheres.

This work is the first attempt to prepare NdCaUPO monazite-cheralite with 0 < ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions ( = 110 °C for four days) of the NdCaUPO·HO rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite-cheralite samples were obtained. However, α-UPO was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite-cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of = 0.08.