Hydrothermal alteration of BaxMIVxCe2−2x(PO4)2 [MIV=Zr, Hf] as hosts for minor actinides

BaxMIV xCe2−2x (PO4)2 [MIV=Zr, Hf] monazite-like compounds were succesfully synthesized by solid state reaction for x≤0.2 (MIV=Zr) and x≤0.1 (MIV=Hf). The low miscibility of BaMIV(PO4)2 (MIV=Zr, Hf) compounds in CePO4 was explained on the basis of the monoclinic-to-trigonal phase transition that occurs at 733 K in BaZr(PO4)2 and at 798 K in BaHf(PO4)2. The hydrothermal alteration of these compounds was tested using a modified MCC-1 static leaching test in acid (1 mol·dm−3 HCl) and basic (1 mol dm−3 KOH) solutions at 373 K, 473 K and 573 K; both the experimental fluids and the reacted solid specimens were analyzed by different analytical techniques and the reaction mechanisms were elucidated. All the tested compounds are stable in 1 mol·dm−3 HCl until 573 K. The stability of the monazites in 1 mol·dm−3 KOH is a function of the temperature.