Studies on supercritical hydrothermal syntheses of uranium and lanthanide oxide particles and their reaction mechanisms

Abstract In order to develop preparation method of raw metal oxide particles for low decontaminated MOX fuels by supercritical hydrothermal (SH) treatments, we have investigated behavior of aqueous solutions dissolving U(VI), Ln(III) (Ln: lanthanide = Ce, Pr, Nd, Sm, Tb), Cs(I), and Sr(II) nitrate or chloride compounds under SH conditions (temperature = 400–500 °C, pressure = 30–40 MPa). As a result, it was found that Ln(NO 3 ) 3 (Ln = Ce, Pr, Tb) compounds produce LnO 2 , that Ln(NO 3 ) 3 (Ln = Nd, Sm) compounds are hardly converted to their oxides, and that LnCl 3 (Ln = Ce, Pr, Nd, Sm, Tb), CsNO 3 , and Sr(NO 3 ) 2 do not form their oxide compounds. Furthermore, HNO 2 species were detected in the liquid phase obtained after treating HNO 3 aqueous solutions containing Ln(NO 3 ) 3 (Ln = Ce, Pr, Tb) under SH conditions, and also NO 2 and NO compounds were found to be produced by decomposition of HNO 3 . From these results, it was proposed that the Ln oxide (LnO 2 ) particles are directly formed with oxidation of Ln(III) to Ln(IV) by HNO 3 and HNO 2 species in the SH systems. Moreover, the uranyl ions were found to form U 3 O 8 and UO 3 depending on the concentration of HNO 3 . From these results, it is expected that the raw metal oxide particles for low decontaminated MOX fuels are efficiently prepared by the SH method.