Sintering of β-Thorium−Uranium(IV) Phosphate−Diphosphate Solid Solutions from Low-Temperature Precursors

Abstract

Pure and single phase thorium−uranium(IV) phosphate-diphosphate β-TUPD (orthorhombic form) dense pellets were prepared by two wet chemical routes from low-temperature precursors. The first method was based on the “direct evaporation” of a mixture containing tetravalent actinides and phosphoric acid while the second one involved the initial precipitation of the thorium−uranium(IV) phosphate−hydrogenphosphate hydrate (TUPHPH). Both methods led to single phase sintered samples following a two-step procedure using a uniaxial pressing at room temperature and then a heat treatment at high temperature. The dilatometric study and the determination of the linear shrinkage of the pellets showed that the best densification of the samples prepared via the “precipitation” method was obtained after heating the raw pellet at 1250 °C for 5 h, for initial uranium amount lower than 44.5 wt %(U). For samples prepared via “direct evaporation”, the heat treatment must be prolonged for at least 15 h at this temperature. Moreover, the complete characterization of the samples (EPMA, and so forth) confirmed that the homogeneity of the samples was significantly improved when using TUPHPH as a precursor, probably due to the better reactivity of the initial powder (higher specific surface area, smaller grain size, and so forth). Several leaching tests were performed in various acidic media and in natural waters on β-TUPD sintered samples. All the corresponding normalized dissolution rates remained low (10-6 to 10-5 g/(m2 day)) even in aggressive media which confirms the high chemical durability of β-TUPD, thus enhancing the potential use of this ceramic in the field of the efficient immobilization of tetravalent actinides.