Influence of the pH on the dissolution of TPD and associated solid solutions

Synthesis and characterization of low-temperature precursors of thorium–uranium (IV) phosphate–diphosphate solid solutions

“…For all the samples, the apparent and effective relative densities reached 90-95% and 94-99% of the value calculated from XRD data, respectively [24]. Leaching tests achieved in various solutions demonstrated that both powdered and sintered samples doped, or not, with tetravalent actinides, exhibit a strong resistance to aqueous corrosion [21,25,26]. The normalized dissolution rate ranges from (5.8 ± 0.3) · 10 À6 g m À2 day À1 (10 À1 M HNO 3 ) to (4.8 ± 0.3) · 10 À8 g m À2 day À1 (pH = 7) at room temperature, which is several orders of magnitude lower than for other matrices such as basaltic glasses [27].…”

“…Among the radionuclides to be stored, actinides must be considered carefully due to their long-term high radiotoxicity and to their long half-life period. Several phosphate-based materials such as apatites (Ca 10 (PO 4 ) 6 F 2 ) and associated britholites (Ca 9 Nd(PO 4 ) 5 (SiO 4 )F 2 ) [1,2], monazites (LnPO 4 ) and associated brabantites (M II M IV (PO 4 ) 2 ) [3][4][5][6][7][8][9], sodium dizirconium phosphate (NaZr 2 (PO 4 ) 3 , NZP) [10][11][12][13] or uranium and thorium phosphates [14][15][16][17][18][19][20][21] were studied as potential matrices to stabilize these radionuclides in solids (e.g. ceramics).…”

“…The normalized dissolution rate ranges from (5.8 ± 0.3) · 10 À6 g m À2 day À1 (10 À1 M HNO 3 ) to (4.8 ± 0.3) · 10 À8 g m À2 day À1 (pH = 7) at room temperature, which is several orders of magnitude lower than for other matrices such as basaltic glasses [27]. The dissolution of b-TPD is rapidly followed by the formation of neoformed phases containing radionuclides in the back-end of the dissolution of the initial ceramic: they are thorium phosphate-hydrogenphosphate hydrate Th 2 (PO 4 ) 2 (HPO 4 ).H 2 O (TPHPH) for thorium [28] and uranyl phosphate pentahydrate for uranium (tetravalent uranium being oxidized into uranyl during the dissolution process) [21,26]. Due to the different chemical behaviors of thorium and uranium during leaching tests, the heterogeneity of the samples can degrade their chemical durability through the presence of Uenriched secondary phases.…”

Actinide solubility-controlling phases during the dissolution of phosphate ceramics

“…In order to evaluate the contribution of such precipitates to the long-term retention of actinides, the associated solubility constants were estimated at infinite dilution from the elementary concentrations at equilibrium (Table 3, [12][13][14][15][16][17]) considering the Specific Interaction Theory. All the values obtained are characteristic of very low-soluble phases which confirms the significant role they could play in the field of the immobilization of actinides in phosphate ceramics.…”

Temperature effects on the surface acidity properties of zirconium diphosphate

“…For instance, in the near field environment of a spent fuel or vitrified high-level nuclear waste repository, the temperature could be elevated for a long time period: in a bentonite, the temperature may reach 70-90 • C under saturated conditions for 1000 years after repository closure [9]. Whereas the influence of the temperature on the dissolution rates [10][11][12] is rather well studied, very few studies have been devoted to the temperature effects on the surface reactivity [13][14][15][16].…”