Thermodynamic Stabilities of MO2+x(s) (M = U, Np, Pu and Am), Pourbaix diagrams.

The experimental solubilities of the hydrated amorphous freshly precipitated M(OH)z(am) and MO2(OH)z(am) compounds are often used as an upper limit for the safety assessments of deep waste repositories, since these compounds slowly transform to less soluble ones, as typically M(OH)4(am) to MO2(cr). Solubility (vs. redox potential) at pH=8, and E-pH predominance diagrams are plotted in aqueous solutions at 25°C by using thermodynamic data recently selected by the NEA-TDB review, or estimated by using classical chemical analogies for the non-redox reactions. The solubilities and relative stabilities are also calculated for the MO2+x(s) crystalline compounds of known stabilities: U4O9(s), U3O7(s), U3O8(s) and Np2O5(s) where 2+x = 2.25, 2.33, 2.67 and 2.5 respectively. The stabilities of the other MO2+x(s) compounds are estimated by analogy: M4O9(s) (M=U, Np, Pu), M3O7(s) and M3O8(s) (M=U, Pu) ,and M2O5(s) (M=Np, Am) are predicted to be more stable (i.e. less soluble), than the amorphous hydroxides. However their precipitation have never been observed at room temperature possibly for kinetic reasons or difficulties in interpreting solubility experiments.