chemical thermodynamics of neptunium and plutonium - U.S. ...

324 17. Plutonium oxygen and hydrogen compounds and complexesvalue, and the remaining spread of almost three orders of magnitude is possibly withinthe experimental uncertainties.The Gibbs energies of reaction of M 4+ ions (M = U, Np, Pu) with water to formthe corresponding MO 2 crystalline oxides (26.1, 55.7 and 45.8 kJ·mol −1 , respectively),show a large variation that is much greater than the accepted uncertainties. This variationis reflected in the values of log 10 K for the reactionMO 2 (cr) + 2H 2 O(l) Å M(OH) 4 (aq)Values for M = U, Np and Pu are −9.4 [92GRE/FUG] (or−13.4 estimated from[90RAI/FEL]), −19.6 (cf. Table 3.1) and−14.9 respectively. It would be expectedthat this same variation of 5-10 orders of magnitude should be reflected in actinide(IV)“solubilities” measured in near-neutral, reducing solutions over “amorphous” or “hydrated”MO 2 (cf. Sections 8.2.5.2 and 17.6). The observed restricted range of solubilityvalues (or similar “solubility limits”, e.g.,[85RAI/RYA], [90RAI/FEL] and also the recentlypublished [99RAI/HES2]) is most likely due to inadequate detection limits, butis also possibly due to the differences in important experimental variables, one of whichcould be the nature of the solid phases.