Coprecipitation of Pu and Np amorphous oxides with UO2 (am, hyd) and their solubility under reducing conditions at various pH

Artikel i vetenskaplig tidskrift, 2026

In order to study coprecipitation of Pu and Np with UO2(s) observed during spent fuel leaching under repository conditions, coprecipitates were prepared through neutralization of acidic solutions containing U(IV), Pu(III) and Np(IV) at room temperature in glove box atmosphere and in the presence of sodium dithionite. The solubilities of the amorphous solid solutions U1-x-y Pux Npy O2±z with different mole fractions of Pu (x = 0.01 or 0.05) and of Np (y = 0.001 or 0.005) were determined in low ionic strength (10 mM NaCl) solutions between pH 2.1 and 12.0 under reducing conditions. The solids were investigated by a variety of methods (chemical analysis, SEM-EDX, XRD, XANES) to determine the nature of solid solutions formed, their composition and the valence state of Pu, Np and U. XANES confirmed the oxidation state of Pu in the solids both before and after the equilibration as Pu(IV). The amorphous coprecipitates reached equilibrium relatively fast (∼1 week). The releases of Np indicated formation of ideal solid solutions between UO2(s) and NpO2(s). The release of Pu from the coprecipitates was totally dominated by the release of uranium in the whole pH range. The Pu concentrations decrease slightly with the decrease of Pu content in the solid, suggesting that PuxNpyU1-x-yO2±z solids behave thermodynamically as solid solutions. The concentrations of U in equilibrium with the coprecipitate were in excellent agreement with the solubility of UO2(s) under reducing conditions reported in the literature. The XANES data show Pu(IV) in the coprecipitate and the Pu(III) concentrations resulting from the reductive Pu dissolution of the coprecipitate were 2.1–3.6 orders of magnitude lower than those measured during reductive dissolution of pure PuO2(s) in the acidic range and 0.5–1 orders of magnitude lower in the neutral to alkaline range, which indicates a non-ideal solid solution behavior. To test if the oxidation to Pu(IV) occurred during storage or transport of the XANES samples, we have analyzed the solubility data also assuming the hypothetical presence of Pu(III) in the coprecipitate. The conditional solubility product of Pu(OH)3 from the coprecipitate was several orders of magnitude (∼ 2 in the near neutral pH range and ∼14 in the acidic range) lower than that of pure Pu(OH)3(s). The activities and activity coefficients of Pu(OH)3(s) in the coprecipitate were also estimated.