Thermodynamic Studies on the Hydrolysis of Trivalent Plutonium and Solubility of Pu(OH)₃(am)

Abstract

The temperature-dependent reaction properties of actinide elements are of particular interest in the safety assessment of high-level radioactive waste (HLRW) disposal systems. In this study, the hydrolysis of Pu­(III) and the solubility of Pu­(OH)3(am) were investigated at various temperatures (10–40 °C) in 0.1 M NaClO4. A strong reducing condition for maintaining the oxidation state of Pu­(III) while slowly increasing the pH of the solution was realized by electrolysis. The formation constants of the first hydrolysis species, log * β1 ′, and the solubility products of Pu­(OH)3(am), log * K s,0 ′, at 10, 17, and 40 °C were experimentally determined using spectrophotometry, laser-induced breakdown detection, and radiometry. The enthalpy and entropy changes for these reactions were estimated using the van’t Hoff equation. The first hydrolysis of Pu­(III) is endothermic (Δr H m ° = 34.10 ± 4.48 kJ mol–1), and the dissolution of Pu­(OH)3(am) is exothermic (Δr H m ° = −294.29 ± 23.05 kJ mol–1) with negative entropy changes. These thermodynamic data will contribute to improving the reliability of the safety assessment of HLRW disposal facilities and understanding the geochemical behavior of Pu under reducing or anoxic aqueous conditions at elevated temperatures.

Dissolution behaviors of PuO2(cr) in natural waters

“…Data on the dissolution of various Pu solids are insufficient compared to those on the formation of aqueous Pu species. Reaction enthalpy and entropy are scarce and study for determination of the values has recently been encouraged for the reliable estimation of chemical behaviors of Pu at high temperatures (Cho et al, 2022). The geochemical modeling of Pu in repository environments contains a relatively large uncertainty; therefore, sitespecific investigation of the chemical behavior of Pu is often required for the safety assessment of radioactive waste disposal facilities.…”