Reaction Kinetics of Dihydroxyurea With Nitrous Acid and Its Effect on Stabilizing Pu(Ⅲ)

Nitrous acid should be scavenged in the partitioning step of Purex process because it affects the valence of plutonium and some of other nuclides. Dihydroxyurea (DHU) was found to be a promising reductant to stabilize Pu(Ⅲ). The reaction kinetics of DHU with nitrous acid was studied in HClO₄ and HNO₃ media. The reaction rate equation is expressed as -dc(HNO₂)/dt= k·c(HNO₂)¹·c(DHU)⁰·c(H⁺)^-0.15. In HClO₄ and HNO₃ media, the reaction rate constants are (2.37±0.04) mol^0.15/(L^0.15·min) at θ=15 ℃， I=0.5 mol/kg and (1.29±0.06) mol^0.15/(L^0.15·min) （n=8） at θ=10 ℃, I=0.5 mol/kg, respectively. The reaction rate constant decreased slightly with the increase in ion strength. The effect of temperature on the reaction rate was also studied and it was found that the reaction rate increased quickly with the increase in temperature. The reaction activation energies are 68.2 kJ/mol and 76.8 kJ/mol in HClO₄ and HNO₃ media, respectively. In 0.5-3.0 mol/L nitric acid solutions, excess DHU can stabilize Pu (Ⅲ) well over 48 h.