Effects of Metal lon Impurities on PuO₂ Dissolution in Nitric-Hydrofluoric Acid Solution

The effects of Cu²⁺, Hg²⁺, Zn²⁺, La³⁺, Ce³⁺, Al³⁺, Pu⁴⁺, Th⁴⁺, and Zr⁴⁺ metal ion impurities on PuO₂ dissolution in 8.0M HNO₃—0.1M HF solution at 100°C have been investigated. Results based on 1.0 h of dissolution time show that such metal ions as Al³⁺, Pu⁴⁺, Th⁴⁺, and Zr⁴⁺, which form strong fluoride complexes, greatly decrease the dissolution rate, whereas such metal ions as Cu²⁺, Hg²⁺, Zn²⁺, La³⁺, and Ce³⁺, which form relatively weak fluoride complexes, have little or no effect. Fluoride ion activities in the dissolvents were calculated based on an empirical equation, K₁a_F + a_F − 0.10 γ_s = 0, where K₁, a_F, and γ_s denote first metal ion fluoride complex stability constant, fluoride ion activity, and stoichiometric fluoride ion activity coefficient, respectively. The PuO₂ dissolution rates were found to increase linearly with increase in the calculated fluoride ion activities.