The Effects of Zirconium on the Dissolution Rate of Thoria in Nitric-Hydrofluoric Acid Solutions

Dissolution tests using ThO₂ microspheres, ThO₂ pellets, and Zircaloy tubing were performed to establish conditions that can be used for the selective dissolution of thoria from Zircaloy cladding. Although additional work is needed in several areas, the following conditions are tentatively recommended for use in process demonstration tests: 0.02 to 0.04 M HF in refluxing 8 to 12 M HNO₃ with sufficient Al³⁺, or (preferably) Zr⁴⁺, added to prevent major corrosion of the stainless steel dissolver vessel. If Zr⁴⁺ is used, the concentration required is approximately the same molarity as the fluoride concentration.