The Use of Sulfur Chloride Systems in the Digestion and Purification of Zircaloy Nuclear Fuel Cladding

The overarching goal of this research has been to develop and understand a low-temperature protocol for the digestion of zirconium nuclear fuel cladding. The basic goal will be to reduce the used cladding to low-level radiological waste that can be disposed of more easily and at a reduced cost. A "blue sky" goal will be to eventually recycle the cladding to be reused in industrial processes. A hypothesis of this project is that by using various sulfur chloride compounds, the digestion of Zircaloy® can be converted to zirconium(IV) tetrachloride (ZrCl₄) at low temperatures and the purification/decontamination can be accomplished through a crystallization using thionyl chloride or other sulfur chloride reagents. Three sulfur chloride reagents were investigated, thionyl chloride (SOCl₂), sulfur dichloride (SCl₂) and sulfur monochloride (S₂Cl₂). At 150 °C for each of these reactions the speed at which they digest the metal is very different. For SOCl₂ (2.937 g Zr, 1.3 times of excess SOCl₂), 30 - 40 hours; SCl₂ (3.033 g Zr), 28 hours; and S₂Cl₂ (2.950 g Zr, 2.5 times excess of S₂Cl₂), 3 hours. Recrystallization of ZrCl₄ was accomplished using SOCl₂ and preliminary study of ZrCl₄ in SOCl₂ was conducted. A cloud point technique was used to determine the solubility. This was accomplished by dissolving ZrCl₄ in SOCl₂ then removing SOCl₂ at a specific temperature until crystallization occurred. The solubility was high at high temperature and was very temperature dependent over the range of 0 °C to 50 °C, the solubility increased by a factor of almost 4. Understanding the solubility will aid in the development of a purification strategy due to high-temperature dependence.