Formation of zirconium molybdate sludge from an irradiated fuel and its dissolution into mixture of nitric acid and hydrogen peroxide

In the dissolution step of the PUREX process, a sludge of zirconium molybdate can be generated from zirconium and molybdenum dissolved in the solution. In order to avoid accumulation of sludge, a new cleaning method using a mixture of nitric acid and hydrogen peroxide was developed. In this work, the cleaning method was verified using an irradiated fuel. Cubic crystals of the zirconium molybdate ZrMo2O7(OH)2 were formed upon dissolution of an irradiated PWR fuel. The sludge deposition continued after the completion of the fuel dissolution. A small amount of fissile material was incorporated into the sludge. Although almost the same amount of insoluble residue was present, the sludge could be dissolved with a HNO3–H2O2 solution at 80 °C within 5 h. The sludge formation on the clean Zr surface is inhibited.