As originally developed, the voloxidation process is carried out in an air atmosphere at 480°C to 600°C, primarily to remove tritium from the fuel. During oxidation of the UO2-based fuel, tritium, which may be present in the fuel in elemental form, diffuses to the surface of the particles where it reacts with oxygen to form water, which enters the gas stream.6,7 The rate of reaction at 480°C is such that >99.9% of the tritium is released from the fuel in about 4 h.

A significant portion of the tritium in the used fuel assemblies is associ­ated with the cladding as zirconium hydride (ZrTx).5 The fraction of tritium in the cladding has been reported at 40%8 and higher.6 The hydride should readily oxidize, releasing the tritium, if it were accessible by the oxygen. However, experimental data indicate that oxidation has little effect on the tritium held in the cladding when processed for 6 h at a temperature of 480°C.9