Semi-volatile species, which include the elements I/Br, Ru, Rh, Tc, Cs, Rb, Mo, and Se/Te, can be released under certain processing conditions. Early removal of the semi-volatiles simplifies the off-gas treatment compared with recovery from wet NOx-laden off-gas generated from the downstream dissolution process. Iodine is difficult to fully remove from liquid dissolver solutions and can be problematic as a corrosive. Also, the early removal of specific semi-volatiles can simplify the downstream separations processes; for example, the recovery of technetium by means of dry volatilization will eliminate the need to co-extract it with uranium and separate the technetium under conditions that may complicate the uranium recovery. Moreover, the recovery of molybdenum by means of dry volatilization will eliminate the major source of precipitates (e. g., zirconium molybdates) that can foul the evaporation and solvent-extraction equipment.11

In the basic voloxidation process (oxidation in air at 480°C), less than about 0.2% of each of the semi-volatiles, 106Ru, 125Sb, and 134-137Cs, is evolved. Higher temperatures and vacuum operation increase the fraction evolved.

Collaborative tests by Idaho National Laboratory (INL), the Korean Atomic Energy Research Institute, and Oak Ridge National Laboratory (ORNL) showed that nearly 100% of the semi-volatiles can be separated under a variety of conditions using air or oxygen as the oxidant atmosphere, temperatures up to 1250°C, and vacuum operation. The removal effective­ness was highly dependent on the processing conditions. In general, initial oxidation at relatively low temperature (around 500°C) to generate the fine fuel powder with increased surface area is beneficial. Increasing tempera­tures enhanced the amounts of the semi-volatiles released in the following approximate order: Ru + Rh, Tc, Cs, Te, and Mo.1213

Enhanced oxidation, at selected temperatures and with alternative reagents such as ozone, NO2, and steam has been tested at ORNL to com­plete the removal of volatiles (all xenon, krypton, carbon, and iodine) and to remove semi-volatiles (molybdenum, technetium, and others). At low temperature (200-300°C), the U3O8 powder obtained by conventional voloxidation may be further oxidized with ozone (O3) to produce a finer UO3 powder. Also, NO2 can be used and will oxidize UO2 or U3O8 to UO3. The fine UO3 powder may be heated to higher temperatures, in a secondary step if necessary, to remove those species released by diffusion-controlled processes. Although higher temperatures cause UO3 to revert to U3O8, the benefit of the finer particles will remain. Additionally, the process can be cycled between UO3 and U3O8. Further fracture of the particles will enhance the release of volatile species and can be accomplished by cycling between higher and lower oxide species.

For example, in the AIROX1415 and DUPIC161718 processes, the fuel is first subjected to oxidation in air to obtain U3O8 powder. The resulting powder is then subjected to repeated cycles of hydrogen reduction to UO2 (usually at temperatures between 600°C and 800°C) and then re-oxidization of the UO2 to U3O8. Alternatively, a cycle between U3O8 and UO3 can be carried out using O3 or NO2 to generate UO3, then heated above 450°C to decompose the UO3 to U3O8. This is a lower temperature cycle that avoids any potential complication due to alternating hydrogen and oxygen atmospheres.