Development of a low pressure distillation process for fluoride molten salts treatment

Molten inorganic fluorides are materials with high potentialities in nuclear engineering field: Generation IV molten salt reactors, pyrochemical reprocessing, blanket material for the fusion reactor, separation technique of fluorination, etc. In fuel salt reprocessing different possible processes have been proposed for soluble fission products removals such as reductive extraction in liquid metal, electrowinning, low-pressure distillation or precipitation of oxides. The distillation pyrochemical process is used to recover renewable salts from the salt wastes without salt loss and minimize the radioactive wastes. Based on the difference of vapor pressures between carrier salt and rare earth fluoride, distillation is convenient for such removal. In the present work, the feed material, consisting of alkali fluorides containing various rare earth species, are treated by low-pressure distillation which requires temperature from 700・C to 1000・C and pressure from 1Pa to 1000Pa. The experimental results are presented including inactive studies for pure alkali and FLiNaK eutectic salt distillation, feed material composition influence. The demonstration of distillation rates of several ten grams per hour for eutectic FLiNaK and rare earth mixtures were available by lab-scale thermo-gravimetric distillation system. Tests with the mixture give decontamination factor of rare earth higher than 1000. The recovery yield of salt is higher than 95 % for eutectic mixtures using a closed chamber distillation system. Low-pressure distillation development is still in progress as a large variety of technical challenges have to be solved. Future works will focus on the qualification of molten salt distillation for the whole composition domain of spent salts from MSR.