Patent Number: 048448389
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of treatment of a radioactive liquid waste including high-level liquid waste and medium- or low-level liquid waste produced from a reprocessing plant, to decompose sodium compounds contained in the liquid waste for the recovery and reuse of sodium and for the volume reduction of the radioactive liquid waste to be disposed, thus facilitating the vitrification or bitumen solidification of the liquid waste. High-level liquid waste produced from a reprocessing plant essentially comprises a large amount of sodium nitrate and a small amount of fission products. This type of liquid waste is generally vitrified by melting it together with a large amount of a glass forming agent. On the other hand, medium- or low-level liquid waste substantially comprises sodium nitrate and a slight amount of fission products. This type of liquid waste is generally mixed with bitumen under heating and the mixture is solidified and discarded. In order to obtain a vitrified product having excellent properties, the sodium content in the product must be below a specified limit. Therefore, in order to convert a large amount of high-level liquid waste into a vitrified product having excellent properties, a large amount of a glass forming agent must be used, which disadvantageously brings about an increase in the amount of waste. Further, in the treatment of medium- or low-level liquid waste, the mixing of sodium nitrate with bitumen under heating involves risk of fire or explosion. Thus much attention is required and the working efficiency is poor. SUMMARY OF THE INVENTION It is therefore an object of the present invention to overcome the above-described disadvantages in the prior art and to provide an improved method of treatment of a radioactive liquid waste, which permits the recovery and reuse of sodium by decomposing thermally decomposable sodium compounds contained in the liquid waste, while the resulting impurity residue essentially comprising fission products is converted into a stable form exhibiting a high safety. Another object of the present invention is to provide a method of treatment of a radioactive liquid waste, which permits the removal of sodium in the liquid waste, so that the remarkable volume reduction of waste to be disposed of can be accomplished, a vitrified product having excellent properties can be produced and the risk of fire or explosion in the bitumen solidification can be greatly reduced. According to the present invention, there is provided a method of treatment of a radioactive liquid waste comprising the following steps: (1) a heating step wherein a radioactive liquid waste containing fission products and a thermally decomposable sodium compound is heated to convert the sodium compound into oxides of sodium, so that a calcination product comprising fission products and oxides of sodium is formed, (2) a reaction step wherein the oxides of sodium in the calcination product are converted into sodium hydroxide, (3) a purification step wherein the sodium hydroxide in the calcination product is reacted with an alcohol to form a sodium alcoholate, and the thus formed sodium alcoholate is separated by solid-liquid separation from an impurity residue comprising the fission products, and (4) a decomposition step wherein the sodium alcoholate is decomposed to form and recover sodium hydroxide. The impurity residue obtained in the above step (3) may further be subjected with vitrification or bitumen solidification according to a conventional solidification process. In the method of the present invention, the thermally decomposable sodium compound contained in the radioactive liquid waste is converted into sodium oxide and sodium peroxide by heating. The oxide and peroxide are converted into sodium hydroxide by the reaction with water vapor or the like. The sodium hydroxide, which contains therein fission products, is reacted with alcohol to convert the sodium hydroxide into sodium alcoholate. The thus formed alcoholate is then separated from the impurity residue comprising fission products. The separated sodium alcoholate is decomposed with water to form sodium hydroxide, which may then be recovered. As described above, the sodium contained in the radioactive liquid waste can be separated and recovered, while only impurities essentially comprising fission products remain as a residue. Therefore, the volume of radioactive waste to be solidified can be remarkably reduced. Further, the impurity residue produced in the present invention hardly contains sodium, so that it can be vitrified into a solidified product having excellent properties or can be safely treated by bitumen solidification.