Patent Number: 052522580
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a method, used in a spent nuclear fuel retreatment process, of rendering mineral an iodine compound containing recovered radioactive iodine and storing it, the radioactive iodine being recovered as an iodine compound without causing release of long half-life .sup.129 I radioactive iodine into the environment, and disposing of scrub liquids from which radioactive iodine has been collected through freeze vacuum drying. In a spent nuclear fuel retreatment process, elimination of radioactive iodine generated in the process of dissolving and shearing spent fuel is particularly vital due to a high iodine toxicity, a high iodine content of fission products, and a high rate of nuclear fission realized in the mass index of the two primary types of radioactive iodines, 1% .sup.129 I at a long (1.7.times.10.sup.7 years) half-life and 2.09% .sup.131 I, a brief (eight day) half-life. When the iodine in the liquids supplied to solvent extraction remains to react with the solvent, it produces a chemical compound resistant to removal with a resultant contamination throughout the entire treatment process and makes important the removal of as much iodine as is practicable before it comes into contact with the solvent. In the off-gas phase, aqueous phase or organic phase, iodine appears in I.sub.2, HI or HIO or organic forms and appears as HIO.sub.3 in dense nitrate solutions. Conventional methods of removing iodine are removal by immersion in alkaline solutions such as Na.sub.2 CO.sub.3 or NaOH or removal also through use of silver zeolite loaded with silver in a zeolite catalyst. In methods employing alkaline solutions, however, removal of organic iodine is impossible and there is furthermore no way to recover iodine and treat it while maintaining it in solution. Radioactive iodine incapable of maintenance is released into the atmosphere and the accumulation of .sup.129 I particularly with its long half life becomes a problem. In recovering radioactive iodine it is further necessary to utilize such valuable absorbents as silver zeolite. For this purpose a process making possible the suspension of radioactive iodine in a liquid is desirable. As it is vital to store long half-life .sup.129 I over long periods, a process making possible safe storage of radioactive materials such as recovered .sup.129 I is desired. In addition, liquid waste containing radioactive iodines generated in the retreatment process does not cause vaporization of radioactive iodines, so it is necessary to control the alkalinity. However, if this liquid waste becomes mixed with acidic liquid wastes, there is the danger that radioactive iodine may be irretrievably released. For this reason, there is need of a process in which liquid containing radioactive iodines may be treated individually without becoming mixed in with liquid wastes from another process. SUMMARY OF THE INVENTION An object of the present invention is to do away with the release of long-half life .sup.129 I radioactive iodine into the environment through concentrating liquids containing radioactive iodine by a freeze vacuum drying process and recovering the iodine as iodine compounds. Another object of the present invention is the long term stable storage of radioactive iodine by rendering iodine compounds containing recovered radioactive iodine mineral. Yet another object of the present invention is to treat radioactive iodine while maintaining it in solution and reduce the consumption of an iodine collecting agent such as silver zeolite. Still another object of the present invention is to avoid the mistaken release of radioactive iodines by separate processing solely of liquids containing radioactive iodine without allowing mixing with waste liquids from other processes. Lastly, a final object of the present invention is to avoid vaporization of radioactive iodine from liquids by adding silver and copper ions to liquids containing radioactive iodine, causing the precipitation of radioactive iodine compounds and additives and separating the radioactive iodine from solution. In order to attain these objects, the present invention is characterized in that, in a spent fuel retreatment process, a generated off-gas is scrubbed, after which the cleaned scrub solution is precipitated alone or with the addition of an additive, followed by freeze drying the radioactive iodine-containing waste solution by a freeze vacuum drying process and recovering the radioactive iodine as an iodine compound. Scrubbing of the off-gas is carried out by using sodium hydroxide solution, and copper and silver ions are added as the additive in the precipitation process. Further, the invention is characterized by solidifying and mineralizing, by a high-pressure press or the like, the iodine compound containing the recovered radioactive iodine directly or after adjusting the compound into a composition of a naturally occurring iodine-containing mineral. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.