Patent Number: 052522580
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the invention will now be described with reference to the FIGURE. The FIGURE is a view showing an example of the flow in a case where the iodine recovery and storage method of the invention is applied to retreatment processing of spent nuclear fuel. In the FIGURE, (1) is a shearing step, (2) a dissolving step, (3) off-gas, (4) a scrub solution such as a sodium hydroxide solution, (5) steps in the process of scrubbing the off-gas, (6) scrub solution after completion of off-gas scrubbing, (7) additives, (8) a precipitation step, (9) a freeze vacuum drying step, (10) an iodine compound recovery step, (11) a storage step, (12) an ingredient preparation step, (13) a mineralizing step, (14) a preservation step, (15) a water and nitric acid treatment step, (16) a reutilization step, (17) a discharge step, (18) scrubbed off-gas, (19) a radioactivity capture step, and (20) a discharge step. In the FIGURE, spend fuel is sheared at step (1) at a retreatment facility. Off-gas containing the radioactive iodines .sup.129 I of long half-life and .sup.131 I of short half-life produced at the dissolving step (2) is scrubbed at the off-gas scrubbing step (5) by the scrub solution (4) such as sodium hydroxide and recovered in cleaned off-gas scrub solution (6) through generation of radioactive iodines NaI, NaIO.sub.3. Meanwhile cleaned off-gas (18) containing unrecovered radioactive iodines in scrub solution (6) is released into the environment at the discharge step (20) after removal of iodines at step (19) through a radioactive iodine capturing substance such as silver zeolite. The solution in the dissolving step (2) is treated in a solvent extraction process. Clean off-gas scrub solution (6) containing radioactive iodines is put through the scrub solution precipitating step (8) by adding copper and silver additives (7) as necessary, and radioactive iodines are separated from the solution by the formation of such precipitates as copper iodide or silver iodide. It should be noted that this treatment can be omitted since the compound of Na and iodine will undergo almost no decomposition by a low-temperature treatment below. Next, when either cleaned scrub solution (6) containing radioactive iodine or liquids containing precipitate remaining after settling of the scrub solution is treated by freeze drying at the freeze vacuum drying step (9), water and nitrates sublime and are separated, and radioactive iodine (10) is recovered as iodine compounds (NaI, NaIO.sub.3, CuI, AgI, etc.) in the form of residue. Under these conditions, freezing is eliminated, but no vaporization takes place since recovery is achieved in the form of iodine compounds. When necessary, the recovered iodine compounds containing radioactive iodine either directly or after being temporarily stored at the storage step (11) are made to have the same composition by adding suitable compounds to, e.g., natural, stable iodine-containing minerals (Chile saltpeter, Dictzeit, Jodyrite, Mitertite, etc.). After adjustment of ingredients at step (12), or without such adjustment, mineralization is performed by a high-pressure press or the like at the mineralization step (13), and the results are preserved as a mineral at step (14). Storage is thus possible in a stable mineral state. Meanwhile, sublimation is carried out at the freeze vacuum drying step (9), and water and nitric acid trapped and recovered at the water and nitric acid treatment step (15) containing almost no radioactive iodine are either utilized again at step (16) or released at step (17). The following advantages are achieved according to the present invention: (i) When processing liquids containing radioactive iodines by a freeze vacuum drying process, the radioactive iodines are not vaporized. Therefore, it is possible to eliminate release of .sup.129 I radioactive iodine into the environment. Owing to its long half-life, such release of radioactive iodine into the environment would cause problems. (ii) When treating liquids containing radioactive iodines by freeze vacuum drying, radioactive iodine is not vaporized. This makes it possible to reduce the consumption of collecting agents such as silver zeolite that are used to collect vaporized radioactive iodine. (iii) By treating liquids containing radioactive iodines individually without allowing mixing with waste liquids that do not radioactive iodine generated at a retreatment process, it is possible to maintain alkalinity at all times so there is not chance of causing vaporization of radioactive iodine by mistake. (iv) It is possible to store long half-life .sup.129 I radioactive iodine safely for an extended period by sealing recovered radioactive iodine in stable minerals for a long period of time. (v) It is possible to prevent vaporization of radioactive iodine and recover the same by adding silver ions, copper ions or the like to the liquid containing the radioactive iodine, causing precipitation of the compounds of additive and radioactive iodine and separating the radioative iodine from solution. As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.