Patent Number: 043494536
Section: summary

This invention relates to a method for processing alkaline solutions containing radioactive iodine during reprocessing of nuclear fuels. It is known that radioactive iodine is formed during irradiation of fuel in nuclear reactors and that the greater part of the iodine is trapped within the fuel elements. At the moment of shearing of the fuel can at the beginning of the fuel-reprocessing operation, only about 10% of the iodine escapes and the remainder is released only during dissolution of fuel in a nitric acid medium. Since radioactive iodine is a contaminant and is produced in quantities which steadily increase with the number of nuclear reactors in service, several attempts have already been made to recover said iodine under the best possible conditions. Among the known methods already employed for this purpose, mention can be made of the method described in French Pat. No. 2,277,415 filed on July 3, 1974 and entitled "Method of extraction, trapping and storage of radioactive iodine contained in irradiated nuclear fuels". In this method, the irradiated fuels are dissolved in a nitric acid solution, the vapors consisting essentially of water, nitrogen oxide and iodine and formed during dissolution are passed into a condenser, then into a first absorption column in which is formed recombined nitric acid containing nitrous ions and iodine, the recombined acid is passed into a desorption column in couterflow to a carrier gas which is loaded with iodine. Said iodine-loaded gas is passed into a second absorption column in counterflow to an alkaline solution to which a reducing agent may or may not be added, whereupon the alkaline solution discharged from the second absorption column receives an addition of a copper or lead salt which precipitates the iodine in the form of iodide. In the case of precipitation of iodine in the form of lead iodide, this method is subject to a disadvantage which lies in the fact that the iodine discharge from the second absorption column is not completely freed from the nitrous ions. In a reducing medium and during precipitation of the lead iodide, said nitrous ions are liable to combined with the lead and thus produce lead nitride which is an unstable compound and presents an explosion hazard. The present invention makes it possible to overcome this disadvantage by providing a method for removing the nitrous ions which accompany the iodine in the alkaline solution discharged from the second absorption column. The invention is directed to a method for processing alkaline solutions containing radioactive iodine during reprocessing of nuclear fuels in which the irradiated fuels are dissolved in a nitric acid solution, the vapors constituted essentially by nitrogen oxides, iodine and water formed during dissolution are passed into a condenser and then into a first absorption column in which the recombined nitric acid is formed. The method is distinguished by the fact that the recombined acid is returned to the dissolver whilst the gases discharged from said absorption column are passed into a second absorption column in a counterflow to an alkaline solution which is loaded with iodine and with nitrous ions. A further distinctive feature lies in the fact that the alkaline solution discharged from the second absorption column is passed into a reaction vessel containing a mixture of nitric acid and sulphamic acid which destroys the nitrous products and releases the iodine. In accordance with a known method, the iodine can then be separated-out in complete safety in the form of lead iodide.