Patent Number: 042750458
Section: summary

This invention relates to a method of extraction, trapping and storage of the radioactive iodine contained in irradiated nuclear fuels. At the time of irradiation of nuclear fuels, there are obtained among the different fission products the isotopes 127, 129 and 131 of iodine, the quantity and relative percentage of which depend on the neutron flux and on the time of residence of the fuel elements within the reactor. The isotope 131 is a .beta..gamma.-emitter having a half-life of 8.1 days whereas iodine-129 which is also a .beta..gamma.-emitter can be considered as stable on the human scale since its half-life is 17.2.times.10.sup.6 years. By way of example, in the case of an oxide fuel of the type employed in LWR's (light-water reactors) and irradiated to 33,000 MWd per tonne, the total quantity of iodine per tonne of fuel is 270 g, 20% of which consists of iodine-127 and 80% consists of iodine-129, the proportion of iodine-131 being negligible. After 150 days of cooling in the case of one tonne of fuel, the activity is 0.038 curie in the case of iodine-129 and 2.61 curie in the case of iodine-131. Taking into account the considerable increase in the quantities and in the mean burnup level of fuels to be processed in the coming years as well as the trend in standards or permissible rejects towards values which are as low as possible, it proves necessary to trap iodine in a reliable and efficient manner in proportions which exceed 99.9%. Tests concerned with processing of irradiated fuels have shown that a percentage of iodine of less than 10% escapes at the time of shearing of the fuel element whilst the greater part remains included in the oxide and is released only at the time of dissolution. Studies pursued up to the present time have been chiefly concerned with trapping of iodine in the gaseous effluents. There are thus many alternative methods in which recourse is had to: scrubbing of gas with alkaline solutions having a base of sodium hydroxide, thiosulphate, hydrazine or the like, PA0 scrubbing with solutions of mercuric or mercurous-mercuric nitrate in a weakly acid (10.sup.-3 to 1 M) nitric acid medium. The mercuric iodide HgI.sub.2 or mercurous-mercuric iodide H.sub.g.sbsb.2 I.sub.2 -HgI.sub.2 is thus precipitated in the scrubbing tower, PA0 solutions of (0.1 to 0.4 M) mercuric nitrate in a 7 to 9 M nitric acid medium. There is thus formed an iodomercuric complex which is then destroyed in the hot state by concentrating the solution so as to precipitate the mercuric iodate Hg (IO.sub.3).sub.2, PA0 highly concentrated solutions of 16 to 22 M nitric acid which oxidize the iodine to the iodic state IO.sub.3.sup.- and the iodic anhydride I.sub.2 O.sub.5 is then precipitated by concentration externally of the column. There are finally a considerable number of methods of trapping on solid adsorbents and among these can be mentioned the active carbons, the zeolites or molecular sieves exchanged with silver and the catalytic supports impregnated with silver nitrate AgNO.sub.3. While they permit good iodine decontamination of the gaseous effluents, all these methods do not prevent the transfer of a substantial fraction of the iodine into the fuel dissolving solution, then into the solvent at the time of subsequent solvent extraction operations. The invention is directed to a method of extraction, trapping and storage of radioactive iodine contained in irradiated nuclear fuels and is distinguished by the fact that, after dissolving the fuels in a nitric acid medium, the vapors resulting from this dissolution and consisting of water, nitrogen oxides and iodine are passed into a condenser, then into a column for the absorption of the nitrous vapors in which is formed recombined nitric acid containing iodine and nitrous ions, the iodine contained in the recombined acid being then separated-out under the favorable influence of the nitrous ions present in the recombined acid. In a first embodiment of the method, separation of the iodine contained in the recombined acid is carried out by passing the recombined acid solution into a desorption column in counterflow to a carrier gas which is thereby loaded with iodine and the iodine thus trapped in said gas is then recovered. The iodine can be extracted from the carrier gas by circulating this latter in counterflow to an alkaline solution to which is added a reducing agent in an absorption column. The alkaline solution which leaves the absorption column receives an addition of a lead salt which precipitates the iodine in the form of lead iodide. As an alternative to the above two steps, the iodine-loaded carrier gas may be passed into a scrubbing tower in counterflow to an alkaline and reducing solution containing a lead salt so as to form lead iodide in a single step. As an alternative, the iodine-loaded carrier gas may be passed through a column containing a solid adsorbent exchanged with silver and chosen from the zeolites, the molecular sieves, the catalytic supports. As still another alternative, iodine-loaded carrier gas is passed through a column in counterflow to a wash liquor which contains Pb.sup.++ ions and to which is added a make-up quantity of lead nitrate and hydrazine nitrate so as to recover the iodine in the form of lead iodide crystals. In a second embodiment of the method, in order to separate the iodine contained in the recombined acid, the solution which leaves the recombination column is passed into a distillation unit in which the iodine distils quantitatively in the presence of nitrous ions contained in the solution and is then separated-out by cooling. The iodine crystals formed at the time of cooling are separated from the distillate for the purpose of longterm storage. The iodine crystals are redissolved in an alkaline solution to which is added a reducing agent in order to transfer this solution to the effluent treatment station. The alkaline solution containing the dissolved iodine receives an addition of a lead salt so as to precipitate the lead iodide which is conditioned for long-term storage in accordance with a third embodiment of the method. The recombined acid is introduced into an organic solvent extractor which is then treated by being passed in counterflow to a wash liquor which contains Pb.sup.++ ions and to which is added a make-up quantity of lead nitrate and of hydrazine nitrate so as to recover the iodine in the form of lead iodide crystals. A wash liquor containing a copper salt can also be employed, in which case the iodine is precipitated in the form of coppr iodide. In a third embodiment, the iodine is extracted from the carrier gas by an organic solvent. Processes which are similar in some respects to the method in accordance with the invention are described in a number of patents and among these can be mentioned Belgian patent No. 788,022 and French patent No. 1,537,292. The method described in the Belgian patent makes it possible to remove iodine from a gaseous atmosphere by countercurrent contacting with an aqueous nitric acid solution which can subsequently be distilled. It is worthy of note that, in this method, the iodine is converted to iodate IO.sub.3.sup.- which cannot readily be stored and that there is also formed I.sub.2 O.sub.5 which is both unstable, hygroscopic and corrosive. On the other hand, in the method according to the invention and as will again be pointed out later, the presence of NO.sup.-.sub.2 ions in the recombined acid has the effect of stabilizing the iodine in the elementary form I.sub.2 which distils quantitatively. The method described in the French patent which relates to the production of iodine-131 by irradiation of tellurium is closely related to the method according to the invention by the fact that is permits the recovery of iodine by distillation. However, this distillation can be carried out only by continuously adding hydrogen peroxide so as to convert the iodine to the elementary state and thus to permit quantitative distillation of this latter.