Patent Number: 042750458
Section: description

At the time of dissolution of irradiated fuels in the dissolver 1, the nitrous vapors produced are recombined with a view to recovering the nitric acid which, when recovered, is passed back into the dissolver 1 either continuously or in a batch process. The equilibrium constant [I] gas/[I] liquid and the relative flow rates of gas and of solution are such that the greater part of the iodine passes back into the recombined acid AR. Direct recycling 2 of this acid to the dissolver 1 therefore results in an accumulation of iodine which crystallizes locally within the condensation circuit 3 and absorption circuit 4 for the nitrous vapors, no matter which mode of continuous or periodic recycling is contemplated for the recombined acid. The method according to the invention permits complete removal of the iodine contained in the recovered acid as well as trapping and storage of said iodine in a stable form. In a first embodiment (FIG. 1), the recombined acid AR which leaves the nitrous vapor absorption column 4 is passed by means of a pump 5 either continuously or batchwise from a collecting tank 4a to the top of a desorption column 6 of the packing type (Raschig rings, Berl saddles and the like) or of the plate type in counterflow to a "desorbent" gas which can be air or an inert gas and which enters the column through the pipe 7. The column 6 can be heated by means of a jacket 8. A knowledge of the partition coefficients of the iodine between the recombined acid solution and the gas as a function of the operating temperature makes it possible to adjust the flow rates of gas and of solution so as to obtain a desorption factor which is considerably larger than 1. The residual quantity of iodine in the acid after desorption depends only on the number of theoretical plates of the column and therefore both on the efficiency of the column packing and on the column height. This accordingly makes it possible to obtain an iodine removal efficiency which is higher than 95%. The acid thus treated can then be transferred to the dissolver or to any other point of the installation without any difficulty. The iodine which is entrained in the gas stream can then be trapped in accordance with different variants. VARIANT A The gas circulates within a scrubbing tower 9 in counterflow to an alkaline and reducing solution A (sodium hydroxide+hydrazine or thiosulphate, etc.) which is fed to the tower by means of a pump 11 and the pipe 10. The gas is then evacuated through a pipe 12 to a purifying column 13 before being discharged. The alkaline solution is then either sent to the effluent treatment unit 14 in which it is neutralized (pH 8.5) and in which the addition of a reducing agent stabilizes the iodine in solution in the form of iodide or treated at 15 by addition of a salt which produces an insoluble compound. This is followed by precipitation of the lead iodide PbI.sub.2 which is very sparingly soluble and highly stable since its decomposition temperature exceeds 950.degree. C. The lead iodide is retained on a filter 16. After separation from the mother-liquors by filtration, said precipitate is passed to a solid-waste conditioning station 17 for long-term storage in the form of bituminous coated products. The precipitation mother-liquors 18 are sent to the effluent treatment unit 14. VARIANT B The gas to be freed from iodine is passed in counterflow to a solution B containing a salt which forms an insoluble compound such as lead iodide, for example. The column 9 can be heated in order to prevent formation of the precipitate within this latter, precipitation of the insoluble compound being obtained by cooling the solution outside the column. Separation of the precipitate of the mother liquors and their respective treatment take place as in the previous case. VARIANT C The gas containing the iodine passes within a column 19 over a bed of catalytic supports impregnated with silver nitrate or with silver-exchanged zeolites. This variant permits high trapping efficiency but is undoubtedly the most costly. In short, the advantage of this embodiment of the method lies in the fact that the operation is carried out on a small gas stream. This makes it possible to employ devices of small size and to increse their efficiency by virtue of the higher concentration of iodine in the gases to be treated. VARIANT D The gas containing the iodine passes within a column 19' heated to more than 50.degree. C. in counterflow to a wash liquor 20' containing Pb.sup.++ ions. This liquor 20' which passes successively through the heat exchanger 21' and through the heater 22' receives at 23' a make-up quantity of lead nitrate and hydrazine nitrate prior to introduction into the top of the column 19'. At the bottom of this column, the liquor 20' is partly fed back to the top of the column and partly directed to the heat exchanger 21' and the cooler 24' in which the iodine precipitates in the form of PbI.sub.2. In a second embodiment of the method (FIG. 2), the recombined acid AR containing the iodine is passed into a distillation unit 20 in which it is brought to the boil. The iodine can be removed quantitatively by distilling less than 10% of the volume of acid solution by reason of the high vapor tension of iodine. It has been possible to demonstrate the predominant influence of the nitrous NO.sub.2.sup.- ions on the volatilization of iodine. In point of fact, these ions play the part of reducing agents and stabilize the iodine in the elementary form I.sub.2 and prevent the formation of oxidized species such as the hypoiodites IO.sup.- or iodates IO.sub.3.sup.-. The table hereunder gives a record of the decontamination factors obtained as a function of the volume percentage of distilled acid and of the proportion of nitrous ions, the initial proportion of iodine being 3.times.10.sup.-3 M. TABLE ______________________________________ Volume percentage of initial initial NO.sub.2.sup.- concentration distilled 2.1.times. 10.sup.-6 1.1.times. 10.sup.-2 2.2.times. 10.sup.-2 4.4.times. 10.sup.-2 8.8.times. 10.sup.-2 solution M M M M M ______________________________________ 0 1 1 1 1 1 0.05 28 31 36 39 43 0.1 47.5 102 123 218 354 0.15 50 152 202 365 785 0.20 50 158 251 454 1230 0.25 50 158 252 500 1470 ______________________________________ It can be noted that, in the case of 10% of distilled acid, there remains only 2% of iodine in respect of a proportion of 2.10.sup.-6 M of nitrous acid and less than 0.3% in respect of 8.8.times.10.sup.-2 M of nitrous acid. It should be pointed out that these nitrous ions exist naturally in the recombined acid, their concentration being dependent on the efficiency and conditions of oxidation and absorption of the nitrous vapors. Recovery of iodine from the recombined acid takes place in different ways, a number of variants being given hereunder. VARIANT E The iodine is separated directly from the distillate by crystallization in the cold state at 21 followed by filtration at 22. In fact, the iodine has very low solubility (300 mg/1 at 30.degree. C.). There are thus obtained iodine crystals which can be treated at 23 either in order to be stored in leak-tight containers or to be conditioned for subsequent fabrication of sources of radioactive iodine 129.sub.I. The crystallization mother-liquors 24 are recycled in the distillation unit 20. VARIANT F The iodine crystals collected at the time of filtration at 22 are dissolved in an alkaline solution 25 with or without reducing agent. This solution is neutralized and sent to the effluent treatment unit 26 as in the case of the variant A of the first embodiment of the method. VARIANT G Starting from the alkaline solution in which the iodine has been dissolved, an insoluble compound such as lead iodide PbI.sub.2 is precipitated at 27 as in the first embodiment of the method. The mother liquors are sent to the effluent treatment unit at 28. VARIANT H The water vapor containing iodine is condensed directly at the exit of the distillation unit 20 in a solution containing a reducing agent and a salt which forms with the iodine an insoluble compound (such as PbI.sub.2) in order to precipitate the formed compound at 29 and separate this latter directly by filtration at 30, the precipitation mother-liquors being recycled in the precipitation reactor at 29. In accordance with a third embodiment of the method (FIG. 3), the aqueous solution containing the recombined acid AR loaded with iodine is introduced by means of the pump 5 into an extractor 31 in which the solution is contacted with an organic solvent 32 which is thereby loaded with iodine. The iodine-loaded solvent is then introduced into a stripper 33 in which it is treated with the liquor 20' as in the variant D. The unloaded solvent is then directed to a treatment unit 34 prior to re-use in the extractor 31.