Patent Number: 043494536
Section: description

Two tons of irradiated fuels containing radioactive iodine are dissolved within a vessel 1. The vapors formed and consisting essentially of nitrogen oxides, iodine and water are passed into a condenser 2 in which a fraction of the iodine is removed as a dissolved component of the water condensate, then into a first absorption column 3. Recombined acid is formed in said column and collected in a vessel 4 before being returned to the dissolver by means of a pump 5. The gases discharged from the first absorption column are passed into a second absorption column 6 in couterflow to an alkaline solution supplied from a tank 7. The gases discharged from the column 6 at 15 can be recycled to the head of the dissolver or directed to silver traps. The quantity of alkaline solution which leaves the column 6 and has been loaded both with iodine and with nitrous ions is approximately 1000 liters and contains: ______________________________________ NaOH 0.4M NaNO.sub.3 0.45M NaNO.sub.2 0.45M NaI 4.25 .times. 10.sup.-3 M ______________________________________ This solution is passed at a low rate of flow over a period of approximately 1 hour into a reactor 8 containing 250 l of a 2 M sulphamic and nitric acid solution, the temperature of which is brought to a value between 50.degree. and 70.degree. C. The concentrations of HNO.sub.3 are 5.5 M at 50.degree. C. and 2.4 at 70.degree. C. The sulphamic acid performs a function both of destruction of nitrous products and oxidation of the iodine according to the reactions: EQU HNO.sub.2 +SO.sub.3 HNH.sub.2 .fwdarw.N.sub.2 +H.sub.2 SO.sub.4 +H.sub.2 O EQU 2SO.sub.3 HNH.sub.2 +3NaI+H.sub.2 O.fwdarw.NaI.sub.3 +Na.sub.2 SO.sub.4 '(NH.sub.4).sub.2 SO.sub.3 It can be noted that desorption of the iodine contained in NaI.sub.3 which can be written NaI, I.sub.2 is assisted by the presence of nitric acid. In order to ensure destruction of the nitrous products, it must be ensured that the ratio [SO.sub.3 HNH.sub.2 ]/[NO.sub.2 ](entering) is either higher than or equal to 1. When the temperature of the reaction vessel is 50.degree. C., it must further be ensured that the initial concentration of HNO.sub.3 is equal to 5.5 M and this initial concentration of HNO.sub.3 must be equal to 2.4 M when the temperature is 70.degree. C. In order to serve as carrier gas for the released iodine, a gas (such as air, for example) is injected into the bottom of the reaction vessel by means of a pump 9. The gas flow rate is maintained at 100 m.sup.3 per hour after the end of injection of the alkaline solution until the presence of iodine within the reaction vessel 8 is no longer detectable. The desorption operation requires 1 to 2 hours after injection of the alkaline solution has been stopped. The greater part of the iodine is entrained in the form of elementary iodine within the reaction vessel 10 in which it is precipitated in the form of lead iodide. After removal of the iodine from the reaction vessel 8 which contains 1250 l of solution, the concentration of the remaining iodine is lower than 10.sup.-6 M, the concentration of Na+ is 1.04 M and the concentration of SO.sub.4.sup.-- is 0.4 M. The carrier gas which entrains the iodine is admitted into a reaction vessel 10 above which is mounted a scrubbing column 11 in which the upflowing gas is sprayed with a solution consisting of a mixture of 10.sup.-2 M lead nitrate and 2.times.10.sup.-2 M hydrazine nitrate. Satisfactory precipitation takes place when the ratio [Pb.sup.++ ]/[2I.sup.- ] is higher than or equal to 2, when the concentration of hydrazine nitrate is of the order of 2.10.sup.-2 M and when the pH value is between 3 and 1. The assembly consisting of reaction vessel and column must be maintained at 60.degree. C. The gases discharged from the column 11 at 12 can be recycled to the head of the dissolver or directed to silver traps. The solution which leaves the reaction vessel is cooled to a value between 15.degree. and 20.degree. C. in order to obtain precipitation of lead iodide and is then passed into a settler 13 which is in turn cooled to a value between 15.degree. and 20.degree. C. The precipitate settles rapidly and can be collected at the bottom of the settler 13. The supernatant solution is filtered, reheated to 60.degree. C. within the reheater 14 and readjusted by means of lead oxide and hydrazine before being passed to the head of the gas-scrubbing column 11. The lead iodide is recovered in the solid state and conditioned in known manner.