Patent Number: 050698273
Section: description

EXAMPLE 1 Dissolving PuO.sub.2 by Cr.sup.2+ in 5M H.sub.2 SO.sub.4 Use is made of an electrolyzer having a platinum anode and a cathode constituted by a solid copper support covered with gold and which has been amalgamed by soaking in mercury. The electrolyzer is subdivided into two compartments by a porous wall. Into the cathode compartment are introduced 1134 mg of PuO.sub.2 powder calcined at 500.degree. C. and 100 ml of a sulphuric acid solution with 5 mol/l of H.sub.2 SO.sub.4 containing 0.1 mol/l of Cr.sup.3+ ions. The solution is stirred with a magnetic stirrer and between the anode and the cathode is applied a potential difference such that the current density on the cathode is 0.036 A/cm.sup.2, whilst maintaining the electrolyzer at 85.degree. C. After stirring for 16 minutes, it is found that the dissolving rate is 100%, 85% of the plutonium being present in the solution in the form of Pu(III), which corresponds to 8.5 g/l and the remaining 15% are present in the form of a blue precipitate of plutonium (III) sulphate. The corresponding dissolving rate is 6.25%/min, which corresponds to a faradic yield of 37.7%. COMPARATIVE EXAMPLE 1 Dissolving PuO.sub.2 by 5M H.sub.2 SO.sub.4 The same operating procedure as in example 1 is adopted in order to attempt to dissolve 113 mg of the same PuO.sub.2 powder in the same solution operating at a temperature of 85.degree. C. and accompanied by stirring, but without applying a potential difference between the anode and the cathode. Thus, it is not possible to produce and then regenerate in the solution the Cr.sup.2+ ions, which serve as the plutonium reducing agent. Under these conditions, after stirring for 2 hours, the dissolving rate does not exceed 10%, which corresponds to a rate of 0.1%/min. Thus, sulphuric acid is ineffective in the absence of the reducing agent for the reduction of plutonium dioxide. EXAMPLE 2 Dissolving PuO.sub.2 by Cr.sup.2+ in 1M H.sub.2 SO.sub.4 The same operating procedure as in example 1 is used for dissolving 1134 mg of a PuO.sub.2 powder identical to that of example 1 and using 100 ml of sulphuric acid solution at 1 mol/l of sulphuric acid and containing 0.1 mol/l of Cr.sup.3+. The same temperature, current density and stirring conditions are used. After stirring for 8 minutes, the dissolving rate is 100% and a 10 g/l Pu.sup.3+ solution is obtained. Thus, the reaction rate is equal to 12.5%/min, which corresponds to a faradic yield of 80%. EXAMPLE 3 Dissolving PuO.sub.2 by Cr.sup.2+ in 1M H.sub.2 SO.sub.4 In this example, PuO.sub.2 is dissolved without electrolytically regenerating the reducing species, i.e. the Cr.sup.2+ ions. A mass of 266 mg of PuO.sub.2 powder calcined at 450.degree. C. is introduced into a thermostatically controlled reactor under an inert atmosphere. Into the reactor are introduced 120 ml of a sulphuric acid solution at 1 mol/l and containing 0.1 mol/l of Cr.sup.2+ ions. After stirring for 9 min at 70.degree. C., the dissolving rate is 100%, which corresponds to a dissolving rate of 11%/min and to a final plutonium concentration in the solution of 2 g/l. Thus, when using an adequate chrome ion quantity for reducing all the plutonium, it is not necessary to electrolytically regenerate the Cr.sup.2+ ions and PuO.sub.2 can be dissolved under good conditions. EXAMPLE 4 Dissolving PuO.sub.2 by U(IV) The same operating procedure as in example 2 is followed for dissolving 1134 mg of PuO.sub.2 powder calcined at 500.degree. C., but using 100 ml of a uranyl sulphate solution having a sulphuric acid concentration of 1 mol/l and a UO.sub.2.sup.2+ ion concentration of 0.1 mol/l. The reducing agent U.sup.4+ is produced and regenerated electrolytically and the same temperature, cathode current density and stirring conditions as in example 1 are followed. After stirring for 40 min, the dissolving rate is 100%, which corresponds to a rate of 2.5%/min and to a faradic efficiency of 13%. Thus, uranium IV is effective for dissolving PuO.sub.2 in sulphuric solution, but the reaction is slightly slower. EXAMPLE 5 Dissolving PuO.sub.2 in the presence of cellulose by Cr.sup.2+ in 1M H.sub.2 SO.sub.4 The same operating conditions as in example 2 are used for dissolving 1134 mg of PuO.sub.2 identical to that of example 2 and to which is added 1 g of finely divided cellulose. The same solution as in example 2 is used, as well as the same temperature, stirring and cathode current density conditions. After stirring for 8 min, the dissolving rate is 100% as in example 2 and the plutonium (III) content of the solution is 10 g/l. COMPARATIVE EXAMPLE 5 Oxidizing dissolving using silver of the PuO.sub.2 /cellulose mixture In this example, use is made of the oxidizing dissolving process described in European patent 158 555 for dissolving PuO.sub.2 in the presence of cellulose. Thus, into the electrolyzer are introduced 1134 mg of PuO.sub.2 identical to that of example 1 and 1 g of finely divided cellulose, followed by the addition of 100 ml of a 5 mol/l nitric acid solution containing 0.1 mol/l of Ag.sup.+ and an anode current density of 0.036 A/cm.sup.2 is applied in order to regenerate the Ag.sup.+ ions and working at 25.degree. C. Under these conditions, after stirring for 100 min, the PuO.sub.2 dissolving rate is only approximately 40%, which corresponds to a dissolving rate of 0.4%/min and a faradic efficiency of 5%. Thus, oxidizing dissolving using silver is much less effective for dissolving PuO.sub.2 in the presence of organic materials with reducing properties. EXAMPLE 6 Decontamination of cryobroyats by reducing dissolving In this example, dissolving takes place of the plutonium present in nuclear waste formed from cryobroyats, which are waste materials mainly constituted by contaminated, crushed or ground organic substances. In the reactor is placed a mass of 6.33 g of cryobroyats and 100 ml of a 1 mol/l sulphuric acid solution containing 0.1 mol/l of Cr.sup.3+ ions is added. A temperature of 85.degree. C. is maintained and the same cathode current density (0.036 A/cm.sup.2) and the same stirring conditions as in example 1 are used. After reacting for 62 min, the decontamination rate is 100%. Thus, the inventive process makes it possible to bring about a plutonium decontamination of the organic waste under good conditions and with relatively short treatment periods. EXAMPLE 7 Decontamination of crushed organic waste by reducing dissolving using V(II) and V(III) In the electrolyzer is placed a mass of 12.44 g of plutoniferous waste and 200 ml of 1.05 mol/l sulphuric acid containing 0.1 mol/l of VOSO.sub.4. The mixture is raised to a temperature of 85.degree. C. and electrolysis is carried out by using a current density equal to 0.029 A/cm.sup.2. After 135 min electrolysis, it is found that 90% of the plutonium initially present in the waste has dissolved. The curve translating the solubilization kinetics of the plutonium shows that V(III) (generated at the start of electrolysis) and V(II) (generated when all the initial V(IV) had disappeared) are effective for solubilizing the plutonium contained in the waste. EXAMPLE 8 Decontamination of crushed organic waste by reducing dissolving using U(IV) and U(III) The same operating procedure as in example 7 is adopted but using UO.sub.2 SO.sub.4 in place of VOSO.sub.4. The waste has the same origin as example 7 and the electrolysis conditions are the same as in the latter. After electrolysis for 210 min a solubilization rate of the plutonium equal to 90% is reached. The solubilization kinetics of the plutonium shows that U(IV) and U(III) are effective. Nevertheless the action of U(III) leads to a faster solubilization of the plutonium.