Patent Number: 050698273
Section: summary

The present invention relates to a process for dissolving a refractory compound of plutonium which it is difficult to dissolve, namely plutonium dioxide. It more particularly applies to the dissolving of said compound present in solid waste and in particular in organic waste. One of the main problems which is frequently encountered in nuclear installations is the recovery of the plutonium in the form of PuO.sub.2, which is substantially insoluble in acid solutions. This compound is more particularly present in solid waste resulting either from the production of nuclear fuel elements, or from the processing of irradiated nuclear fuels. Such waste can be constituted by ash resulting from incineration at 800.degree. to 900.degree. C. of highly plutonium contaminated combustible waste in which the plutonium is in oxide form. Other waste is constituted by laboratory waste, particularly organic material waste, such as plastic and cellulose material waste contaminated by plutonium in the form of oxide which it is difficult to dissolve. Hitherto four methods have been used for dissolving plutonium dioxide. The first method consists of maintaining the plutonium in the tetravalent state during its dissolving. One process is based on the catalytic action of fluoride ions and the formation of a complex anion PuF.sup.3+ making it possible to solubilize the plutonium in a nitric solution. This dissolving can be accelerated by adding a silver compound which oxidizes the complex ion PuF.sup.3+ and thus regenerates the F.sup.- ions. This process is more particularly described in U.S. Pat. Nos. 3,976,775 and 4,069,293. This process suffers from the disadvantage of requiring the use of corrosive reagents and of producing fluorinated effluents, whose treatment causes problems. Moreover, when the plutonium is present in reducing organic waste, the latter can be oxidized by Ag.sup.2+ ions, which no longer fulfil their function of accelerating the plutonium oxide dissolving reaction. Thus, this process is not very suitable for the treatment of organic waste. Another possible way of dissolving plutonium whilst maintaining it in the tetravalent state is to transform it into plutonium sulphate by the action of concentrated sulphuric acid at a high temperature of approximately 250.degree. C. and as is described by B. Stojanik et al in Radiochimica Acta, 36, pp 155-157, 1984. Thus, this process suffers from the disadvantage of requiring the use of high temperatures and concentrated acid solutions. Moreover, it does not permit the complete dissolving of PuO.sub.2, when the latter has been exposed to high temperatures. The second procedure for dissolving plutonium dioxide consists of oxidizing dissolving in which the plutonium is brought into the soluble state by oxidizing at valence VI. This can be obtained by using a powerful oxidizing agent such as the ion Ag.sup.2+ and as is described in European patents 160 589 and 158 555. This procedure is very interesting because it makes it possible to obtain high dissolving rates. However, it is not very suitable for the treatment of organic waste having reducing properties because, in this case, the reducing organic materials can be oxidized by the Ag.sup.2+ ions, which can therefore no longer participate in the dissolving reaction. This reduces the effectiveness of the process and greatly increases the waste treatment time, which constitutes a handicap for the performance of this type of treatment on an industrial scale. A third plutonium dioxide dissolving procedure consists of treating the plutonium dioxide by a concentrated 6M hydroiodic acid solution brought to the boiling temperature and as is described in U.S. Pat. No. 4,134,960. However, in such a process, the dissolving rate is relatively low and the effluent obtained is difficult to treat. A fourth procedure based on the reduction of plutonium described in FR-A-2 553 560 consists of dissolving the plutonium dioxide in a nitric solution brought to boiling point and which contains uranium IV and hydrazine. Thus, in this process, the stabilization of the uranium IV by hydrazine is necessary. In addition, a hydrazine depletion at the end of dissolving can lead to an autocatalytic oxidation of the uranium IV excess, i.e. an explosive reaction making its performance on an industrial scale difficult. Thus, the presently known processes do not make it possible to ensure under good conditions the dissolving of a refractory compound of plutonium, such as plutonium dioxide and which is present in waste having reducing properties. The present invention specifically relates to a process for the reducing dissolving of plutonium dioxide, which obviates this disadvantage. This process consists of contacting the plutonium dioxide with a hydrazine-free, acid aqueous solution, which contains a reducing agent, whose redox potential is below +3.5 V/ENH in order to reduce the plutonium and to dissolve the same. The inventive use of reducing agents having a potential below +0.5 V/ENH makes it possible to carry out the reduction of the plutonium from valence IV to valence III and thus solubilize the same in the acid solution. Thus, the standard potential of the transformation ##STR1## is E.sub.o =0.544 V/ENH at 25.degree. C. and E.sub.o =0.487 V/ENH at 85.degree. C. Moreover, the use of reducing agents having a redox potential below +0.5 V/ENH make it possible to obtain said transformation. Thus, contrary to what is indicated in FR-A-2 553 560, the ferrous ion Fe.sup.2+, whose redox potential is equal to +0.77 V/ENH is not suitable for carrying out this transformation. Examples of suitable reducing agents are Cr.sup.2+, U.sup.4+, U.sup.3+, Eu.sup.2+, V.sup.3+, V.sup.2+, Ti.sup.3+ and Ti.sup.2+. According to the invention, the reducing agent is chosen in such a way as to obtain a favourable dissolving kinetics as a function of the solution and operating conditions used. According to a first embodiment of the inventive process, the reducing agent is present in the acid aqueous solution in a quantity adequate to reduce all the plutonium to be dissolved. When the reducing agent is one of the aforementioned ions, it can be introduced in the form of a soluble salt, e.g. sulphate. It is also possible to introduce it in solution in oxidized form and to reduce the solution, e.g. by electrochemistry or by zinc amalgam. When the oxidation reaction of the reducing agent only uses a single electron, it is necessary for the molar concentration in the reducing agent of the solution to at least be equal to the molar concentration of the plutonium to be dissolved. If the oxidation reaction of the reducing agent uses several electrons, the molar concentrations of the reducing agent can be lower. This embodiment of the process can be used when there are only small plutonium quantities to be dissolved. However, in the case of relatively large quantities, it is preferable to regenerate the reducing agent in solution to limit the reducing agent quantities used. According to a second embodiment of the inventive process, the reducing agent in solution is regenerated by electrolysis. This can be carried out by using the oxidizing dissolving installations described in European patents 158 555 and 160 589, where the oxidizing agent Ag.sup.2+ is regenerated by electrolysis. In the invention, regeneration takes place on the cathode and the anode and cathode compartments are reversed as compared with what is used in installations for oxidizing dissolving. In this second embodiment of the inventive process, the reducing agent concentration in the solution is generally 0.02 to 0.2 mol/l. The acid aqueous solutions used for dissolving can be of different types, the only condition being that the acid is compatible with the chemical species which can occur in solution and in particular with the plutonium and reducing agent. For example, it is possible to use sulphuric acid, preferably at a molar concentration of 0.5 to 7 mol/l. It would also be possible to use formic acid. It is preferable to avoid using nitric acid due to possible reactions between the nitrous acid and the reducing agent, particularly in the case of U(IV). On regenerating the reducing agent by electrolysis, the plutonium to be dissolved, optionally included in waste in the presence of the dissolving solution is brought into the cathode compartment of an electrolyzer having an anode and a cathode. The reducing agent can be introduced in oxidized form and can be generated at the start of the reaction by applying an adequate potential difference to reduce its oxidized form. Preferably, the solution is vigorously stirred, on the one hand to facilitate exchanges between the solution and the cathode and on the other in order to perfectly impregnate the plutonium dioxide or the waste containing the same with the solution. In this way the reactions are accelerated and in particular the reduction of the oxidized species of the reducing agent, because the reducing kinetics are mainly dependent on the speed at which the ions to be reduced can reach the cathode. Generally, the potential difference is applied in such a way as to impose a constant current in the cell, which permits the permanent regeneration of the reducing agent. The material from which the cathode is made must comply with the electrochemical properties of the reducing agent and have adequate mechanical and chemical characteristics to remain unimpaired under the adopted operating conditions. When the regeneration of the reducing agent takes place at a potential higher than that of the reduction of the solution, the cathode can be made from platinum. In the case of more powerful reducing agents requiring a high cathode overvoltage, it is possible to use an electrode having a solid copper support covered with gold, whereby it is amalgamed by soaking in mercury. Thus, such an electrode roughly has the cathode overvoltage of mercury due to the use of gold amalgam as the electro-active material. The copper support contributes to the rigidity of the electrode. The use of such an electrode is particularly suitable for the regeneration of powerful reducing agents such as Cr.sup.2+, U.sup.4+, U.sup.3+, Eu.sup.2+, V.sup.2+, Ti.sup.3+ and Ti.sup.2+. In order to improve the efficiency of electrolysis, as well as the reaction rate between the reducing agent and the plutonium compound, it is preferable to work at a temperature above ambient temperature, e.g. at 50.degree. to 100.degree. C. Moreover, when the reducing agent used reacts with the oxygen, the cathode compartment is placed under an inert atmosphere to prevent said reaction. A condenser is also added to the cathode compartment in order to limit the vaporization of the solution. Other features and advantages of the invention can be better gathered from reading the following examples concerning the performance of the inventive process and which are obviously given in an illustrative and non-limitative manner. Comparative examples 1 and 5 reveal the advantages provided by the inventive process.