Patent Number: 055235140
Section: summary

DESCRIPTION The present invention relates to a process for dissolving plutonium and/or radioactive elements present in solid or liquid products more particularly usable for dissolving plutonium or plutonium dioxide in an aqueous solution and for treating plutonium-contaminated liquid or solid products. It more particularly applies to the treatment of organic radioactive waste from nuclear installations, as well as the direct dissolving of plutonium and plutonium dioxide which are difficult to dissolve in an aqueous solution. Among the presently known processes for dissolving in an aqueous solution the plutonium present in plutonium dioxide or in radioactive waste, EP-A-158 555, EP-A-160 589 and EP-A-297 738 describe processes based on the use of electrolytically regeneratable Ag.sup.2+ ions for catalyzing the dissolving of plutonium in an acid solution. Such processes are interesting, but they require the use of an electrochemical cell having a particular construction in order to bring about a rapid regeneration of the Ag.sup.2+ ions necessary for dissolving purposes. Moreover, the dissolving solution is polluted by the Ag.sup.2+ ions. According to the present invention, it has been discovered that it was also possible to obtain good results for the treatment of radioactive waste materials without using Ag.sup.2+ ions, but instead carrying out a microwave heating operation in order to mineralize the waste materials and bring about the dissolving of the plutonium. For some years now microwave heating has been used in different fields. Thus, microwave heating equipment has been developed for analysis processes involving a wet mineralization stage, as described in FR-A-2 560 529 and FR-A-2 560 686. However, up to now, nobody has considered using this heating procedure for treating waste with a view to the decontamination thereof. The present invention specifically relates to the use of a microwave mineralization method for the treatment of products containing plutonium and/or other radioactive elements with a view to dissolving the plutonium and/or said elements in an aqueous solution, which makes it possible in the case where the products are waste materials to limit the volume thereof, whilst improving the possibilities of recovering the plutonium present therein. According to the invention, the process for the treatment of solid or liquid products containing plutonium and/or other radioactive elements comprises: a) introducing the solid or liquid products into aqueous, acid solution and b) microwave heating the acid, aqueous solution containing these products to a temperature and for a time adequate for destroying the solid products and for dissolving the plutonium and/or radioactive elements in the aqueous solution, Generally, the acid, aqueous solution used is a solution incorporating at least one acid chosen from among nitric, sulphuric., hydrochloric, hydrobromic and hydrofluoric acid. Preference is given to the use of a nitric acid solution, because the citric solution obtained at the end of the operation is compatible with the solutions presently treated in nuclear installations and in particular irradiated fuel reprocessing plants using the PUREX process. In addition, the plutonium can be recovered from these solutions using conventional methods. In certain cases, it is possible to add to the acid, aqueous solution an oxidizing agent such as H.sub.2 O.sub.2, a persulphate or a permanganate, e.g. kMnO.sub.4, in order to improve the efficiency of the mineralization reaction. In the process according to the invention, the choice of the acid used is more particularly dependent on the nature of the treated products. According to the invention, the treated liquid or solid products can either be organic or mineral waste materials contaminated by plutonium, or metallic plutonium, or difficultly dissolvable plutonium dioxide, as is e.g. the case with the dissolving fines obtained during the dissolving of irradiated fuels. In the process according to the invention, the use of microwave heating in particular makes it possible to bring about the dissolving of the plutonium using shorter times and less aggressive reagents than in the conventionally used processes. Moreover, when the treated products are solid products such as organic waste, e.g. ion exchange resins, gloves, pockets, cottons and handkerchiefs, it is easy to bring about a complete destruction of the waste, which was not possible with conventional processes. In the case where the solid products are organic waste materials, preference is given to an acid, aqueous solution constituted by a sulphuric or nitric acid solution. Nitric acid solutions are more particularly suitable in the case of polymer-based organic waste, such rubber or cellulose products e.g. cotton, KLEENEX, etc. Sulphuric acid solutions are preferred in the case of organic waste constituted by ion exchange resins and vinyl polymers. When the treated products are liquid products, e.g. contaminated organic solvents such as tributyl phosphate in dodecane, it is possible to use an aqueous nitric acid solution. When the product to be treated is plutonium or plutonium dioxide, preference is given to an aqueous solution of nitric acid and hydrofluoric acid, which catalyzes plutonium dissolving. As examples of appropriate aqueous solutions reference can be made to solutions containing 5 to 14 mole/l of nitric acid and 0.01 to 0.1 mole/l of hydrofluoric acid, such as the aqueous solution of 7 mole/l of HNO.sub.3 and 0.05 mole/l of HF. The invention also relates to an apparatus making it possible to carry out the microwave heating of such radioactive, liquid or solid products in a shielded enclosure or in a glove box. This apparatus for the microwave heating of radioactive products comprises a tight enclosure provided with a biological protection wall, a chamber made from electricity conducting material placed in the enclosure and able to receive a container containing the product to be heated, a microwave generator located outside the enclosure, means for connecting the microwave generator to the chamber and for supplying the latter with microwaves passing through the enclosure and the biological protection wall of the enclosure by means of a passage opening and an alpha sealing means positioned level with said passage opening. In this apparatus, the tight enclosure can be constituted by a glove box or a shielded enclosure, and the fitting of means for supplying the enclosure with microwaves from a generator located on the outside, makes it possible to adapt the microwave heating to the nuclear operating conditions and simplify the operation of the apparatus by facilitating access for an operator to the microwave heating control and regulating means. According to a first embodiment of the apparatus according to the invention, the means for connecting the microwave generator to the chamber and for supplying the latter with microwaves are constituted by a waveguide, which traverses the biological protection wall of the enclosure by the passage opening and which has at least one bent portion between said passage opening and the microwave generator. The apparatus also comprises a biological protection wedge positioned behind the bent portion of the waveguide at the passage opening in order to ensure the continuity of the biological protection at said opening. The waveguide used can have a square or rectangular section. Preferably, the section of said waveguide is as small as possible, e.g. less than 25 cm.sup.2. Preferably, the waveguide has two portions each bent by 45.degree., which makes it possible to position a biological protection at the passage opening of the waveguide in the enclosure and to position the waveguide generator at 90.degree. from the waveguide very close to the enclosure. Advantageously, the chamber serving as a Faraday cage for focusing the microwaves has means for supporting the container containing the product to heated making it possible to regulate the height position of the container with respect to the chamber microwave supply. According to a second embodiment of the apparatus according to the invention, the electricity conducting material chamber constitutes a waveguide and the means for connecting the waveguide generator to the chamber and for supplying the latter with microwaves are constituted by a coaxial cable traversing the biological protection wall of the enclosure by means of the passage opening. Advantageously, the coaxial cable is a semirigid cable having a small diameter, e.g. 6 mm. Through using a semi-rigid, small diameter coaxial cable, it is possible to form bends having a small radius of curvature, e.g. 50 mm, whilst leading to a significant simplification of the tight passages of the enclosure and the biological protection wall. Thus, the passage opening in the enclosure and in the biological protection wall also has a small diameter, e.g. 10 mm. In addition, the presence of a biological protection wedge at the passage opening is not indispensable, as in the first embodiment of the apparatus. However, when the coaxial cable has at least one bent portion between the passage opening and the microwave generator, a biological protection wedge can be located at said opening. In the second embodiment of the apparatus according to the invention, the chamber constituted by the waveguide is preferably provided with an absorber making it possible to prevent a microwave return to the generator. This absorber can be constituted by a tight compartment of the chamber in which is circulated a cooling fluid such as water. In this second embodiment of the apparatus according to the invention, the chamber serving as a waveguide has a reception cavity for the container containing the product to be heated. It can also have means for supporting the container in the cavity permitting a height regulation of the position of the container with respect to the microwave supply. The coaxial cable has a total length which is a multiple of the wavelength .lambda. of the microwaves and the waveguide chamber is dimensioned in such a way that the distance between the centre of the cavity of the chamber for receiving the container and the end of the coaxial cable entering the chamber is a multiple of .lambda./2.