Patent Number: 055235140
Section: description

FIG. 1 shows that the apparatus comprises a tight enclosure 1, such as a shielded enclosure or a glove box, provided with a biological protection wall 3, an electricity conducting material chamber or case 5 placed in the enclosure 1 and a container 7 containing the product to be heated located within the chamber 5. A microwave generator 19 is located outside the enclosure and is used for supplying microwaves to the chamber 5 via 8 waveguide 11, which traverses the enclosure biological protection wall 3 by means of an opening 13 and tangentially enters the chamber 5 at 12. It would also be possible to locate the microwave supply opening 13 on the axis, so as to direct the microwaves directly onto the axis of the chamber 5. This waveguide, in its portion outside the enclosure 1, is formed from three sections 11a, 11b and 11c, the section 11b having two segments bent by 45.degree., in order to place behind said section 11b at the passage opening 13 a biological protection wedge 15 ensuring the continuity of the biological protection at said opening 13. The interest of producing the waveguide in three sections is to permit its use in enclosures not requiring a biological protection wall 3. Thus, under these conditions, it would be possible to eliminate the section 11b and directly fit the microwave generator 9 by means of the section 11a to the section 11c. At the enclosure 1, within the section 11c is provided a known alpha sealing means 17, e.g. a polytetrafluoroethylene membrane fitted on an O-ring. The chamber 5 is provided with a detachable base 5a making it possible to introduce into it the container 7 for containing the product to be heated and is provided with a gas extraction duct 5b. This mobile base 5a is associated with a support 19 provided with a rod 20, which can slide in the base 5a with a view to regulating the height of the support 19 with respect to the bottom of the chamber 5a, so as to position the container 7 relative to the chamber microwave supply 12. FIG. 2 shows in cross-sectional form a microwave heating apparatus according to the second embodiment of the invention. FIG. 3 is a vertical section of the apparatus of FIG. 2, In these drawings, the same references for designating the components of the apparatus are used as in FIG. 1. Thus, the apparatus of FIGS. 2 and 3 comprises a tight enclosure 1, such as a shielded enclosure or a glove box, equipped with a biological protection wall 3, a waveguide chamber 5 made from an electricity conducting material located in the enclosure 1 and a container 7 containing the product to be heated positioned in a cavity 50 of the chamber 5. The microwave generator 9 is located outside the enclosure 1 and supplies the waveguide chamber 5 by means of a coaxial cable 21, which traverses the biological protection wall 3 by an opening 13 and enters the chamber 5 at 12. The waveguide chamber 5 comprises a tight compartment 51 in which circulates a fluid, e.g. water, for absorbing energy losses. The distance between the centre 52 of the cavity 50 of the waveguide chamber and the end 12 of the coaxial cable is proportional to .lambda./2, .lambda. being the wavelength of the microwaves. The length of the coaxial cable between its two ends 12 and 53 is proportional to .lambda.. Bearing in mind the small cross-section of the passage opening 13 in the biological protection wall 3 of e.g. 10 mm, the biological protection wedge 15 located outside the enclosure and at the passage opening is optional. FIG. 3 shows that the reception cavity 50 of the container 7 is extended on either side of the waveguide 5 and that it is provided with a detachable base 5a, a gas extraction duct 5b and a height-regularable support 19 for the container 7, as in FIG. 1. In the two embodiments described here, the container 7 can be of different types. It can in particular be constituted by a tight cylinder, optionally provided with gas discharge means 18. The container is made from a material which does not conduct electricity, e.g. from a plastics material such as polytetrafluoroethylene or glass. The apparatus also has a control case 23 making it possible to regulate the operating conditions (power, time, etc.) of the microwave generator 9. In the apparatuses described hereinbefore, use is only made of corrosion-resisting materials, such as steel, stainless steel or polytetrafluoroethylene, within the enclosure 1. The biological protection wall 3 can be made from lead or steel and have a thickness below 25 cm. The wedge 15 is preferably made from lead. The microwave generator 9 can be a conventional power-modulatable generator, e.g. having a maximum power of 300 to 800 watt and able to operate for more than 2 hours. For performing the process according to the invention in said apparatus, into the container 7 is introduced the desired acid, aqueous solution quantity and the desired quantity of solid waste or product containing plutonium. Good results can be obtained by using quantities such that the waste/solution ratio (in weight/volume) is approximately 1/80 (1 g of waste for 80 ml of acid solution). The container 7 is then placed in the chamber 5 at the desired height for ensuring that the liquid level is satisfactorily positioned with respect to the waveguide. The microwave generator 9 is then started up by supplying an appropriate power for the desired time for obtaining the complete dissolving and destruction of the waste. The power applied and the treatment time are chosen as a function of the nature of the treated product and the reagent used. Generally, the power applied is decreased on obtaining the boiling of the solution in order to maintain boiling with a lower power. Thus, at the end of the operation a plutonium-containing solution is recovered and this can, if necessary, be concentrated and then treated with a view to recovering the plutonium. The following examples illustrate the performance of the process according to the invention. EXAMPLE 1 In this example, 1 g of waste coming from contaminated rubber gloves is treated with 80 ml of 12N nitric acid for 16 min, initially using 25% of power of the microwave generator at a frequency of 2.45 GH or 75 W and whilst then maintaining boiling at a 15% power for 3 min and then 20% for 11 min, i.e. power levels of 45 and 60 W. Therefore the total power used is 15.7 W.multidot.h. After 16 min of treatment, there is a virtually complete destruction of the rubber and a 95 to 99% yield or efficiency is obtained. The latter is defined by the ratio ##EQU1## EXAMPLE 2 In this example treatment takes place of 0.4 g of DOWEX 18-type anionic resin contaminated by plutonium in 32 ml of concentrated sulphuric acid using the same generator as in example 1. After 23 min of microwave exposure, i.e. an energy of 23 W.multidot.h, there is a 98% efficiency, i.e. a complete destruction of the resin and the total release of the plutonium. EXAMPLES 3 TO 7 In these examples treatment takes place of different types of waste using the same microwave generator as in example 1. The treated waste, the reagents used, the treatment conditions and the results obtained appear in the attached Table, which also gives the results obtained in examples 1 and 2. This table makes it clear that the process according to the invention makes it possible to obtain a mineralization of rubber gloves, ion exchange resins, cottons, KLEENEX and other cellulose articles, as well as polymer elements. It is also suitable for the treatment of liquid products such as tributyl phosphate in dodecane. In this case very good results are obtained in a nitric medium, despite the problem of the immiscibility of the organic waste and the reagent and the solvent evaporation problems at high temperatures. Thus, the process according to the invention is of great interest for these different waste products. COMPARATIVE EXAMPLES 8 AND 9 These examples use the same operating procedure as in example 2 for treating the waste appearing in the Table using the reagents and treatment conditions given in the Table. The results obtained also appear in the Table. The results of examples 8 and 9 demonstrate that the choice of the reaction medium is important, because for anionic resins, it is not possible to bring about mineralization with concentrated nitric acid, whereas a 98% efficiency is obtained with sulphuric acid (example 2). The same applies with respect to the polymer (polyvinyl chloride) in example 9, despite the addition of an oxidizing agent constituted by persulphate, whereas the latter is partly attacked by the sulphuric acid in example 3. EXAMPLE 10 This example uses the process according to the invention for dissolving PuO.sub.2 in an aqueous solution of nitric acid and hydrofluoric acid. To this end, 1 g of PuO.sub.2 is introduced into 20 ml of an aqueous solution with 14 mole/l of HNO.sub.3 and 0.05 mole/l of HF, followed by heating the solution to the boiling point using microwaves having a frequency of 2.45 GHz and a power of 50 W for 30 min. This leads to a complete dissolving of the plutonium dioxide. EXAMPLE 11 The same operating procedure as in example 10 is used for dissolving plutonium dioxide, but use is made of an aqueous solution with 7 mole/l of nitric acid and 0.05 mole/l of hydrofluoric acid. Under these conditions total dissolving is brought about in 1 hour. COMPARATIVE EXAMPLE 12 This example uses the same operating procedure as in example 10 for dissolving 1 g of PuO.sub.2 in 20 ml of a solution with 14 mole/l of HNO.sub.3 and 0.05 mole/l of HF, except for using Joule effect heating in place of microwave heating. Under these conditions the PuO.sub.2 is completely dissolved after approximately 3 h. On comparing examples 10 to 12, it can be seen that as a result of using microwave heating, it is possible to dissolve more rapidly and use a solution having a lower nitric acidity and which is therefore less corrosive. EXAMPLE 13 In this example 0.2 g of plutonium are dissolved in an aqueous solution with 7 mole/l of nitric acid and 0.05 mole/l of hydrofluoric acid performing the microwave heating for 15 min, at a power of 50 W.multidot.h and a frequency of 2.45 GHz. This leads to a complete dissolving of the metal plutonium. COMPARATIVE EXAMPLE 14 This example follows the conventional operating procedure for dissolving 0.2 g of plutonium using 10 ml of a 3 mole/l hydrobromic acid solution, followed by the elimination of the bromine by distillation and taking up the residue by an aqueous solution of 7 mole/l nitric acid and 0.05 mole/l hydrofluoric acid. Under these conditions, the total treatment time is 2 hours, whereas it is only 15 min in the previous example. Thus, the use according to the invention of microwave heating makes it possible to improve the radioactive waste treatment processes, as well as the processes for dissolving plutonium from plutonium dioxide or metallic plutonium. TABLE __________________________________________________________________________ QUANTITY INTRO- REACTION % Time Energy EX WASTE SOURCE DUCED MEDIUM Yield in min Supplied __________________________________________________________________________ 1 Rubber Pregloves 1 g 80 ml of 12N 95-99 16 min 16 W.H HNO.sub.3 2 Anionic resin DOWEX 1*8 0.4 g 32 ml of 98% 23 min 23 W.H conc. H.sub.2 SO.sub.4 3 Polymer 18 ml jugs 1 g 80 ml of 30-40 28 min 24 W.H conc. H.sub.2 SO.sub.4 4 Cellulose KLEENEX 1 g 80 ml of 12N 100% 10 min 10 W.H HNO.sub.3 5 Cotton DECONTAMINATION 0.6 g 48 ml of 12N 100% 18 min 16 W.H HNO.sub.3 6 Solvent TBP/DODECANE 1 ml 80 ml of 12N 60-90 42 min 36 W.H HNO.sub.3 7 Rubber VENITEX gloves 1 g 80 ml of 12N 95-99 30 min 28 W.H HNO.sub.3 8 Anionic resin DOWEX 1*8 0.2 g 16 ml of 12N 0 22 min 17 W.H HNO.sub.3 9 Polymer 18 ml jugs 0.5 g 40 ml of S.sub.2 O.sub.8 -- 0 30 min 32 W.H HNO.sub.3 __________________________________________________________________________