Patent Number: 043448720
Section: summary

The present invention relates to a method of removing waste products proceeding on the basis of solutions of fission products, particularly nitric solutions, which contain ruthenium. This method utilizes concentration and solidification, particularly vitrification. The present invention also relates to an apparatus which is suitable for carrying out this method. During the reworking of highly burned up nuclear fuel, a nitric aqueous phase with the entire fission products usually remains after the separation of uranium and thorium with organic extraction agents. In so doing, 7.3 m.sup.3 solution having a residue on ignition of 1.6% and an acidity of 1.45 M/l nitric acid is obtained, for example, per metric ton of heavy metal. These solutions, which due to their high radioactivity cannot be removed in a simple manner, are concentrated for a final storage, and the concentrate obtained is then preferably solidified by vitrification. This operation though simple in principle is complicated by the presence of nitrate, and particularly nitric acid, since the oxidizing conditions caused by their presence, at high temperatures, lead to evaporation of ruthenium-106 in the form of ruthenium tetroxide. In addition, toxic waste gases result, which contain nitrous fumes (NO.sub.x) and have a corrosive effect, causing additional problems. The ruthenium evaporation occurs particularly already during concentration of the solutions by distillation or similar methods of concentration. For this reason, the concentration and solidification are generally preceded by a denitration by adding reducing agents such as formic acid, formaldehyde, or sugar, according to which nitrous gases are liberated from the nitric acid and the easily decomposable nitrates. In subsequent condensers and washers, these nitrous gases can be reconverted into nitric acid. The very stable alkali and earth alkali nitrates decompose only at increased oven temperatures during the solidification or vitrification. The thereby acidic corrosive oven waste gases can therefore only be discharged into the atmosphere after costly washing. In addition to this customary denitration, concentration and solidification or vitrification, a method of treating chiefly neutral or alkali solutions of fission products which contain nitrates and nitrites is known. According to this known method, the solution of fission products is mixed with a quantity of urea which is stoichiometric relative to the nitrite and nitrate content of the solution; the solution is heated up, particularly to temperatures of at least 130.degree. C. to 180.degree. C., until the water content is removed. Such a dehydrating denitration in a closed receptacle is not usable with nitric solutions of fission products, because these solutions tend to foam very strongly without a corresponding decomposition. It is an object of the present invention to obtain a good separation of nitric acid from solutions of fission products without evaporation of ruthenium. This object, and other objects and advantages of the present invention, will appear more clearly from the following specification in connection with the accompanying examples and drawing, which is a flow diagram of the apparatus used to carry out the method of the present invention. The method of the present invention is characterized primarily in that the solution is concentrated to a solid content of at least about 15 to 20% in a vacuum at a pressure of at most about 50 mm Hg. It was unexpectedly discovered that an extensive separation of the nitric acid without evaporation of ruthenium could be achieved if the fission product solution was concentrated in a vacuum at the corresponding pertaining low temperatures. With such a vacuum concentration, less than 1 ppm ruthenium is present in the acid condensate, and the separated nitric acid can be recovered with relative ease. The concentrate which is obtained from the vacuum concentration can be temporarily stored. A particularly favorable refinement of the method of removing waste products according to the present invention consists however in a combination of vacuum concentration and subsequent solidification, particularly vitrification, accompanied by the addition of ammonia derivatives, such as calcium cyanamide or especially urea, which react with nitrate along with the formation of reaction products, such as nitrogen, laughing gas (nitrous oxide), carbon monoxide, dioxide, etc, which comprise no nitrogen oxides rich in oxygen, so that the nitrous fumes content of the waste gas remains low. By such a vitrification of the concentrate along with the addition of vitrifiers and, as the case may be, intermediate drying, the occurrence of nitrous gases in the oven waste gas is avoided. Pursuant to the present invention, a considerable simplification of the treatment of solutions of fission products, particularly nitric solutions, is obtained in that the normally customary denitration, concentration, drying, and fusion along with restraint of nitrous gases is replaced by the steps of vacuum concentration accompanied by simple recovery of nitric acid--followed, as the case may be, by drying--and fusing with ammonia derivatives, particularly addition of urea.