Patent Number: 043448720
Section: description

A vacuum evaporator 1, especially a wiping blade evaporator, possibly a two-step evaporator, is charged at 2 with the solution of fission products, particularly a nitric solution. The solution is concentrated in the evaporator with recovery of acid condensate at 3, which condensate can be returned for fuel dissolving. The concentrate is mixed in the mixing device 4 with vitrifiers (added at 5) and a nitrate decomposing agent, particularly urea (added at 6), and passes either directly or after intermediate drying at 7 into the vitrifying oven 8. The vitrification of the mass along with simultaneous decomposition of nitrates takes place in the oven 8. The vitrified material leaves the oven at 9, while the practically nitrous-free waste gases are liberated at 10. The method proceeds as follows: (1) Vacuum Concentration The separation of the nitric acid from the fission product solution is preferably effected in a wiping blade evaporator, System-Sambay, at about 20 mm Hg pressure (corresponding to 35.degree. to 40.degree. C. vapor temperature). During concentration to 1/10 of the starting volume, a good 40% of the total nitrate quantity is separated out as nitric acid. Preferably, the concentrate residue obtained is rediluted with water (approximately in the ratio of 1:1), and is again concentrated as before, resulting in an increase of the nitric acid separation to a good 70%. Ruthenium was detectable in the acid condensate only in quantities below 1 ppm (decontamination factor: 10.sup.3). Up to a solid content of 25%, the concentrate flows off freely and without forming a crust. The nitrate content is about 9 M/l, of which about 1/3 is difficult to decompose nitrate. The preceding concentration can be utilized to reduce the volume of fresh solutions of fission products which are first to be temporarily stored. As a result, the space which is required and the corrosiveness are reduced. The degree of concentration depends upon the maximum allowable decay energy of the fission products per volume and the chemical characteristics of the solution. However, after the addition of urea and the corresponding vitrifiers, the concentrate can also be supplied directly to a subsequent solidification process, particularly a drying and vitrifying process. To adapt to reliable and practiced technologies, a drying by means of a roller or drum dryer can be provided. The addition of urea can take place prior to the drying or directly before the fusing. A fluid dosing of waste-vitrifier-urea-suspension into the smelting or vitrifying oven is also possible. After being concentrated, the separated-off acid can be returned to the dissolving process. (2) Vitrification, particularly with the addition of urea The fission product concentrate obtained pursuant to step (1) is, in conformity with the respective formula, mixed with vitrifiers as well as with a quantity of urea which, according to the total nitrate content and the ratio of nitrate to free acid, is about 20 to 300% of the nitrate content. Determinative is the nitrous fumes content of the melting off gas, which should be below 3% by volume NO.sub.x. The thus obtained mixtures can readily be dehydrated, for example, with a roller dryer, and yield a less powdery, more granular product, which can be easily and freely melted in a melting device to a clean glass mass. EXAMPLE 12 l fission product solution (with about 1.5 M/l nitrate and 1.6% fission product oxide) with 17.9 Mol nitrate were concentrated in a wiping blade evaporator at 40 mm Hg pressure and a corresponding vapor temperature of 44.degree. C. 1.083 l concentrate (with 9.35 M/l nitrate and 11.8% fission product oxide) as well as 10.4 l distillate (with an acid content of 0.7 M/l) was obtained. The degree of concentration (12 l:1.083 l) was about 11. The nitrate balance is computed as follows: From the starting amount of 17.9 Moles nitrate 10.1 Moles nitrate remained in the concentrate corresponding to a reduction of about 44%; 7.28 Moles nitrate are found in the distillate. The concentrate was rediluted with water (1:1), and was again evaporated under the same conditions, yielding 520 ml concentrate (with 9.82 M/l nitrate and 24.6% fission product oxide) as well as 1460 ml distillate (with an acid content of 2.6 M/l) at a degree of concentration (altogether) of 23. The nitrate balance of this two-step treatment is as follows: ______________________________________ Starting solution Concentrate 2 Distillate 1 and 2 ______________________________________ 17.9 M nitrate 5.1 M nitrate 7.28 + 3.80 M = 11.1 M nitrate ______________________________________ i.e., a reduction of 71.5% of the starting nitrate quantity. 100 g fission product concentrate (with 9.36 M/l nitrate and 20% fission product oxide) was, with the addition of water, mixed with 38 g silicic acid (SiO.sub.2), 30 g borax, 11 g boron oxide, and 15 g calcium oxide and, in the ratio nitrate:urea=3:1, was mixed with 15 g urea. The mixture was dried and continuously dosed into a melting crucible which was at 1100.degree. C. The collected waste gas contained 3% by volume NO.sub.x, 10% CO.sub.2 and 10.8% CO, as well as unknown quantities of N.sub.2 and N.sub.2 O from the reaction between nitrate and urea and was nearly colorless. The fused glass was yellowish-gray, ceramic-like, and "homogeneous" (i.e. in itself uniform). The advantages of the method of the present invention, which were in part already expanded upon above, comprise a reduction of the method steps, an extensive recovery of acid, a lower usage of reduction agents, and a simplification of the waste gas treatment. The present invention is, of course, in no way restricted to the specific disclosure of the specification, examples, and drawing, but also encompasses any modifications within the scope of the appended claims.