Patent Number: 042973040
Section: description

The invention will now be explained by way of the examples which follow without, however, being limited to these examples. EXAMPLE 1 The chemical composition of a highly active nitric acid waste solution which is obtained during reprocessing of spent fuel elements in a light water nuclear reactor where 33,000 MWd/t fuel are burnt was simulated in its main components by chemically similar inactive isotopes. The nitric acid waste solution was denitrated with formic acid such that a pH of 2.5 resulted. The pH of the solution was then adjusted by the addition of 1 M sodium liquor, to a value of 6 and was concentrated by way of distillation. After this pretreatment, the solution or suspension, respectively, had the following chemical composition: ______________________________________ (a) water content: 700 g H.sub.2 O (b) residual nitrate content: 109 g NO.sub.3.sup.- (c) residue after heating: 91.2 g Gd.sub.2 O.sub.3 35.0 g ZrO.sub.2 36.5 g MoO.sub.3 23.4 g Na.sub.2 O 18.8 g BaO 28.5 g Ag 8.0 g MnO.sub.2 4.0 g Te 12.8 g Pb.sub.3 O.sub.4 20.8 g Fe.sub.3 O.sub.4 3.7 g Cr.sub.2 O.sub.3 g NiO 285.9 g Total ______________________________________ This solution or suspension, respectively was kneaded in a kneading vessel together with 981 grams of a mixture of portland cement and Hirschau kaolin (weight ratio 1:8) to form a dough. By pressing the doughy mixture through a tube, molded bodies were produced having a diameter of 25 mm and a height of about 20 mm. Further, cylindrical bodies having a diameter of 80 mm and a height of 80 mm were shaped from the doughy mixture in polyethylene beakers. These latter bodies could be removed from the beakers after only 3 days due to shrinkage of the mass. All of the molded bodies were dried in air for 20 to 30 days then rinsed with water and thereafter dried, calcined and sintered in a furnace at increasing temperatures. The heating schedule for the furnace is shown in the table below. ______________________________________ Temperature Time (.degree.C.) (Hours) ______________________________________ 20-150 15 150-800 30 800-1150 5 1150-1280 10 ______________________________________ The sintered end product exhibited an elevated hardness of 6 to 7 according to the Mohs scale, and poor solubility in water. The water solubility at room temperature is less than 10.sup.-6 g of the product with reference to 1 cm.sup.2 of surface per day. The table below gives the chemical composition of the product. ______________________________________ Components Weight - % ______________________________________ SiO.sub.2 38.8 Al.sub.2 O.sub.3 26.8 CaO 6.2 Fe.sub.3 O.sub.4 0.9 TiO.sub.2 0.3 MgO 0.7 Na.sub.2 O 0.1 K.sub.2 O 1.9 Fission product oxides 24.3 (See heat treatment residue (c)) ______________________________________ The stoichiometry of this sintered body end product corresponds approximately to that of anorthite or nepheline, respectively, which are known as very stable natural minerals. EXAMPLE 2 Medium activity waste solutions which are the result of the reprocessing of nuclear fuels contain up to 90% sodium nitrate as salt ballast. To simulate this category of waste, a sodium nitrate solution was made into a dough with a cement/kaolin mixture (weight ratio of 1:10). The chemical compositions of the dough was as follows: ______________________________________ simulated waste solution: 184 g NaNO.sub.3 300 g Water portland cement: 50 g Geisenheim kaolin 500 g ______________________________________ As in Example 1, molded bodies were again produced by extruding and molding. Air drying and rinsing with water were effected as in Example 1. The heating schedule for the drying, calcination and sintering steps is given in the table below: ______________________________________ Temperature Time (.degree.C.) (Hours) ______________________________________ 20-150 12 150-400 20 400-450 25 450-800 15 800-1150 10 1150-1200 10 ______________________________________ The sintered end product has a hardness of 5 to 6 on the Mohs scale. The water solubility at room temperature is about 10.sup.-6 g of the product with reference to 1 cm.sup.2 of surface per day. The chemical composition of the sintered end product is shown in the table below: ______________________________________ Component Weight - % ______________________________________ SiO.sub.2 45.8 Al.sub.2 O.sub.3 31.9 CaO 5.9 Fe.sub.2 O.sub.3 1.0 TiO.sub.2 0.4 MgO 0.4 K.sub.2 O 2.3 Na.sub.2 O (From sodium nitrate solution) 12.3 ______________________________________ The stoichiometry of this sintered body end product corresponds approximately to that of nepheline which is known as a very stable natural mineral. EXAMPLE 3 Actinide concentrates which are formed as radioactive wastes during the manufacture of plutonium containing fuel elements are either evaporated solutions or combustion ashes obtained from the combustion of organic materials. They contain as radioactive components relatively large quantities of plutonium and americium which is produced during the radioactive decay of the relatively short-lived plutonium isotope Pu.sup.241. These actinide concentrates can be easily bound into a ceramic matrix according to the present invention since the chemical nature of the waste components themselves comes close to the heat treatment residues of the high activity waste solutions listed in Example 1. To simulate this category of waste, a suspension of 2.94 g americium dioxide powder in 7 g water was mixed into a dough with a mixture of 10 g portland cement and Hirschau kaolin (weight ratio 1:10). The doughy mass was pressed through a polyethylene tube so that a cylindrical molded body resulted which had a diameter of 20 mm and a height of 30 mm. The molded body was dried for 10 days at room temperature. In the same manner, a molded body was produced from 7 g of water and 10 g of a mixture of portland cement and Hirschau kaolin, (weight ratio 1:10) but without americium dioxide and likewise dried for 10 days at room temperature. Both molded bodies exhibited the same shrinkage of 28.+-.2% after drying compared to the starting volume during manufacture. This proves that radiolysis gas development and decomposition heat are without influence on the manufacturing process. The molded body containing AmO.sub.2 was dried in a furnace at increasing temperatures and sintered according to the heating program below: ______________________________________ Temperature Time (.degree.C.) (Hours) ______________________________________ 20-150 8 150-800 24 800-1150 10 1150-1300 10 ______________________________________ After sintering, a product resulted having a stoichiometry corresponding to anorthite or nepheline. The weight loss of the sintered product during leaching with water at room temperature is less than 10.sup.-7 g per cm.sup.2 surface. per day. The chemical composition of the sintered product is shown in the table below: ______________________________________ Component Weight - % ______________________________________ SiO.sub.2 39.2 Al.sub.2 O.sub.3 27.3 CaO 5.0 Fe.sub.2 O.sub.3 0.8 TiO.sub.2 0.3 MgO 0.4 Na.sub.2 O 0.1 K.sub.2 O 1.9 AmO.sub.2 25.0 ______________________________________ The specific alpha activity of the sintered body is 715 mCi/g, the specific decay heat is about 22 mW/g. EXAMPLE 4 In addition to dry combustion with oxygen from the air, the organic radioactive wastes from the production of plutonium containing fuel elements can also be concentrated by wet combustion methods. One of these methods is based on the carbonization of organic wastes in concentrated sulfuric acid at temperatures above 200.degree. C. and subsequent oxidation of the carbon with chemical oxidation means such as nitric acid. This produces combustion residues having high sulfate contents which, mainly after neutralization with sodium liquor, contain large amounts of sodium sulfate but also sodium chloride from the combustion of polyvinyl chloride. In Example 2 it was shown that sodium nitrate solutions can be solidified into a sintered body to form a chemical compound which stoichiometrically corresponds to the natural mineral nepheline. It is, moreover, possible to solidify sodium sulfate and sodium chloride containing sodium nitrate solutions in the same manner where the absorption capability of nepheline for sodium sulfate is limited to 14 percent by weight and for sodium chloride to 12 percent by weight. The crystallic phases formed thereby correspond to the natural stable minerals noselite which contains sodium sulfate and sodalite which contains sodium chloride. EXAMPLE 5 100 ml of a solution containing 5 weight-% sodium sulfate were mixed with 180 g of kaolin with and without addition of 4 weight-% BaO with respect to the kneaded mass and solidified to a sintered body. It was qualitatively demonstrated by condensing the foam evolved upon sintering that the BaO-containing sample had a very low release of sulfate with respect to the reference sample. EXAMPLE 6 100 ml of a solution containing 10 weight-% cesium nitrate were kneaded with 200 g of kaolin. To half of this batch, 10 g of TiO.sub.2 -powder were additionally added. Both samples were solidified to sintered bodies in the same manner. The foam evolved during the sintering process was condensed and analyzed for its cesium content. The TiO.sub.2 containing product had a cesium volatility of less than two orders of magnitude lower than the reference sample. It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.