Patent Number: 051851040
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of treatment of a high-level radioactive waste generated, for example, from reprocessing of spent nuclear fuels. In particular, it relates to a method for treating a high-level radioactive waste which comprises heating the radioactive waste at a high temperature, separating part of the elements contained in the radioactive waste by utilizing sublimation or boiling of each element in its various chemical forms during the heating step, and recovering a resultant residue as a solidified material. The high-level radioactive waste generated from reprocessing of the spent fuels contains transuranium elements and Tc (technetium) having long half-lives; Cs (cesium) and Sr (strontium) that are noteworthy elements from the aspect of treatment, storage and disposal because they are responsible for the major proportion of heat generation; and valuable platinum group metals such as Ru(ruthenium), Rh(rhodium) and Pd(palladium). It is therefore very important to separate and recover them prior to solidification of the waste, and to utilize them as a radiation source, a heat generation member and a noble metal, from the point of view of effectively utilizing resources. The following three methods are heretofore known as prior art techniques for recovering these elements from the high-level radioactive waste: 1) A solvent extraction method wherein the intended nuclides are separated by using a special solvent from the high-level radioactive waste generated from the reprocessing steps; 2) An ion-exchange method wherein the intended nuclides are separated by using an ion-exchange resin from the high-level radioactive waste generated from the reprocessing steps; and 3) A lead extraction method for platinum group elements wherein lead is added to glass at the time of the glass melting step of a vitrification process to thereby move platinum group elements to molten lead and separate them with the molten lead. However, these prior art techniques described above are not free from the following disadvantages, respectively: 1) Since a new-type solvent is introduced to the reprocessing step in the additional solvent extraction method, the solvent treatment step becomes complicated and efficiency of the main solvent extraction step lowers conseqently. 2) Flammable materials are produced when the ion-exchange resin comes into contact with a nitric acid solution of the radioactive waste. Therefore, the ion-exchange method involves safety problems. 3) The lead extraction method for platinum group elements in the vitrification process can separate the platinum group elements but secondary treatment is necessary in order to extract them from lead. Furthermore, none of these prior art methods can reduce the volume of the high-level radioactive waste at a high rate, whichever method may be employed. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method for treatment of a high-level radioactive waste which solves the problems with the above-described prior art techniques and can separate and recover valuable elements in the radioactive waste in an extremely simple manner. It is another object of the present invention to provide a method of treatment of a high-level radioactive waste which does not generate a secondary waste and can obtain a highly volume-reduced solidified material. According to the present invention, in order to accomplish the above-described objects, there is provided a method of treatment of a high-level radioactive waste comprising heating the radioactive waste at a high temperature to vaporize part of the elements contained in the radioactive waste, and cooling the resultant vapor to collect the elements. In one embodiment of the present invention, the radioactive waste is reduction-heated at a high temperature to vaporize part of the elements contained in the radioactive waste, and the resultant vapor is then cooled to collect the elements. The high-level radioactive waste is ordinarily a nitric acid solution obtained as an extraction residue in the reprocessing step of spent nuclear fuels, and contains almost all of the nuclear fission products and actinides in spent nuclear fuels. In the present invention, the nitric acid solution is heat-treated so as to evaporate the moisture and nitric acid in the solution and to obtain a calcined material, which is further heated at a temperature ranging from about 500.degree. to about 3,000.degree. C. and more preferably, from about 1,000.degree. to about 2,500.degree. C. According to another embodiment of the present invention, in a first stage treatment, those elements which sublimate or boil in the form of oxides are heat-treated at a normal or reduced pressure to vaporize those elements. The resultant vapor is then cooled by a plurality of cooling/collecting units whose temperatures are differently set so as to correspond to sublimation or boiling points of each compound or element, thereby collecting the respective elements separately. In a second stage treatment, the remaining high-level radioactive waste is heated in the presence of a reducing agent such as hydrogen to reduce the radioactive waste, and those elements which sublimate or boil in the form of metal are vaporized. The resultant vapor is then cooled, in the same manner as in the first stage treatment, by the cooling/collecting units whose temperatures are set so as to correspond to sublimation or boiling points of the respective elements, thereby collecting the respective elements separately. Needless to say, those elements which are reduced to metals during heating in the first stage treatment can be separated by sublimation or boiling without reduction in the second stage treatment. A voloxidation method is known as a technique for removing radioactive materials from spent fuels but this method is merely directed to non-metallic elements such as krypton, iodine, tritium and the like. The present invention is directed to metallic elements and not only removes radioactive materials with high boiling points by heating the high-level radioactive waste at a high temperature, but also can remove both Cs and Sr, that are high heat-generation elements and pose problems during disposal, by combining the heat-treatment with the reduction reaction. The resultant residue comprises metals or a mixture of the metals and oxides, and can be recovered as a volume-reduced high-level radioactive solid. Almost all of the elements have boiling points or sublimation points different from those of other elements. Some elements contained in the high-level radioactive waste have a relatively low sublimation point or boiling point in the form of an oxide or metal. For example, the boiling point is 690.degree. C. for metallic cesium, 311.degree. C. for technetium oxide, 765.degree. C. for metallic cadmium and 1,384.degree. C. for metallic strontium. By utilizing the difference in these boiling points, therefore, each valuable element can be separated and recovered by heat-treating the high-level radioactive waste at a high temperature to obtain the oxides thereof or by reducing them by hydrogen or the like to obtain metals, causing their sublimation or boiling, and cooling stepwise the resulting vapor ,mixture at the predetermined temperatures. After the removal of Cs and Sr, the amount of heat generated from the high-level solid waste is reduced to about 10% and therefore the burying density for disposal can be improved drastically. Incidentally, if Cs alone is removed, the amount of heat generation becomes only 50% and a large effect cannot be expected. The boiling points of oxides of Sr are at least 2,430.degree. C. and that of metallic Sr is 1,384.degree. C. as described above. Accordingly, strontium can only be separated by the method of the present invention wherein the heating step is combined with the reduction reaction. Incidentally, vaporization of each element can be effected at a lower temperature if the heating step or the reduction-heating step is carried out under a reduced pressure.