Patent Number: 051851040
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION FIG. 1 is a conceptual view of an apparatus used for practising the method of the present invention. The apparatus is equipped with a heat-treatment unit 10 and a plurality of cooling/collecting units 12a, . . . , 12n connected to the former. The heat-treatment unit 10 includes a heating vessel 14 and a heat-generation member 16. A feed port 18 for a reducing agent is provided at the upper part of the heating vessel 14 and a vapor passage 20 is interposed between the vessel 14 and the cooling/collecting unit 12a. A heat-generation and insulating member 22 is fitted around the vapor passage 20. The heating vessel 14 may be made of a refractory metal such as tungsten or a ceramic material such as alumina or high chromium refractory brick, depending on the heat-treatment temperatures. Besides external heating by supplying power to the heat generation member 16 shown in FIG. 1, high-frequency heating, microwave heating, heating by directly flowing electric current through the high-level radioactive waste or the like may be employed as the heating method. It is also important to utilize effectively the heating due to the decay heat of the high-level radioactive waste to be treated. The high-level radioactive waste 24 to be treated is charged into the heating vessel 14 and heated. This radioactive waste 24 is, for example, a calcined material obtained by heating a nitric acid solution generated from the reprocessing step of the spent nuclear fuels to evaporate the moisture and nitric acid. The heat-treatment in the heating vessel can of course be carried out continuously from the state of the nitric acid solution. The calcined material is heated to about 500.degree. C. to about 3,000.degree. C., more preferably to about 1,000.degree. C. to about 2,500.degree. C. The elements contained in the calcined material are vaporized due to heating at their sublimation or boiling points in accordance with their chemical forms and are sent to the cooling/collecting units 12a, . . . , 12n through the vapor passage 20. Each of these elements that are vaporized is individually cooled and collected by each of cooling/collecting units 12a, . . . , 12n whose temperature is controlled so as to correspond to a sublimation or boiling point of each compound or element. Though heating may be carried out at a normal pressure, it is preferably carried out under a reduced pressure from the aspect of energy efficiency because the sublimation or boiling point drops and heat-treatment can be made at a lower temperature. In a preferred embodiment of the present invention, those elements which sublimate or boil in the form of oxides are heat-treated under a normal or reduced pressure and separated in the first stage treatment. The remaining high-level radioactive material is then heated in the second stage treatment while a reducing agent is introduced through the feed port 18 to reduce the radioactive material and to separate those elements which sublimate or boil in the form of metal. Finally, the resultant residue inside the heating vessel 14 is recovered. Hydrogen gas, carbon, carbon monoxide or the like may be used as the reducing agent to be introduced through the feed port 18. The discharge method of the residual molten material 25 from the heating vessel 14 may be of a bottom flow system such as shown in FIG. 2 or of an overflow system such as shown in FIG. 3. In either case, the residual molten material 25 is discharged into a vessel 26 for solidification and is left for cooling to obtain a highly volume-reduced solidified material. EXAMPLE 1 A simulated nitric acid solution of a high-level radioactive waste in which radioactive nuclides were simulated by stable elements was prepared and was subjected to evaporation treatment to obtain a calcined material. The calcined material was then heated and reduced at a high temperature of 1,000.degree. C. for 4 hours in a mixed gas stream of H.sub.2 -He(1:4). In the interim, Te, Cd, Se, Cs and Na were deposited in the cooling/collecting units and could be collected. The respective temperatures in the cooling/collecting units with respect to these elements were 200.degree. to 600.degree. C. for Te, 200.degree. to 300.degree. C. for Cd, about 600.degree. C. for Se, 900.degree. to 1,000.degree. C. for Cs and 600.degree. to 1,000.degree. C. for Na. EXAMPLE 2 The calcined material obtained after the heating and reducing treatment at the high temperature in Example 1 was further heat-treated at 850.degree. to 1,050.degree. C. in a vacuum. It was confirmed that Pd and Ru were deposited in the cooling/collecting units. As is apparent from the foregoing, according to the method of the present invention, the high-level radioactive waste is heated, or reduction-heated, at a temperature to vaporize part of the elements contained in the radioactive waste and the resultant vapor was separated and collected. Therefore, in comparison with the prior art methods described hereinbefore, the method of the present invention has simplified treating steps, and does not need to add any special reagent or ion-exchange resin in the subsequent reprocessing or solidification step. Furthermore, since the collected elements are solids in the form of oxides or metals, they can be used as radiation sources or valuable metals, and can be subjected to transmutation without the need for complicated secondary treatment. In addition, the solidified material obtained by the present invention hardly contains additives other than the nuclear fission products and actinides and has an extremely smaller occupying volume for storage and disposal than the conventional solidified materials and can drastically reduce the costs for storage and disposal. The solidified material can preferably be used as a radiation source for nuclear transformation by neutron irradiation, since its volume is small and the irradiation efficiency is high. Although the present invention has been described with reference to the preferred embodiments thereof, many modifications and alterations may be made within the scope of the appended claims.