Patent Number: 050826034
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION A high-level radioactive waste is usually in the form of a nitric acid solution thereof obtained as an extraction residue in the step of reprocessing spent fuels and contains almost all of fission products in the spent fuels. In the present invention, as shown in FIG. 1, the high-level radioactive waste is heated to evaporate water and nitric acid, thereby obtaining a calcined material or a calcination product. Boron or a boron compound is added to the calcined material, and the resultant mixture is melted by heating in a reduction condition at a high temperature of 1000.degree. C. or above. This causes platinum group elements in the calcined material to alloy with boron, and a layer of the resultant platinum group alloys settles down and therefore can be separated from a layer of residual oxides. Examples of the boron compound to be added to the calcined material include sodium boron hydride, boron nitride and boron carbide. It is a matter of course that the boron compound is not limited to those only. In particular, boron nitride is most suitable because it is easy to handle and low in the cost. The use of boron or a boron compound in an amount of 10% by weight or less in terms of boron as a simple substance will suffice. The addition of boron or a boron compound in a larger amount brings about an increase in the amount of waste and therefore is unfavorable. The amount is preferably 5% or less. The aim of the present invention is to lower the melting point of the platinum group alloy. Although the formation of a eutectic crystal is most desirable for this purpose, an effect can be attained even when boron is added in an amount of 0.5%. Therefore, the amount of addition of boron may be 0.5% or more, preferably 1% or more. The oxidation-reduction state of the calcined material of the high-level radioactive waste in the heat treatment is controlled by the temperature, atmosphere and addition of a reducing agent. The heating temperature is 1000.degree. C. or above. When the temperature is below 1000.degree. C., Ru and Mo cannot be reduced to metallic state although Pd and Rh are reduced. The temperature is thus preferably 1500.degree. C. or above. Since Ru-, Pd-, Rh-, Mo- and B-base alloys melt at 2000.degree. C. or below, there is no need to employ a temperature above 2000.degree. C. The control of the atmosphere is conducted for accelerating the reduction reaction. In the present invention, the reaction is preferably carried out in an atmosphere of air having a reduced oxygen content, nitrogen or argon. A reducing agent as well is used for accelerating the reduction reaction. Gaseous reducing agents such as hydrogen and carbon monoxide, reducing agents such as carbon which gasify in a redox reaction, and reducing agents such as alkaline earth metals and rare earth elements which are elements constitute the residual oxide layer are used for the purpose of avoiding the occurrence of a secondary waste. It is also possible to use as a reducing agent, substances such as aluminum, which do not have any adverse effect on the residual oxide phase even when it remains as an oxide. The above-described temperature, atmosphere and reducing agent are properly combined with one another depending upon the reaction conditions. Fission products in spent fuels are broadly classified into (1) metallic elements, (2) non-metallic elements, and (3) rare earth elements. Examples of the metallic elements include alkaline earth metals, transition metals such as Mo, and platinum group elements Most of the non-metallic elements described in the above item (2) and the alkaline earth metals in the metallic elements described in the above item (1) are removed by heating the high-level radioactive waste. Examples thereof include Sb, Te, Cs, and Rb. As a result, in the case of spent fuels of 45000MWD/MTU in the burnup and five years in the cooling time, major components of the calcined material except for elements having a content of 100g/MTU or less are as follows: Alkaline earth metals (Sr, Ba) ..... 3.3Kg/MTU: 8.7% by weight PA1 Transition metals (Zr, Mo, Tc) ..... 10.5Kg/MTU: 27.9% by weight PA1 Platinum group elements (Ru, Rh, Pd) ..... 5.4Kg/ MTU: 14.3% by weight PA1 Rare earth elements (Y, La, Ce, etc.) ..... 18.5Kg/MTU: 49.1% by weight PA1 Total ..... 37.7Kg/MTU The heat-melting of this calcined material provides a high-level radioactive residual solidified waste having a higher degree of volume reduction than that of a usual solidification product (fission products content: about 10% by weight) of the high-level radioactive waste. It is to be noted that in the case of a vitrification product, the weight thereof is 10 times that of the fission products and the volume thereof is several hundreds of liters per ton of spent fuel, while in the present invention the volume of the volume-reduced residual solidified waste is several tens of liters. Further, in the present invention, platinum group elements are separated and recovered. As is known, the platinum group element has a small free energy of formation of its oxide and is reduced into a metallic state when heated. The melting point of the platinum group element is 1554.degree. C. for Pd, 1963.degree. C. for Rh, and 2254.degree. C. for Ru. Ru and Rh do not form a solid solution perfectly because they are different from each other in the crystal form. Pd does not form an alloy having a eutectic point with Rh and Ru. Therefore, in the platinum group element and its alloy system, the melting point often exceeds 2000.degree. C., which makes it difficult to separate the platinum group element alone or in the form of an alloy from the residual oxides through melting of the calcined material Namely, even when they can be separated as a phase, a very high melting temperature is required for separating the two layers from each other in the molten state Mo in the calcined material has a relatively small free energy of formation of an oxide and forms an alloy having a low melting point with the platinum group elements. However, the content of Mo and the platinum group elements in the fission products is determined by the burnup of spent fuels. Therefore, it is difficult to realize a composition having the lowest melting point in the respective alloy systems comprising four components. In the heating step of the present invention, boron or a boron compound is added to the calcined material This causes alloys of Mo or the platinum group elements with boron to be formed, and these alloys melt at a low temperature. In general, numerous elements (M) combine with boron (B) to form an M/B or 2M/B compound. This compound forms a eutectic crystal together with the element (M). The melting point of the eutectic crystal is much lower than those of the original elements. Since the atomic weight of boron is as small as about 11, the weight content of boron in a eutection point with other element is 5% at the highest. Therefore, the amount of boron to be added for the purpose of lowering the melting temperature of the platinum group elements and Mo may be very small. Thus, the platinum group elements and Mo are reduced at a temperature of 2000.degree. C. or below into an easily meltable form, so that a layer of the molten alloys is formed. Since the molten alloy layer separates from the residual oxide layer, the platinum group elements can be recovered and the residual oxide layer becomes a high-level radioactive solidified waste of a high degree of volume reduction. FIG. 2 is a schematic view of one embodiment of an apparatus for practicing the method of the present invention. This apparatus exemplifies a bottom flow type apparatus. A calcined high-level radioactive waste and boron or a boron compound are placed in a melting container 10. The calcined waste is reduced under heating and separated into a layer 12 of platinum group element alloys having a higher specific gravity and a layer 14 of residual oxides having a smaller specific gravity. The platinum group element alloy layer 12 and the oxide layer 14 successively flow down through a flow-down nozzle 16 to be poured into separate containers for solidification. FIG. 3 is a schematic view of another embodiment of an apparatus used for practicing the method of the present invention. This apparatus exemplifies an overflow type apparatus. A calcined high-level radioactive waste and boron or a boron compound are placed in the central part of a melting container 20 to be heat melted. A layer 12 of platinum group element alloys located in the lower part and a layer 14 of residual oxides located in the upper part respectively pass through passages 22, 24, flow down through flow-down nozzles 26 and 28, and are poured into separate containers for solidification. The construction of the apparatus is not limited to the two types above-described and may be a compromise between the bottom flow type and the overlow type. Namely, the platinum group element alloy layer is flowed down from the bottom and poured into one container for solidification, while the oxide layer is flowed down by overflow and poured into another container for solidification. For the calcination of the high-level radioactive waste, a rotary kiln system, a microwave heating system, etc., which are under research in relation to vitrification, can be used. For the heat treatment of the calcined waste, a heater system, a direct energization system, a high-frequency heating system, etc., may be employed. Particular Experimental Examples will now be described hereinbelow. EXPERIMENTAL EXAMPLE 1 A composition of fission products in a spent fuel of 45000MWD/MTU in the burnup and 5 years in the cooling time was calculated by using ORIGEN code to prepare a simulated waste solution of the corresponding high-level radioactive waste solution. The simulated waste solution was heated to 600.degree. C. to prepare a calcined material. A mixture of 45g of the calcined material and 5g of boron nitride (BN) were placed in a crucible and heat-treated in an argon atmosphere at 1800.degree. C. for 1 hr. The contents of the container were observed after cooling to reveal that the upper surface is smooth, indicating that the mixture had melted. The crucible was broken and the contents were taken out. The contents were separated into two phases, and a metal mass was present in the bottom and could easily be separated from the residual portion. The metal mass was analyzed with an X-ray micro-analyzer (EPMA). As a result, Ru, Rh, Pd, Mo and B were detected. The oxide as the residue was subjected to measurement of the leaching rate in water according to JIS R3502. The leaching rate was 8.times.10.sup.-5 g/cm.sup.2 d and substantially the same as that of the vitrification product. Thus it has been confirmed that the residue has a chemical durability sufficient as a high-level radioactive solid waste. EXPERIMENTAL EXAMPLE 2 The simulated waste solution was treated in the same manner as that of Experimental Example 1, except that the amount of addition of boron nitride was change to 2.5g. The results of observation after the treatment were the same as those of Experimental Example 1. COMPARATIVE EXAMPLE An experiment was conducted under the same conditions as those of Experimental Example 1, except that no boron nitride was added. The contents were observed after cooling to reveal that they were in a baked state and no evidence of melting was observed. The mass could easily be taken out of the crucible but did not separate into two phases, so that no metallic mass could be formed. As described above, the method of the present invention comprises adding boron or a boron compound to a calcined high-level radioactive waste and heat-melting the mixture in a reduction condition at a high temperature of 1000.degree. C. or above. This method makes it possible to separate and recover useful platinum group elements, simplify the treatment process and reduce the size of an apparatus for the treatment. Further, since the resulting residue of oxides is solidified as it is, the solidification is accompanied by such a remarkable volume reduction that the volume is below one-tenth of that of the conventional vitrification. This enables the cost of storage and disposal of the high-level radioactive waste to be remarkably reduced. In the present invention, the heat-treatment can be conducted at a temperature of 2000.degree. C. or below because boron or a boron compound is added to the waste. Therefore, it becomes possible to adopt a heat-treatment wherein heating is conducted with a heater without the necessity for using a special heating system (e.g., electron beam heating, plasma heating, etc.), and the material for the melting furnace may be zirconia, etc. without the necessity for using special high-melting materials (e.g., thorium oxide), which enables the facilities for treatment to be easily constructed at a low cost.