Patent Number: 055641024
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 illustrates an example of an apparatus for practicing the method according to the present invention. A cold-crucible induction melting apparatus 10 has a slit-divided water-cooling type melting furnace 12 of copper disposed on the inner side of a water-cooled high-frequency coil 14. An object material to be melted 16 (radioactive liquid waste and glass material) and a conductor (silicon carbide rod 18 in this embodiment) the melting point of which is higher than that of the glass material are inserted into this melting furnace 12. The cooling water 20 is then circulated in the apparatus to cool the same, and a high-frequency current is supplied from a high-frequency power source board 22 to the high-frequency coil 14. Consequently, the silicon carbide rod 18 is heated first, and the surrounding object material to be melted 16 is then heated indirectly with the heat generated by the rod, so that the portion of the material 16 around the rod is put in a molten state. The molten glass material has a conductivity. After the glass material has then become able to continue to be in a molten state owing to the heat generated by itself, the silicon carbide rod 18 is withdrawn. The portion of the glass material which is in a molten state is then enlarged by keeping supplying the current to the high-frequency coil 14, until the whole of this material is melted completely. According to this method, the melting furnace 12 corresponding to the conventional refractory furnace material and crucible is cooled with water, and the surface of the molten glass material contacting the melting furnace 12 turns into a solid layer (skull) due to cooling. Therefore, the molten material does not directly contact the inner surface of the furnace, so that the high-temperature corrosion of the melting furnace 12 does not occur. Since the melting furnace is cooled, the melting operation is not restricted by the heat resisting temperature thereof, and an object material can be melted at an arbitrary temperature with required electric power supplied thereto. The molten material thus obtained is then poured into a canister (stainless steel container), in which it turns into vitrified waste. The general constitution of the waste treatment process to which the method according to the present invention is applied is shown in FIG. 2. A high level radioactive liquid waste generated from a reprocessing plant is subjected to a pretreatment, such as condensation or composition regulation in a reception-pretreatment step. The pretreated high level radioactive liquid waste and a glass material are melted in a melting step according to the present invention in which the cold-crucible induction melting technique is utilized. An offgas generated in this step is processed in an offgas processing system. After the melting step has been carried out, the canister in which vitrified waste is packed is capped and welded in a vitrified waste handling step so as to clean the outer side of the canister. The canister with the vitrified waste is then inspected and stored in a vitrified waste storage facility. An example of an experiment using simulated glass and the results of the experiment will now be described. An apparatus used has construction shown in FIG. 1. A melting furnace is constructed so as to have an inner diameter of 100 mm and a depth of 150 mm and so as to be divided into ten segments. A high-frequency coil has an outer diameter of about 170 mm, a height of about 100 mm and 7 turns. The frequency of a high-frequency power source is 4 MHz. A melting test was conducted by using simulated glass cullet (having a particle diameter of not more than 2 mm), the composition of which was shown in Table 1, as an object material to be melted. First, 600 g glass sample was inserted into the melting furnace. A hollow tubular silicon carbide rod of 30 mm in outer diameter and 20 mm in inner diameter was then inserted into the melting furnace to a depth of 90 mm measured from an upper surface thereof. After a plate voltage of 5 kV being applied, it was increased at a rate of 1 kV/2 minutes and set to 8 kV. When a cathode current then attained 4.5 A, the silicon carbide rod was withdrawn to ascertain that even the glass alone continued to be induction heated. When a similar operation of the apparatus was carried out with 1200 g of glass sample inserted therein, it was also ascertained that the glass kept being induction heated by itself. The melting temperature after the starting of the melting of the glass could be set freely in the range of 1100.degree. to 1600.degree. C. by regulating the plate voltage. These facts proved that glass of 1200 g at most could be melted completely under the conditions of 1100.degree. to 1600.degree. C. in this test. TABLE 1 ______________________________________ Components Composition (wt. %) ______________________________________ Glass SiO.sub.2 46.7 Additives B.sub.2 O.sub.3 14.3 Al.sub.2 O.sub.3 5.0 Li.sub.2 O 3.0 CaO 3.0 ZnO 3.0 Waste Na.sub.2 O 9.6 P.sub.2 O.sub.5 0.3 Fe.sub.2 O.sub.3 1.9 NiO 0.5 Cr.sub.2 O.sub.3 0.5 Oxide of F. P. 9.8 Oxide of actinides 2.4 Total 100.0 ______________________________________ According to the present invention, an unmelted solid layer called a skull is formed between a molten material and a melting furnace as mentioned above, and the molten material does not directly contact in a molted state the structural materials (refractories) of the melting apparatus, so that the high-temperature corrosion of the structural materials of the apparatus does not occur. Therefore, the lifetime of the melting furnace is prolonged, and the amount of generation of secondary waste can be reduced. In the conventional melting method, the heat resisting temperature of the structural materials of the melting apparatus is an upper limit of the operational temperature of the apparatus. On the contrary, in the present invention, the object material to be melted is directly induction heated, and the structural materials of the apparatus are water-cooled. Therefore, such a limitation is not placed on the operational temperature, so that the glass can be melted at a high temperature.