Examples of processing problematic waste and material. A-5. High temperature processing of asbestos waste

In direct arc heating melters the relatively high temperature of the process (approx.1400 degree centigrade) leads to accelerated corrosion of the furnace lining and electrodes and, as a consequence, to a reduction of the inter-repair time of such facilities. Application of induction crucible melting furnaces is limited by the crucible campaign: about 30 fuses. Owing to the finite lifetime of the melters, both technologies are characterized by the large volumes of difficult to treat secondary waste in the form of radioactively contaminated cap moulds and ceramic material. In order to prolong the operating time of the melters and to diminish the volumes of secondary waste, it is necessary to decrease the temperature of the process. This can be done, for example, by mixing the initial material with FeO (10-25% mass) or with the products of pressurized water reactor liquid radioactive waste calcinations, which contain oxides of potassium and boron (20-40% mass). This approach enables the operator to decrease the temperature of the melting process to about 200 degree centigrade. However, it complicates the technological scheme of the process and leads to an increase in the volume of the final waste form. It is considered that some disadvantages of the methods described above can be avoided by employing an induction heated, water cooled skull melter made of stainless steel ('cold crucible') or an electroslag skull furnace with direct resistance heating. At present, equipment commercially produced in the Russian Federation provides an opportunity for manufacturing cold crucibles with an output of up to 70 kg (melt)/h or 1 m3 (HIM)/h. However, the theoretical efficiency of such melters is up to 500 kg(melt)/h. A simplified version of an electrothermal facility for HIM melting has been developed and tested by the All-Russia Design and Research Institute for Integrated Power Technology (VNIPIET). This is an electroslag skull furnace with direct resistance heating with submersible electrodes. A standard metallic drum is used as a water cooled crucible and, concurrently, as the waste package. A thin layer of slag skull, generated on the interior surface of the crucible, guards it against burning and diminishes the heat loss. This experimental facility used a 160 kW-50 Hz heater, a 200 L drum as the crucible container and various types of electrode - graphite and metallic tubes with various fillers. It has been shown that the paint on the interior surface of the crucible under the skull layer is preserved in its initial state, which indicates the normal temperature regime of the furnace. The obvious advantages of such melters are their simplicity, their low cost, their small dimensions, their high specific output, the small volume of the secondary waste and the broad range of permissible temperatures. The latter advantage enables, in principle, the operator to employ the facility for conditioning (along with HIM) radioactively contaminated concrete, asbestos, brick, building debris, ash, furnace black and metals generated in decommissioning activities. At present, pilot facilities with three melters of 500 kW total power are under construction both in fixed and mobile variants