Engineering and manufacture of oblong-shaped metallic melters for enhancing vitrification throughputs

Most countries have taken the vitrification route for managing High Level Waste (HLW). Depending on composition of site-specific HLW, suitable glass forming oxides are added to it in a melting furnace designed for converting the mixture to a matrix of borosilicate glass. After thorough homogenization, the product is poured into stainless steel canisters as an inert solidified form. Canisters are seal welded and 2-3 such containers are then packed and sealed in another bigger container for providing an additional engineered barrier. This product form is considered safe for long-term storage. The concurrent vitrification technologies employ either induction-heated metallic melters or joule-heated direct resistance-heated ceramic melters. In India, both technologies have been adopted. While older facilities in Trombay and Tarapur employ metallic melters, a retrofitted facility at Tarapur and the new plant at Kalpakkam are based on ceramic melter operations. Compared to the ceramic melters, the processing capacity of metallic melters is significantly lower. However, replacement of melter pot, decontamination and decommissioning are much simpler in case of metallic melters. The throughput of existing metallic melters can be enhanced by replacing the cylindrical process pot with an oblong or oval shaped pot. Boil up rates of up to three times can be achieved by using oblong shaped melters. However, taking recourse to this option presents difficulties in fabricating the pot as also in designing the appropriate electrical power supply for the system. Though facilities in Sellafield in UK and Marcoule in France are equipped with elliptical melters, finer details on such designs are not available. Therefore the work of designing oblong-shaped melters was pursued afresh in India as a development activity. One such pot suitable for pilot-scale operation has been manufactured with an appropriately designed power supply for uniformly heating the oblong shaped process pot. Since the development is aimed at using the oblong metallic melters for future vitrification plants, a full-scale facility is ready for inactive demonstration in Trombay. The facility employs oval shaped 15 mm thick Inconel-690 process pot and susceptor surrounded with the new power supply configuration. It is expected that the system will increase the processing throughput of the existing metallic melter on account of the enhanced boil-up rate. It is also expected that the commonly encountered conditions of frothing and foaming could be effectively minimized. This paper presents the efforts of the designers in overcoming fabrication difficulties encountered in manufacturing the process pot and susceptor assembly. The paper also brings out how an appropriate design of power supply system for non-regular induction heating requirement was evolved. (authors)