Table I Comparison of Process Variables for Ceramic and Cold
Crucible Melters.
Igor Sobolev, Serguei Dmitriev, Fedor Lifanov, and Vadim
Tarasov
RADON - Moscow Scientific and Industrial Firm
Robert Judovits and James Mayberry
Foster Wheeler
Environmental Corporation.
ABSTRACT
Vitrifying radioactive waste is an effective technology for immobilizing radioactive constituents and converting the waste to a manageable form. Conventional vitrification systems employ melters with internal electrodes and refractory linings. A new system, developed by RADON of Moscow, uses inductive-heated melters, eliminating the need for the refractory and internal electrodes. This "cold crucible" vitrification process allows for higher melt temperatures without the concern of refractory and electrode corrosion. The higher melt temperatures means that wastes can be incorporated into a variety of glass and crystalline matrices. This advanced technology is capable of solving many of the U.S. Department of Energy's waste management challenges.
INTRODUCTION
One of the challenges facing the U.S. Department of Energy (DOE) is to manage the backlog of waste generated by the nuclear weapons complex. The key component of a waste management solution is a technically feasible process that is cost effective, while providing the necessary isolation of the waste constituents from the environment. The DOE has turned to vitrification as one solution to its waste management needs.
Conventional vitrification, which employs Joule-heated ceramic melters, provides a waste form that is chemically durable and radioactively stable, providing long-term isolation of the waste constituents from the environment. Unfortunately, these vitrification systems tend to be large and the immersed electrodes and ceramic refractory linings prone to corrosion. These drawbacks make vitrification costly and, for some waste forms, not feasible.
The RADON - Moscow Scientific and Industrial Firm has developed a vitrification system that overcomes the drawbacks of conventional systems. This technology employs inductively-heated, or "cold crucible", melters, which are small-sized units capable of long-term operations at very high temperatures without corrosion.
COLD CRUCIBLE TECHNOLOGY
The key element behind the success of cold crucible vitrification is the inductively-heated melter, or crucible. The crucible is manufactured from stainless steel tubes surrounded by an inductor. The inductor replaces the immersed electrodes of a conventional melter. The crucible is charged with a batch of glass forming components and waste. Magnetite paste is poured on the batch surface and serves as the inductive heat source. A layer of partially melted batch forms between the crucible walls and the high temperature melt. This layer provides the refractory function for the melter and prevents corrosion of the crucible walls. This melter system can achieve operating temperatures to 3000°C, compared to the 1300°C temperatures of ceramic melters (1).
The high temperatures achieved by cold crucible vitrification allows radioactive waste to be incorporated not only in borosilicate and phosphate glass, but in boron-free glass and crystalline materials similar to igneous rock. This variety of vitrified waste forms that can be produced allows for developing the optimum final waste form in terms of leachability for a given waste type.
In the 1980's, RADON of Moscow has had great success in applying the cold crucible technology to both liquid and solid radioactive wastes on laboratory and pilot-plant scales.
FULL-SCALE VITRIFICATION SYSTEMS
RADON of Moscow has translated their laboratory-scale and pilot plant experience with cold crucible vitrification into full scale plants for liquid and solid radioactive waste. The large-scale plants use cold crucible melters in parallel to achieve a large waste throughput. An advantage of the small crucible melters is that the number of melters employed in a full scale plant can be adjusted to meet the specific waste management needs.
The full-scale plant developed by RADON of Moscow has been applied to a variety of liquid wastes, including streams from a reprocessing facility and two reactor types (RBMK and VVER).
Liquid waste, concentrated by passing it through an evaporator, is added to the batch mixer along with the glass forming additives. The ratio of the glass forming additives - datolite, sandstone, and bentonite, is adjusted based on the salt content of the waste to optimize the final waste form performance. The batch mix is added to the crucible melter. Three or four melters can be operated in parallel within one plant. The molten glass is poured into containers, which are annealed in a furnace.
An elaborate off-gas system is an integral component of the plant. The system employs a series of scrubbers, filters, and chemical reactors, which removes entrained particles, volatilized radionuclides, and other chemicals from the off-gas stream.
Table I compares process variables for RADON's Joule-heated ceramic melters and cold crucible melters. Note that the melt capacity for the cold crucible system, 25 kg/h, represents a single crucible. For a system with four units, waste throughputs of 100 kg/h could be achieved. As seen in Table I, the compact crucible delivers a much greater melt capacity per unit surface area of the melter.
Table I Comparison of Process Variables for Ceramic and Cold
Crucible Melters.
RADON of Moscow has applied the cold crucible technology on a full scale to manage a solid waste - incinerator ash. In this process, metal components of the ash are removed with a magnet system. Then, the large particle size fraction of the ash is separated from the fine ash. The fine ash is charged into the crucible and incorporated into the glass matrix. The large fraction is placed directly into containers and the molten glass with the fine particle fraction is added to the container. The melt capacities reach 50 kg/h, including the large fraction material (1). The resulting waste form is highly durable and suitable for disposal. Waste volume reduction factors approaching 10 have been achieved (1).
FACING THE U.S. DEPARTMENT OF ENERGY WASTE MANAGEMENT CHALLENGE
RADON of Moscow has proven that cold crucible vitrification can successfully serve as a component in a waste management system for radioactive waste. This technology is also well suited for the challenges facing the DOE's weapons complex.
One specific application of cold crucible vitrification is managing transuranic wastes. Many of the DOE facilities have liquid and solid transuranic waste streams. Some of these streams represented process steps that were frozen in-place when production efforts stopped. Do to the long half-lives and relatively high radiotoxicity of transuranic radionuclides, highly leach-resistant waste forms are desirable. Vitrification is a preferred technology for managing this material. Advantages of cold crucible vitrification over conventional vitrification is the modular nature of the facility, the reliability of the system, and the ability to incorporate the waste into a variety of glass and crystalline matrices. Systems can be built to size and waste forms optimized for the specific waste type, yielding the highest performance waste type for the lowest cost.
Another proven application is solidifying incinerator ash. Incinerators provide a means for reducing the volume of solid waste and destroying the hazardous components of some mixed waste. The acceptable radionuclide content of the waste sent to the incinerator facility is often limited by the acceptable concentrations of radionuclides in the final waste form, usually cemented ash. A vitrified ash waste form would provide greater resistance to leaching, allowing higher concentrations of radionuclides in the waste. Also, cementing the incinerator ash increases the waste volume compared to the ash itself. With vitrification, the ash waste volume is further reduced.
The two examples above represent only a sampling of the utility of cold crucible vitrification to the DOE's waste management effort. The system can be applied to many different liquid and solid waste streams when a durable final waste form is needed and the limitations of conventional vitrification makes that process not feasible.
REFERENCES