A. Jouan
Commissariat à l’Energie Atomique (CEA),
Rhône Valley Research Center, Bagnols-sur-Cèze, France

R. Do Quang
COGEMA, Vélizy, France

S. Merlin
SGN, St. Quentin en Yvelines, France

More than 1500 metric tons of spent fuel are reprocessed per year in France by COGEMA at the La Hague plant. The separated uranium and plutonium are partially recycled in the fabrication of new fuels, including MOX, while the fission products are currently vitrified in two facilities designated R7 and T7, designed and built by SGN. This situation was made possible through research supported and implemented by the CEA since 1957, and today conducted jointly with COGEMA.

The Cold Crucible Melter (CCM) is the result of many years of research and development. It uses induction heating to contain the molten glass in a solidified shell of the same material in contact with the water-cooled crucible wall. Molten glass exhibits sufficient electrical conductivity (up to a few tens of W ·cm) to allow induced currents to heat the glass by Joule effect. This requires melter dimensions of a few tens of centimeters to one meter in diameter and a current frequency of several hundred kHz.

Cold crucible melting is now a fully mature technique with both military and nuclear industrial applications. Examples include the high-level waste solutions at Saluggia, Italy, which will be vitrified by SGN for ENEA; incineration-vitrification of reactor wastes as part of a joint development between the CEA and SGN with the Korean firm KEPCO; as well as the eventuality of using this technique one day to vitrify the high-level waste solutions at Hanford in view of the increasingly promising results of the feasibility tests conducted to date.

Induction heating in a cold vessel is not a new idea, and has long been used to melt metals. Although reserved for noble metals and alloys, the process is used in most industrialized countries – but without fanfare so as not to divulge the state of their art. It has not been widely used to melt glasses and/or molten salts, which require higher voltage and higher frequency currents that have been difficult to generate until recently. Recent progress in heavy electrical engineering has opened the way to developments in both nuclear and non-nuclear applications.

The CEA, initially alone^[1] and today in collaboration with COGEMA and its subsidiary SGN, continues to develop the technology to enhance its reliability and to extend the range of applications notably by increasing the production capacity.

The CCM process has been frequently discussed in the literature^[1,2]. The technique is based on the use of a water-cooled structure that is transparent to the electric field produced by an induction coil surrounding it; this allows currents to be generated inside the material contained in the structure. Figure 1 illustrates the principle and includes a photo of a cold crucible melter in operation.

The salient feature of the process lies in the fact that all the equipment components may be cooled. The molten material at the center of the crucible thus solidifies near the melter walls, which never exceed temperatures of about 200°C. This has a number of implications:

• The crucible is protected against any risk of corrosion, ensuring a very long operating lifetime and allowing it to be used to process a wide range of materials containing even highly corrosive elements: glass with high P₂O₅ and even SO₃ concentrations may be produced without difficulty. A glass melter has been used at Marcoule for more than a decade, during which it has produced a broad spectrum of glass compositions and highly corrosive salts, and yet it still appears virtually new today.
• Extremely high – practically unlimited – core temperatures may be reached in the process material. Crucibles of this type have been used to melt uranium and zirconium oxides at temperatures exceeding 2500°C, as well as zirconium metal at 1800°C and even hafnium at over 2200°C.
• A cold crucible melter may be easily dismantled. Neither glass nor metal adheres to the cooled wall, which is therefore never subject to strong contamination.

The melter is highly compact, considering its ability to generate extremely high power densities in the glass – much higher than in an electrode furnace, where the power density is limited by electrode wear. Cold crucible melters are thus particularly well suited for obtaining high throughput in a small volume, and extend the range of potential compositions for solidifying fission product solutions or any other type of waste for which this type of treatment is applicable.

The CEA, responsible for developing vitrification techniques, undertook the first large-scale induction heating tests with glass in the 1960s. The glass was contained in electromelted refractory crucibles, which quickly proved to be too short-lived, hence the idea of cooling the wall, and subsequently of eliminating it altogether and conserving only the water-cooled structure itself. The result was the first "cold crucible", consisting of sectorized metallic structures (initially copper was used to limit the electrical losses, but stainless steel was quickly adopted when the electrical criterion proved to be less important than corrosion resistance and mechanical strength).

The induction source comprises a high-frequency converter, an inductor surrounding the cold crucible, and a high-frequency link between them. The link itself includes a penetration through the concrete wall of the melting cell, and telemanipulable couplings for connection to the inductor. Tests were conducted to select a suitable material for this line, to ensure the necessary electrical insulation between the plates carrying the current and resist severe irradiation. The high-frequency converter is a standard solid-state design developed for conventional industrial purposes, notably heat treatment of metals in the mechanical industries, or of graphite. For glass heating, the frequency converters are provided with frequency and impedance matching units to ensure compatibility with the requirements of each application.

Cold crucibles 550 mm in diameter are supplied by 300 kW converters at a frequency adjustable from 10 to 350 kHz at a maximum voltage of 400 V. Ferro, a producer of enamels, has been using this type of converter for over two years at industrial scale with a 500 mm diameter cold crucible.

Three pilot facilities are now available at Marcoule; each is used for a specific application.

• The oldest, shown in Figure 2, is used primarily for simulated HLW vitrification tests. It comprises a stainless steel cold crucible 55 cm in diameter and 70 cm high. Powered by a 300 kW solid-state generator, it is supplied at several thousand volts and at frequencies of several hundred kHz. It may be coupled with a calciner (as shown) and supplied with calcine and glass frit; it may also be fed directly with the fission product solution, in which case it is operated like a liquid-fed ceramic melter and could be called a liquid-fed cold crucible melter.

• The second unit is used for development of the incineration-vitrification process, in which the organic matter is burned on the surface of the glass melt and the residual inorganic compounds are directly incorporated into the vitreous phase. The cylindrical stainless steel cold crucible is 30 cm in diameter, and is supplied by a vacuum-tube generator rated at 240 kW, supplying several thousand volts at a few hundred kilohertz.

The main problems yet to be solved concern the anions (Cl^-, SO₄⁼, F^-) in the feed stream, which may combine with cations to form a very wide range of chemical species generally entrained as fine particles in the off-gas lines. Nearly one ton of plastics and moist or previously dried resins have been burned in this unit, providing valuable information for the off-gas processing system, including the startup and shutdown transients.

The third device, modestly named after the mighty Antarctic volcano EREBUS, is reserved for melting exotic oxides and glasses, and for the development of new systems. It is supplied by a 160 kW solid-state generator, and can accommodate crucibles of various diameters. Its primary use has been to determine the melting parameters for oxide materials with poorly known electrical properties. It was successfully used in collaboration with the Australian Nuclear Scientific and Technological Organisation (ANSTO) to test the possibility of producing Synroc by melting.

Three projects are now underway in which a cold crucible melter will be used to vitrify liquid and solid radioactive wastes.

The Italian national agency for new technologies, energy and the environment (ENEA) has chosen SGN to build the facility that will vitrify some 200 m³ of fission product solutions generated by reprocessing CANDU and MTR reactor fuel, and currently stored at the Saluggia site^[4]. About a hundred canisters of glass will be produced. The formulation adopted is a borosilicate glass containing 21 wt% waste material. The mean solution activity is about 1 Ci·l^-1.

The liquid waste stream will be supplied directly to the cold crucible at a rate of about 10 l·h^-1, making a calciner unnecessary. The radioactive liquid waste will thus be vitrified in a facility where no components are operating at high temperature. The 10 l·h^-1 feed rate allows for considerable flexibility in operation, as rates of 20 to 30 l·h^-1 can be obtained with a cold crucible 55 cm in diameter.

Moreover, the compact equipment design allows them to be installed in the cramped existing cells. All the ancillary process functions (preparation and metered transfer of feed solutions, off-gas treatment, canister handling) are performed using equipment similar to the devices implemented in the COGEMA plants at La Hague and Marcoule.

The facility is scheduled to begin operation with radioactive solutions at the end of 2001, and will be operated for 2.5 years.

Under the Hanford tank waste remediation program, the USDOE has awarded a contract to SGN covering the design of a vitrification facility for the high-level waste solutions stored in four identified tanks on the Hanford site, as well as the proof of principles tests. The following step, to be accomplished under a privatization contract, will cover the construction and operation of the facility initially with the solutions contained in these tanks.

The proposed solution comprises a vitrification line including a rotating calciner with an evaporation capacity of 150 l·h^-1 and a cold crucible approximately 800 mm in diameter capable of processing 50 kg of glass per hour. According to the DOE specifications the waste oxide concentration in the glass must exceed 25%, not including the Na and Si in solution. In view of this requirement, and considering the elements present in the waste, the glass will be a poor electrical conductor. Cold crucible operation at relatively high temperatures allows the glass to be produced easily at the required rate, as already demonstrated by the tests conducted at Marcoule.

The Korea Electric Power Research Institute (KEPRI), a subsidiary of the Korea Electric Power Company (KEPCO), has undertaken a joint development program with SGN and with support from the CEA for a cold crucible vitrification process for low-level waste from nuclear power plants. This three-year program will involve direct vitrification of organic waste such as ion exchange resins and solid combustible wastes.

The 3-step program began with preliminary testing in France at a pilot facility comprising a 300 mm diameter cold crucible and a simplified off-gas system. The next step will involve the construction of a larger pilot facility on the KEPRI site at Taejon, Korea, with a 550 mm diameter cold crucible and a complete off-gas system including high-temperature filtration, afterburning and gas scrubbing. The final step will cover testing in the Korean pilot unit.

Preliminary testing is now nearing completion. The initial results have been highly encouraging, notably with regard to the high processing capacity of the 300 mm diameter cold crucible. The tests have also confirmed the advantages of the cold crucible over other high-temperature technologies.

Cold crucible vitrification of radioactive liquid and solid waste is an innovative technology that is now suitable for industrial implementation with a number of applications in several domains. This situation is largely due to the experience acquired in France by the CEA, COGEMA and SGN in vitrifying nuclear waste.

The technological options proposed for all the process functions (with the exception of the cold crucible itself) are similar to those used in the industrial-scale vitrification facilities operated by COGEMA for high-level radioactive liquid waste, as well as in other less complex industrial applications such as cement manufacturing or incineration, when the application involves low-level liquid or solid waste vitrification. The proposed options are all proven solutions, capable of meeting customer requirements as part of an overall cost-effective solution. Intensive test programs have been carried out by the CEA to verify the feasibility of the technological choices involved in the cold crucible process.

CCM technology continues to be the subject of major test programs designed not only to increase the limits of our fundamental knowledge, but also in the short term to extend the scope of industrial applications to organic and inorganic low-level waste of all types.

1. A. JOUAN, JP. MONCOUYOUX, S. MERLIN and P. ROUX, "Multiple Applications of Cold Crucible Melting", Waste Management ’96, Tucson (1996).
2. A. JOUAN, R. BOEN, S. MERLIN and P. ROUX, "A Warm Heart in a Cold Body – Melting Technology Tomorrow", SPECTRUM ’96, Seattle (August 18-23, 1996).
3. A. JOUAN, R. BOEN, S. MERLIN and V. PUJADAS, "New Development for Medium and Low Level Waste Vitrification", NUTHOS-5, Beijing (April 14-18, 1997).
4. C. CALLE and A. LUCE, "CORA Project – The Italian Way for Conditioning High and Low Level Liquid Radioactive Waste", ICEM ’97, Singapore (October 12-16, 1997).

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