Renaud LIBERGE
SGN
1, rue des Hérons
Montigny-le-Bretonneux
78 182 Saint Quentin-en-Yvelines Cedex

Jean-Louis DESVAUX
COGEMA La Hague
50 444 Beaumont-Hague Cedex

Daniel PAGERON, Christophe SALICETI
COGEMA Marcoule
BP 170
30 206 Bagnols-sur-Cèze Cedex

After a brief history of the R&D work which led to the definition of the borosilicate glass formulation and to the industrialisation of the French two-step vitrification process, this paper reviews the industrial experience of Marcoule and La Hague vitrification plants.

The AVM plant in Marcoule which is still operating can be considered as the first industrial in-line vitrification experience in the world. This small capacity unit, matching the needs of the Marcoule reprocessing plant, was an unique feature to demonstrate and validate the major principles that still governs the French vitrification facilities. The description of the process and technologies is followed by a quick look to the main evolution of the facility since its start-up.

Since their start of operation in 1989 and 1992, the vitrification plants of La Hague R7 and T7 have demonstrated the capability of the process to fit the requirements of modern and high capacity reprocessing plants. Nevertheless, the constant will of improving availability and quality and to minimize the operating costs while keeping safety at its highest level has led to a continuous development of these facilities. The paper outlines these different improvements whose a major example is the work performed to extend the service life of the melting pots.

Since 1978, the French vitrification facilities of Marcoule and La Hague have produced more than 3,000 tons of HLW glass in more than 8,000 canisters.

1998 marks in France the 20th anniversary of the beginning of industrial activity in the field of high-level waste (HLW) vitrification. More than a technical success, in-line vitrification of HLW produced by operating reprocessing plants has become a commercial reality, that led in 1995 to the first return of glass canisters to COGEMA's customers.

When looking back on these twenty years of industrial operation, two main lessons can be drawn to explain this success:

• a constant respect of some basic principles that are at the heart of the French two-step process;
• a not less constant will of improvement in terms of safety, availability, quality product, and waste minimization.

From the start-up of AVM plant at Marcoule in 1978, twenty years of industrial experience have been accumulated in France in the field of vitrification of radioactive HLW solutions. Based on the successful experience of the AVM plant, the first vitrification plant of La Hague, R7, entered active service in 1989. It was followed in 1992 by the start-up of the second La Hague vitrification plant, T7. A third facility using the same process (AVH process, derived from the AVM process) was commissioned at the Sellafield reprocessing plant (WVP: Windscale Vitrification Plant) and began operation in 1990.

These industrial successes have been possible thanks to a major program of research and development conducted by the Atomic Energy Commission (CEA) since 1957. The two most important results of this work were:

• the development of borosilicate glass formulation, which was later world-wide recognised and adopted for immobilizing HLW waste resulting from reprocessing operations;
• the development of the French two-step process which has since then proven its great flexibility and reliability through the different industrial experiences quoted before.

The main objectives of solidification of fission products (FP) are to reduce the volume and stabilize the HLW waste resulting from reprocessing operations in a solid matrix as much inert and leaching resistant as possible, with a view of long-term geological storage. Among the different processes of HLW waste solidification, only vitrification came out from the industrial point of view.

The CEA began research on the vitrification of HLW in 1957. Several types of matrices were investigated, among which crystalline materials, phosphate and borosilicate glasses. Due to their amorphous structure, glass appeared to be a very good candidate to incorporate easily most of the FP oxides in the vitrous network. In the mid-60, borosilicate glasses were chosen for vitrification of HLW solutions, as the best compromize in terms of confinement (chemical, thermal and irradiation stability), technological feasibility and economical criteria (volume reduction factor). Today, borosilicate glasses are a world-wide reference and have been chosen for nearly all vitrification processes of HLW solutions.

As it is well-known, the two-step French process consists in first converting the HLW solutions to a solid form in a rotary calciner and then to vitrify it in an induction-heated metallic melter. The basic principles that still governs that choice are the following:

• Separation of the functions, in order to simplify the equipment: in particular, the metallic melter is a small equipment and all induction power is used for the fusion of calcinate and glass frit materials. In case of a liquid-fed alimentation of the melter, about 60% of the power would be used to evaporate water.
• Easy remote replacement of all process equipment, which means small and light equipment, that can be assembled and disassembled by using overhead cranes, master-slave manipulators and remote-controlled tools. Another advantage of small equipment and hold-ups is an easy reaching of steady-state conditions allowing a quick response of the process.
• Flexibility of multi-lines facilities: for high-capacity plants like La Hague, the 3-line design of vitrification facilities enables to go on operation on two lines if one is in maintenance or in stand-by. Only this design is suitable for the in-line vitrification of HLW solutions produced by an operating high-capacity reprocessing plant.

The best justification of these main choices lies in the following features: more than 8,000 glass canisters produced in France at La Hague and Marcoule plants since 1978, corresponding to more than 3,000 tons of glass and nearly 100 millions of TBq immobilized, and no major trouble encountered in vitrification facilities since that time.

From 1960, small vitrification pilots were built. This early work cumulated in the pilot-scale facility PIVER, featuring a single induction-heated pot where the three operations of evaporation of the HLW solutions, calcination of the residue and glass elaboration were performed. PIVER operated successfully from 1969 to 1973 to produce 12 tons of glass containing 5 millions curies of activity. PIVER resumed operation in 1979 to vitrify HLW from the reprocessing of fast-breeder fuels. It was decommissioned between 1988 and 1990.

In parallel, separate calcination was investigated, which led to the test of a rotary calciner in inactive conditions in 1972. Following these successful tests, the design of AVM, the first vitrification facility working in-line with a reprocessing plant was undertaken.

The AVM (Atelier de Vitrification de Marcoule) started active operation in June 1978. This facility (nominal capacity: 15 kg/h) was scheduled to match the needs of the Marcoule reprocessing plant. At the first time, it vitrified the backlog of stored waste. Then, it began to treat in-line the HLW solutions resulting from the reprocessing of GCR fuels (mainly) and research reactor fuels by the UP1 plant.

The AVM process is presented in Fig. 1.

After analysing, the concentrated FP solution is adjusted in a stirred vessel (about 60 m³ batches) to make its chemical composition compatible with specifications for the glass product. The solutions are then sent to a rotary calciner heated through 4 heating zones where they are evaporated and nitrates transformed into oxides. Additives are added to the solution for ensuring a good calcinate form to feed the melter. The evaporating capacity of the calciner is 40 l/h.

At the outlet of the calciner, a mixing of calcinate and borosilicate glass frit feed the metallic crucible.

The metallic crucible of the AVM facility is a cylindrical pot in Inconel 601 (external diameter: 0.35 m; overall height: 1,7 m) inducted-heated through 4 induction zones (10 kHz - 60 kW). Two additional inductors enable to heat the glass lid which ensures the tightness between the calciner and the melter, and to heat the glass plug which controls the glass pouring in the refractory steel canister. The main inductors can heat the glass up to a temperature of 1,150°C.

Glass pourings are performed every 8 to 12 hours depending on the composition of HLW solutions being treated. The filling of a canister ( volume: 150 l) is performed in 3 pourings of 125 kg each. At the end of filling, a lid is welded onto the canister by means of a plasma torch. After external decontamination with pressurized water (250 bar), the canister is transferred to the air-cooled interim storage facility. This facility, initially 3 modules containing 60 or 80 shafts of 10 canister per shaft (2,200 canisters) has been extended to 4 modules (3,000 canisters), and the fifth module, now under construction, will be available at the end of 1998.

Off-gas treatment comprises a hot scrubber with baffles, a water vapour condenser, an absorption column, a washing column, a ruthenium filter and 3 HEPA filters. As told before, the most active gas washing solutions are recycled to the calciner.

The AVM unit is built around a vitrification hot cell containing almost all process equipment, including FP solution feeding system, rotary calciner, hot crucible, glass canister lid welding equipment, as well as the primary components of the off-gas treatment system (scrubber and condenser). Solid wastes such as worn components are dismantled in the same cell and packaged into containers of the same size as the glass canisters. Surrounding rooms contain auxiliary equipment.

All mechanical and process equipment subject to maintenance is design for remote assembly and disassembly by means of a 20 kN overhead crane and master-slave manipulators. Replacement equipment is introduced by the overhead crane from its hoist park outside the crane.

Special jumpers have been designed for radioactive fluid piping connections, that are operated by remotely-controlled electric tools.

The production records as of October 1st 1997 are given in Table I.

With an availability of about 70 % for only one process line, AVM has proven the great reliability of the two-step vitrification process.

During these 20 years of operating, no major trouble susceptible of causing long-length shutdowns has been encountered. On the other hand, every important maintenance have been tested: the metallic crucible is periodically replaced, the calciner tube, the driving motor, the inductor, the scrubber and its recycling pot, the condenser have been changed. All these exceptional in-cell maintenance operations have confirmed the efficiency of remote maintenance equipment. They have been performed without major difficulty, thanks to the facility design, the precautions taken for inactive testing and a perfect check of interchangeability before introduction of new equipment into the cell.

No major evolution concerning the vitrification process is worth to be mentioned, which is another favourable argument for this process.

One great evolution concerns the metallic crucible lifetime. Initially of 2,000 hours, this lifetime has been progressively increased up to more than 7,000 hours, with a mean of 6,000 hours for the 4 last pots. Two main reasons explain that progression: technological improvements in the fabrication of the pots, which are still in Inconel 601, and better heat process control in order to avoid power jumps which are very prejudicial for corrosion resistance of the crucible.

Concerning the calciner subassemblies, design improvements have been achieved in calciner end sealing and greasing technique (including oil recovery).

The replacement of the original overhead crane by a modular one was also an important evolution. In fact, the first crane was of a conventional design, with mechanical or electrical elements that cannot be remotely replaced. With time it became more and more radioactive, leading to growing difficulties in crane maintenance. Associated to a new crane servicing room, the modular crane enabled a significant reduction in the radiation doses received by maintenance staff. After 10 years of operation, a revamping of the modular crane was performed in March 1997, without any problem.

In 1995, the quality insurance and quality control (QA/QC) system defined for La Hague plants has also been implemented in AVM. This system ensured glass quality according to ISO 9002 through three main elements: raw material control, process control and quality control division.

Finally, it is worth to mention a set of modification on annex equipment, which participate to the general and constant improvement of the facility.

In mid-97, UP1 plant stopped its reprocessing activity and entered in a first phase of rinsing, prefiguring its decommissioning. During this future decommissioning, AVM will have an important role to play: vitrify all the HLW solutions produced by the decontamination of UP1 equipment. So, AVM is scheduled to operate until 2002 and to produce between 350 to 400 glass canisters.

Beyond its role of in-line vitrification facility for UP1 plant, AVM has been a unique opportunity to define major process and design choices for La Hague vitrification facilities. Indeed, the first lesson of AVM experience relies on the confidence it gave upon the two-step vitrification process, which can ensure a continuous and safe production, thanks to a reliable process equipment and well adapted maintenance capabilities. So, the selection of the process for the two facilities planned for La Hague was clear. Nevertheless, it was necessary to adapt the design to the new requirements.

• Increased capacity: in order to in-line vitrify HLW solutions produced by a 800-ton capacity reprocessing plant, each facility had to ensure a production of 50 kg/h. According to basic principles defined before, a 3-line design was retained, each line capable of producing 25 kg/h of glass. Considering 2 lines in service and one in stand-by, it fits the requirements and give a great flexibility of operation.
  To obtain the required capacity on each vitrification line, the following solutions have been adopted:
  - scale-up of the calciner;
  - avoid metallic crucible (Fig. 2): this shape enables to limit the maximal distance between the pot wall and the glass, which is important in case of a conduction/convection heat transfer, while giving access to an increased hold-up.
  Glass pourings are performed every 10 to 12 hours with nominal solutions and the filling of a canister is performed in 2 pourings of 200 kg each.

• Glass formulation adapted to LWR fission product solutions, including alkaline liquid waste concentrates as well as platinoid-rich clarification fines. An important work was performed by the CEA (cold and hot laboratory tests, pilot tests), which led to the definition of the R7/T7 formulation. It must be noticed that the AVM/AVH process is well-adapted to the incorporation of clarification fines in the glass. Indeed, limited dwell-time and external heating avoid the troubles that can be encountered with other vitrification processes (platinoid accumulation at the bottom of melters, causing clogging or short-circuits in the glass bath when joule-heating is used : case of PAMELA experience for instance).
• Layout design and maintenance design concept and technologies adapted to the principles developed for the new La Hague facilities. The main process equipment of each vitrification line is located in a separate cell, while pouring and cooling cells are common to the three lines. These cells are equipped with cranes, master slaves manipulators and shielded windows for remote maintenance. They are associated with hoist parking cells allowing crane maintenance as well as introduction of new equipment. In process cells, lay-out is optimized in order to facilitate access, modifications, and even addition of new equipment, and modular design of equipment allows partial replacement as much as possible.
  As a result, maintenance operations are fully integrated in process operations, which is of utmost importance to minimize downtime and to increase availability for production.

Apart from these adaptations, the AVH process is very close to the AVM one. So, it is not described here in more details.

R7 entered active service in June 1989 and began to treat the backlog of the HLW solutions that have been accumulated since the start of the first La Hague plant, UP2. These solutions included neither clarification fines nor alkaline liquid wastes concentrates, but represented an important volume (about 1200 m³), nearly saturating the storage capacities for HLW solutions. So, the challenge of R7's start-up was to reach very quickly a sustained rate.

From this point of view, the start-up was a complete success, since the FP stock began decreasing from the first campaigns. This result is all the more noteworthy because operators had to cope with two significant problems:

• the crucible lifetime was lower than expected (this point will be discussed further), requiring frequent replacements;
• the containment at the connection between the pouring nozzle and the canister was not enough gastight during pourings leading, as a main consequence, to an increased activity of the cell ventilation filters. First countermeasures in R7 included the improvement of the tightness of the connection.

In spite of these problems, FP stock resorption and production goals were met thanks to the great facility of maintenance that has been described before.

The T7 facility is devoted to treat the HLW solutions produced by UP3 plant. It entered in active service in July 1992, 3 years after its "twin" facility R7.

Of course, the design of T7 took advantage of R7's experience by including the following improvements, that have been identified on R7 but could not have been implemented in active conditions:

• implementation of a new connecting device between the pouring nozzle and the canister, associated with an improved pouring off-gas system;
• addition of an in-cell washable pre-filtering device on the ventilation of main hot cells (vitrification, pouring, dismantling) in order to protect HEPA filters from contamination;
• modification of the cranes to improve the reliability of some components and to reduce the need for their maintenance;
• improvement of the canister decontamination device (higher water pressure and throughput).

Moreover, T7's operators had the unique opportunity to make a training period on R7 before T7's active start-up.

As a consequence, T7 was able to reach very quickly its production goals, and the modifications mentioned above proved to be very beneficial in terms of reduction of operating costs, reduction of the volume of waste, reduction of doses to personnel, outstanding availability. For instance, the use of washable metallic pre-filters enabled to divide by about 10 the number of HEPA filters to replace.

T7 was also the first vitrification facility in France including clarification fines and alkaline wastes to the HLW solution to be vitrified.

At the beginning of 1994, the backlog of FP solutions from UP2-400 that R7 vitrified since its start-up was exhausted. At mid-94, the new plant UP2-800 was scheduled to start, but one storage year would be necessary to let HLW activity decrease before their vitrification. So, the decision was taken to upgrade R7 to the same level as T7, by implementing the same improvements.

The project, which included significant in-cell operations in the most active part of the plant, was possible only thanks to a careful preparation work which lasted more than one year. In particular, all the most difficult operations (especially those located in limited-access cell even after decontamination) were rehearsed beforehand using inactive mock-ups.

The main objective during the project was to minimize the doses to the personnel and the careful training on inactive mock-ups was an essential element to reach it. The two others goals were to minimize the volume of waste and to comply with the deadlines in order to do not interfere with production schedule of the facility.

The work was conducted in two steps. During the first one, from February to June 1994, only one line was stopped, while vitrification operation continued on the other ones. During the second step, from July 1994 to March 1995, all the lines were stopped. But as it was told before, this production stop was anyhow necessary before beginning the treatment of FP solutions coming from UP2-800.

All the previous objectives were reached: the doses to the personnel were 10 % lower than expected (themselves far below initial estimations); the volume of generated wastes as well as modification costs were very close to the forecasts; and R7 resumed operation on March 21th 1995, 10 days early than expected.

As a conclusion, the experience gained on T7 with the same improvement allows to forecast a return on investment (financial and radiological) of 2 or 3 years, thanks to the reduction of maintenance work and volume of high-active wastes generated during operation.

Since their start of operation, R7 and T7 have demonstrated the industrial maturity of the process. Nevertheless, the constant will of improving availability and quality and to minimize the operating costs while keeping safety at its high level has led to a continuous development of these facilities.

From this point of view, the modular concept of La Hague vitrification facilities is of prime importance. Indeed, in a first time, any improvement can be tested and optimized on one line without significantly disturbing the production. After that, the modification can be extended to the other lines, before being reported to the "twin" facility. This process of reporting on one facility the benefits acquired on the other is also possible thanks to extensive information exchanges between the two operating teams.

One of the best example of continuous improvement is given by the work performed to extend the melting pot lifetime.

At the start of R7, the lifetime of the oval-shaped metallic crucible was far less than expected (2,000 hours) due to the combined effects of thermal, electrical, chemical and mechanical stresses applied to the pot. The induced corrosion led to a prohibitive replacement frequency, even if their design concept is well-adapted to this operation.

An important R&D work was carried out, involving experts in various fields (metallurgy, materials, induction heat, heat engineering, fluid mechanics, glass technology,...). Results of this R&D work were discussed and induced successive changes, taking advantages of periodical replacements of the melting pots without disturbing the production. The major improvements concern:

• mechanical performances and corrosion resistance of the material;
• temperature measurements by thermocouples;
• heating distribution at the crucible surface;
• induction power control.

These changes allows a sharp increase in the lifetime of the crucibles, as illustrated on Fig. 3. At present, with about 3,000 h average lifetime, the initial objective of 2,000 h has been largely exceeded, and the number of glass canisters per pot has been doubled with regard to the initial forecast. Additional improvements are still being studied.

It must be noticed that the lifetime of melting pot is still higher in AVM than in R7/T7, because HLW solutions of La Hague are more corrosive (presence of molybden in particular). Furthermore, thermal stress is higher in oval-shaped crucible than in cylindrical one.

The main benefits of this extended lifetime are the reduction of maintenance operations (for crucibles replacements but also for washable pre-filters since the breaches of confinement during crucible replacements are the main source of cell contamination), the reduction of waste and the economical gain due to saved crucibles. Due to the reduction of maintenance operations, a reduction is also obtained in doses to the personnel since only these operations require a constant human presence in front of the shielded windows to operate the telemanipulators.

The production records as of October 1st 1997 are given in Table II.

For twenty years, French vitrification of HLW has been proving its efficiency and availability through Marcoule and La Hague industrial experiences. The main actors of this success are:

• The CEA, who has been conducting R&D work on vitrification processes and glass formulations for more than 40 years;
• SGN, the engineering part of COGEMA's group, who designed and built vitrification facilities well-adapted to the main concepts of French vitrification process;
• COGEMA, who operates these facilities and contributes to their continuous improvement in close collaboration with the others actors.

Small and reliable equipement, remote maintenance and flexible capacities remain the main options of French vitrification. Designed for in-line vitrification of a reprocessing plant, this process has taken into account production and commercial requirements from its conception stage. Today, these achievements can be considered as a significant advantage regarding any HLW vitrification need.

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