WM 00 Conference, February 27 March 2, 2000, Tucson, AZ EVALUATION OF GLASS MELTER OPERATION IN TOKAI VITRIFICATION FACILITY

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1 EVALUATION OF GLASS MELTER OPERATION IN TOKAI VITRIFICATION FACILITY Masahiro Yoshioka and Noboru Endo Japan Nuclear Cycle Development Institute Tokai-Works 4-33 Muramatsu, Tokai-mura Ibaraki Japan ABSTRACT The Tokai Vitrification Facility (TVF) was constructed as a first plant in Japan to immobilize the high-level liquid waste (HLLW) to the borosilicate glass in April The TVF finished successfully the test operation being carried out since then using both the simulated waste and HLLW transferred from the Tokai Reprocessing Plant (TRP). After getting an operational license on December 1, 1995, the TVF shifted from the stage of test operation to the full operation. The total HLLW of 52 m 3 was treated, and 62 vitrified wastes were produced through the operation of 1996 including 22 during the test operation. Since then, the operation of TVF including TRP have been stopped up to now due to the accident of the Bituminization Facility occurred in March In future, however, the TVF will have to continue the operation of producing more than 1000 vitrified wastes from the HLLW in TRP, including the HLLW generated in a next decade. The behavior of electro-conductive sludge like noble metals in the glass melter must be evaluated for stable operation of TVF glass melter over a long period. This paper describes operational results and its evaluation related to the noble metals through the glass melter operation with the feed of actual HLLW. It was found that noble metals concentrated in the middle of the melt and then settled to the bottom due to the change of operation where they accumulated. A modification of melter operation and design is presented in this paper to prevent this accumulation. INTRODUCTION The TVF is designed to solidify the HLLW generated from the TRP which treats the LWR spent fuels burned-up 28,000 MWD/MTU in average. The vitrification technology of TVF is based on the liquid fed joule-heated ceramic melter (LFCM) process which has been developed in Power Reactor and Nuclear Fuel Development Corporation (at present, Japan Nuclear Cycle Development Institute (JNC)) through the cooperation with the United State and Germany since Many developmental works were carried out on the glass melter and relative process for the TVF(1). The ground preparation for the TVF was started in mid of The construction of TVF building was completed in February of Subsequently, the installations of process equipment to the cell were finished in April of The test operations had been gone along step-by-step through the cold test and the radioactive test since The cold tests divided into two major tests, the operation test of process equipment and the remote maintenance test, had been conducted alternately. The improvements

2 of some process equipment were carried out based on the result of the cold test for getting more good operability. Each operation result showed that the TVF has enough performance for the safety and process operation, and also for the quality control of glass products. For the remote maintenance of process equipment in the vitrification cell, the remote maintenance capability by two-armed servo-manipulators and in-cell cranes had been confirmed for all remote equipment more than 1000 objects including a glass melter and the racks mounting the process equipment. Subsequently the TVF commenced the radioactive test operations in January 1995 after the preparation for start-up of operation like a cell closing, an establishment of control area, and a connection of HLLW transfer pipe with the TRP. At the beginning of this test operation, the draining glass accumulated in the coupling device between the glass melter and a canister during the third glass draining. After some improvements of both the instrument and glass temperature control were carried out, the test operation was restarted. And radioactive test operation of TVF was finished successfully to produce more 20 vitrified wastes in October After taking the final inspection before the use by government during this test operation, the TVF got an operational license for full operation in December 1, OUTLINE OF TVF Vitrification Process The HLLW is transferred from the TRP to a receiving tank of the TVF after the cooling more than five years. Elemental and radioactive analysis is carried out for process and product quality control. The HLLW is pretreated to adjust the composition by the addition of sodium nitrate and by concentration using an evaporator when required. After the pretreatment, HLLW is transferred to a glass melter continuously using a two-stage airlift system for accurate quantitative feed. Glass fiber cylinders are used as a glass additive for melting. The HLLW transferred to a glass melter is soaked into the cylinder just before it is fed into the melter pool. The glass melted at the temperature of 1100 C to 1200 C is discharged periodically through a metallic nozzle located at the bottom of the melter into a canister. During the discharge, the weight and volume of the glass in the canister are successively measured by load cells and by the gamma-ray method, respectively. The filled canister is subsequently cooled, and transferred to the welding position, and a lid is welded by a TIG welder to seal the canister. After being decontaminated by high-pressure water jet spray and wire brushing and being inspected such as smear test, leak test from lid of a canister, the glass products are stored in forced-air cooling pits. Melter off-gas is cleaned by wet scrubbing process such as a submerged bed scrubber, a venturi scrubber, a perforated plate type water scrubber, and subsequent filtration process such as a high efficiency mist eliminator (HEME), a ruthenium adsorber (silica gel), an iodine adsorber and HEPA filters. The secondary waste like a washed water after treating the off-gas, water used in the decontamination of canister, and condensed waste from the vessels is treated using three stage of evaporation in the TVF. The concentrated waste is mixed to the HLLW at the time of receipt from the TRP, is treated to the vitrified waste. The very low active waste is transferred to the TRP to release to the sea after neutralization. Figure 1 shows a main process flow of the TVF.

3 Specification and Features of the Plant Fig. 1 Main Process Flow in TVF The capacity of TVF, the HLLW treatment rate of 0.35 m 3 /d, is equivalent to the TRP capacity of 0.7 metric tons of uranium per day. The vitrified waste of 300 kg has a heat generation rate of 1.4 kw and radioactivity of 1.5E16 Bq in the design. The TVF incorporates a new concept of fully remote maintenance system for the process equipment in accordance with the adoption of a large vitrification cell so that the plant availability is increased and personnel exposure decreased. In order to carry out this concept, over head system like the bilateral typed two-armed servo-manipulators (BSM), in-cell cranes, and rack system which mount the process equipment on the modularized frames are disposed in a vitrification cell(2). Also the low flow ventilation system is adopted as a cell exhaust treatment system. This system contributes to reduce the release of radio-nuclides from the vitrification cell to atmosphere in the case of release from the equipment to the cell because that exhaust gas is treated by the melter off-gas and vessel off-gas treatment system installed in the vitrification cell(3). The TVF has two main cells; one is a vitrification cell with 27m length, 12m width, and 13m height where most of vitrification process equipment such as the vessels and an evaporator treating HLLW, a glass melter, and the off-gas treatment equipment are installed. Another cell is a transfer cell with storage pits where the glass products are inspected and stored.

4 DEVELOPMENT OF NOBLE METAL COMPATIBLE GLASS MELTER SYSTEM Glass Melter Structure for Noble Metal The noble metals such as Ru, Rh, Pd in the HLLW have high density and small solubility to the glass, so they have a tendency of sedimentation to the bottom once they are fed into the glass melter. If they are not discharged thoroughly from the glass melter and accumulate on the bottom area, the current between electrodes will make a preferable shortcut through the accumulation of noble metals due to its high electro-conductivity. As a result, the glass melter based on the LFCM process has a possibility that the melting capability may fall down, and in the worst case, a part of electrode or refractory will be melted down by the heat generated through the excessive current between the accumulation and electrode. Many developmental works for discharging the noble metals effectively through the glass pouring were carried out due to the characteristics in TVF such as the high concentration of noble metals, a long staying time of it in the melt making big sedimentation of it. Several experiments were performed in order to look into the effect of melter bottom slope on the melter operation and discharge of noble metals. The result showed that a melter bottom slope of 45 would eliminate the operational difficulties by facilitating the discharge of deposits. However, the operational result of glass melter with 45 sloped bottom carried out using highly simulated waste showed that the accumulation of 10% of noble metals in every batch might cause operational problems in the glass melter over a long period(1). Therefore, a technique was needed for the TVF that discharge the total amount of noble metals fed into the melter. Glass Melter Operation Mode for Noble Metal Tests to prevent the accumulation of noble metals and to discharge them more effectively from the glass melter were carried out through the operation of Mock-up glass melter over a long period. One of the methods to make this attempt operationally was to keep the melter bottom at a low temperature. The operational method to control the melter bottom temperature was done by controlling the power of auxiliary electrodes located at the melter bottom, as shown in Figure 2. The melter operation with a low bottom temperature kept the resistance between main electrodes stable. No significant change was observed during the operation using this method. Also it was confirmed by evaluating the balance of noble metals after finishing the run, that the amount of noble metals left in the melter was less than 1% of total amount fed into the glass melter(4). On the other hand, the resistance between auxiliary electrodes decreased with the increase of noble metals fed into the glass melter. However, this low resistance was recovered after pouring the glass through the bottom drain nozzle every batch. This operation result of Mock-up glass melter using highly simulated waste meant that the noble metals fed into the glass melter were discharged effectively through the operation mode of a low bottom temperature over a long period. So the cold bottom mode of operation was expected to be one of the most promising methods to eliminate the operational difficulties caused by noble metals for the operation of TVF.

5 Fig. 2 Glass Melter Structure and Operation Mode RESULT AND EVALUATION OF GLASS MELTER OPERATION Result of Glass Melter Operation The glass melter has been operated for producing 62 vitrified wastes by treating 52 m 3 of HLLW containing the noble metals through the divided runs. The amount of noble metals fed into a glass melter was about 260 kg in oxide total, and concentration of them was slightly higher than standard composition (RuO 2 : 0.74%, Rh 2 O 3 : 0.14%, PdO: 0.35%) used in the feed of simulated waste during cold test operation. The accumulation of noble metals on the slope of melter bottom that develops to the decrease of melting capability or the damage of electrodes can be detected by observing the change of resistance between main electrodes during the melter operation. As a result of all glass melter operations, the drop of resistance taken account of the contribution of glass temperature was not observed even in gradually or abruptly during all the operation, so the accumulation of noble metals on the slope of the bottom is evaluated to be prevented by the cold bottom operation mode. The accumulation of noble metals on the bottom can be detected by observing the change of resistance between auxiliary electrodes located at the melter bottom as well as the accumulation on the bottom slope. In usual glass melter operation, the resistance between auxiliary electrodes drops due to the sedimentation of noble metals at the bottom in accordance with the feed operation. However this resistance is recovered to its original value through the discharge of noble metals by glass pouring operation. As a result of all glass melter operations, this resistance dropped gradually with glass melter operation shown in Figure 3. In the first half of operation for producing about 40 vitrified wastes, the dropped resistance due to the sedimentation of noble metals, even big drop in some batches, recovered to nearly former basis after the glass pouring, although the resistance dropped little by little. In the latter of operation for producing 20 vitrified wastes, however, the resistance did not recovered even after glass pouring, especially in big drop at the first batch after the start-up of the glass melter.

6 It was found that there was a difference in the pouring rate at the just beginning of glass pouring in the latter of operation. Abnormal pouring rate was more than 50kg/h at the start of glass pouring in comparison with the normal pouring rate less than 10kg/h. This meant that the glass just upper the draining nozzle was heated by much current of auxiliary electrodes collected through the accumulation of noble metals. This accumulation did not affect to the operation of glass melter except for the heating of melter bottom using auxiliary electrodes. However the bottom heating was confirmed to be made through the current between main electrode and draining nozzle. Consequently the noble metals fed into the glass melter is evaluated to accumulate around the top of draining nozzle at the melter bottom. Fig. 3 Relationship between Drop of Resistance and Operation Evaluation of Noble Metal Accumulation This kind of accumulation was never observed through the operation of Mock-up glass melter and TVF glass melter by feeding the simulated waste containing the noble metals. The difference between both these operations of glass melters is the glass temperature that makes the sedimentation rate of noble metals change. The glass temperature at upper level in the glass melter treating the actual HLLW was relatively low less than 1150 C as compared with being maintained at about 1200 C during the feeding of simulated waste. The low glass temperature is attributed to the good melting capability in feeding the actual HLLW as compared with the simulated waste, that makes large exposure of glass surface to release the heat from the glass to the plenum space. Also the limitation in regulation of HLLW feeding rate less than 0.35 m 3 /day in spite of HLLW concentration contributed to the release of heat from the glass due to increasing the exposure of glass surface.

7 If the glass temperature drops from 1200 C to 1100 or 1000 C, the sedimentation rate of noble metals will be decreased by 1/5 and 1/50 respectively. So the noble metals stayed at the middle of glass melter, and are concentrated there during the operation of treating the HLLW. During the operation with simulated waste, almost all of noble metals fed into the glass melter in each batch settled to the bottom due to the high glass temperature, and were discharged by glass pouring. In the case of operation with actual HLLW, on the other hand, much of noble metals concentrated at middle level of glass melter, noble metals more than the amount fed into the glass melter each batch, is evaluated to settle on the bottom at a time according to the change of operation condition like upper glass temperature or bottom glass temperature. Some of them were discharged by glass pouring, and some accumulated on the bottom area around the draining nozzle. The change of operational condition that let down the resistance between auxiliary electrodes was the rise of upper glass temperature or bottom to medium glass temperature. The rise of upper glass temperature was made by increasing the feed into the glass melter on purpose for making sedimentation of the staying noble metal at the middle of melter. Because there was a concern that much more amount of noble metal staying in the melter could not be discharged through a glass pouring. The rise of bottom glass temperature was made as the result for the big heating of the bottom area before the glass pouring for getting smooth glass stream. So, in the case of idling operation for a long time and start-up operation of the glass containing melter that made the bottom temperature drop greatly, bottom and middle level of glass melter was heated very much. The relationship between the rise of glass temperature or the input power for bottom heating due to the drop of bottom temperature and the drop of resistance is evaluated to have a good coincidence through the analysis of operation data shown in Figure 3. Further, the TVF glass melter has a plane area consisting of the step-up structure around the draining nozzle for preventing the blockade of nozzle by the broken pieces of refractory as shown in Figure 4. So the accumulation of noble metals is estimated to be proceeding little by little, sometimes greatly on a plane area of melter bottom. Step-up Structure Refractory Protection Draining Nozzle PRESENT MELTER NEW MELTER Fig. 4 Modification of Melter Bottom Structure

8 CONCLUSION As a result of operational evaluation, the mechanism of noble metal accumulation on the bottom area was estimated as follows: The noble metals fed into the glass melter were staying and concentrating at the middle of glass melter without making the sedimentation to the bottom every batch of feeding. The change of operation, the rise of glass temperature at middle level of the melter, caused a great deal of sedimentation to the bottom at a time. All of the noble metals settled on the bottom were not discharged due to a plane with step-up structure on the bottom in addition to the excessive amount of noble metal. The accumulation of noble metals proceeded gradually along with the decrease of slope angle to the draining nozzle. In order to discharge all the noble metals even in the case of a great deal of sedimentation, the melter bottom must be improved to the bottom structure with no plane, the structure with straight slope to the nozzle, in addition to making the proper sedimentation of noble metals fed into the glass melter every batch. A step-up structure in the present melter is for preventing the draining nozzle from being blocked by broken pieces of refractory. New melter will adopt the protection system instead of the step-up structure as shown in Figure 4. References 1.Yoshioka, M., et al., Glass Melter and Process Development for PNC Tokai Vitrification Facility, WASTE MANAGEMENT, Vol. 12, pp. 7-16, Senba, Y., et al., Fully Remote Maintenance System in Tokai Vitrification Facility, Proc. of International Conference on Remote Techniques for Hazardous Environments at London, April 19-20, Tsuboya, T., et al., The Japanese Vitrification Program, Proc. of the symposium on waste Management at Tucson Arizona, Feb.28-Mar.3, 1988, Vol.2 pp Yoshioka, M., et al., Evaluation of glass melter operation using highly simulated waste for TVF, Proc. of International Topical Meeting on the Nuclear and Hazardous Waste Management (SPECTRUM'90), Knoxville, TN, Sep.30-Oct.4, 1990.