JP9950237

PROGRESS IN NUCLEAR WASTE VITRIFICATION BY MELTER TECHNIQUE

S. WEISENBURGER Forschungszentrum Karlsruhe Institut fur Nukleare Entsorgungstechnik P.O.BOX 3640, 76021 Karlsruhe, Germany

ABSTRACT Nuclear waste vitrification by using the -fed ceramic-lined waste melter process started in 1973 with the pioneering development at Batelle Pacific Northwest Laboratory. The first radioactive plant applying this technique was the PAMELA plant in Mol/ Belgium which was put into hot operation in 1985. A main part of the technology for this plant including the melter was developed by the Institut fur Nukleare Entsorgungstechnik (INE) of Forschungszentrum Karlsruhe (FZK)! For the time being there is an increasing demand for the availability of small-scale vitrification units for processing of small stocks of high level liquid wastes (HLLW). Limited quantities of HLLW solutions were obtained during the period of development of reprocessing techniques at various international sites. One example is the former WAK (Wiederaufarbeitungs-anlage Karlsruhe) reprocessing plant. It is located at the site of Forschungszentrum Karlsruhe and is now under decommissioning. The overall decommissioning program includes vitrification of 70 m3 of stored HLLW with a total p7y radioactivity of 8.9 x E17 Bq. This paper focuses on progress achieved in the design of small-scale liquid-fed ceramic glass melters for these purposes. Improvements are described regarding extension of power electrode life time by optimized air cooling, glass pouring operation, off- pipe cleaning, glass level detection system in the melt tank, and arrangement of a small-scale melter in a hot a cell. Some test results achieved with the new melter are also outlined.

INTRODUCTION Ceramic-lined waste glass melters are currently in use in several radioactive vitrification plants like Tokai Vitrification Facility in Japan, Savannah River Defence Waste Processing Facility, and West Valley Demonstration Plant in the US. The PAMELA plant, using this technique as well vitrified 907 m3 of high level waste into 4901 of glass between 1985 and 1991. In Germany the development of the vitrification technology is currently focused on the special aspects of wastes stored at former nuclear reprocessing sites. These include mainly (1) appropriate simplification and scale down of the vitrification system while maintaining or even improving the functional safety, and (2) keeping the overall plant size and thus costs as low as possible particularly in view of the usually limited waste volume to be processed and, hence the short operation time. The VEK-project (Verglasungseinrichtung Karlsruhe) is one example of application of a small-scale vitrification plant for conditioning of small HLLW stocks stored at reprocessing sites under decommissioning [1]. A prototype non-radioactive test facility with such features was designed in 1995 at the Institut fur Nukleare Entsorgungstechnik (INE) of the Forschungszentrum Karlsruhe. After two years of construction the facility has been put into operation in May 1998. The paper outlines the progress which had been achieved referred to the glass system. Improvements are described concerning extension of power electrode life time, glass pouring operation, noble metals compatibility, off-gas pipe cleaning, glass level detection in the glass pool, and optimised arrangement of the melter in the hot cell.

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SMALL-SCALE MELTER TECHNOLOGY The process of converting high-level liquid waste into glass in a liquid-fed ceramic melter is characterized by the fact that individual process steps of vitrification (drying, calcining, and reaction of the waste residue with glass formers) take place simultaneously within the glass melter (Fig. 1). HLLW and glass forming additives - glass frit in the form of beads - are directly fed onto the hot glass melt surface. Vitrification occurs in a limited reaction layer formed on the melt pool surface. The glass melt is current-heated through immersed electrodes according to the Joule principle. The melting tank is thermally insulated and the whole system is encased by a stainless steel containment box. The off-gas leaving the melter through a melter roof nozzle is cleaned in a multistage off-gas treatment system and contains steam, NOx, particles, volatiles and inleakage air. The melter type developed for use in a small-scale vitrification facility is a conspicuous novelty in melter technology. The design and characteristics of the small-scale glass melter can be seen in Fig. 2. The novel type features cylinder shape, bottom drain (no additional glass overflow system) and reduced size which was primarily achieved by a reduced melt tank content, and substantial simplifications concerning the glass heating system. The compact melter design has considerable impacts on the size of the melter cell, the arrangement of the melter within this cell, and the remote handling technique. The melter has a design throughput of 12 1/h corresponding to a glass production rate of approximately 8.5 kg/h. The glass tank made of high temperature resistant ceramic refractory contains about 150 1 glass melt which corresponds to the volume capacity of one canister with a glass capacity of 400 kg and dimensions 430 mm in dia. and 1335 mm height. The main features of the melt tank are its sloped side walls and the bottom design. The slope forces the noble metals

High active ^Gl^frit liquid waste ^

Stainless steel box

Alternating el. current -50 Hz

pouring channel Canister

Fig. 1: Simplified scheme of the liquid-fed ceramic melter process

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CZZJ Fig. 2: Layout and characteristics of INE's today small-scale melter technology Melter outside dimension: 1500 mm in diameter, 1700 mm height

Table 1: Main characteristics of the small-scale glass melter

Characteristic Data

Nominal throughput 121/h Glass production rate 8.5kg/h Glass pool surface 0.44 m2 Melt tank content 1501

Residence time of glass in the pool 44 h Glass pool heating Direct electric heating

Heating power release 40 kW maximum

Glass pouring system Bottom drain Quantity of glass per pouring 100 kg, flow rate appr. 100-130 kg/h Frequency of pouring Every 11-12 h

Melter size 1.5 m dia., 1.7 m height Weight 8.5 t

- 243 - JAERI-Conf 99-004 ruthenium, palladium and rhodium - not soluble in the borosilicate melt and settling to the melter floor - to flow towards the glass discharge area at the melt tank bottom. A special glass channel design in the discharge zone assures the complete outflow of any noble metal sludges arriving in this area. The melt is heated in the melt tank center to !200°C by a single pair of electrodes made of high temperature resistant INCONEL 690. Towards the side walls and the melter bottom, temperature drops between 100-200°C exist. Additional energy can be released, if needed, by a pair of auxiliary electrodes installed in the bottom area of the melt tank. The first melter start-up and also restarts are performed by means of several external SiC-resistance heaters introduced temporarily through openings in the melter roof. At sufficiently high temperatures the electric conductivity of the glass for direct heating is attained. For discharge of the glass the melter is equipped with a draining system at its bottom. The system works on the principle of a thermal valve. For initiation of pouring, the glass solidified in the discharge channel is remelted by external energy supply from two heating systems (direct electric heating and medium frequency induction heating of the pouring pipe). The glass flow rate of approximately 120 kg/h is controlled via energy input of the pipe heating. The flow rate is measured by monitoring the weight increase of the canister vs. time. Some characteristic data of this small- scale melter design are summarised in Table 1.

IMPROVEMENTS IN MELTER DESIGN Improvements of the melter design has been made in recent years. They refer to the extension of electrode lifetime by an improved air-cooling, to the glass pouring system and its operation mode, to the cleaning of the melter off-gas pipe, and to a reliable glass level detection system in the melter.

Extension of power electrode life time The life time of the power electrodes is an important factor of the overall life time of the melter system. The submerged surface of the INCONEL 690 power electrodes undergoes only negligible corrosion attack by the glass melt as long as the surface is at or below approximately 1000°C. The details of corrosion mechanism and kinetics and the impact of temperature had been investigated and reported in Ref. [2]. The air cooling of the electrodes of the small-scale melter has been optimised in order to keep the submerged electrode surface at or below 1000°C. The cooling air (flow rate around 20 m3/h each electrode) is released into the melter cell after passing the air cooling channels of the electrode. Air cooling lowers the submerged electrode surface temperature by approximately 100°C compared to non-cooling. The results of numerical modelling of the melter system confirms the experimental data. Fig. 3 gives the numerical modelling results of the isotherms in the power electrode area under conditions of air-cooling compared to non-air-cooling conditions. Thus, permanent air-cooling during glass production operation extends the life time of the power electrodes considerably. At INE, a melter is in use since more than 8 years without indications of significant corrosion attack of the air-cooled power electrodes.

Glass pouring system Glass pouring into the stainless steel canisters is one of the most important steps in HLLW vitrification. It requires a system with a safe function and a long service life. It must be compatible with the service life of other main components of the melter like the ceramic refractory material of the glass tank/melter plenum, or the power electrodes. A glass pouring system in the melter bottom for discharge of nuclear waste glass melt requires a number of features including a. Absolute glass-tight integration of the pouring pipe into the melter bottom structure

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1075°C sotherm field (At25°C)

Non-cooled power electrodes (9ncon@9 690)

975°C

Air-cooled power electrodes Fig.3: Decrease of submerged power electrode surface temperature by air cooling of the electrode

b. Easy control for start-up of the pouring c. Reliable stop of the glass flow when the target content in the canister is achieved d. Avoiding blocking of the glass pouring pipe by materials e. Assuring easy remove of highly viscous and highly electrically conductive noble metals sludge from the melter (when processing HLLWs with significant concentration of these metals) f. Allowing reliable glass flow rate measurement by force free coupling of the pouring system to the canister g. Allowing efficient remove of volatile radioactive species escaping from the glass pouring stream and from the hot melt in the canister.

Function In Fig. 4 the design principle of the glass pouring system is shown. It consists in the upper part of an INCONEL 690 block placed in the melter bottom. It contains one central glass exit channel and 12 side channels. Beneath this block a ceramic refractory block follows with the glass channel in its center. Glass material in this channel can be remelted by passing electric current through it. For this purpose the INCONEL 690 block at the melter bottom and the induction-heated drain pipe are used to establish this electrical circuit. Power release below 1 kW is sufficient for remelting the glass. The outlet channel below this ceramic retractory block forms a thick-walled pouring pipe made of INCONEL 690. It is induction heated to about 950°C which is high enough to start the glass pouring.

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Sedimentation of M. Rh, Pd

Bottom of stainless steel Glass into canister

Fig. 4: Design principles of melter bottom and glass pouring system to achieve both noble metals compatibility of the melter and protection of the pouring system against blockage by solid pieces

Glass tightness Due to the special geometry of the pipe, and the firm integration of the pipe flange into the melter bottom, an absolute glass tightness has been achieved. There is no way for the glass melt to migrate into the melter bottom structure. The only way the glass can leave the melt tank is the pipe channel. In earlier melter designs this feature was less rigorously taken into account. The present design guarantees that this very important requirement of an induction heated glass pouring system, coupled to a ceramic-lined glass pool, is achieved.

Start-up of pouring Start-up of glass pouring requires within about 40 min gradual increase of induction heating of the pouring pipe until about 950°C. Then direct electrical heating of the glass in the ceramic part of the outlet channel follows. A power release of typically 0.6 kW (6A, 100V) for 10 min initiates finally the pouring start. After start of the glass flow the direct electrical heating circuit is switched off. In this way, two independent heating devices are required to be able to start the glass flow. This contributes to the safety control of the start-up of pouring operation. At the beginning of the pouring the temperature of the glass stream is at 950 - 1000°C. Later in the pouring the glass stream temperature increases to about 1050°C.

Reliable stop of the glass pouring The glass flow stops reliably when the induction-power for the heating of the pouring pipe is reduced. Gradual reduction of induction power causes gradual decrease of the glass flow rate from 120 kg/h to about 40 kg/h. From that point of pouring conditions complete switch off of induction heating leads to the stop of the glass flow in less than 1 min. Canisters can be filled to 400 kg content within a precision of ± 2-3kg.

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Blockage protection, easy remove of noble metal sludge The new optimised bottom drain design, as shown above, has additional to the central channel, 12 subchannels trough which the glass melt flows into the induction heated pouring pipe. During a long overall service life of the melter solid material could fall onto the drain area (e.g. an irregularly formed piece of ceramic refractory). However, no blockage of the pouring pipe must be feared because of the multi-channel system. In former melter designs, only one channel was available conducting the melt into the drain pipe. Thus the risk for a blockage was substantially higher. The new design additionally minimises the blockage risk by the fact that the diameter of the side channels is smaller than that of the drain pipe. Hence, solid pieces which could once pass one of the subchannel can pass the pouring pipe in any case. Another main reason for the design of the subchannel system in the bottom area was due to the aspect that the noble metals of the HLLW - ruthenium, rhodium, and palladium - do practically not dissolve in the melt. They form isolated particles which - driven by their high density- settle towards the melter bottom. There they form together with the adjacent glass an extremely viscous and also electrically well conductive sludge. This sludge flows along the sloped melter side walls towards the bottom area of the glass tank. The subchannels in this area support the flow of the sludge down into the pouring pipe. In this way the slope of the melter walls ensures the collection of the viscous noble metals sludge, and the subchannels in the melter bottom provide easy, safe and complete discharge of the sludge. An accumulation of noble metal sludge would cause serious problems regarding power release in the glass pool. The electric field and thus the current field in the melt tank would be unfavourably changed. The portion of electric current passing through the sludge would increase to undesirable high values compared to that through the bulk of glass.

Reliable glass flow rate measurements and removal of volatile (radioactive) species During glass pouring the increase of glass mass in the canister is monitored by weight measurement. From the data the glass filling degree of the canister and the glass flow rate are derived. To minimise loss of volatile radioactive species from the hot glass pouring stream into the melter cell, the canister should be rather gas-tight coupled to the bottom drain system On the other hand, the canister length increases during glass filling due to thermal expansion. Gas tightness between bottom drain and canister sustained by a bellow system can have the disadvantage of influencing the weight measurement by uncontrollable forces caused by canister expansion. These problems have been solved by application of a labyrinth seal and removal of the volatiles by suction into the melter plenum and thus the off-gas according to Fig. 5. Among others, particularly Cs-compounds are increasingly volatile from glass melt with enhanced temperatures. Measurement data of partial vapour pressure of such types of species proving this effect, are also given in Fig. 5 according to Refs. [3,4]. The loss of radioactivity into the melter cell during glass pouring without coupling of the canister to the pouring system has been tested in the PAMELA-plant (see upper diagram of Fig. 5). The specific radioactivity of the melter cell increased significantly during the glass pouring operation. The data has been the base to improve the melter design in this respect.

Cleaning of off gas pipe The melter off-gas carries among others particulate material which can partly deposit inside the off-gas pipe connecting the melter off-gas exit with the first off-gas treatment component. The pipe requires therefore periodic cleaning in order to avoid accumulation of deposited material. The cleaning method had been improved in respect to simplicity as well as effectiveness and safety. Two so-called air blasters are used for cleaning the vertical and horizontal section of the pipe as shown in Fig. 6. - 247 - JAERIConf 99-004

Off-gas Data from PAMELA plant Specific 10 Glass Glass radioactivity 4 pouring ^ pouring of the air in 10 melter cell Bq • , 10 1 m3 10

1 10 V 12 16 18 24 28 Time (h) 10 c ': Partial _ pressure 10 (mftor)

Cdliir •c 10 Af NaCs(B 'A , 10 600 800 1000 1200 1400 Canister Temperature (*C)

Fig. 5: Minimizing loss of volatile radioactive species from glass stream into the meiter cell during glass pouring. Data of partial vapor pressure of some Cs-compound are from Ref. [3] (at lower temperatures) and Ref. [4] (at higher temperature). Data of the increase of radioactivity in the air of the melter ce!l are from PAMELA plant (operated from 1985-1991)

netter roof insulation !

Fig. 6: Off-gas pipe with two air blasters for cleaning the pipe from deposits

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The blasters consists of a pressure vessel and a valve system which allows (a) filling of the pressure vessel with pressurised air and (b) sudden release of the air (within some milliseconds). This sudden release of the air is associated with air wave generation. These waves removes the deposits from the pipe wall. The pressure vessel volume and pressure have been adjusted to the small-scale melter here described. Additional to this cleaning procedure, the vertical section of the off-gas pipe is designed to allow the release of rinsing air at the entrance of the off-gas into the pipe. The rinsing air cools the pipe in this section and helps to avoid that deposits become by a sintering process. Furthermore the complete off-gas pipe has been designed for remote replacement if necessary.

Level detection in the melt tank For easy and safe operation reliable information is required concerning the time when the glass pouring operation should be started. For this purpose the small-scale melter has been equipped with a glass level detection probe. The principle function of this device is shown in Fig. 7. It consists basically of an electrically isolated INCONEL 690 rod which is introduced trough a nozzle of the melter roof. When the glass level arrives at this rod, an electric signal (voltage) is delivered which maintains as long as the rod is in contact with the melt. If glass pouring takes place and the glass level decreases, the electric signal disappears as soon as it looses contact with the melt (see schematic diagram in Fig. 7). The detection rod is protected by a kind of globe in order to avoid undefined contacts of the rod with waste processing material present on top of the glass pool. The device is designed for remote replacement.

EL Signal (voltage) when contact

Melter plenum Iraconel 690 (el. isolated) max.level minJevel

Fig. 7: Principle of level detection probe used for initiating the glass pouring procedure.Time period ® glass pouring operation, © normal gradual increase of glass level

PROTOTYPE NON-RADIOACTIVE TEST FACILITY Tests of the small-scale melter have been performed and are continued in a non-radioactive prototype test facility designed and constructed at the Institut fur Nukleare Entsorgungstechnik (INE) of Forschungszentrum Karlsruhe (FZK). This facility comprises a HLLW-simulant receipt area, a section for feeding glass frit and waste solution into the melter, a melter cell equipped with

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Fig. 8: New small-scale melter in the vitrification cell of the non-radioactive prototype test facility at the Institut fur Nukleare Entsorgungstechnik (INE)

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remote technique, an off-gas line with wet cleaning and dry filtration of the off-gas , and a remotely operable canister handling cell (canister cooling station, lid welding). The work is done to support the VEK-project described elsewhere [1]. Until now two continuos long-term test trials have been performed each lasting 4-5 weeks with folly simulated waste solution including the noble metals. A photograph of the small-scale melter inside the melter cell of this facility is given in Fig. 8. The small-scale melter allows its installation in the hot cell in a hanging position. This solution is advantageous as the area beneath the melter can completely be used for remote handling operations. The melter proved to be easy and safely operable including process control, glass pouring, off-gas pipe cleaning, and glass level detection. Remote handling operation of subcomponents of the melter has also been tested. The glass produced in the first test run has been investigated in the glass lab. It showed the predicted data including waste glass loading and physical and chemical properties. The noble metals balance revealed that the small scale melter is fully noble metals compatible. Further test with the facility are foreseen in 1999 including simulation of defined off-standard conditions and waste solution compositions. The test results will also be provided to the VEK- project. The active operation of the VEK facility is scheduled for 2003/2004.

REFERENCES

[1] J. Fleisch, W. Griinewald, W. Lumpp, G. Roth, W. Tobie, "Status of planing and licencing of the German HLLW vitrification plant", Proceedings of the Tucson Conference on WM, Tucson, Arizona, March 1-5,1998, session 14-7 [2] R. Knitter, "Stoffliche Wechselwirkungen in einem keramischen Schmelzofen zwischen Systemkomponenten und Glasproduktschmelzen", Dissertation University of Munster, Germany, 1988, p. 40-70 [3] M. Asano et al, J. of Non-Crystalline Solids, 92, (1987), p. 245-260 [4] D.W. Bonnell et al, J. of Non-Crystalline Solids, 84, (1986), p. 268-275

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