Electric Melting of Nuclear Waste Glasses

226 Journal of Non-Crystalline Solids 84 (1986) 226-240 North-Holland, Amsterdam

ELECTRIC MELTING OF NUCLEAR WASTE GLASSES State of the art

Chris C. CHAPMAN, James M. POPE and Steve M. BARNES

West Valley Nuclear Services Co., Inc., West Valley, New York, USA

Vitrification of high-level liquid radioactive wastes has become the international approach for converting highly radioactive wastes into a durable solid. With the exception of France and the first production facility in England, the technology for converting the liquid wastes into glass is similar to "all-electric" melters used in the commercial glass industry. However, the preferred melter feed is a 25-40 wt.% slurry rather than dry batch.

The general criteria and bases for design of these melters are presented with a brief review of the historical development. The primary focus of this paper is to review the current status of the designs of the melters in the United States, West Germany, Japan and England. The primary lessons learned in these countries' development programs will be summarized. The influence of melting problems such as corrosion, crystalline sludge, reboil and metal precipitation on the melter designs is reviewed. Future refinements and potential problems that may influence future designs are identified.

1. Introduction and background

The generation of high-level radioactive waste evolves from the reprocessing of nuclear fuel discharged from nuclear reactors. Two primary sources of high-level radioactive waste are from the commercial power reactor fuel cycle and from reprocessing of spent fuel to recover materials for the various countries' defense programs. Typical high-level waste compositions can vary significantly depending on the fuel type and the recovery process used in recovering the nuclear materials. Table 1 provides a brief list of the various reference waste compositions in which the radioactive waste is present. Processes from the early generation reprocessing plants typical of the defense sites also include a large quantity of nonradioactive components. Cleaner, more highly concentrated radioactive wastes are produced in more recent reprocessing plants and are indicative of the future waste compositions. The fission product content and the radioactivity of the recent flow sheets are substantially higher than the present existing wastes in the United States and in Europe. The most voluminous amount of high-level nuclear waste is in the United States at the various Department of Energy facilities. The total volume approaches 300 000 m 3. Existing and future high-level wastes are contained in a solution or slurry. The motivation for converting these wastes into a borosilicate glass is to transform the liquids into a durable solid which will

0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

C.C. Chapman et al. / Electric melting of nuclear waste glasses

Table 1 Typical radioactive waste compositions (approximate)

227

Waste as oxides (wt.%)

Existing wastes Future wastes

Savannah West Valley, Mol, Tokai, West River, USA, USA, Belgium, Japan, Germany DWPF SFCM PAMELA PNC (reference)

Fe203 36.7 47.5 15.5 8.1 4.8 AI203 16.3 5.0 8.5 Na20 6.1 3.9 30.3 39.7 Cr203 0.5 0.8 2.5 0.4 1.3 SiO 2 10.0 1.1 NiO 3.0 1.2 3.9 0.9 0.6 ZrO 2 1.2 7.3 5.8 11.2 SO 4 1.0 1.5 5.9 F 6.9 UO 2 7.1 2.3 0.7 2.0 4.7 MnO~ 10.8 5.4 5.6 1.5 CaO 3.3 2.0 P205 0.1 10.5 1.2 ThO 2 0.9 15.0 BaO 0.9 1.9 4.5 CeO 2 1.2 Cs20 1.2 3.0 5.7 MoO 3 2.6 5.6 13.3 RuO 2 1.1 2.9 6.8 PdO 0.5 1.4 3.7 Re203 0.7 0.5 6.2 19.8 23.3 (Actinides)

Other bal. bal. bal. bal. bal.

minimize the poten t ia l for release of the rad ioac t iv i ty into the env i ronment dur ing its haza rdous period.

2. History of technology for melting radioactive waste glasses

The deve lopmen t of vi t r i f icat ion technology for conver t ing rad ioac t ive wastes into a glass solid began in the ear ly 1960s. The ear ly technologies used the d isposa l con ta iner as the process ing unit. Liquid waste and glass forming chemicals were slowly fed into a hea ted canister , d i rect ly conver t ing the waste l iquid into a mol ten glass. Subsequent ly the canis ter would be cooled and be m a d e ready for disposal . The technology for the convers ion process evolved away from this a p p r o a c h as the process capac i ty requi rements increased. The next process was ca lc ina t ion of the l iquid high-level wastes ( H L W ) fol lowed by mel t ing in ei ther metal l ic crucibles or the d isposa l canister. This process became the reference th roughout the in te rna t iona l programs. A n excel lent his tor ical review is given by McEl roy [1]. The technology has since evolved to

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C.C. Chapman et al. / Electric melting of nuclear waste glasses 229

all-electric glass melters with slurry feeding of the HLW. The primary focus of this paper is to discuss the state-of-the-art of this dominant preference throughout the world.

The generalized design bases for a technology for converting high-level liquid wastes into glass is based on the peculiar aspect of radioactive operations. The process and equipment is housed inside a heavily shielded facility which protects the operating personnel from the high radiation fields inside the "hot cell". Because manned entry into these cells is precluded by the radiation levels, the technology and equipment used in these facilities must be highly reliable to avoid expensive maintenance activities. Thus, the dominant basis for selection of a technology in this field is to minimize the amount of equipment and its complexity while attempting to achieve a long, reliable operating life. Although evaporation of a slurry feed in an all-electric glass melter may be considered less efficient in commercial glass production, the preference for this approach in nuclear waste management is based predomi- nantly upon the simplification in the number of equipment pieces and services required in the "hot cell". The glass throughput capacity for these processes is in general quite modest when compared to commercial glass industry production rates. Table 2 contains the generalized characteristics of these operating facilities and reference melter designs for converting slurries into glasses.

3. Current reference melter designs

The following paragraphs describe the current melter design concepts that are being used. In a following section the similarity and differences between the various designs are reviewed together with the apparent design consensus. The listing and review of the melter designs are in chronological order based on when the melter design has or is projected to be placed into operation.

3.1. Radioactioe liquid-fed ceramic melter (RLFCM)

The radioactive liquid-fed ceramic melter (RLFCM) is the only melter in the United States that is operating radioactively. It is operated by Battelle, Pacific Northwest Laboratories at Hanford, Washington. The details of this melter design are explained more fully in a report by Holton [2]. The schematic of this operating unit is provided in fig. 1. The purpose of this melter is to demonstrate various flow sheets in fully radioactive, pilot-plant conditions.

3.2. PAMELA melter of West GermanY

The PAMELA melter design, shown in fig. 2, is to go into hot operations in the fall of 1985. The objective of this melter is to convert existing waste generated by the former reprocessing plant at Mol, Belgium. The verification

230 C.C. Chapman et al. / Electric melting of nuclear waste glasses

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of this design in hot operations will likely form the basis for proceeding with a production melter in conjunction with a new reprocessing plant within West Germany. The objective of this melter is to convert the existing waste into two glass products. The first product is glass cast into canisters which are approximately 1 m tall and 30 cm in diam. The second product is called Vitromet. For this product glass beads are formed by dripping glass onto a rotating cooled plate. Then the beads are scrapped off into a canister. This glass product is then surrounded by a lead matrix inside a canister. The PAMELA process and melter design are more fully explained by Hohlein [3] and Weisenburger [4].

3.3. West Valley Demonstration Project (SFCM)

The reference design for the West Valley Demonstration Project (WVDP), located in Western New York, is the SFCM, an acronym for the slurry-fed ceramic melter. A schematic of this melter is shown in fig. 3. The project has

C.C. Chapman et al. / Electric melting of nuclear waste glasses 231

H L L W G l a s s m a r b l e s

-"~ '~ " ( : i ~'3 tt.~.~.~2.: S t a r t - u p <',.? " • ~ ' ~ :' "~ " . ~ ~ : h e a t e r s .

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,[1 I Canister

Fig. 2. Schematic of the PAMELA melter design.

been presented in greater detail by Knabenschuh [5]. Specifics about the melter design have been presented by Chapman [6]. The objective of the SFCM is to convert the existing wastes that were generated from the reprocessing of about 640 tons of nuclear fuel. This is a remedial action project in which the existing waste will be converted into borosilicate glass in a campaign of about two years. Subsequent to the completion of waste vitrification, the facility will be decommissioned.

3.4. Defense waste processing facility (D WPF)

In contrast to the previously identified projects and programs, the Defense Waste Processing Facility (DWPF) located in Savannah River, South Carolina, is a long-term production facility. The reference design for the DWPF is shown in fig. 4. The DWPF melter design is the largest production melter currently planned for operation in the world. The function of this facility is to

232 C. C. Chapman et a L / Electric melting of nuclear waste glasses

Plan Section of Refractory

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7'11-112" (HOTI

C.C. Chapman et a L / Electric melting of nuclear waste glasses 233

work off the existing wastes and the future generated wastes from the Savannah River operations in the next 30 years. The details of this plant are explained in greater detail by Raudenbush [7].

3.5. Japanese vitrification plant of PNC

The Japanese have been operating a reprocessing facility at Tokai and have a liquid waste inventory of over 150 m 3. The vitrification facility which will

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process and vitrify the high-level wastes which have been and will be generated from the reprocessing plant will be in operation in about 1992. The current, advanced concept that is scheduled for their vitrification plant is shown in fig. 5. The details of the design are explained in a paper by Sasaki [8].

3.6. Hanford waste vitrification plant

The Hanford waste vitrification plant is scheduled to come on line in about 1995. The reference design, shown in fig. 6, is the basis for this vitrification facility; however, the reference design may change to the reference design of either the DWPF at Savannah River or the SFCM at the West Valley Demonstration Project, depending on the operational success of these designs. The current plans are explained in a paper by Gurley [9].

3. 7. United Kingdom Harwell melter (HJCM)

Although the first vitrification facility that will go into operation in the United Kingdom is the French AVH rotary calciner-metallic melter, the United Kingdom has supported development of an advanced vitrification

PASSIVE COOLED

Fig. 6. HWVP reference melter design.

CI C" Chapman et al. / Electric melting of nuclear waste glasses

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concept which uses the technique of joule heating similar to the technologies presented in the previous paragraphs. The reference pilot plant design is given in fig. 7 and explained in detail in a technical report by Robinson [10].

4. Comparison of melter design technology

The melter design concepts presented in the previous section vary in detail but have many similarities. Some of the design differences are a result of more recent findings in the process and due to problems encountered during pilot plant experiments after early designs were "frozen". Other differences between the melter designs are more a function of the waste that is being vitrified rather than a difference in technology. In the following paragraphs the major general design features for a melter are reviewed and compared between the various melter design concepts.

4.1. Melting cavity geometry

The majority of the glass melter designs presented have a rectangular or square cavity geometry. The major exceptions are the DWPF design which is cylindrical and the SFCM which is prismoidal with the side walls and floor sloping towards a bottom electrode. The primary reason for the DWPF melter

236 C. C. Chapman et al. / Electric melting of nuclear waste glasses

having a cylindrical configuration is the concern with refractory integrity after long operating time. A cylindrical geometry assures that cave-in of the refractory is unlikely. The geometry of the West Valley melter design was selected primarily because of concerns over molten metal or conductive sludges precipitating from the molten glass. The significance of precipating conductive material and its influence on the melter design cavity is not yet clear but may be a major factor in future melter designs.

4.2. Electrode configuration

The dominant electrode configuration is that of large plates made of a high nickel alloy material, Inconel-690 ® (60% Ni, 30% Cr, 9.5% Fe). The more recent preference is to provide at least two zones for power control. This is accomplished by having at least two sets of electrodes in the molten glass body. The DWPF, HWVP and the PAMELA designs have this feature. The SFCM system provides for zone control but with a three-electrode configuration. This is accomplished by adjusting the power dissipated between the three power circuits. Similarly, a separate set of rod electrodes is used near the bottom of the PNC melter design. Thus, the consensus is that the electrodes should be made of large plates opposing one another in the melting cavity and to have at least two sets, enabling zone heating of the glass.

In most of the electrode designs, the predominant method of assuring long operating life, even with relatively corrosive glasses, is to provide ~tirect air cooling of the electrode. In nearly all cases cooling channels are machined in the back face of the plate electrode. By passing the cooling fluid through the channels, the power electrodes are maintained at a temperature lower than the glass. The exception is the DWPF design which uses indirect water cooling to maintain a cool electrode.

4. 3. Refractory structure and composition

Almost without exception the preferred glass contact refractory is a high chromium oxide material, typically 15 cm or more in thickness. The melters in the US employ a product of the Carborundum Company, Monofrax K-3 ® (27% Cr203, 60% A1203, 6% MgO). Many of the foreign countries also use this material or a product made in Europe, ER 2161 ® (28% Cr203, 28% AI203, 30% ZrO 2, 14% SiO2). The dominant refractory structure in these melters is to use a high-integrity glass contact material backed up by a material that has less glass corrosion resistance but has higher thermal resistance. Two or more layers of the secondary refractories are typical. The final layer of refractory is usually a highly insulating material used to reduce heat losses. Various high alumina and conventional glass furnace refractories are used in this multi- layered approach. The only exception to this general approach is the DWPF design. The back side of the glass contact material is insulated by a relatively thin layer (less than 2 cm) of fibrous insulating product which is chilled by a water-cooled containment vessel.

C. C. Chapman et al. / Electric melting of nuclear waste glasses 237

An unusual characteristic of this technology when compared to commercial glass industry practice is the encasement of the melting furnace within an air-tight vessel. The general approach is to use a water-cooled jacket beneath glass level to assure that migrating molten glass cannot penetrate the refractories to contact the metal shell and cause an electrical short.

4. 4. Draining techniques

The draining technique for the various melter designs has broken into two preferences. In the US the approach is to use a bottom takeoff, overflow type drain for routine pouring. The method of starting and stopping the outflow of glass during the canister change-out period differs in technique but all use an overflow structure. For the DWPF, a differential pressure is introduced between the melting cavity and the drain spout. To achieve filling of the canister a slight underpressure is induced in the canister and glass overflows into the canister. For the other style of overflow drain designs an airlift is used to control glass draining. This is achieved by injecting air bubbles into the glass column of the riser which causes the pumping out of molten glass into the receiving canister. Termination of outflow is implemented by stopping the air flow.

The PAMELA design preference is to remove molten glass from the floor of the glass melter. Flow is stopped by cooling the drain pipe which cools and freezes the glass in the drain pipe. The apparent advantage for this approach is to remove material such as refractory sludge and molten metals from the melting cavity during routine draining. Although PNC has an airlift overflow routine drain, they plan to use the bottom freeze drain periodically ( - weekly) to remove potential precipitates.

Final draining of the melting cavity is also different between the various melter designs. For the bottom drain systems of PAMELA and PNC, the final drain-out is the normal draining system. The DWPF design also uses a bottom-drain total glass removal. However, for the other designs final draining is achieved by evacuated canisters which siphon the glass from the melting cavity. The evacuated canisters are suspended from the crane over the glass melter during the evacuation process. The Harwell JCM design plans to use this approach even for routine filling of canisters.

4. 5. Startup techniques

The dominant approach for starting up the glass melters is to use resistance heaters in the plenum area of the melting cavity. The major difference between the various melter design concepts is whether the resistance heaters are permanently installed or removed after joule heating is achieved.

The PAMELA melter uses molybdenum disilicide resistance heaters in blind holes behind the interior plenum refractory. The DWPF melter design uses Inconel pipes which pass horizontally through the plenum and are used


during startup as well as to boost throughput. The PNC design concept is to use silicon carbide heaters in blind holes similar to that used by PAMELA. For the SFCM, the RLFCM and HWVP designs, heaters are lowered into the melting cavity through flanges in the roof and are removed after joule heating is achieved.

A summary comparison of different melters is provided in table 2.

4. 6. Difficulties in the melting process

During the development programs at the various sites, difficulties relating to melting of these complex waste glasses have occurred. The major problems relate to the feed streams and glass chemistry more than to the melter design. However, melter design concepts have been modified to attempt to accom- modate the process upsets observed during pilot plant experiments. The difficulties encountered include:

Reaction layer foaming; Glass reboil; Sulfidation corrosion of interior metal components; Phase separation: crystalline phases, molten metals. Reaction layer foaming and sluggish melting can be a consequence of

batching anomalies. This problem causes the processing rate to be decreased and is best resolved by making changes in the composition of the feed to the melter. Several approaches have been used to improve the production rate even under these conditions. The DWPF, PAMELA and PNC designs can and have used the startup heaters to boost the throughput by providing heating from the plenum to the top surface of the molten glass.

Glass reboil encountered in the commercial glass industry can also be a problem in the radioactive process. Adjustment of the redox state of the glass has been found to be successful but this phenomenon must be considered in the design. To account for glass reboil foaming, the plenum above the molten pool is made as large as practical. Most of the designs include an in-plenum TV camera to provide a continuous monitor of the melting process. If excessive foaming is observed through the TV monitor, the operator can then take the appropriate counter measures.

Phase separations in the melter feed have caused significant problems in pilot plant melters. As can be noted from table 1, many of the waste glasses approach the solubility limit for the transition metals. This has resulted in the formation of crystals which can precipitate to the floor of the melter and result in sludge filling in the melting cavity. Chemistry control of the process is needed to avoid this potential problem. However, providing the capability to mobilize and remove these crystalline materials should be incorporated into the melter design in case a process upset should occur. This unusual situation has been accommodated by providing air agitation of the molten pool by an air lance and draining system.

To avoid glass reboil, the oxidation state is adjusted with reducing agents.

C. C. Chapman et al. / Electric melting of nuclear waste glasses 239

If the quantity of reducing agent is excessive, reduction of precious metals and transition metals could occur and did so in pilot plant experiments. This can be quite troublesome for flat bottom melters because electric shorting between the electrodes through the molten metal can occur and did. This can effectively stop the operation of the melter. Close chemistry control and sloped bottoms to the drain outlet have been used to address this potential problem.

The low solubility of ruthenium oxide within molten glass is known. High burnup nuclear fuels contain relatively high concentrations of ruthenium. In recent, full-compositional simulations of wastes containing higher ruthenium oxide, precipitation of the ruthenium oxide to the floor of the melter has been observed. Because the oxides of ruthenium are electrically conductive, shorting out of the melter has occurred. The design response to this problem is the same as for molten metal precipitation.

Another life shortening phenomenon that has been observed in the pilot plant experiments is sulfidation corrosion of metal components. When typical construction alloys are exposed to a sulfur-containing environment above the molten glass in the temperature range of approximately 4 0 0 -9 5 0 ° C , rapid sulfidation attack of the metal components has been observed. This may result in reducing the operating life of the exposed metal to as little as 4-12 weeks. The counter measure that is currently being employed is to maintain these metal parts above or below the range mentioned.

5. Potential melter design refinements

The most troublesome issue for this technology is precipitation of metals a n d / o r oxide constituents that can cause electric shorting between the power electrodes. This concern is more important for high burnup fuel wastes because of higher precious metal and ruthenium content. Sloping the floor to a central collection area or to a bottom drain is the current design preference for avoiding this potential life-limiting problem. The effectiveness of these design approaches is not yet fully demonstrated but appears to provide an adequate solution.

The primary refinements of this technology will center on extending the melter's operating life. Improved corrosion-resistant materials both beneath the glass level and in the vapor space will be the likely focus of future research and development. The current operating temperatures for the glass melters are in the range of 1150-1250°C in the bulk of the glass. Improved chemical durability may be achieved with glasses of higher melting temperatures and this is an additional area for refinement. Use of higher melting temperature electrode materials that are oxidation-resistant, such as tin oxide or higher chromium alloys, appears to provide an opportunity for increasing the melting temperature.

Future refinements will center on the above objectives until results from production facilities are obtained. Many of the melters that have been de-

240 c. c. Chapman et al. / Electric melting of nuclear waste glasses

signed are going into product ion in the very near future or their designs have been frozen because of schedule constraints. Thus the major refinements and design changes will likely be a result of product ion experience in the operat ing plants. The large amount of pilot plant experience and prel iminary radioactive pilot plant experience in the US and throughout the world provides high assurance that radioactive waste can be safely and effectively converted into a durable glass material through use of this fundamental technology.

References

[1] J.L. McEIroy, in: Prec. ANS Topical Meeting on Treatment and Handling of Radioactive Wastes, Richland, WA (April 1982) (Battelle Press, Columbus, Richland, 1982) p. 177.

[2] L.K. Holton, PNL-SA-12282, Pacific Northwest Lab., Richland, WA (June 1984). [3] G. Hohlein, Nuclear Europe 2 (1985) 16. [4] S. Weisenburger, ibid., ref. 1, p. 190. [5] J.L. Knabenschuh, in: Prec. Symp. on Waste Management, Tucson, Arizona, Vol. 1 (March

1985) p. 41. [6] C.C. Chapman, Advan. Ceram. 8 (1984) 149. [7] M.H. Raudenbush, Nucl. Eng. Int. (June 1985) p. 47. [8] N. Sasaki, in: Prec. ANS Topical Meeting on Fuel Reprocessing and Waste Management,

Jackson, Wyoming (August 1984) Vol. 1 (Amer. Nucl. Soc., LaGrange Park, IL, 1984) p. 1. [9] R.N. Gurley, ibid., ref. 5, p. 179.

[10] K.S. Robinson, AERE-Rl1082, AERE Harwell, Oxfordshire, United Kingdom (July 1984).

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Electric Melting of Nuclear Waste Glasses