7
Other Waste Forms

This chapter contains general descriptions of several procedures or processes for producing waste forms from the Idaho National Engineering and Environmental Laboratory (INEEL) calcine other than the vitrified (see Chapter 5) and cementitious (see Chapter 6) waste forms already discussed. Except for the hot isostatic press (HIP) option, these processes come from sources other than the Department of Energy (DOE) literature that was presented to the committee. In most cases, these procedures are at an early stage of conception and will require a significant amount of additional investigation to assess their feasibility for practical use with the compositionally unique INEEL calcines. These descriptions are, therefore, necessarily general and are included primarily for purposes of completeness.

Some of these processes, such as those involving sintered glass (Gahlert and Ondracek, 1988) and glass-ceramics (Hayward, 1988), are not new, having been first suggested in the early 1980s. Unlike fully vitrified forms, which are composed ideally of a single-phase, chemically homogeneous glass, the glass/ceramic waste forms are composed of a mixture of one or more crystalline phases bonded together, and the partially vitrified waste forms surround these crystals with a glassy phase.

Most of the waste forms discussed in this chapter will be physically and chemically heterogeneous on a microscale, so that the overall properties such as leach resistance will depend on the chemical properties of each crystalline and glassy phase. However, there are no scientific reasons preventing a properly prepared waste form of this heterogeneous character from having a chemical durability equivalent or superior to that of a fully vitrified waste form. In certain cases, there are important manufacturing advantages in producing heterogeneous, rather than homogeneous, waste forms.

A feature all of the following procedures have in common is that they utilize the existing INEEL calcine in its present state. No dissolution of the calcine or separation of the radioactive elements from the calcine is anticipated. These procedures fall into two categories identified below as simulated spent fuel (SSF) and partial vitrification.

SIMULATED SPENT FUEL

The simulated spend fuel (SSF) process consists of forming the existing alumina and zirconia calcine into strong, cylindrical pellets that are then packed into a suitable metal tube such as zircaloy or stainless steel and sealed (basically the way that ordinary light water reactor



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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory 7 Other Waste Forms This chapter contains general descriptions of several procedures or processes for producing waste forms from the Idaho National Engineering and Environmental Laboratory (INEEL) calcine other than the vitrified (see Chapter 5) and cementitious (see Chapter 6) waste forms already discussed. Except for the hot isostatic press (HIP) option, these processes come from sources other than the Department of Energy (DOE) literature that was presented to the committee. In most cases, these procedures are at an early stage of conception and will require a significant amount of additional investigation to assess their feasibility for practical use with the compositionally unique INEEL calcines. These descriptions are, therefore, necessarily general and are included primarily for purposes of completeness. Some of these processes, such as those involving sintered glass (Gahlert and Ondracek, 1988) and glass-ceramics (Hayward, 1988), are not new, having been first suggested in the early 1980s. Unlike fully vitrified forms, which are composed ideally of a single-phase, chemically homogeneous glass, the glass/ceramic waste forms are composed of a mixture of one or more crystalline phases bonded together, and the partially vitrified waste forms surround these crystals with a glassy phase. Most of the waste forms discussed in this chapter will be physically and chemically heterogeneous on a microscale, so that the overall properties such as leach resistance will depend on the chemical properties of each crystalline and glassy phase. However, there are no scientific reasons preventing a properly prepared waste form of this heterogeneous character from having a chemical durability equivalent or superior to that of a fully vitrified waste form. In certain cases, there are important manufacturing advantages in producing heterogeneous, rather than homogeneous, waste forms. A feature all of the following procedures have in common is that they utilize the existing INEEL calcine in its present state. No dissolution of the calcine or separation of the radioactive elements from the calcine is anticipated. These procedures fall into two categories identified below as simulated spent fuel (SSF) and partial vitrification. SIMULATED SPENT FUEL The simulated spend fuel (SSF) process consists of forming the existing alumina and zirconia calcine into strong, cylindrical pellets that are then packed into a suitable metal tube such as zircaloy or stainless steel and sealed (basically the way that ordinary light water reactor

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory (LWR) fuel rods have been prepared for decades). The final step is to bundle the tubes and seal them in a second (metal) protective canister for final storage. The premise of this procedure is that if the Calcined Solids Storage Facility (CSSF) calcine is packaged to satisfy the acceptance requirements for commercial spent nuclear fuel (SNF) and can be made to emulate the internal properties and containment features of SNF, then the regulatory issues for permanent disposal should be greatly simplified. This premise is unverified at this time, and, of course, Resource Conservation and Recovery Act (RCRA) issues would still have to be addressed. Much of the equipment already exists for forming pellets approximately 2.5 cm (1 inch) in diameter and height by pressing the existing calcine, sintering the pellets at elevated temperature to increase their strength and chemical durability, and packing and sealing the pellets into metal tubing. These procedures are similar to those that have been used for many years to manufacture commercial fuel rods. These processes would have to be modified and optimized, however, to the INEEL powdered calcine mixture, which might also require additives to improve its sintering properties, strength, and chemical properties. There is very little information available at this time as to what cold pressing (pressure and time) and sintering (time/temperature profile) conditions will be needed for the existing calcine, along with any additives that might be needed, to produce a high-strength, chemically durable pellet with the desired properties. Another issue for which information would have to be developed is that of the calcine blending requirements. The degree of calcine homogeneity required for this physical pressing process is likely to be different than for a chemical process using materials in solution. Investigations would be needed to address what level of inhomogeneity the SSF process could tolerate, whether blending different ages and types of calcine is an adequate strategy, and whether blending during retrieval would pose any problems. There are no readily identifiable obstacles to producing a metal-clad pellet from the existing calcine, but considerable developmental work is still needed to determine if pellets of sintered calcine, with acceptable properties, can be produced by this procedure. In those calcines containing fluorides and nitrates, the behavior of these materials during the pressing and sintering steps will need to be determined, as would any tendency of the compacted pellets to release gases or to corrode the metal container.1 There is no reason why the SSF pellet should have the exact dimensions of actual fuel. The diameter of a nuclear fuel element is set by heat transfer considerations that would not be the same for a SSF waste form. SSF rods could be considerably greater in diameter than actual fuel rods, thus reducing the number required. Or, they could be packaged in odd shapes, to fill void volumes in a repository. 1   Testing for these behaviors undoubtedly would be required to certify the waste form, but reasons can be advanced for expecting them to be a minor issue. Although NRC (1997) reported on radiolytic degradation of fluorides in a fluoride salt, with generation of fluorine gas and possible corrosion products, the chemical environment in the INEEL high-level waste calcine differs from the environment of the Molten Salt Reactor Experiment (MSRE) fuel salts in its thermodynamic activity for fluorine and oxygen, and hence in the expected reactivity and recombination of fluoride compounds. Because the calcium fluoride (CaF2) salt in the calcine is structurally stable, the fluorite crystal lattice would tolerate defects. Although radiolysis can break the ionic bonds to create fluoride ions in the crystal, these stripped ions can readily drift to a vacancy, crystal defect, or interstitial site and recombine. The effectiveness of this recombination, which prevents the release of gases such as fluorine, is enhanced by the ionic character of the calcium fluoride bonds. The high electronegativity of fluorine atoms, the lack of oxidizable or volatilizable ions, and the well-known retention of atomic species in strong crystalline matrices assures that not much (likely none) of the elemental fluorine or volatile fluorides would be released. This contrasts with the behavior of the fluoride compounds in the MSRE fuel salts, which have bonds of more covalent character and in which less effective recombination would be expected. There, oxidizable UF4 was present and formed a volatile product (UF6), and a mechanism existed for its migration and reconcentration outside the salts. These MSRE conditions are not analogous to those of the SSF waste form considered here.

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory A potential advantage of SSF is that it does not add any more inert material to the calcined waste. This minimizes waste volume, and also makes this process simpler than the alternatives. This simplicity should translate into lower cost and radiation exposure. Another potential advantage is that transportation issues might be simpler, insofar as the safe transport of properly packaged spent fuel has already been established. Radioisotopes are more easily leached from calcine than from some other waste forms. An unknown issue is the leach rate of radioisotopes from SSF, and whether this leach rate is acceptable. For example, a high leach rate of fission products such as Cs and Sr may not be relevant to repository performance because these radioisotopes will likely have been diluted and suffered natural decay by the time they would navel to the accessible environment. The leach rates of longer-lived radioisotopes would be more important limitations. Another unknown is the degree to which the waste form would exclude moisture. Fabrication Issues SSF pressing and sintering operations for calcine would have similarities and differences compared to these operations for fresh nuclear fuel (UO2) fabrication. Some of these comparisons are noted below. The firing temperature and redox conditions would differ. Conditions for UO2 are 1650-1700 °C in a hydrogen atmosphere, with controls on dimensions, density, and stoichiometry. In contrast, the calcine can be heated in oxidizing conditions (e.g., air) since the heavy element valence states are stable. The sintering temperature would be approximately 1000 °C. Residual uranium will be in the high oxidation states, which provide the highest affinity for cesium retention. Silica could be added as a sintering aid to promote bonding and to capture cesium. A variety of commercial furnace systems are available for these conditions and can be utilized relatively inexpensively. Shielded cell and remote-handled operations would be required due to the radioactivity levels of the calcine. Remote-handled technology is already in use in UO2 and some PuO2 fuel fabrication plants. Automated powder pressing, sintering, rod loading, and bundle fabrication are also already used. The activity of the calcine implies that these operations be done in a shielded cell, with possible complexities. Nevertheless, such operations would probably impose fewer facility requirements and less cost than chemical processing. Thermal management issues would have to be investigated for SSF production and storage. Although calcine temperatures in the bins are quoted in this report as 190 °C for the zirconia calcine and 440 °C for the alumina calcine, the temperature elevation in a smaller size sample during its production and storage could be controlled to be smaller, as with air streams and cooling channels that can be incorporated in equipment design features to effect adequate heat transfer. Were this temperature rise to be controllable to only a few degrees, heat generation would not be a dominant issue affecting the process design. High temperatures could pose difficulties in blending suitable binders and/or plasticizers into the feed material for pressing. Operational issues in blending and combining with material additives would have to be developed for SSF, but again, fuel fabrication techniques exist as a useful reference to guide this development. Agglomerates of UO2 and PuO2 used in fuel manufacturing have sizes similar to the calcine particles. Organic binders are commonly used for pressing and are readily burned away in sintering. Zoned furnace heating or multi-staged treatments at controlled temperatures could be used to provide desired waste form properties.

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory Off-gas treatment would differ due to the presence of fission products and other volatile species. In the SSF concept, fluorides, ruthenium, iodine, and mercury are among those species that should be planned for off-gas entrapment. Since the oxidative sintering of calcine does not require high gas flows, as is required for UO2 sintering, the off-gas control could be simpler. In summary, the committee believes that fuel fabrication techniques can be appropriated and modified to develop a suitable SSF process, although significant development and testing would be required. The overall merit of a SSF approach would depend on the qualification and regulatory approval of the final waste form, as discussed next. SSF Waste Form The anticipated but unverified advantage of this SSF waste form is an expected ease in its qualification compared with other types of waste forms, insofar as it would be analogous to an already qualified waste form (i.e., commercial spent fuel, which must qualify technically if direct disposal is pursued).2 Other significant advantages are the large waste loading (which could be up to 70 to 90 weight percent depending upon the additives needed), the availability of existing commercial equipment, the extensive experience in fabricating commercial nuclear fuel by this procedure, an expected high throughput (several presses can be used simultaneously), and the double encapsulation in corrosion resistant materials whose performance is well known. All of these factors should contribute to a relatively inexpensive waste form (as compared, for example, with complete vitrification of the existing calcine). These potential advantages make the SSF concept an attractive one for further study at this stage of development of the INEEL HLW program. However, as stated above, the overall merit of a SSF approach would depend on the qualification and regulatory approval of the final waste form. PARTIAL VITRIFICATION A common feature of the following five partial vitrification processes is that the existing calcine, plus additives, will be only partially vitrified so that the final waste form consists of crystalline material(s) embedded in a glassy matrix. This type of partially vitrified solid is loosely referred to as a "glass-ceramic," but a true glass-ceramic is a material that at one time was totally glassy and later partially or fully crystallized in a controlled manner. Unlike full vitrification, where the final waste form is composed ideally of a chemically homogeneous glass free of crystalline particles, these processes yield a chemically inhomogeneous waste form that is only partially glassy. There are no scientific reasons why a partially vitrified waste form cannot be made that will meet all of the existing performance requirements for fully vitrified waste forms. However, at this time, the absence of current certification for partially vitrified waste forms is a perceived disadvantage for partial vitrification. A common feature of all five procedures listed below is that, as with the vitrification options discussed in Chapter 5, they all use the same type of feed stock, namely, a mixture of 2   That is, a SSF waste form would be analogous to spent fuel material forms primarily in the way it would be produced (as a sintered oxide), and not necessarily in its detailed material form, composition, physical properties, and chemical properties, all of which would be different. Although the radionuclide-bearing phases might differ from those of SNF forms, a SSF waste form might provide sufficient similarity to SNF waste forms that are to be disposed of in a geologic repository, particularly if production similarities were to translate into similarities in material form properties to permit a straightforward comparison of waste form performance.

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory the existing and future calcines at INEEL. Thus, the first step will be to recover the calcine and blend it together, with or without additives, to form a sufficiently large supply (e.g., enough to last 1 to 2 years) of a mixture of calcine sufficiently homogeneous to bound its important physical and chemical characteristics within an acceptable range. Cold Pressing Followed By Sintering In the cold pressing followed by sintering (CPS) procedure, a mixture of calcine and additives (glass formers) is cold-pressed into a shape of the desired dimensions. The cold-pressed, partially porous compact is then heated through a controlled temperature/time profile to form (sinter) a dense, mechanically strong, and chemically durable solid. During the heat treatment, numerous chemical reactions occur between the calcine particles and any additives to form a heterogeneous composite of crystals embedded in a glassy matrix. The purpose of the additives is to control the type(s) of crystal(s) present and the properties of the glassy matrix. The objective is to form a partially vitrified waste form composed of crystals and glass, both of which have a chemical durability that meets existing requirements. The end result of the SSF and CPS procedures is basically the same—namely, a partially vitrified waste form that should be dense, strong, and chemically durable. However, the size and shape of the waste form wood probably be different, and no encapsulation of the waste form in protective metal tubing is envisioned for the CPS procedure. Three advantages of the CPS procedure are that (1) no further processing of the existing calcine, apart from physical blending operations to achieve sufficient homogeneity, is necessary; (2) the expected high waste loading (greater than 50 percent by weight in the final waste form) reduces the volume of waste to be permanently stored; and (3) the procedure is based on well-established technology for pressing and firing consolidated powders. All of these factors should reduce the cost of a waste form made by CPS compared to complete vitrification. Hot Uniaxial Pressing Hot pressing differs from the CPS procedures in that the pressing and firing (sintering) steps are combined into a single step, so that the mixture of calcine and any additives is consolidated under unidirectional pressure in a heated mold. This process again produces a partially vitrified waste form composed of crystals embedded in a glassy matrix. The hot pressing process has essentially the same advantages and disadvantages as noted for the CPS process. In general, hot pressing is more difficult and requires more complex equipment than cold pressing followed by firing (sintering) in a separate furnace. There are reports of previous work in Germany (Gahlert and Ondracek, 1988) where waste forms up to 30 cm in diameter have been successfully prepared using hot pressing. In a presentation to the committee, Professor Werner Lutze of the University of New Mexico presented encouraging data from his experiments on a simulant of INEEL zirconia calcine (a surrogate representing the calcine type in CSSF #5) and on a simulated Hartford tank waste form (Lutze, 1998). In these experiments, hot pressing a mixture of the nonradioactive calcine surrogate (30 and 40 percent by weight) with silica and soda produced partially vitrified products. The partially vitrified simulated waste form he produced from the simulated CSSF #5 calcine contained zirconia (ZrO2) and fluorite (CaF2) crystals embedded in a chemically durable sodium-zirconia-silicate glass matrix. Table 7.1 summarizes the properties of this partially vitrified "sintered glass" waste form and compares them to properties reported in INEEL literature for partially vitrified waste forms produced and studied in the past

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory at INEEL. Although he used hot isostatic pressing (HIP) to produce partially vitrified waste forms of good chemical durability, he believed that the same results could be obtained for partially vitrified waste forms made by unidirectional hot pressing (i.e., hot uniaxial pressing, or HUP), which is simpler and less hazardous than HIP, discussed next. Hot Isostatic Pressing INEEL has HIP experience (Russell and Taylor, 1998) and has produced many simulated waste forms (Staples et al., 1997) by this procedure, starting in the 1970s. The process consists of placing the starting materials (calcine plus any additives) in a suitable (usually metal) container that can withstand the temperature and pressure to be used, and then heating the contents in a pressurized chamber to a desired temperature for the necessary time. The chamber is then depressurized, and perhaps cooled, whereupon the reacted material is removed and the procedure repeated. There are no scientific reasons why a partially vitrified waste form with the required properties cannot be produced by the HIP process. However, the committee does not consider the HIP process to be a practical method for consolidating the INEEL calcine for three reasons: First and most important are the safety aspects of the HIP process, which uses a highly compressed gas. Because of the high temperatures needed to react and densify the INEEL calcine (at least 1000 °C and probably higher), some type of pressurized gas must be used instead of a less dangerous liquid. In prior experiments at INEEL on samples less than 15 cm (6 inches) in diameter, simulated wastes have been typically hot pressed in argon at 20,000 psi and at 1050 °C (higher temperatures have been used and may be needed depending on the waste composition). The potential consequences of HIP facility accidents are significant because of the pressures and temperatures involved in the process, and these risks from HIP operations were not evaluated in the site risk assessment (Slaughterbeck et al., 1995). The engineering and metallurgical problems that would be encountered in designing a large-scale HIP facility to meet applicable safety standards and codes are probably too large to make HIP a viable option.3 A second factor is the estimated low-output rate for producing a waste form by HIP. Even with several HIP production lines operating simultaneously, a relatively low-output is anticipated since HIP, as with the HUP process described previously, is a batch process. The possibility of scale-up [50-cm (20-inch) diameter samples have been mentioned] is limited by the problem of finding materials from which large molds can be made (to operate at high temperatures and pressure), as well as the increased safety risk presented by the larger volume of compressed gas as the size of the pressure chamber is increased for higher output. Finally, the HIP process is inherently more expensive than the cold-or hot-pressing processes mentioned above. 3   To expand on the safety distinction between the HUP and HIP processes, HIP requires gas for pressurization because of the high temperatures involved, whereas HUP can be done under high pressure applied hydraulically with metal pistons. This latter method of applying pressure is much safer because of the much lower stored energy of the contents under pressure, as represented in the product of pressure and volume. Although the pressure is comparable in both cases, the volume change after expansion is small for liquids but large for gases. The committee concludes that the HUP process could be safely performed in a hot cell, without the pressurized gas in a HIP process that would pose an additional safety hazard.

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory TABLE 7.1 Comparison of a HIP Sintered Glass Waste Form Produced at the University of New Mexico to Partially Vitrified (Glass-Ceramic) Waste Forms Produced at INEEL   Sintered glass produced via HIP at the University of New Mexico, using a nonradioactive surrogate to simulate INEEL calcine Glass-ceramic produced via HIP at INEEL Additives Amorphous silica or/and Na2O Amorphous silica and MgO Waste loading Up to 50 wt%; demonstrated higher loading is possible 60 to 80 wt% Processing conditions Pressure: I to 30 Mpa Pressure: 138 MPa   Temperature: 750 to 850 °C Temperature: 1000 °C   Hot isostatic (could also use hot uniaxial) pressing Hot isostatic pressing Processing experience Prototype plant in Karlsruhe, Germany; up to 30 cm glass cylinders Laboratory-size samples only Waste components 1. Most waste components (CaO, Na2O, Fe2O3, MgO, P2O5, and (B2O3) are completely dissolved in the glass phase, 1. Waste components partially occur as calcine relic (baddeleyite, fluorite, etc.).   2. Al2O3 and ZrO2 are embedded in the glass phase and occur as crystalline phases, 2. New crystalline phases form, e.g., apatite, zir con, greenockite, Ca-Mg borate, plagioclase, etc.   3. CaF2 present as undissolved fluorite, 3. Components incompatible with ceramic phases partition into the glass phase. Glass phase 1. The glass phase forms a continuous matrix to hold dissolved waste components, 1. The glass phase occurs as discontinuous islands and ceramic phases dominate the waste form.   2. Crystalline phases are completely embedded in the glass matrix. 2. Glass phase is 0-22 wt% of the waste form.   3. Glass phase accounts for >50 wt% of the waste form.   Chemical durability Forward rate for Na is < 1 gm-2d-1. Forward rate for Na is up to ~37 gm-2d-1   Chemical durability determined by the high silica glass matrix (65-75 wt% SiO2). Chemical durability determined by hydrothermally unstable phases, e.g., surefire (6.6-15 wt%) and nepheline Microstructure of waste form Simple and easily manipulated Complicated and difficult to manipulate and to characterize NOTE: Prepared by W. Lutze, University of New Mexico, 1998.

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory Based on the information available, the committee sees no future merit in the HIP process as a practical method for processing the large amounts (volumes) of calcine located at INEEL. The committee knows of no identifiable reason for DOE to continue investigating the HIP process. Embedding Calcine in Glass This procedure (Sombret, 1998a; Sombret, 1998b; Bonniaud, Labe, and Sombret, 1968) involves mixing the existing alumina or zirconia calcine with glass particles ("frit") and melting the mixture in a suitable furnace. The partially vitrified end product would consist of particles of the melted calcine distributed and embedded in a chemically durable glass matrix. Anticipated advantages of this technique are a high waste loading estimated at 50 weight percent or more, a high throughput similar to that of full vitrification processing, and no requirement for high pressure steps. Unresolved issues are the lack of information for the flow characteristics of the molten mixture, the potential settling of the unmelted (higher density) calcine in the furnace, and the acceptability of a partially vitrified waste form. Synthetic Rock Waste Forms A collaboration of investigators at the Lawrence Livermore National Laboratory and the Australian Nuclear Science and Technology Organization has adapted a synthetic rock (Synroc) medium (with a titanate pyrochlore host phase) for potential use in mobilizing excess weapons-grade plutonium. The material produced has a long-term durability (with the appropriate time scale set by the 24,000-year half-life of plutonium-239). In this material, the chemical recovery of plutonium is difficult enough to provide a measure of proliferation resistance. This Synroc approach in principle could be applied to the INEEL HLW calcine (Godfrey, 1999; Jostsons et al., 1996, 1997). The relatively low (i.e., compared to the characteristics of DOE HLW inventories at Hanford and Savannah River) sodium content and relatively high aluminum and zirconium content of the INEEL HLW calcines are compositional features that would favor good product stability and high waste loading. However, any application of Synroc technology to the INEEL HLW calcines would require development of a process that could accommodate the compositional variations of INEEL calcine. As with all of the nonvitrification options discussed in this chapter, qualification of the waste form would be required. SINGLE-USE MELTERS If a partial vitrification waste form (i.e., one consisting of crystalline particles embedded in a glassy matrix) is considered acceptable for disposal, then a "single-use melter" offers some advantages over the continuous melters now in use. In this concept, a mixture of calcine and powdered glass frit would be partially melted in a container (i.e., the interior crucible) that could become part of the final waste container.4 Some of the advantages offered by this concept are as follows: 4   This "single-use melter" concept differs from the "in-can" melting concept that was designed in the 1960s.

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory Much higher waste loadings could be achieved than are possible for a fully vitrified product made in a continuous melter. The viscosity and conductivity of the melt and the formation and settling of any precipitates are of major concern in a continuous melter, in which the melt must flow properly to exit the furnace. In contrast, waste loadings of 50 weight percent or more could be expected in a single-use melter, since the Objective is only to form a viscous mass of unmelted solids surrounded by a glassy matrix that has an acceptable chemical durability. Higher temperatures could be used, if needed, to accelerate the melting process (i.e., increase solubilities to shorten the melting time) since the service life of the refractory would not be a critical factor. Although higher temperatures cause (a) more rapid corrosion of the furnace, resulting in a shorter operating life, and Co) increased volatilization of species such as Cs and Hg, these disadvantages are not critical in the single-use concept.5 Shorter ''processing" times—on the order of a few hours as compared to times greater than 48 hours now used in continuous melters—are possible because melting requirements are reduced. To meet the objective set forth in number 1. above, it is not necessary to melt chemically durable components such as alumina or zirconia, but to merely surround them by glass. Shorter processing times hold the potential for faster throughput depending on the design of the single-use melter. The relative insolubility of alumina and zirconia in glass does not pose the same problem in partial vitrification as it does in full vitrification. The host phase(s) should be able to accommodate a volume fraction of undissolved oxide and still possess adequate durability. Depending upon the detailed microstructure, separate phases would be present, which might affect the physical and chemical durability of the waste form. For example, localized stresses in the event of thermal-expansion mismatch are possible, and their effect on waste form performance would need to be explored by suitable testing. Greater ease in handling an accident scenario. If something goes wrong with the melter, it is easier to remove a broken crucible and refractory container or liner from a batch furnace than to attempt repairs to a continuous melter. SUMMARY All of the waste forms described in this chapter are composed of several glass or crystalline phases, as opposed to the single glassy phase for a fully vitrified product. A blend of the INEEL calcines could probably be used successfully in its present state (i.e., with no dissolution and chemical processing) to achieve waste loadings of greater than 50 percent by weight in a glass/ceramic waste form whose chemical durability would satisfy those criteria now in place to qualify HLW forms. Glass/ceramic waste forms offer potential advantages of higher waste loading, ease of processing, and potentially lower costs than options involving chemical processing and complete vitrification. Whether these potential advantages can be realized will depend on regulatory and policy decisions for acceptance of heterogeneous waste forms in permanent repositories. That is, a potential disadvantage of the heterogeneous waste forms discussed in this chapter is the unknown cost and time to qualify them for permanent storage. This disadvantage may be more of a perception than a real technical issue, insofar as (1) the performance of heterogeneous waste forms can in principle and in practice be assessed, and (2) vitrified waste forms such as the HLW glass made at Savannah River are also hetero- 5   For example, Canadian experiments in the 1960s used aluminosilicate ("nepheline syenite") glass for immobilizing HLW (Lutze and Ewing, 1988, and references therein). The in-cell meltings were performed in mullite crucibles at relatively high temperatures (1500-1600 °C) to give products that are far more durable than the present generation of borosilicate glasses.

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Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory geneous due to recrystallization upon cooling. The conclusion is that most, if not all, waste forms are heterogeneous, and insofar as the Savannah River glass formulation cannot be used on INEEL HLW due to its significantly different composition, qualification of the INEEL HLW form will have to be done regardless of which waste form is produced.