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4 Waste Processing and Waste Form Production T he focus of this chapter is on the fourth charge of the statement of task for this study (see Box 2.1 in Chapter 2), which calls for the identification and description of “potential modifications of waste form production methods that may lead to more efficient1 production of waste forms to meet their performance requirements.” Waste form produc- tion involves many complex operations, a comprehensive review of which is well beyond the scope of this study. Instead, this chapter presents some key unit operations/technologies that have been or could potentially be used to produce nuclear waste forms. The following waste processing technologies are described in this chap- ter, and their key attributes are summarized in Table 4.1: • Joule-heated melters • Cold crucible induction melters • In-container vitrification • Self-sustaining vitrification • Cold pressing and sintering • Hot uniaxial pressing • Hot isostatic pressing 1 A given waste processing technology is “more efficient” compared to a baseline technology when, for example, it enables higher material throughputs or higher waste loadings; accom- modates higher levels of feed stream variability or liquid feed streams; accommodates higher levels of incompatible elements; results in reduced secondary wastes; is more operationally robust; or is less expensive. 87
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88 WASTE FORMS TECHNOLOGY AND PERFORMANCE TABLE 4.1 Waste Form Processing Technologies Processing Treatment and Processing Mode B=Batch Waste Stream Scalea Technology C=Continuous Waste Forms Produced Joule-Heated, Melter C Large Borosilicate glass other (JHM) glass types (LaBs, FeP, AIP, chalcognide, and others) Advanced Joule C Large Borosilicate glass, glass- Heated Melter ceramic materials, other (AJHM) glass types (LaBs, FeP, AIP, chalcognide, and others) Cold Crucible C Large Borosilicate glass, glass- Induction Melter ceramic materials, other (CCIM) glass types (LaBs, FeP, AIP, chalcognide, and others), crystalline ceramics/simple oxides, metal matrix In-Container B Depends on Borosilicate glass, glass- Vitrification (ICV) container ceramic materials, other (also known as bulk size (could be glasses (LaBs, FeP, AIP, vitrification) medium to large) chalcognide, and others) Cold Press and B Small Glass-ceramic materials, Sinter crystalline ceramic/simple oxides, metal matrix, zeolites, hydroceramics Self-Sustaining B Small Glass-ceramic materials Vitrification (SSV) Hot Uniaxial B Small Glass-ceramic materials, Pressing (HUP) crystalline ceramics/simple oxides, metal matrix, zeolites, hydroceramics Hot Isostatic B Small Borosilicate glass (lab Pressing (HIP) demonstration only), glass- ceramic materials, crystalline ceramics/simple oxides, metal matrix, zeolites, hydroceramics
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89 WASTE PROCESSING AND WASTE FORM PRODUCTION Advantages Disadvantages Proven technology; typically operates with Materials of construction can be problematic a “cold cap” to minimize volatility of for some wastes; solubility control of certain species of concern species (e.g., chromium spinels) critical; excessive spinels may seize-up melter; electrode erosion may be a problem Increases throughput and melt rate Operates with minimal or no “cold cap” with compared to JHM associated increases in volatility of species of concern Allows the processing of corrosive glasses; Higher temperature operation can increase no refractories; no metallic or oxide volatilization of species of concern but “cold electrodes; water cooled; self-cleaning; high cap” coverage minimizes these impacts purity; can be stirred if needed; increases capacity compared to JHM and AJHM; can operate at higher temperatures than JHM and AJHM; operates with a “cold cap” to minimize volatility Relatively cheap and simple for low- Inhomogeneous waste forms produced; activity wastes or contaminated soils; not no temperature control so radionuclide applicable to high-level radioactive waste vaporization is high; little to no convection in melt causes processing problems Higher waste loadings; minimum disposal Usually small scale; may require precalcining volumes or pretreating waste to an oxide form to avoid shrinkage of form Low capital requirements, can be used to May require some pre-processing, for process small amounts of wastes at remote example, grinding of the waste and locations pre-mixing Higher waste loadings; minimum disposal Usually small scale; may require precalcining volumes; mature flexible technology; or pretreating waste to an oxide form for mature industrial process shrinkage control Zero off-gas emissions; higher waste Processes small quantities; can overpressurize loadings; minimum disposal volumes; if large amounts of volatiles (e.g., nitrates/ mature flexible technology; no major hydrates) are present; may require secondary wastes; mature industrial precalcining or pretreating waste to an oxide process form (shrinkage handled by bellows like canisters) continued
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90 WASTE FORMS TECHNOLOGY AND PERFORMANCE TABLE 4.1 Continued Processing Treatment and Processing Mode B=Batch Waste Stream Scalea Technology C=Continuous Waste Forms Produced Fluidized Bed Steam C Large Crystalline ceramics/simple Reforming (FBSR) oxides, zeolites as formed, hydroceramics, geopolymers (as encapsulated) a All processes characterized as “small” are batch processes. The size of the batch varies by technology. For example, HIP and SSV batches can range from less than a liter to several tens of liters in volume. The processing capacity depends on a multiplicity of factors, including the time scales of various physical process (e.g., time to charge, melt, and cool). Processes charac- terized as “large” are continuous processes that have the potential to handle several kilograms to several hundreds of kilograms of waste per hour. In HLW applications, for example, JHMs can typically process 3 tonnes per day per melter or 125 kilograms per hour; AJHMs can process up to about 4 tonnes per day. For LAW the production rate could be up to 25 tonnes per day. Similarly, a CCIM can double the capacity of waste processing. A fluidized bed, on the other hand, does not have the geometrical physical limitations imposed by JHMs or CCIMs and can process several hundred kilograms to several tonnes of waste per hour. • Fluidized bed steam reforming • Other thermal technologies • Mix and set technologies Cold crucible induction melters, hot isostatic pressing, and fluidized bed steam reforming technologies were discussed in the committee’s interim report (NRC, 2010). This list of technologies is not exhaustive. Rather, the committee selected technologies that it judged were potentially most relevant to the U.S. Department of Energy, Office of Environmental Management (DOE- EM) cleanup program. 4.1 JOULE-HEATED MELTERS The term Joule heating refers to heating obtained by passing an electri- cal current through a resistively conducting material. The electrical resis- tance of the material causes the electrical energy to be converted to heat, with power dissipation following Ohm’s Law. A Joule-Heated Melter (JHM) is a refractory-lined container with nickel-chromium alloy electrodes, usually Inconel™. It is loaded with a
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91 WASTE PROCESSING AND WASTE FORM PRODUCTION Advantages Disadvantages Industrially proven technology; acidic or Product is granular and requires a high- basic tank wastes can be processed without integrity container or encapsulation in a neutralization; destroy organics and binder to make a glass ceramic material, a nitrates; convert aqueous components in geopolymer, or a hydroceramic; radionuclide a waste to stable, water-insoluble mineral partitioning among the phases needs to be products in single step; immobilize sulfur, further studied chlorine, and fluorine in a stable mineral form with no secondary waste calcine or slurry containing waste and glass frit and is melted by applying voltage across the electrodes. The nominal melt temperature is 1,150°C, which is only 200°C lower than the melting point of the Inconel™ elec- trodes. A waste form is produced by pouring the molten material into a container and allowing it to cool. The melting and pouring processes can be operated in continuous or semi-continuous modes. JHM technology is being tested or used successfully to immobilize high-level radioactive waste (HLW) in several countries, including Belgium, France, Japan, Russia, UK, and the United States (see Table 3.3 in Chapter 3). JHMs are part of the current DOE-EM baseline for immobilizing HLW at West Valley, Savannah River Site (SRS), and Hanford Site. Details on melter designs in use in different countries can be found in recent com- pendiums by Caurant et al. (2009), Jain (1998), Jantzen (in press), Ojovan (2011), and Ojovan and Lee (2005, 2007). The size of a JHM is usually limited only by capacity of the cranes that are used to install and, if necessary, replace it; structural support is provided by a stainless steel shell of the melter, which contains the refractory. The Defense Waste Processing Facility (DWPF) at the Savannah River Site is the largest production JHM ever built. Larger JHMs are under construction for use at the Hanford Site; these large melters will be mounted on rails for ease of replacement.
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92 WASTE FORMS TECHNOLOGY AND PERFORMANCE Several improvements have been made to conventional JHM designs to increase throughputs and waste loadings. These improved designs are referred to as Advanced Joule Heated Melters, or AJHMs. These designs have been proven at the pilot scale with simulated wastes, as well as operated at the plant scale with mixed radioactive/hazardous waste at SRS where main- tenance and repairs could be handled manually. They have not been operated with HLW where maintenance and repairs have to be performed remotely. JHMs have production rates that are approximately proportional to the surface area of the melt; convection caused by the Joule heating is enhanced as the size of the melter is increased. So larger melters, with larger surface areas, have proportionately higher melt rates. The melt temperature is limited by the materials of construction of the electrodes, generally Inconel™ 690. Melt rates can also be increased without increasing melter size through the following means: • Adding lid heaters to increase the temperature of the melter plenum and enhance melting of the cold cap (i.e., the unmelted feed mate- rial on top of the melted mass). • Adjusting the proportions of frit and cold chemical additions. • Increasing the use of reducing agents (e.g., formic acid/sugar) to control oxygen foaming. • Adding surface-active materials such as sulfates and halides. • Increasing melter convection by using lower-viscosity glass, power skewing the bottom electrodes, or mechanical agitation (stirrer/ bubblers/airlift pumps). • Dry feeding instead of slurry feeding. • Increasing the operating temperature. • Bubbling to enhance feed to glass conversion. Melt rates can also be increased by increasing melt temperatures. JHMs can be operated at ~1,200°C before different materials of construction are required. However, increased melt pool volatility, refractory corrosion, and electrode corrosion are to be expected at higher operating temperatures. In order to limit volatilization of radionuclides such as techentium-99 and ruthenium-104 in borosilicate melts, the melter can be operated with a cold cap and under reducing conditions to keep the technetium as sodium pertechnatate (NaTcO4) and ruthenium as an oxide (RuO2). The cold cap also helps minimize volatilization of cesium (as CsBO2) and other alkali salts such as sodium chloride (NaCl), sodium fluoride (NaF), and sodium iodide (NaI) (Jantzen, 1991). The increasing production capability is offset by increasing complexity of the melter system. The DWPF is considered to be a JHM with a few AJHM design fea- tures. It is round with a slightly sloped floor to avoid cold corners and
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93 WASTE PROCESSING AND WASTE FORM PRODUCTION improve melt pool convection. Wet feeding is used to avoid dusting and the need for a dust collection system. This melter also has lid heaters, adjusts the proportions of frit to waste, uses reducing agents to control oxygen foaming and convert nitrates to N2, uses lower melting viscosity glasses, power skews the bottom electrodes, and uses an airlift pump to improve melt rate. The planned Hanford HLW melter is a square AJHM melter.2 It will allow the proportions of cold chemical additions to be adjusted, use of reducing agents to convert nitrates to N2, and use of lower viscosity glasses. Additionally, it will allow use of power skewing of the bottom electrodes and use of bubblers3 to improve melt rate and melt pool convection. Higher temperatures will be used. Partial dry feeding will also be used to improve melt rate. The primary advantages of JHMs for waste immobilization are their high production rates and ability to produce waste form material of consis- tent quality. They can be designed with sloped bottoms and bottom drains with or without mixing to facilitate periodic draining of noble metals that may precipitate (as is done with the JHM in use at Tokai, Japan). The overall problems with noble metals in waste streams have been addressed by Bibler (2005) and some specifics of noble metals in a JHM have been treated by Jantzen and Lambert (1999). JHMs also have several disadvantages: They are large compared to some other types of melters (see Section 4.2), and the electrodes and refrac- tories require maintenance and periodic replacement. They are also intoler- ant of crystal growth in the melt, which causes slag formation. For example, the induction heating melter at Sellafield (Riley et al., 2009), which is not a JHM design, has shown the ability to increase waste loading from 25 weight percent to 38 weight percent by allowing spinel formation in the melt.4 In comparison, the melter planned for Hanford will allow for only 1-2 percent crystallization of spinels; it is anticipated that these crystals will remain buoyant because of melt pool agitation by the bubblers. This strat- egy will likely work unless the crystals grow larger than can be sustained by the bubblers—as might happen, for example, during long maintenance outages. 2 Square melters have cold corners where glass can crystallize unless the melt pool is agi- tated. Round melters such as the DWPF and Japanese designs require no melt pool agitation except as needed to improve melt rate. 3 The melter will be outfitted with eight twin-orifice melt bubblers that bubble air and thus improve melt rate and convection. The use of multiple bubblers with the capability for frequent replacements is considered to be an AJHM design. 4 Transition metal-containing spinels form from the iron generated during fuel reprocessing and from corrosion products. Usually the least soluble component in the wastes is chromium, which readily precipitates in a melt as (Fe,Ni)(Fe,Cr)2O4 (Hrma et al., 2002).
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94 WASTE FORMS TECHNOLOGY AND PERFORMANCE 4.2 COLD CRUCIBLE INDUCTION MELTING The Cold Crucible Induction Melter (CCIM) consists of water-cooled tubes that are arranged to form a crucible, the contents of which are heated by induction. An inductor surrounding the crucible is energized by a high- frequency alternating current that induces eddy currents (and resultant Joule heating) of materials contained in the crucible. The melting process is usu- ally initiated by inserting a resistive heating element into the crucible. This element couples with the fields to form an initial melt; the initial melt then couples with the electromagnetic field. At that point, the resistive element can be removed so that no foreign materials are in contact with the melt. A solid “skull” of quenched waste material, typically a few millimeters in thickness, forms along the inside crucible wall, protecting it from degrada- tion and corrosion. The shell isolates the melt from the crucible so that the latter can be maintained at ambient conditions. This prevents molten mate- rials from bonding to the crucible, allowing residuals to be removed at the end of the melting campaign. CCIMs are potential replacements for JHMs and AJHMs. As noted previously, JHMs with some AJHM features are now part of the current DOE baseline for production of high-activity and low- activity waste glass (see Chapter 2). CCIMs have several advantages over both current-generation and advanced JHMs. They allow for higher throughputs and waste loadings. They are operationally simpler and allow for faster recoveries from sys- tem upsets.5 The absence of internal electrodes and refractories allows for increased melter longevity and permits higher-temperature operation com- pared to current-generation JHMs. As a consequence, CCIMs can be used to process a wider range of waste compositions, including corrosive wastes that are incompatible with current-generation JHMs. Additionally, they can more easily accommodate differing glass compositions, including iron phosphate glasses, that are incompatible with many types of electrodes used in JHMs. CCIMs can be cycled frequently with varying feed compositions without thermal damage or loss of compositional control. And they are capable of producing crystalline ceramics through controlled or spontane- ous crystallization. They can be fitted with stirrers to facilitate melt pool convection and homogenization so that crystals do not accumulate in one location in the melt pool. CCIMs can also be used in conjunction with other technologies. For example, an integrated process that combines an oxygen plasma and induc- tion-heated cold crucible is reported by Vernaz and Poinssot (2008). This process, which is still under development, is referred to as the Advanced 5 Simpler and more robust processing technologies are generally preferable because system upsets can pose critical bottlenecks for operations that must be conducted in hot-cell environ- ments to protect workers from high radiation fields.
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95 WASTE PROCESSING AND WASTE FORM PRODUCTION Hybrid System for Incineration and Vitrification (SHIVA). It consists of a single reaction vessel that has three functions: (1) incineration, (2) vitrifica- tion, and (3) gas post-combustion. This is a promising technology for pro- cessing mixed wastes containing radioactive, organic, and other hazardous chemical constituents that are difficult to separate by other processes. The plasma decomposes organic material, significantly reducing its volume, and produces a high-quality containment material (glass in this instance). CCIM development began in France and Russia in the 1970s (Elliott, 1996). Russia is currently using CCIMs to process radioactive waste at the Mayak Plant (Demine et al., 2001; Kushnikov et al., 1997; Lifanov et al., 2003; Polyakov et al., 1997; Stefanovsky, 2009; Toumanov, 2003), and the French are using a CCIM to vitrify HLW at an industrial scale at the La Hague plant. DOE-EM is currently investigating CCIM technology for possible use in its HLW immobilization programs. CCIM is an emerging technology for the vitrification of fission product solutions and decontamination waste streams. It also has potential applica- tions for processing metallic waste streams (Vernaz, 2009). The underlying technology is proven, but operational experience in large-scale waste stream processing environments is limited in comparison to JHMs. Its deployment in DOE-EM applications may require some up-front development work to ensure its compatibility with specific process flow sheets, but no basic research is likely to be required. Of course, a CCIM system must be engi- neered for safety, for example, to prevent the loss of coolant. Gombert and Richardson (2001) have discussed such safety aspects for CCIM design. Because CCIMs are smaller per unit of throughput and operation- ally more robust than JHMs, they could potentially be back-fitted to the Defense Waste Processing Facility at SRS (Barnes et al., 2008) and the Waste Treatment Plant at Hanford. For example, the DOE Independent Project Evaluation Team (IPET, 2003) examined the feasibility of replacing the JHMs in the Waste Treatment Plant at Hanford with CCIMs. It concluded that two CCIMs could be retrofitted into each of the two melter cells in the plant. If the melters were installed before the plant was hot commis- sioned, only about four months would be required to modify the melter cells and install the new equipment (IPET, 2003, p. 4.70). Additional time would be required to install the melters after hot commissioning—either to decontaminate the melter cells prior to installation of the new equipment or to construct a new melter facility. Although CCIMs have a track record of successful deployment inter- nationally, the experience and understanding for DOE applications are limited. Furthermore, the design of these melters is challenging in that, for example, one needs to understand the material properties, particularly the electrical resistivity of the melt, for determining the optimal operating conditions for a given crucible diameter. Some of these design challenges
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96 WASTE FORMS TECHNOLOGY AND PERFORMANCE are summarized by Gombert and Richardson (2001). However, these are the kinds of challenges that are normally faced when implementing any new technology, and they can be overcome with appropriate development work. 4.3 IN-CONTAINER VITRIFICATION In-Container Vitrification (ICV) (Thompson et al., 2003) is a batch process by which contaminated soil, liquid waste mixed with soil, and glass formers are vitrified in situ in a refractory-lined steel vessel. The energy for melting is generated by passing an electrical current through graphite electrodes installed in the vessel. The vitrified waste can be disposed of with or without the container; in the latter case the container can be reused. A similar concept for immobilizing contaminated soil in the ground, In-Situ Vitrification, was developed earlier (Thompson et al., 1992). ICV requires soil- or glass-formers to establish the melt and create a stable, vitrified waste form. Therefore, soil or soil-like materials must be added to the waste stream being processed. (The silica and alumina contained in the soil are the glass formers for the process.) Because neither soil nor frit is a good electrical conductor at ambient conditions, melting is initiated by placing a conductive pathway between the electrodes.6 The heat generated from graphite electrodes provides the electrical current in the vicinity of the conductive pathway and melts the soil or frit, which increases its electrical conductivity and establishes Joule heating. The melted zone gradually grows outward toward the refractory liner, and eventually the entire vessel contents become molten. The temperature of the melt varies between 1,400°C and 1,800°C depending on the composition of the soil and waste materials. It usually takes up to three days to melt and process a single batch. The melting process destroys organic contaminants contained in the soil/waste mixture by pyrolysis or dechlorination. ICV can be equipped with an off-gas treatment system to capture any residual organic or other volatile materials released from the melt. The removal efficiency of the organics is typically greater than 99.9999 percent (Thompson et al., 2003). The retention efficiency of most metals and radionuclides in the melt is greater than 99.99 percent for non-volatile species (Thomspon et al., 2003). The residuals released with the off-gas stream are captured either by filtration or scrubbing; radionuclide volatility is high because of the high processing temperatures and the inability to monitor melt temperatures. This is one of the major drawbacks of the process. The vitrified product usually consists of a mixture of glass and crystal- line materials. Convective currents generated during melting help mix the 6 A thin layer of graphite is added to the top of the waste-glass former mixture and touches the graphite electrodes to initiate Joule heating.
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97 WASTE PROCESSING AND WASTE FORM PRODUCTION melt. In theory, if the melt is well mixed, the high content of glass-former materials such as SiO2 and Al2O3 contained in the starting mixture can produce a durable waste form. However, compositional non-uniformities may exist that may greatly reduce durability. The ICV technology was considered to treat low-activity waste (LAW) at Hanford and has been piloted at the Demonstration Bulk Vitrification System (DBVS), a full-scale test facility at the site. The process used soil from the Hanford Site and glass-former additives as the starting mixture. Several problems were encountered during the demonstration: High melting temperatures caused vaporization of all of the technetium into the off-gas stream. The Fe2O3 in the soil was reduced to Fe metal, which penetrated the ceramic liners and the outer metal shell of the container. These operational challenges are currently under review (Gerdes et al., 2007), and the project is on hold (GAO, 2007). 4.4 SELF-SUSTAINING VITRIFICATION Self-Sustaining Vitrification (SSV) utilizes exothermic chemical reac- tions to produce glass, glass-ceramic, or ceramic waste forms. This technol- ogy was initially developed in the Soviet Union for producing high-quality ceramics and other refractory compounds (Borisov et al., 2002). The appli- cation of this technology to radioactive waste immobilization is discussed in Ojovan and Lee (2007). The exothermic chemical reaction can be obtained by mixing waste and powder metal fuels (PMFs) such as Mg, Al, Si, or Ca. The oxidation of these fuels releases heat. The process is controlled by waste and PMF compositions, which are established based on computer simulations. A number of pre-processing steps may be required to develop the appropri- ate starting compositions and properties. For example, water removal is required to avoid excess gas generation and ensure the uniformity of the resulting waste form. Because it does not require expensive equipment, SSV can be particu- larly useful for immobilizing small-volume waste streams or wastes that are difficult to immobilize by other methods. The feasibility of producing glass- ceramic waste forms using this technology has been demonstrated for a number of waste materials, for example, contaminated clay soils and ashes, spent inorganic ion exchangers, and calcined HLW (see Ojovan and Lee, 2007; Ojovan et al., 1999). 4.5 COLD PRESSING AND SINTERING Cold Pressing and Sintering (CP&S) is one of the earliest technologies used for forming technical crystalline ceramics. The technology itself is
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108 WASTE FORMS TECHNOLOGY AND PERFORMANCE 4.10.4 Bitumen Bitumen, a viscous hydrocarbon and a major component of asphalt, has been used in Europe and Canada and to a lesser extent in the United States to solidify and stabilize radioactive materials. Bitumen immobilizes waste by encapsulation; it does not bind the waste chemically. The advan- tages of bitumen as a waste form are simplicity of production, low operat- ing cost, and leach resistant characteristics. However, bitumen does not perform well with dehydrated salts, such as sodium sulfate, sodium nitrate, magnesium chloride, and aluminum sulfate (Noyes, 1995). It can also be a fire hazard (Zakharova and Masanov, 2000), especially when oxidizing wastes like nitrates are involved. There are several processes for solidifying bitumen with waste streams (Ojovan and Lee, 2005). The most common are the use of a screw extruder or a rotary thin-film evaporator. The screw extruder, which is commonly used to mix pastes and plastics, consists of a screw, barrel, drive mecha- nism, bitumen, and waste feed point. The constantly turning screw augers the waste and bitumen through the heated barrel where it is heated to form a homogeneous melt. The volatile gases are allowed to vent. The thin-film evaporator consists of vertical vessel with a rotated shaft at its center equipped with wiper blades, which help create a thin film on the wall and help mix the bitumen-waste mixture. Preheated bitumen and partially evaporated liquid waste are fed to the top of the evaporator, a distributor spreads the mixture around the inner wall of the vessel where heat is supplied, while the action of the wiper assemblies create a thin liquid film on the wall and also help to mix the molten material. The evaporated liquid passes through a series of condensers, and a molten bitumen waste mixture is discharged from the bottom of the evaporator. 4.11 DISCUSSION This chapter has provided a brief overview of processing technologies used to produce a variety of waste forms, emphasizing technologies that are currently used, planned to be used, or in development stage that may be considered advancements. These technologies are generally well established through their use in other industries. The attributes, the advantages, and disadvantages of these processes are summarized in Table 4.1. The committee highlighted the advantages of three technologies in its interim report (NRC, 2010): fluidized bed steam reforming, cold crucible induction melting, and hot isostatic pressing. These waste form production technologies are being used commercially in both nuclear and/or non- nuclear applications, as described elsewhere in this chapter, and appear to be applicable for processing and immobilizing a range of DOE-EM waste
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109 WASTE PROCESSING AND WASTE FORM PRODUCTION streams, especially HLW streams, which are summarized in Table 4.2. DOE-EM is already planning to apply these technologies to some of its waste streams, as discussed in the interim report and elsewhere in this chap- ter. The committee concurs with DOE-EM about the applicability of these technologies in the cleanup program. Even though these are mature tech- nologies and have applications in different industries, some development work will be needed to use them in processing nuclear waste as described in the interim report (NRC, 2010) and in this chapter.
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110 WASTE FORMS TECHNOLOGY AND PERFORMANCE TABLE 4.2 Waste Form Production by Various Processing Technologies Other Glasses Minerals/ (LaBs, FeP, Glass- Crystalline AlP, chalco- Ceramic Ceramics/ Borosilicate gnide, and Materials Simple Glass others) (GCMs) Oxides Joule-Heated Melters Y Y Y N (JHMs) Advanced JHMs Y Y Y N Heated Melters (bubblers, etc.) Cold Crucible Y Y Y Y* Induction Melters THERMAL (CCIM) Press + Sinter N N Y Y Hot Isostatic Pressing N N Y Y (HIPing) Fluidized Bed Steam N N Y Y Reforming (FBSR) Calcining N N N Y Hydrothermal N N Y Y Processing Mix and Set N N N N NONTHERMAL
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111 WASTE PROCESSING AND WASTE FORM PRODUCTION Cements Zeolites (OPC, Metal Hydro- Ceramicrete, and Matrix ceramic Others) Bitumen Geopolymers N N N N N N N N N N Y N N N N Y Y N N N Y Y N N N N Y N N N N N N N N N Y Y N Y N N Y [to include Y (heating Y [to include use of as waste necessary to use of as waste forms and as melt bitumen) forms and as binders (macro- binders (macro- encapsulation)] encapsulation)]
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112 WASTE FORMS TECHNOLOGY AND PERFORMANCE APPENDIX 4.A FLUIDIZED BED STEAM REFORMING The Fluidized Bed Steam Reforming (FBSR) of nuclear waste is a rela- tively new technology, though the fluidization phenomenon and steam reform- ing are well established in the chemical engineering field. Steam reforming is a method for generating hydrogen by reacting fossil fuels with water. For example, for natural gas: CH4(g) + H2O(g) → CO(g) + 3H2(g) If coal is used as a carbon source, it first undergoes pyrolysis or devola- tilization then the char (C) reacts with steam according to the following reaction: C(s) + H2O(g) → CO(g) + H2(g) The H2 is combined with O2 so that no excess H2 exists in the system at any one time. This combination is exothermic and provides energy in the form of heat for the autocatalytic operation of the FBSR. The FBSR consists of two fluidized beds. The first one operates in a reducing environment, and its function is to evaporate the liquid nuclear waste stream; destroy organics; reduce nitrates, nitrites, and nitric acid to nitrogen gas; and form a stable solid waste product. The first-stage fluid- ized bed of the FBSR process is referred to as the Denitration and Min- eralization Reformer, or DMR. The DMR uses superheated steam as the fluidizing media. The bed material consists of granular solid additives and co-reactant(s), such as carbon, clay, silica, and/or catalysts. Liquid waste is directly fed to the fluidized bed after minor pretreatment (e.g., to concen- trate or dilute solubles) except the addition of clay. By analogy to the above steam reforming chemistry, the carbon fed to FBSR (coal in this instance) produces H2 and CO. For organic compounds in the waste stream, which undergo pyrolysis to form various hydrocar- bons, the reducing environment is generated by the following reaction: CnHm(g) + nH2O(g) → nCO(g) + (n + m/2)H2(g) Similarly, the nitrates contained in the liquid waste are reduced to 2NaNO3(g) + 3C(s) → 2NO(g) + 3CO(g) + Na2O(s) In the steam environment, the sodium oxide is transferred to sodium hydroxide:
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113 WASTE PROCESSING AND WASTE FORM PRODUCTION Na2O(s) + H2O(g) → 2NaOH(s,l) yielding the overall reaction 2NaNO3(g) + 3C(s) + H2O(g) → 2NO(g) + 3CO(g) + 2NaOH(s,l) 2NaNO3(g) + 2C(s) + H2O(g) → 2NO2(g) + 2CO(g) + 2NaOH(l,s) The NO and NO2 are further reduced to nitrogen gas by the reaction of CO, C, or H2 generated from the reaction of the organic material with steam as shown above. The nitrates can also be reduced by the addition of a catalyst or a metal. For example: 2NaNO3(g) + 5Fe(s) + H2O(g) → N2(g) + 5FeO(s) + 2NaOH(s,l) The second fluidized bed of the FBSR process operates in an oxidizing environment and is referred to as the Carbon Reduction Reformer, or CRR. The fluidizing gases are the off-gas from the first stage and added oxygen. Its function is to gasify carbon fines carried over in the process gases from the DMR, oxidize CO and H2 to CO2 and water, and convert trace acid gases to stable alkali compounds by reacting these acids with the bed media consisting of calcium carbonate and/or calcium silicate particles. The addition of bulk aluminosilicates to the fluidized bed results in the production of various phases including anhydrous feldspathoid mineral analogue phases such as sodalite. The sodalite family of minerals (includ- ing nosean) are unique because they have cage-like structures formed of aluminosilicate tetrahedra. The remaining feldspathoid mineral analogues, such as nepheline, have a silica “stuffed derivative” ring-type structure. The cage structures are typical of sodalite and/or nosean phases where leach testing has indicated that the cavities in the cage structure retain anions and/or radionuclides, which are ionically bonded to the aluminosilicate tetrahedra and to sodium cation. Sodalite has the formula Na8[Al6Si6O24](Cl2). In sodalites and ana- logues with sodalite topologies, the cage is occupied by two sodium and two chlorine ions. When the 2NaCl are replaced by Na2SO4, the mineral phase is known as nosean, (Na6[Al6Si6O24](Na2SO4)). Because the Cl, SO4, and/or S2 are chemically bonded and physically restricted inside the soda- lite cage structure, these species do not readily leach out of the respective FBSR waste form mineral phases. Thus, FBSR waste forms can be useful for immobilizing these species to prevent their leaching into groundwater. Other minerals in the sodalite family, namely hauyne and lazurite, which are also cage-structured minerals, can accommodate either SO42– or S2–. They are potential products of the steam reforming depending on the
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114 WASTE FORMS TECHNOLOGY AND PERFORMANCE REDOX of the sulfur during the process. Sodalite minerals are known to accommodate Be in place of Al and S2 in the cage structure along with Fe, Mn, and Zn, e.g., helvite (Mn4[Be3Si3O12]S), danalite (Fe4[Be3Si3O12]S), and genthelvite (Zn4[Be3Si3O12]S). These cage-structured sodalites were minor phases in HLW supercalcine waste forms and were found to retain Cs, Sr, and Mo into the cage-like structure, e.g., Mo as Na6[Al6Si6O24] (NaMoO4)2. In addition, sodalite structures are known to retain B, Ge, I, and Br in the cage-like structures. Indeed, waste stabilization at Idaho National Laboratory currently uses a glass-bonded sodalite ceramic waste form (CWF) for disposal of electrorefiner wastes for sodium-bonded metal- lic spent nuclear fuel from the EBR II fast breeder reactor.
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