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Waste Forms Technology and Performance: Final Report (2011)

Chapter: 4 Waste Processing and Waste Form Production

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Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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4

Waste Processing and Waste Form Production

The 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 production 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 chapter, 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

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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; accommodates 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.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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TABLE 4.1 Waste Form Processing Technologies

Processing Technology Processing Mode B=Batch C=Continuous Treatment and Waste Stream Scalea Waste Forms Produced
Joule-Heated, Melter (JHM) C Large Borosilicate glass other glass types (LaBs, FeP, AIP, chalcognide, and others)
 
Advanced Joule Heated Melter (AJHM) C Large Borosilicate glass, glass-ceramic materials, other glass types (LaBs, FeP, AIP, chalcognide, and others)
 
Cold Crucible Induction Melter (CCIM) C Large Borosilicate glass, glass-ceramic materials, other glass types (LaBs, FeP, AIP, chalcognide, and others), crystalline ceramics/simple oxides, metal matrix
 
In-Container Vitrification (ICV) (also known as bulk vitrification) B Depends on container size (could be medium to large) Borosilicate glass, glass-ceramic materials, other glasses (LaBs, FeP, AIP, chalcognide, and others)
 
Cold Press and Sinter B Small Glass-ceramic materials, crystalline ceramic/simple oxides, metal matrix, zeolites, hydroceramics
 
Self-Sustaining Vitrification (SSV) B Small Glass-ceramic materials
 
Hot Uniaxial Pressing (HUP) B Small Glass-ceramic materials, crystalline ceramics/simple oxides, metal matrix, zeolites, hydroceramics
 
Hot Isostatic Pressing (HIP) B Small Borosilicate glass (lab demonstration only), glass-ceramic materials, crystalline ceramics/simple oxides, metal matrix, zeolites, hydroceramics
Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×
Advantages Disadvantages
Proven technology; typically operates with a “cold cap” to minimize volatility of species of concern Materials of construction can be problematic for some wastes; solubility control of certain species (e.g., chromium spinels) critical; excessive spinels may seize-up melter; electrode erosion may be a problem
 
Increases throughput and melt rate compared to JHM Operates with minimal or no “cold cap” with associated increases in volatility of species of concern
 
Allows the processing of corrosive glasses; no refractories; no metallic or oxide electrodes; water cooled; self-cleaning; high 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 Higher temperature operation can increase volatilization of species of concern but “cold cap” coverage minimizes these impacts
 
Relatively cheap and simple for low-activity wastes or contaminated soils; not applicable to high-level radioactive waste Inhomogeneous waste forms produced; no temperature control so radionuclide vaporization is high; little to no convection in melt causes processing problems
 
Higher waste loadings; minimum disposal volumes Usually small scale; may require precalcining or pretreating waste to an oxide form to avoid shrinkage of form
 
Low capital requirements, can be used to process small amounts of wastes at remote locations May require some pre-processing, for example, grinding of the waste and pre-mixing
 
Higher waste loadings; minimum disposal volumes; mature flexible technology; mature industrial process Usually small scale; may require precalcining or pretreating waste to an oxide form for shrinkage control
 
Zero off-gas emissions; higher waste loadings; minimum disposal volumes; mature flexible technology; no major secondary wastes; mature industrial process Processes small quantities; can overpressurize if large amounts of volatiles (e.g., nitrates/hydrates) are present; may require precalcining or pretreating waste to an oxide form (shrinkage handled by bellows like canisters)
Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×
Processing Technology Processing Mode B=Batch C=Continuous Treatment and Waste Stream Scalea Waste Forms Produced
Fluidized Bed Steam Reforming (FBSR) C Large Crystalline ceramics/simple 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 characterized 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 electrical current through a resistively conducting material. The electrical resistance 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

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×
Advantages Disadvantages
Industrially proven technology; acidic or basic tank wastes can be processed without neutralization; destroy organics and nitrates; convert aqueous components in a waste to stable, water-insoluble mineral products in single step; immobilize sulfur, chlorine, and fluorine in a stable mineral form with no secondary waste Product is granular and requires a high-integrity container or encapsulation in a binder to make a glass ceramic material, a geopolymer, or a hydroceramic; radionuclide partitioning among the phases needs to be further studied

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™ electrodes. 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 compendiums 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.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

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 maintenance 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 material 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 (NaT cO4) 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 features. It is round with a slightly sloped floor to avoid cold corners and

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

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 consistent 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 refractories require maintenance and periodic replacement. They are also intolerant 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 strategy will likely work unless the crystals grow larger than can be sustained by the bubblers—as might happen, for example, during long maintenance outages.

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2 Square melters have cold corners where glass can crystallize unless the melt pool is agitated. 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).

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

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 usually 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 degradation and corrosion. The shell isolates the melt from the crucible so that the latter can be maintained at ambient conditions. This prevents molten materials 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 system upsets.5 The absence of internal electrodes and refractories allows for increased melter longevity and permits higher-temperature operation compared 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 spontaneous 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 induction-heated cold crucible is reported by Vernaz and Poinssot (2008). This process, which is still under development, is referred to as the Advanced

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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 environments to protect workers from high radiation fields.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

Hybrid System for Incineration and Vitrification (SHIVA). It consists of a single reaction vessel that has three functions: (1) incineration, (2) vitrification, and (3) gas post-combustion. This is a promising technology for processing 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 applications 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 engineered 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 operationally 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 commissioned, 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 internationally, 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

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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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 crystalline materials. Convective currents generated during melting help mix the

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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.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

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 reactions to produce glass, glass-ceramic, or ceramic waste forms. This technology was initially developed in the Soviet Union for producing high-quality ceramics and other refractory compounds (Borisov et al., 2002). The application 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 appropriate 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 particularly 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

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

simple; it involves pouring ceramic powder, often with small amounts of organic binders, into a die. The simplest die geometry consists of a cylindrical body with top and bottom plungers. The application of uniaxial compressive stress along the axis of the plungers leads to compaction of the powder into a cylindrical pellet. The pellet is ejected from the die and then sintered in a conventional kiln.

Alternatively, cold pressing can be done in an isostatic press. The powder is loaded into an elastomer mold, which is sealed and inserted into a liquid bath (generally oil). The liquid is then pressurized to compact the powder, then the pressed part is removed from the mold and sintered. The mold can be reused or discarded.

Uniaxial pressing is susceptible to die wall friction, which can result in density gradients in unsintered pellets. Sintering can produce distortions in the shape of the pellet or cracking. Isostatic pressing has the advantage of avoiding density gradients, thereby leading to more uniform and predictable shrinkage during sintering. The use of organic binders and lubricants with the powder will also minimize density gradients but requires processing of the waste stream to introduce these materials and thermal treatment of the pressed products to remove the organic components.

For the development of waste forms, the CP&S offers several advantages: it uses inexpensive equipment, is easily adaptable to small batches of waste, and is particularly well suited to laboratory studies of potential phases for waste forms such as glass ceramic material (Juoi et al., 2008; Staples et al., 2007) or crystalline ceramic material (Meyers et al., 1998; Oversby and Vance, 1994; Ringwood et al., 1988).

4.6 HOT UNIAXIAL PRESSING

Hot Uniaxial Pressing (HUP) has been in common use in the ceramic processing industry for decades and is often referred to simply as hot pressing. It involves loading of powder into a die, much as in CP&S. However, the die and plungers are inserted into a furnace and heated while pressure is applied. The process can produce high-density ceramics at lower temperatures than the two-step CP&S process described previously. However, the process is relatively slow because the die set and sample must be heated before pressing. Also, the dies can be expensive and, depending on the degree of reaction between the dies and powders, they can have a limited life.

HUP has been investigated as a technology for producing nuclear waste forms (Oversby and Vance, 1994; Ringwood et al., 1988; Staples et al., 2007). As with CP&S, the process is frequently used for laboratory evaluation of potential waste forms. For large-scale production of waste forms, however, consideration must be given to the volume of waste resulting from worn and damaged dies, which could be relatively large.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

4.7 HOT ISOSTATIC PRESSING

Hot Isostatic Pressing7 (HIP) also produces waste forms through the simultaneous application of heat and isostatic pressure. The waste and other materials to be processed are loaded into a metal can, typically stainless steel, which is sealed by welding and placed into a pressure vessel inside an electrically heated furnace. Pressure is then applied by compressing a gas such as argon, either by heating or by pumping, which compresses the waste isostatically while sintering into a solid monolithic waste form.

The HIP process, originally referred to as gas-pressure bonding, was first developed by Battelle Memorial Institute in the mid-1950s (ASME, 1985). Its initial use was for manufacturing nuclear fuels, but it is now a well-established technology used by a wide range of industries for castings, tool making, and manufacturing of ceramic components. The Australian Nuclear Science and Technology Organisation has developed and demonstrated HIP for immobilizing radioactive wastes from medical isotope production; it plans to commence the commissioning of a HIP facility for this purpose at the end of 2011. In January 2010, DOE announced its formal decision to use HIP to convert HLW calcine at the Idaho Site into “ceramic-like” waste forms (DOE, 2010). However, the technology readiness assessment is still in progress, the details of which had not been released by the time that this report was being finalized. Additionally, a safety assessment also had not yet been completed.

HIP is a mature and safe technology as demonstrated by its wide use outside the nuclear industry. The pressure vessels are designed with stringent codes such as those developed by the TÜV (Technischer Überwachungs-Verein [Technical Inspection Association], a German product safety and quality assurance testing firm) and the American Society of Mechanical Engineers (ASME). The conservative ASME code and inspection regime are designed to ensure that vessel integrity is maintained over its service life. Other safety features include active and passive over-pressure control systems and safety shields.

HIP also has many potential advantages for processing nuclear waste. Notably, it produces monolithic waste forms with substantially reduced volumes compared to untreated waste streams. Because the waste is processed in a sealed can, there are no volatile emissions. (If volatiles are produced they may not be retained in the solid.) There is no direct contact between the waste and the HIP apparatus, so secondary waste generation is minimized. HIP is compatible with a wide range of waste compositions, although it has a limited tolerance for gases and volatiles—for example, if copious amounts of hydrates or nitrates are present they will cause over

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7 See http://www.synrocansto.com/Download/files/synrocANSTO_HIP_FactSheet.pdf.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

pressurization unless the waste is calcined before HIPing. The HIP technology can produce glass (Harker et al., 1984), glass composite, and crystalline ceramic waste forms.

Unlike many other consolidation technologies, HIP does not require stringent control of physical properties such as viscosity, melt temperature, or melt conductivity, therefore permitting significantly higher waste loadings. However, for making ceramic waste forms, waste form additives must be tailored to sequester radionuclides in specified host phases. For production of SYNROC,8 for example, redox conditions must be controlled to form the desired phase assemblages. In addition, processing conditions (pressure and temperature) must be closely controlled.

Although HIP is a flexible technology it does have some limitations. Crystalline ceramic waste forms produced by HIP (as well as conventional press and sinter technology) may contain intergranular glassy phases, especially when incorporating waste containing alkali or alkaline earth species in the presence of glass formers such as silicon or boron. Also, the distribution of volatiles in the can (when present) is not well understood. This intergranular glass can limit product stability and durability (Clarke, 1981; Cooper et al., 1986; Zhang and Carter, 2010). Additionally, HIP has been demonstrated only at small scales to date. The small size of the waste cans and long times required for heating currently limits the application of this technology to volumetrically small waste streams.

Given its flexibility, HIP is potentially applicable to a range of DOE-EM waste streams, including orphan waste streams and metallic waste streams whose diversity requires versatile methods for treatment and immobilization, as well as waste streams that are difficult or inefficient to process by other technologies because of physical or chemical heterogeneity. However, additional studies are needed to demonstrate the safety and compatibility of this technology with specific waste streams and also to address its scalability to high-volume waste streams.

4.8 FLUIDIZED BED STEAM REFORMING

A bed of granular material can be made to exhibit fluid-like properties by passing a liquid or gas through it. This process is referred to as fluidization, and the apparatus that supports this process is referred to as a fluidized bed. An 1879 patent appears to be the first instance to describe this phenomenon and its advantages. However, fluidization came of age during World War II, when the urgent need for aviation gasoline in the United States led to the development and construction of the first Fluid Bed Cata-

________________________

8 SYNROC (Synthetic Rock) is a monolithic crystalline ceramic containing hollandite, zirconolite, perovskite, and other minor constituents.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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lytic Cracker (FCC). The fluidized bed offered an easy way of circulating the catalyst between regeneration and reaction cycles and was much easier to operate than the conventional process. Today, there are more than 400 FCCs operating worldwide. In addition to gasoline production, fluidization technology is broadly used in coal gasification and combustion, mineral processing, food processing, pharmaceuticals, soil washing, manufacturing of polymers, waste treatment, and environmental remediation. Its applications include several unit operations such as drying, heating/cooling, particle coating, and chemical reactions.

Applications of fluidized bed technology in nuclear fuel production, recovery, and waste processing date back to the late 1950s and early 1960s. For example, fluidization was used for the reduction and hydrofluorination of uranium concentrates (Sutton et al., 1966) and fluidized bed calcinations of high-level radioactive waste (Buckham et al., 1966). In the calcination process, liquid wastes are sprayed using atomizing nozzles into a fluidized bed of heated spherical calcine particles, evaporating water and nitric acid in the wastes and leaving behind solid-phase metal oxides. Two calcination facilities were successfully operated at the Idaho National Laboratory from 1961 to 1981 and from 1981 to 2000 (Newby and O’Brien, 2000). The calcine product is being stored in bins at the site. It may undergo further processing, possibly by HIP, to put it into a form that is suitable for disposal.

Fluidized Bed Steam Reforming (FBSR) is being used commercially for processing nuclear waste. A commercial facility to continuously process organic radioactive wastes at moderate temperatures in a hydrothermal steam environment was built by Studsvik in Erwin, Tennessee, in 1999. The Erwin facility uses a steam reforming technology, referred to as THermal Organic Reduction (THOR®), to pyrolyze organic resins loaded with cesium-137 and cobalt-60 from commercial nuclear facilities. The Erwin facility has the current capability to process a wide variety of solid and liquid streams including ion exchange resins, charcoal, graphite, sludge, oils, solvents, and cleaning solutions at radiation levels of up to 400 rads per hour (Mason et al., 1999).

FBSR is a thermal treatment technology and therefore must comply with a number of regulations. The process has been shown to be Clean Air Act (CAA) compliant. It has also been shown to be Hazardous Waste Combustor (HWC) Maximum Achievable Control Technology (MACT) compliant for mercury, chlorine, carbon monoxide, total hydrocarbons, and heavy metals (Soelberg et al., 2004). A significant benefit of the FBSR process is that liquid secondary wastes are not produced (Mason et al., 1999). (Secondary waste solids such as fines from high-temperature filters and the bag house can be mixed with the bed product and monolithed for disposal.) Many years of operating and design experience with fluidized

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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beds in the chemical industry and the availability of computational fluid dynamics tools significantly reduce development and operating risks for potential DOE-EM applications.

Depending on the starting material feeds, FBSR produces a range of waste form compositions. If kaolinite is added to an alkali-rich waste (e.g., neutralized HLW) during processing,9 a crystalline ceramic waste form is produced that is composed of sodium-aluminum-silicon feldspathoid mineral analogues (e.g., sodalite) that serve as potential hosts or a number of radionuclides (see Appendix 4.A and Chapter 3). Bench-scale, pilot-scale, and engineering-scale tests have all produced this mineral assemblage using a variety of DOE waste simulants as feed materials. Additionally, an illite-type clay additive has been tested at the bench scale and shown to form dehydroxylated mica, which is a good host for lanthanides, cesium, strontium, barium, rubidium, and thallium (Jantzen and Williams, 2008; Keppler, 1990). It is reasonable to expect that these mineral assemblages would also serve as hosts for the radioactive forms of these elements that are present in DOE-EM waste streams.

DOE-EM plans to apply FBSR to some of its waste streams. An FBSR facility is being designed and constructed at the Idaho Site for treatment of decontamination solutions (referred to as sodium-bearing waste) for potential disposal in the Waste Isolation Pilot Plant (Marshall et al., 2003). Another facility is being designed for use at the Savannah River Site to process HLW in Tank 48, which contains nitrates, nitrites, and organic sodium tetraphenyl borate (NaTPB). This process will produce carbonate or silicate phases, which can possibly be fed to the DWPF for vitrification. DOE-EM has also carried out pilot-scale testing on a variety of simulated wastes to produce aluminosilicate ceramic waste forms (Jantzen, 2003).

As noted in the committee’s interim report (NRC, 2010), there are at least two potential types of applications of FBSR in the DOE-EM cleanup program:

  1. As a front-end process for conditioning waste feed streams:
  • For accelerating liquid evaporation at the front end of the HLW vitrification process. This could enable increased waste throughputs to the JHMs and increased production rates of high-activity and low-activity waste forms.
  • For processing waste streams, including resins, containing large quantities of organic materials and nitrates. The planned application of FBSR to process Savannah River tank waste containing high concentrations of NaTPB is an example of such an

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9 The addition of kaolinite in the FBSR process is somewhat analogous to the addition of glass-forming materials (i.e., glass frit) in the vitrification process.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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application. FBSR also has potential applications for processing waste streams containing organic solvents and radionuclide-loaded organic resins, for example, the technetium-99-loaded resins generated by groundwater cleanup efforts at the Hanford Site.

2.   As a process for production of crystalline ceramic waste forms:

  • For processing alkaline HLW with bulk aluminosilicate additives (e.g., kaolinite clay), which could produce waste forms with good radionuclide retention properties and increased waste loadings relative to borosilicate glass (Jantzen, 2006). This process could also reduce or eliminate the need for recycling of melter off-gas condensates and is potentially applicable to both high-activity and low-activity waste streams.
  • For processing recycle liquids from HLW waste processing operations. This application has already been demonstrated at pilot scale for low-activity secondary waste simulants at Hanford.

FBSR is a mature technology in many industrial applications, including for the treatment of radioactive waste. Its deployment in specific DOE-EM applications may require some up-front development work to tailor it to specific waste streams, but relatively little basic research is likely to be required. Possible needs for basic research include the elucidation of key material structural characteristics (see Chapter 3) and waste form durability (see Chapter 5). Development work might also be required to better understand and ameliorate the attrition of granular bed material present in FBSR. Such attrition can be reduced through the proper design of internal components, dust collection equipment, operating conditions, and selection of additive materials. All of these have well-known solutions in chemical or petroleum industry applications of fluidized beds, and there are many available computational dynamics tools that minimize the risk of scale-up.

Waste forms produced by FBSR are granular and therefore may not be suitable for direct disposal in all cases. If necessary, they can be processed into high-integrity containers or further encapsulated in cement, geopolymers, hydroceramics, or glass to meet waste acceptance criteria (see Chapter 8) to be suitable for disposal.

4.9 OTHER THERMAL TECHNOLOGIES

There are several other technologies for immobilizing nuclear waste. Some of these are established procedures in other industries, whereas others have been demonstrated only at smaller scales. The committee provides

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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brief overviews of these technologies in the following sections. For further details the reader is referred to Noyes (1995).

4.9.1 Gas-fired Technologies

In these technologies heat is produced by burning waste and/or fuel. Consequently, they produce large volumes of non-condensable off-gas products. In some cases, secondary tanks may be required to hold the waste form products that are produced. They can also pose safety issues because of open flames and large quantities of fossil fuels used within remote/enclosed facilities.

  • Cyclone Furnaces can be used to immobilize highly contaminated wastes containing heavy metals and organics (and low-volatility radionuclides such as strontium and transuranics) in oil and sludge. Fuel and waste are fed in a spiral manner into a combustion chamber for maximum combustion and contacting efficiency. The waste form, a vitrified slag, can be withdrawn from the bottom of the cyclone.
  • Rotary Kilns are slightly inclined, cylindrical, refractory-lined vessels that rotate about their axis. They find applications in a variety of industries to produce materials (e.g., Portland cement) or dry solids. Fossil fuel-fired glass furnaces have also been used in the glass industry. The same technology may be applicable for producing vitrified waste products. For a rotary kiln to produce a vitrified product it has to operate in a slugging mode. At high temperatures the kiln material becomes amorphous, and molten slag can be withdrawn from the kiln (Noyes, 1995).

4.9.2 Electric Arc Furnaces

Electric Arc Furnaces are primarily used in metallurgy and can also produce vitrified waste forms (O’Connor and Turner, 1999). They consist of refractory-lined vessels, usually water-cooled in larger sizes, covered with retractable roofs through which one or more graphite electrodes enter the furnaces. They can operate either by DC or AC current. The electric arc provides energy for heating and melting the material contained in the furnace. The DOE Albany Research Center in Oregon operated an electric arc furnace demonstration unit with simulated low-level radioactive, high combustible-bearing mixed wastes, and simulated low-level radioactive liquid tank wastes (O’Connor and Turner, 1999). The operation temperature was around 1,600°C, necessitating significant gas treatment equipment to scrub particulate matter and volatile radionuclides.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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4.9.3 Plasma Heating

Plasma Heating is an electrical heating process in which plasma is created by passing a gas, commonly containing nitrogen, oxygen, or noble gases, through an electrical arc. It is suitable for processing low-level mixed waste to produce a vitrified waste form. One such process, the Plasma Hearth Process (PHP) (McFarlane et al., 1997), uses an ultrahigh temperature plasma (in excess of 5,000°C) generated by a direct current torch, which is remotely directed at containers of waste materials that are fed into a refractory-lined processing chamber.

Plasma heating results in oxidation, pyrolysis, and volatilization of the waste. Combustible pyrolysis gases generated in the primary process are oxidized in a propane-fired secondary combustion chamber. The remainder of the off-gas is filtered through HEPA filters. Non-volatile materials, such as metals, ash, and inert materials are then melted in the plasma chamber to produce a molten pool of metal and oxidized materials that form a slag. The molten material is captured in a shallow, refractory-lined hearth.

There have been pilot-scale tests of this technology (DOE, 1998; Wahlquist, 1996), but to the committee’s knowledge there have been no large-scale demonstrations. The electrodes used to produce the plasma have short life times, which can be a significant drawback to this technology.

4.9.4 Microwave Heating

Microwave Heating takes place in dielectric materials because of losses from the polarization effect of electromagnetic radiation at frequencies between 300 MHz and 300 GHz. The successive distortion of the molecules causes heating. Microwave melting has been developed as a batch process and thus has a limited throughput. In this process drums of waste are heated in microwave chambers that function as ovens. Because of the nature of the process, uneven heating within the volume of the drum may occur, producing unacceptable waste form characteristics. Process scale-up of this technology has not been demonstrated at the scale necessary to process large amounts of wastes.

4.10 MIX AND SET TECHNOLOGIES

Waste forms can be generated by mixing wastes with materials that cure and solidify, encapsulating the waste and also binding some waste constituents in hydration product phases. Several binding materials can serve as waste forms.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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4.10.1 Cement

Cement is an inorganic binder that sets and hardens to produce a rock-like material. Portland cement, which is the most commonly used cement type, is mainly comprised of lime, silica, alumina, and ferric oxide. Other oxides such as magnesia and sulfur are also introduced during various manufacturing processes. Portland cement comes in a variety of compositions.

Supplementary cement materials are sometimes added to Portland cement for either cost or availability reasons. These additions may significantly affect the properties of the resulting composite. Typically, cement in nuclear waste forms comprises much less than 50 percent of the total cementitious matrix with other materials, for example fly ash, blast furnace slag, and sand, comprising the remainder.

Two technologies are used for preparing waste-cement mixtures: (1) in-container mixing and (2) in-line mixing. In facilities designed for in-container cement production, the cement and additives are stored in silos and are usually pneumatically transferred to a batching station where they are added to the container. The waste liquid is metered to the container and the material is mixed and allowed to cure. For in-line mixing, the cement and liquid waste are mixed and then pumped into containers as thick slurries.

Cementitious waste forms are being used for the immobilization of radioactive waste in the DOE-EM cleanup program; two noteworthy examples are the Saltstone process at Savannah River Site, where waste salt solutions are mixed with fly ash, slag, and Portland cements and pumped into cement vaults for disposal. The other is the tank closures at Idaho National Laboratory, where the tanks are first washed to remove residues and then filled with grout made of sand, fly ash, blast furnace slag, and Portland cement. An overview of the cementation technology is given in Noyes (1995) and Ojovan and Lee (2007).

Some examples of the more commonly used cement technologies are described in the following sections. Fuhrmann (1981) provides a summary of other cement technologies that may be of utility for minor waste streams. These include sulfur-polymer, urea-formaldehyde, and polymer-impregnated cements.

4.10.2 Geopolymers

Geopolymers are ceramic-like, inorganic polymers made from aluminosilicates cross-linked with alkali metal ions (M2O), nominally 4SiO2·Al2O3·M2O. Clay (heat treated to render it amorphous) or amorphous fly ash are used as the aluminosilicate starting material. Alkali or alkaline earths in the waste (or added as sodium hydroxide or sodium

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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silicate) “activates” the amorphous aluminosilicate structure to reorganize it into a cross-linked inorganic polymer.

Geopolymers are made using methods similar to the processing of cement waste forms. One such process is the Geopolytech® (Hermann et al., 1999). In this process, the geopolymer, cement, and additives are mixed with waste and water (if necessary) then transferred into molds. If desired, the geopolymer can be removed from the mold in 2 hours; the waste form cures to its final compressive strength in 28 days.

4.10.3 Hydroceramics

Hydroceramics are predominantly crystalline waste forms. They derive their name partially from the fact that they are directly related to the synthesis of zeolites from metakaolin and sodium hydroxide, and also from the fact that a hardened hydroceramic looks and breaks (exhibiting brittle behavior) like a ceramic. The term hydroceramic is used to distinguish this material from the much broader class of geopolymers, which as noted above are alkali-activated cements rather than zeolites.

A hydroceramic waste form (Bao et al., 2005) is formulated in the same fashion as zeolites (i.e., from metakaolinite and sodium hydroxide) except that less volume but more concentrated sodium hydroxide is used. A typical process for making the hydroceramics involves mixing of low-nitrate wastes with metakaolin in a pug mill (or similar mixer) and then extruding the mixture into a suitable container; the mixture sets up and hardens when precured at slightly elevated temperatures (ambient to 40°C) and is then hydrothermally10 cured at 90°C-180°C for varying periods of time using an autoclave.

For application to wastes with high concentrations of sodium, the nitrate content of the waste needs to be below 25 percent mol NOx (calculated as the ratio of moles of NOx to moles of Na) for hydroceramics to solidify. Waste that has higher nitrate contents must be pre-treated to reduce them to an acceptable range before incorporation into hydroceramic waste forms.

There are currently no large-scale processes for making hydroceramic waste forms.

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10 Hydrothermal synthesis is a method for producing of mineral and inorganic oxide phases in an aqueous system. Reactants are allowed to react in a closed vessel at temperatures of 100°C-250°C. Reaction vessels are generally autoclaves that allow temperature gradients to form, solution to supersaturate at different temperature zones, and crystallization product to form.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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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 advantages of bitumen as a waste form are simplicity of production, low operating 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 mechanism, 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 nonnuclear applications, as described elsewhere in this chapter, and appear to be applicable for processing and immobilizing a range of DOE-EM waste

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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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 chapter. The committee concurs with DOE-EM about the applicability of these technologies in the cleanup program. Even though these are mature technologies 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.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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TABLE 4.2 Waste Form Production by Various Processing Technologies

Borosilicate Glass Other Glasses (LaBs, FeP, AlP, chalcognide, and others) Glass-Ceramic Materials (GCMs) Minerals/Crystalline Ceramics/Simple Oxides
THERMAL
Joule-Heated Melters (JHMs) Y Y Y N
 
Advanced JHMs Y Y Y N
 
Heated Melters (bubblers, etc.)
 
Cold Crucible Induction Melters (CCIM) Y Y Y Y*
 
Press + Sinter N N Y Y
 
Hot Isostatic Pressing (HIPing) N N Y Y
 
Fluidized Bed Steam Reforming (FBSR) N N Y Y
 
Calcining N N N Y
 
Hydrothermal Processing N N Y Y
NONTHERMAL Mix and Set N N N N
Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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Metal Matrix Zeolites Hydro-ceramic Cements (OPC, Ceramicrete, and 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 use of as waste forms and as binders (macro-encapsulation)] Y (heating necessary to melt bitumen) Y [to include use of as waste forms and as binders (macro-encapsulation)]
Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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APPENDIX 4.A
FLUIDIZED BED STEAM REFORMING

The Fluidized Bed Steam Reforming (FBSR) of nuclear waste is a relatively new technology, though the fluidization phenomenon and steam reforming 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 devolatilization 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 fluidized bed of the FBSR process is referred to as the Denitration and Mineralization 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 concentrate 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 hydrocarbons, 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:

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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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 (including 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 analogues 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 sodalite 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

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
×

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 metallic spent nuclear fuel from the EBR II fast breeder reactor.

Suggested Citation:"4 Waste Processing and Waste Form Production." National Research Council. 2011. Waste Forms Technology and Performance: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/13100.
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×

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×

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The Department of Energy's Office of Environmental Management (DOE-EM) is responsible for cleaning up radioactive waste and environmental contamination resulting from five decades of nuclear weapons production and testing. A major focus of this program involves the retrieval, processing, and immobilization of waste into stable, solid waste forms for disposal. Waste Forms Technology and Performance, a report requested by DOE-EM, examines requirements for waste form technology and performance in the cleanup program. The report provides information to DOE-EM to support improvements in methods for processing waste and selecting and fabricating waste forms. Waste Forms Technology and Performance places particular emphasis on processing technologies for high-level radioactive waste, DOE's most expensive and arguably most difficult cleanup challenge. The report's key messages are presented in ten findings and one recommendation.

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