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9
Possible Opportunities in Waste Form
Science and Technology
T
he previous chapters of this report have focused on the current state
of development of waste form science and technology. The focus of
this chapter is on the future: It describes some exciting trends and
recent developments in materials science, processing technologies, and com-
putational simulation and their potential applications to DOE programs,
especially the Department of Energy-Office of Environmental Manage-
ment’s (DOE-EM’s) cleanup program. This chapter is intended to address
the last two charges of the committee’s statement of task (see Box 2.1.
in Chapter 2) by providing examples of how scientific and technological
advances may improve the DOE-EM cleanup program.
Advances in waste form science and technology could have important
applications in other DOE programs as well. For example, the develop-
ment of advanced nuclear fuel cycles by DOE’s Office of Nuclear Energy
(DOE-NE) and others will require the design of new materials for recycling
or immobilizing radionuclide streams that are unlike DOE’s legacy wastes,
and also the development of new approaches for modeling nuclear fuels
(e.g., Devanathan et al., 2010). Inert matrix fuels or new target materials,
which are contemplated for use in reactors designed to “burn” transuranium
elements, could be designed not only for their performance in those reac-
tors, but also for ease of recycling and disposal (Peters and Ewing, 2007).
Some of the examples provided in this chapter are potentially useful for
these applications, especially for managing actinides.
Several recent workshops and studies have identified exciting new
research opportunities in materials science, including the development of
219
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220 WASTE FORMS TECHNOLOGY AND PERFORMANCE
improved waste forms, processing technologies, and computational capa-
bilities. The reports include the following:
• Summary Report of the Nuclear Energy Research Initiative Work-
shop, April 23-25, 1998 (see the report of working group #4).
Available at http://www.ne.doe.gov/pdfFiles/nerachWorkshop.pdf.
• Basic Research Needs for Advanced Nuclear Energy Systems,
July 31-August 3, 2006 (see the panel #5 report on advanced
waste forms). Available at http://www.er.doe.gov/bes/reports/files/
ANES_rpt.pdf.
• Basic Research Needs for Geosciences: Facilitating 21st Century
Energy Systems, February 21-23, 2007 (see sections related to
subsurface geologic storage and modeling/simulation of geologic
systems). Available at http://www.er.doe.gov/bes/reports/files/GEO_
rpt.pdf.
• Basic Research Needs for Materials under Extreme Environments,
June 11-13, 2007 (see section on nuclear energy). Available at
http://www.er.doe.gov/bes/reports/files/MUEE_rpt.pdf.
• Global Nuclear Energy Partnership Integrated Waste Manage-
ment Strategy Waste Treatment Baseline Study. GNEP-WAST-
AI-RT-2007-00034. 2007 (see vol. 1 sections on processing and
stabilization with different types of waste forms). Available at
http://www.engconfintl.org/9arIWMS.pdf.
• Directing Matter and Energy: Five Challenges for Science and the
Imagination, A Report from the Basic Energy Sciences Advisory
Committee, 2007 (see chapter 7 on designing new materials). Avail-
able at http://www.er.doe.gov/bes/reports/files/GC_rpt.pdf.
• Alternative Waste Forms: Aqueous Processing (Ryan et al., 2009).
• Alternative Waste Forms for Electro-Chemical Salt Waste (Crum
and Vienna, 2009).
Additionally, the recent National Research Council report Frontiers in
Crystalline Matter from Discovery to Technology (NRC, 2009), which was
sponsored in part by DOE, outlines an exciting agenda for the development
of new materials for special applications. Although most of the examples
in this report are for high-technology applications (e.g., microelectronics,
superconductivity, and heterostructures), opportunities also exist for the
development of new waste form materials.
The committee sees possible innovations developing from at least three
directions:
• New waste form materials designed for specific performance func-
tions (e.g., high durability in specific disposal environments; com-
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POSSIBLE OPPORTUNITIES IN WASTE FORM SCIENCE & TECHNOLOGY
patibility with specific waste streams) or designed to remain stable
over different ranges of time, depending on the half-life of the
radionuclide.
• New processing technologies that can handle complex, highly
radioactive waste streams and produce more consistent waste form
products.
• Advanced techniques for understanding and modeling waste form–
near-field interactions.
In the sections that follow, the committee provides some examples of
potential innovations in each of the three categories (i.e., materials, pro-
cessing technologies, and models) mentioned above. These examples are
presented to illustrate the wide variety of possibilities; they should not be
viewed as inclusive or as recommendations for specific investigations. The
examples are provided primarily to illustrate what might be developed by
DOE-EM even with modest investments. Some of these examples are incre-
mental in that they build on research programs that have obvious relations
to the ongoing DOE-EM cleanup mission. Others are simply “outside-the-
box” ideas that may warrant attention by DOE-EM.
9.1 NEW DEVELOPMENTS IN MATERIALS SCIENCE
The following examples illustrate how advances in materials science
can be used to develop new and improved waste form materials for specific
applications, for example, for immobilizing specific waste streams or for dis-
posal in specific geological environments. These materials may benefit from
further development or application; some have not been fully explored by
the waste management community. These examples are intended to be illus-
trative, not comprehensive. The committee has made no effort to determine
whether these materials are suitable for particular DOE-EM waste streams.
9.1.1 Amorphous Materials Designed with
Short-Range and Intermediate-Range Order
There continues to be substantial progress in the characterization and
understanding of the structure of glass and the interplay between glass
composition, structure, and properties. This progress is the direct result
of advances in materials characterization techniques, mainly spectroscopic
techniques (Hawthorne, 1992; Pierce et al., 2010), including the use of syn-
chrotron sources for X-ray scattering and X-ray absorption spectroscopy
(Brown et al., 1995). A recent workshop has reviewed the possible applica-
tions of synchrotron radiation techniques to materials that contain radio-
nuclides (ANL, 2010).
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222 WASTE FORMS TECHNOLOGY AND PERFORMANCE
The application of advanced spectroscopy techniques has provided a
greatly improved understanding of the structural properties of glasses, par-
ticularly short-range (nearest-neighbor atomic spacing) and intermediate-
range (the connectivity extending across several metal-metal distances)
order (Calas et al., 2010) (Figure 9.1). As a result, it now appears feasible
to use amorphous network engineering to tailor glass compositions with
specific atomic sites for incorporating radionuclides (e.g., Martin et al.,
2002). Great progress has also been made in simulating glass structures and
calculating the energetics that control glass durability (Garofalini, 2001;
Poole et al., 1995). The improved knowledge of glass structure and dura-
bility should provide increased confidence in understanding glass behavior
in disposal environments.
FIGURE 9.1 Schematic diagram showing the complexity of the structure of glass
Figure 9.1.eps
with both short-range (the individual coordination polyhedra) and intermediate
range (extending across the different rings of polyhedra). With the increased under-
bitmap
standing of the structure of glass, one can use the intermediate-range order for the
atomic-scale incorporation of specific radionuclides.
SOURCE: Calas et al. (2010), Figure 2.
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POSSIBLE OPPORTUNITIES IN WASTE FORM SCIENCE & TECHNOLOGY
9.1.2 Glass-Ceramic Materials
Although glass may accommodate waste loadings of up to 38 weight
percent, there are certain constituents (e.g., chromium, sulfate [SO4=], tita-
nium, zirconium, phosphorus, and actinides) that have limited solubilities
in certain glass compositions. Similarly, crystalline ceramics often have thin
selvages of amorphous material along their grain boundaries. As noted in
Chapter 3, there is a continuum of glass and crystalline phases within many
materials. In fact, such glass-ceramic materials (GCMs) are probably more
common than is generally appreciated because processing technologies are
generally limited in their ability to provide phase-pure materials.
The multi-phase nature of GCMs makes them useful for immobiliz-
ing radioactive waste. As noted in Chapter 3, GCMs could be designed to
incorporate long-lived radionuclides (e.g., actinides) into crystalline phases
of greater durability and shorter-lived radionuclides (e.g., some fission
products) into less durable glass phases. This approach was proposed more
than 20 years ago (e.g., Hayward, 1988). However, recent advances in
materials processing technologies may make it feasible to actually produce
these materials at reasonable scales and costs.
There still remain a number of challenges for designing GCMs for
radionuclide immobilization. The physical properties and leaching behav-
ior of GCMs may differ from either pure glass or an assemblage of fully
crystalline phases because of the coupling of processes between the phases.
For example, crystalline phases that contain actinides may expand as radia-
tion damage accumulates; this expansion may cause microcracking in the
surrounding glass (Figure 9.2). The interface between the glass and crystal
may be the point of maximum leach rate and radionuclide release on initial
contact with water, resulting in a high instantaneous release followed by
slower matrix dissolution.
Nevertheless, GCMs have already demonstrated utility as waste forms
for iodine (Garino et al., in press). They can accommodate a greater range
of radionuclides and achieve higher waste loadings (see Figure 3.1 in Chap-
ter 3) than many materials now in use. They can also be produced at lower
temperatures.
9.1.3 AOx Isometric Structures
Incremental changes in the composition and structure of a material
can result in substantial improvements in its performance as a waste form.
For example, uranium dioxide (UO2), which comprises the matrix of com-
mercial nuclear fuels, has a simple, isometric fluorite (CaF2) structure, the
same structure as many actinide oxides. During the past decade, derivative
structures such as A2B2O7 pyrochlore, which contains two cation sites
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224 WASTE FORMS TECHNOLOGY AND PERFORMANCE
FIGURE 9.2 Effect of alpha-decay in crystalline phases on microstructure in a par-
tially devitrified glass: (a) alpha-autoradiograph showing curium-244 concentrated
Figure 9.2.eps
in crystalline phases; (b) optical micrograph of region in (a) indicating no micro-
bitmap
15 alpha-decays/g; and (c) microcracking in same region from
cracks after 6 × 10
amorphization of crystalline phases after 2.4 × 1017 alpha decays/g.
SOURCE: (a) Courtesy of William J. Weber, University of Tennessee; (b) and (c)
Weber et al. (1998), Figure 6.
(A, B) and one missing oxygen, have been examined for possible use for
incorporating actinides, either as part of an inert matrix fuel or for direct
disposal in a repository. A typical composition that has been investigated
for this purpose is titanate pyrochlore (Gd,Pu)2(Ti,Hf)2O7. This material is
very susceptible to radiation damage because of alpha-decay from the incor-
porated actinides (in this case plutonium), which causes the material to be
transformed to an amorphous state. However, if the composition is changed
by substituting zirconium for titanium to produce (Gd,Pu)2(Zr,Hf)2O7, the
material has an entirely different response to radiation damage and does
not become amorphous even at very high radiation doses.
This difference in behavior is illustrated schematically in Figure 9.3. The
curves in the figure show the doses required to amorphize three pyrochlore
materials as a function of temperature. The temperature at which thermal
annealing dominates over damage accumulation—that is, the point at which
the curves become vertical and a material can no longer be amorphized—is
different for these three materials. By selecting a composition for which
the thermal annealing occurs at low temperatures (e.g., Gd2ZrTiO7 in
Figure 9.3), one can ensure that the waste form never becomes amorphous
because of radiation damage.
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POSSIBLE OPPORTUNITIES IN WASTE FORM SCIENCE & TECHNOLOGY
FIGURE 9.3 Predicted temperature dependence of amorphization in pyrochlore
phases containing plutonium-239. The curves bend upward at elevated tempera-
tures because of thermal annealing. Where the curves become vertical the material
remains crystalline at high alpha-decay 9.3.eps range of potential repository
Figure doses. The
bitmap
temperatures is indicated by the horizontal line. SOURCE: Ewing et al. (2004).
This annealing behavior could be used to advantage for disposing of
minor actinides immobilized in pyrochlore waste forms. Such waste forms
could be disposed of in boreholes drilled several kilometers into Earth’s
crust, where temperatures are sufficiently elevated (because of the geother-
mal gradient) to prevent waste form amorphization (Figure 9.3).
Pyrochlore materials are potentially useful for immobilizing other
radionuclides besides actinides. For example, Weck et al. (2010) recently
synthesized a series of rare-earth technetate pyrochlores that can potentially
be used to immobilize long-lived fission products such as technetium-99.
Russian researchers are investigating new approaches for technetium immo-
bilization (e.g., Laverov et al., 2010).
9.1.4 Complex Structure-Types
There are a number of more structurally complicated materials—for
example, complex oxides, silicates, phosphates, and vanadates—that have
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226 WASTE FORMS TECHNOLOGY AND PERFORMANCE
not been fully considered or developed as waste forms. Some of these mate-
rials were described in Chapter 3.
For example, murataite and garnet have recently received attention as
potential host phases for actinides (Laverov et al., 2006, 2009a,b, 2010).
Murataite-based ceramics (A6B12C4TX40-x)1 have atomic periodicities that
are multiples of the basic AX2 structure of fluorite (CaF2); however, they
have more complicated compositions and multiple cation sites. These sites
can be used to immobilize waste streams that have complex compositions,
thus eliminating the need for further chemical separation (Laverov et al.,
2006, 2010; Lukinykh et al., 2008). These materials accommodate high
actinide waste loadings and can be designed to remain crystalline over long
periods of disposal, as was shown previously for pyrochlore (Yudintsev et
al., 2007). However, phase-pure murataite is difficult to make. Usually, it
is one phase in a polyphase assemblage containing (mostly) other titanates,
which can help to encapsulate the actinide-bearing murataite.
Garnet (A3B2[TO4]3), which has an isometric structure with three cat-
ion sites that can accommodate actinides and lanthanides, may also be a
useful waste form material. Recent work (Laverov et al., 2010) has shown
that garnet leach rates and radiation response can be changed substantially
by changing its composition. Crystalline phosphates of the NaZr2(PO4)3
(NZP2) family continue to be of interest mainly in the context of high-level
radioactive waste (HLW) immobilization because the unique NZP struc-
ture can incorporate a variety of cations, including plutonium (Hawkins
et al., 1997; Zyryanov and Vance, 1997). The NZP structure is a three-
dimensional network of corner-sharing ZrO6 octahedra and PO4 tetrahedra
in which plutonium can substitute for Zr, as in Na(Zr,Pu)2(PO4)3 (Orlova
et al., 1994).
Apatite has a complicated, low-symmetry crystal structure:
A10(BO4)6(OH, F, Cl)2, where A = calcium (Ca), sodium (Na), rare earths,
fission products, and actinides; and B = silicon (S), phosphorus (P), vana-
dium (V), or chromium (Cr). It has been studied extensively as a host for
toxic metals, and it also has great potential as an advanced waste form for
complex radioactive waste streams because of its complex crystal chem-
istry, structural flexibility, and good chemical durability. A wide range of
waste components (e.g., tri- and tetra-valent actinides, strontium, cesium)
can be incorporated into the apatite structure by coupled substitutions
at the cation and anion sites (Carpéna et al., 1998, 2001; Langmuir and
Apted, 1992; Maddrell and Abratitis, 2004). Iodine, for example, can be
incorporated in the structural channel in a lead vanadate apatite structure
1 Note: A = cations in 8-fold coordination; B = cations in 6-fold coordination; C = cations
in a trigonal bipyramid (6-fold); T = cations in 4-fold coordination.
2 NZP denotes the sodium zirconium phosphate structural family.
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POSSIBLE OPPORTUNITIES IN WASTE FORM SCIENCE & TECHNOLOGY
(Pb10(VO4)6-x(PO4)xI2). The interest in this material has also resulted in the
examination of new technologies, such as spark plasma sintering, for its
synthesis (Campayo et al., 2009).
9.1.5 Metal-Organic Frameworks
Metal-organic frameworks (MOFs) are a relatively new class of porous
materials that consist of metal atoms (ions) linked together by multifunc-
tional organic ligands (Figure 9.4). An incredibly diverse group of MOFs
have been synthesized because of the wide variations in possible linkages
between the organic and inorganic components of each framework. With
the diversity in framework topology and surface moities, MOFs can be
constructed with extremely large surface areas and with surface adsorption
molecules.
The field of “reticular” chemistry (Batten et al., 1995; Hoskins and
Robson, 1990; Yaghi and Li, 1995; Yaghi et al., 2003) and the development
of MOFs are less than a decade old (O’Keeffe et al., 2000). Nevertheless,
MOFs have already been considered for many applications in the energy
field, for example, hydrogen storage, carbon dioxide sequestration, and
methane sequestration. The most recent development (Furukawa et al.,
2010) has been the synthesis of three-dimensional crystal structures that
have exceedingly high internal surface areas. These high surface areas could
be useful for radionuclide separations. Further, pore spaces in MOFs can
be engineered to specific sizes, which could also be useful for separations.
It is easy to imagine the design and use of MOFs for immobiliz-
ing iodine and technetium. In fact, there is ongoing research funded by
DOE-NE on the use of MOFs as separation materials for radio-iodine
(Sava et al., 2011). At Sandia National Laboratories, research is focused on
existing and novel MOFs for high loadings of I2 separated from both liquid
and gas streams. Published results have shown that various MOF/I2 phases
(including ZIF-8 and HKUST-1 MOFs) can be successfully incorporated
into low-temperature glasses to form GCMs.
9.1.6 Self-Assembled Mesoporous Materials
Mesoporous3 materials have attracted considerable attention since
their discovery in the early 1990s (Beck et al., 1992; Kresgie et al., 1992).
An important innovation has been to functionalize the surfaces of these
materials (Anthony et al., 1993; Sayari, 1996) with self-assembled organic
3 Mesoporous materials have regularly arranged pores ranging from 2-50 nanometers in
diameter. They have high surface areas (up to 1,500 square meters per gram) and uniform
pore sizes and shapes.
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228 WASTE FORMS TECHNOLOGY AND PERFORMANCE
FIGURE 9.4 Framework structures are built of a linked framework of “paddle-
wheel” units. The units can assume many different geometries and can accom-
modate a wide variety of elemental and 9.4.eps species. The small spheres are
Figure molecular
carbon, oxygen, bromine, and metal bitmap large spheres illustrate the size of
atoms. The
the cavities in some of the geometric arrangements.
SOURCE: Yaghi et al. (2003).
monolayers that provide a substrate with high chemical selectivity, allowing
these materials to be used as chemical sensors (Kumar et al., 1994) and in
chemical separation processes (Wirth et al., 1997).
Investigators at Pacific Northwest National Laboratory have extended
this technology with the development of self-assembled mercaptan on meso-
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POSSIBLE OPPORTUNITIES IN WASTE FORM SCIENCE & TECHNOLOGY
porous silica (SAMMS) (Feng et al., 1997a,b) for the removal of mercury
from waste water and organic wastes. A further extension of this technol-
ogy would be the development of mesoporous materials that are func-
tionalized for the separation of specific radionuclides and suitable for the
synthesis of waste forms. Advantages include high radionuclide loadings,
high selectivity, and the possibility of a chemically durable final product
(Feng et al., 1997b).
There are many related structures that share the features of MOFs and
mesoporous materials. Recently, a sulfide structure has been synthesized
that is selective for cesium by incorporation into its open framework (Ding
and Kanatzidis, 2010). Thus, even framework chalcogenide4 structures hold
the potential for the sequestration of radionuclides.
9.1.7 Actinide Clusters and Frameworks
Nanoscale control of actinide materials is a new research field with
potential applications in nuclear waste form development. Cage molecules
containing from 20 to 60 uranium atoms as well as peroxide, hydroxyl,
and oxygen have been reported over the past five years (Burns et al., 2005;
Forbes et al., 2008; Sigmon et al., 2009a,b,c; Unruh et al., 2010). These
clusters have diameters up to 3 nanometers. They are built of uranyl
peroxide hydroxide hexagonal bipyramids that are linked through shared
equatorial edges. This linkage results in topological squares, pentagons,
hexagons, and a wide variety of cluster types. Several of these have fullerene
topologies, including a cluster with 60 uranium atoms that is topologi-
cally identical to C60 buckminsterfullerene (Figure 9.5). It may be possible
to tune the compositions and topologies of these structures for use as
precursors in the creation of novel waste forms or nuclear fuels. Such clus-
ters present the possibility of nanoscale control of chemical composition
and properties of materials.
A new complex supertetrahedral framework has been recently discov-
ered which consists of borate and thorium polyhedral (NDTB-1) (Wang et
al., 2010). This compound has a cationic framework that contains anions
within channels and cages that balance the framework charge. Borate
polyhedra occur both in ordered positions within the framework and as
disordered constituents of the channels. This compound has been shown
to rapidly ion exchange these disordered borate groups with a variety of
anionic groups, including TcO4–. This or other custom-designed materials
with similar properties may be used to separate and/or sequester technetium
to reduce its concentration in waste streams.
4 Materials containing sulfur, selenium, and tellurium, usually as sulfides, selenides, and
tellurides.
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230 WASTE FORMS TECHNOLOGY AND PERFORMANCE
FIGURE 9.5 A cluster consisting of 60 uranium polyhedra (yellow) joined across
shared edges of the polyhedra.Figure 9.5.eps the same topology as a C60
This U60 cluster has
cluster, known as a “buckey ball.” bitmap
SOURCE: Courtesy of Peter Burns, University of Notre Dame.
9.1.8 Multi-Scale Computational Simulation of the Properties of Materials
One of the most rapidly developing research areas in materials science
is the use of computational simulation to determine fundamental physical
and chemical properties of materials. In the 1980s, the use of pseudo-
potentials to capture the behavior of chemically active electrons combined
with density functional theory5 allowed the study of systems consisting of
hundreds to thousands of atoms. At the same time, the rapid development
of computer technology (faster processors and more efficient algorithms)
led to the development of new tools for modeling the structure and prop-
erties of materials—and indeed, the new field of computational chemistry
(Cygan, 2001).
There are now a wide variety of standard computational packages that
are routinely used in studies of solid-state materials, including studies of
5This theory is used in chemistry and physics to investigate the electronic structures of
materials.
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nuclear materials (Stan, 2009; Stan et al., 2007) and even potential waste
form materials (Ferriss et al., 2010) at a number of scales (Figure 9.6).
These techniques are particularly useful for the study of highly radioactive
materials for which actual experiments are time consuming and expensive.
Computational simulations can be used to investigate a wide range of com-
positions or structure types and to focus experimental efforts on the most
critical, bench-marking data requirements. Computational simulations can
be extended to study surface reactions and corrosion mechanisms (Rosso,
2001) and radiation effects in materials (Ewing and Weber, 2010).
9.1.9 Design of Materials for Specific Performance Requirements
With the dramatic advances being made in computational chemistry
it is now becoming feasible to use computational techniques for materials
discovery and design (NRC, 2009; see Figure 9.7). This innovation has
FIGURE 9.6 Multi-scale theoretical and computational methods used for materials
Figure 9.6.eps
model development and computer simulations.
bitmap
SOURCE: Stan (2009).
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232 WASTE FORMS TECHNOLOGY AND PERFORMANCE
FIGURE 9.7 Schematic illustration of a methodology for the discovery and design
of new materials.
Figure 9.7.eps
SOURCE: NRC (2009).
bitmap
potentially important applications to radioactive waste form development:
Once a waste form’s performance requirements are established for a partic-
ular disposal environment, one can design materials to meet those require-
ments. As examples, glass waste form compositions might be changed to
enhance chemical durability or crystalline waste form composition might
be adjusted to enhance thermal annealing of radiation damage (as discussed
in Section 9.1.2). Such applications are, in fact, in their infancy, but their
potential is great.
Computational simulations can also be combined with experimental
techniques now in routine use in chemistry and pharmacology. For exam-
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POSSIBLE OPPORTUNITIES IN WASTE FORM SCIENCE & TECHNOLOGY
ple, combinatorial chemistry methods reduce the time required to invent
new catalysts and drugs. Xiang et al. (1995) has applied combinatorial
chemistry techniques to the discovery of new superconducting materials.
These techniques can be used in nuclear waste processing to invent new
waste form formulations, increase theoretical understanding of material
properties, evaluate the waste form performance, and shorten the time it
takes to develop new waste processing technologies. Undoubtedly, adapt-
ing these techniques to waste form development will require additional
development work.
9.2 NEW DEVELOPMENTS IN WASTE FORM
PROCESSING TECHNOLOGIES
New waste form materials will be of use in the DOE-EM cleanup
program only if they can be synthesized and produced at industrial scales.
Fortunately, there have been many incremental improvements as well as
important innovations in processing technologies that are potentially appli-
cable to production of waste forms. Some examples are described in this
section.
9.2.1 Computational Simulation of Material Processing
Recent advances in chemical engineering, materials science, and metal-
lurgy provide the basis for development of new technologies for nuclear
waste processing. Advances in computational science and recently emerging
tools in Computational Fluid Dynamics (CFD) (e.g., the Ansys® suite of
tools, MFIX by National Energy Technology Laboratory, and Barracuda®
by CPFD Software) have led to significant incremental improvements in
processing equipment and enhanced processing capabilities.
For example, such tools are currently in use to study flow patterns in
Joule-heated melters equipped with gas bubblers (Matlack et al., 2008) and
also for hot gas filter cleanup design (VanOsdol et al., 1996). Barracuda®
and MFIX are being used to model hydrodynamic phenomena and chemi-
cal reactions in fluidized beds. This software enables engineers to develop
trouble-free efficient process designs from the start; reduce the risk of scale-
up from pilot to commercial plants; and avoid operational problems and
downtime. These simulation tools are being used to scale-up fluidized bed
gasifiers, polymerization reactors, and combustors.
These state-of-the-art computational capabilities can pave the way for
the development of novel waste form processing techniques. For example,
fluidized bed steam reforming is being developed for the processing of
sodium-bearing waste at the Idaho National Laboratory (see Chapter 4).
The operational challenges for this technology include generation of fines,
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product agglomeration, and particle size control. CFD simulation tools can
help to address such operational challenges, specifically to:
• Reduce attrition of bed material through the proper design and
placement of internal components (e.g., reduce jets by the proper
design of grids; design the internal cyclones and diplegs) and mini-
mize carry-over.
• Optimize operating conditions through the proper choice of the
bed support, which enables smoother fluidization.
• Reduce scale-up risks through the appropriate inclusion of the
kinetics of particle growth combined with the bed hydrodynamics.
Once a simulation tool is built for these purposes it can also be used for
troubleshooting during actual operations.
In recent years there have also been developments in tools to simulate
the steady state and dynamic behavior of chemical processes. For example,
the Aspen® suite of products (which were developed by Aspentech) is used
widely in the chemical industry for such purposes. CFD models, when com-
bined with such simulation software, can be a powerful way to monitor
and control waste processing equipment such as fluidized beds and melters
together with the entire plant associated with these processes.
9.2.2 Innovative Uses of Existing Technologies
Some existing processing technologies in use in other industries are
also finding new applications for waste form production. These technolo-
gies are described in Chapter 4 and in the committee’s interim report
(Appendix C). Fluidization technology, namely steam reforming, is being
considered as a potential technology for the immobilization of a wide
variety of high-sodium, low-activity wastes such as those existing at the
Hanford Site, Idaho National Laboratory, and Savannah River Site. Cold
crucible induction melting (CCIM), which is a well-established process in
other industries, is being considered for immobilization of HLW and pos-
sible low activity waste in glass. As noted in Chapter 4, this technology
can be used to immobilize waste streams containing chromium, aluminum,
zirconium, sulfate, or phosphate, which can cause problems in Joule-heated
melters. CCIM is also being integrated with an oxygen plasma to destroy
organics and reduce post gas processing loads. Hot isostatic pressing (HIP),
another well-established technology in other industries, is currently being
considered for the production of waste forms containing calcine HLW at
the Idaho National Laboratory.
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POSSIBLE OPPORTUNITIES IN WASTE FORM SCIENCE & TECHNOLOGY
9.3 NEW DEVELOPMENTS IN MODELING
WASTE FORM–NEAR-FIELD INTERACTIONS
The ultimate objective of the DOE-EM cleanup program is the safe
disposal of legacy waste. The evaluation of safety requires the ability to
computationally simulate the behavior of radionuclides in the waste form
and other engineered and geologic barriers in the near-field environment
of a disposal facility. Recent advances in computational capacity and new
conceptual approaches to reactive transport modeling (Steefel et al., 2005)
offer new opportunities for understanding and simulating the release and
mobility of radionuclides in disposal environments.
As discussed in some detail in Chapter 7, the development of high-fidelity
models (Steefel et al., 2005) that realistically simulate radionuclide transport
both at local and repository scales represents a conceptual advancement in
simulation capabilities. Such models can be used to evaluate performance
of a disposal facility not only with respect to radiological risk, but also for
other performance metrics such as repository size.
9.4 DISCUSSION
As was noted in Chapter 2, the DOE-EM cleanup program is not
expected to be completed for at least another four decades. Consequently,
DOE-EM will have ample opportunities in the coming decades to incor-
porate advances in science and technology on waste forms, waste form
processing technologies, and waste form–near-field modeling into its base-
line approaches to increase program efficiencies, reduce lifecycle costs and
risks, and advance scientific understanding and stakeholder confidence.
The past 30 years have seen a steady increase in scientific and techno-
logical advances, perhaps best exemplified by the successful application of
vitrification technologies to immobilize HLW. Still, these past successes do
not preclude the exciting possibilities of new and innovative strategies for
improving waste management systems.
The committee realizes that DOE-EM is already successfully immo-
bilizing radioactive waste and making huge financial investments in the
construction of facilities, such as the Waste Treatment Plant at the Hanford
Site, which will be used to vitrify HLW. However, not every waste stream is
a good match for vitrification, nor can all waste streams be accommodated
in presently planned facilities. Consequently, prudence dictates that some
fraction of the DOE-EM program should be devoted to capturing and
using innovative science and technology. In fact, the “strategic initiatives”
of the EM Engineering & Technology Roadmap (DOE, 2008) are entirely
consistent with the development and use of new waste form materials,
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236 WASTE FORMS TECHNOLOGY AND PERFORMANCE
new processing technologies, and improved modeling and computational
capabilities.
• Advances in the science of materials design provide new methods
for efficiently exploring innovative waste form materials. Com-
putational simulation may provide an efficient and rapid means
of surveying the properties of materials for immobilizing highly
radioactive wastes without having to complete the full range of
costly and time-consuming experiments. Once these computational
surveys are completed, key experiments can be performed to con-
firm (or not) the results.
• Advances in materials processing can lead to improvements in
waste form production rates and product quality at reduced pro-
duction costs.
• Advances in computational capabilities, combined with new con-
ceptual models for materials performance in disposal environments,
can provide new insights into long-term materials performance in
specific disposal environments.
In the development of new materials, technologies, and computational
capabilities, the next 50 years is an eternity, and the prospects for innova-
tive improvements are huge. The incorporation of new science and technol-
ogy into the DOE-EM cleanup program need not interrupt the significant
progress that is being made at present but can, if done well, enhance its
prospects for future successes.
It can take decades to develop and introduce new technologies for
processing radioactive waste. For example, it took about two decades from
the decision to use borosilicate glass for HLW immobilization at Savannah
River to the first “hot” (radioactive) operations at the Defense Waste
Processing Facility. Some waste forms materials (e.g., ceramics) have been
studied for almost 30 years but have not yet found widespread implementa-
tion for radioactive waste immobilization. It is imperative for DOE-EM to
get this development of new processing technologies started in earnest
to reap its benefits for the cleanup program in a timely fashion.
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