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5
immobilization
To reduce the risk of radionuclide transport to the environment,
high-level radioactive waste must be immobilized (i.e., converted to a
stable solid form) before its disposal in a geological repository. The
durabi I ity of the immobi I ized waste form, together with the corrosion-
resistant overpack protection agai nst grou ndwater i ntrusion i ncl uded i n
the design of the future repository, prevent or minimize migration of
radioactive elements to the environment. Because repository space is
limited, it is important to achieve the highest waste-volume reduction to
minimize the number of containers needed for the immobilized waste.
Hence, this would lead to reduced costs by more efficient use of waste
immobi I ization, storage and transport faci I ities, and possibly to more
efficient use of repository space.3
The current immobilization technique used by DOE for HLW is
vitrification in a borosi I icate glass matrix (see Sidebar 5.1 ). The DOE
vitrification process involves blending HLW with borosilicate glass frit
or glass precursor chemicals, such as oxides and carbonates. The mix-
ture is heated in a Joule-heated melter (see Sidebar 5.2) to form a
molten glass, which is then poured into stainless steel canisters and
allowed to cool. The DOE is currently operating HLW vitrification
plants at the SRS and at the WVDP. The current plan at the Hanford
Site is to vitrify both HLW and LLW in borosilicate glass.2 Finally, the
INEEL will also immobilize its SBW in borosilicate glass and is now
considering the option of vitrification for its calcined waste.
'The cost of producing an HLW glass log is approximately $1 million. A 1 per-
cent increase in waste loading at the SRS could reduce cleanup costs by $200
mi 11 ion (Hrma et al., 1 998).
2The TPA decision to vitrify the LLW at Hanford is a departure from the strategy
used at the SRS and at the WVDP, where LLW is instead immobilized in a cement-
based material, referred to as "saltstone" at the SRS (TPA, 1998).
H ~ G H - L E V E E W A S T E
48
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SIDEBAR 5.1 SITE-BY-SITE HLW IMMOBILIZATION BASELINES
Immobilization at the Hanford Site
Both the HLW and the LLW will be vitrified at Hanford using Joule-heated melters, commencing in
2007.The Hanford tanks contain a wide range of reprocessing chemicals and wastes from the early bis-
muth phosphate and REDOX processes, as well as PUREX wastes. During Phase I of waste immobiliza-
tion at Hanford, ending in 201 8, approximately 1 0 percent of the HLW is to be retrieved and vitrified.
Little or no blending between tanks to smooth any waste composition variability is planned during
Phase I because of high costs and limited availability of free tank space. Furthermore, although the
Hanford melter will also use Joule-heating to immobilize HLW in borosilicate glass, it is proposed to
use a melter feed containing raw chemicals (oxides, carbonates, etc.), rather than frit, to give greater
flexibility in glass-batch preparation.
Immobilization at INEEL
The site has calcified liquid, acidic HLW to produce approximately 3.8 million liters of granular solid
waste that is currently being stored, pending a decision on final immobilization and/or disposal.The
remaining 5.3 million liters of liquid SEW is to be vitrified (Huntoon, 2000). A previous NRC committee
on the INEEL recommended in its report that the calcified material be stored until the repository loca-
tion and waste form acceptance criteria have been established (NRC, 1999b).That committee also
advocated further investigation of a number of viable candidate waste forms for the calcified wastes,
in addition to borosilicate glass,with the main objective being to increase the waste loading.These
candidate waste forms include (1) alternative glass compositions, including high-waste-loading glasses
prepared in single-use containers; (2) crystalline ceramics prepared by hot uniaxial or isostatic press-
ing; (3) glass-ceramic materials; and (4) cement-based waste forms.
Immobilization at SRS
In the DWPF vitrification facility at the SRS, HLW is immobilized in a borosilicate glass matrix.The
DWPF melter, described in Sidebar 5.2, uses a wet feed (approximately 50 percent water) comprised of
a slurry of waste and frit.The waste originates from a two-year homogeneous batch where HLW,
retrieved from different tanks, is blended. A target "window" of feed compositions, consisting of a
ternary diagram based on mixtures of two waste feeds and a glass frit (Figure 5.1) is used to determine
the composition of the melter feed.This window has been established on the basis of previous melting
trials with slurry feeds of frit plus simulated wastes. In this process control strategy, called Product
Composition Control System (PCCS), portions of the two-year batch of waste are fed into a tank where,
depending on the waste feed characteristics, the amount of frit (of a given composition) is adjusted so
that the frit volume is minimized and the predicted properties of the final glass will fall within the tar-
get window. A statistically designed variability study comprised of approximately 30 glass melts is per-
formed on every waste batch (about every 2 years) to ensure that waste-frit mixtures are correctly pre-
dicted by the PCCS models.The use of this process control strategy and a large homogeneous two-year
sludge batch minimizes the number of actual radioactive glasses that need to be analyzed from the
canisters produced. Therefore, control of the final radioactive glass composition mainly relies on the
PCCS process control strategy by ensuring that the melter feed composition is such that the resulting
glass exceeds, with a 95 percent confidence level, the measured durability of the benchmark
Environmental Assessment (EA) glass.
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49
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Immobilization at WVDP
Immobilization operations at the WVDP involve a Joule-heated melter similar to that of the DWPF.
However, the process control strategy is different from that used at the SRS. The WVDP uses glass form-
ing chemicals rather than premelted frit. During production, numerous analyses are made on large vol-
umes of wastes, to allow the feed composition to be adjusted to fit within the target composition win-
dow. Numerous samples on every canister of the final glass product are also required to characterize
glass-waste properties.
·eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
Frit
A
.8
.6
.4
.. -
.~ /
.-
~,%....~
.2
/
.,
..
.
.
',:
, ~
.. ...\\ .6
. ... -
/
.
.
it\
~' '.
.
-
.
.
.
..
.
-
\
Waste 1
\
.8 .6 .4 .2
Waste 2
FIGURE 5. 1 The PCCS window for the DWPF melter feed, based on a mixture of two types of HLWstreams, Waste 1 and
Waste2, and glass frit. The solid white area in the diagram, called "Process Acceptable Region (PA R),"is defined by the
mechanistic models in PCCS that bounds the range of allowable waste compositions and frit blends. The "target"point
indicates maximum waste loading with the minimum amount of frit for a given batch of waste being fed to the melter.
Waste-frit blends that fall within the PAR yield a final glass product that meets the appropriate processing and durability
criteria. This PCCS allows minimal sampling on the final glass product while maintaining at least 95 percent confidence
that the glass product falls within the qualified glass region. The window is defined in terms of relevant glass properties,
including such constraints as melt viscosity, product durability and homogeneity, and the temperature for incipient crys-
tallization (the liquidus). The viscosity and liquidus must be low enough to process the feed in the melter and pour it into
the stainless steel canisters. The PCCS models that define the PAR target window have been established on the basis of
H ~ G H - L E V E E W A S T E
50
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data from 300 to 400 non-radioactive and radioactive laboratory melts and pilot-scale melting trials. To verily/ the PCCS
model and the DWPF process composition window, an additional 485 validation glasses, including 237 full-scale canister
glass samples, taken during DWPF non-radioactive startup, were analyzed (Jantzen et al., 1998). The PCCS models were
developed from glasses whose compositions cover the range of waste streams expected to be processed over the lifetime
of the DWPF (Postles and Brown, 1991). The cost of establishing the DWPF process control system was approximately $5
million (Janzlen, personal communication). SOURCE: DOE-Savannah River Site.
SIDEBAR 5.2 OPERATION OF THE DWPF JOULE-HEATED MELTER
The DWPF facility started non-radioactive operations in 1994 and was used to test a wide range of sim-
ulated wastes that covered the range of all wastes anticipated to be processed during the DWPF life-
time. Radioactive vitrification operations began at the DWPF in 1996.The Joule-heated melter in use at
the SRS, shown below, is operated as follows. An initial charge of dry glass-forming ("batch") materials
is fed to the melter and preheated by natural gas burners or electric heaters above the glass pool
("overhead plenum heaters").The batch becomes sufficiently electrically conductive between 600 °C
and 700 °C to allow further heating by electric current.The batch is then heated resistively to approxi-
mately 1150 °C by passage of an alternating current through the submerged melter electrodes (only 2
of the 4 electrodes are shown).The molten glass produced flows through a narrow region and is
removed continuously from a side channel in the melter.The melter is replenished by feeding addition-
al batch material onto the melt surface, where it forms a thick crust of abreacted material (the "cold
cap") that serves to trap condensable volatile emissions and recycle them into the melt.The molten
glass is then poured through the pour spout into stainless steel canisters and allowed to cool.
Successful, continuous, long-term glass production normally requires a constant batch composition,
uniform feed rate, and steady-state operating conditions.
SOURCE: DOE-Savannah River Site.
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The committee identified the following issues related to the choice
of borosilicate glass and Joule-heated melters for the immobilization
of HEW:
· limitations of borosilicate glasses as immobilization medium;
· crystal content of the borosi I icate glass matrix;
· Iong-term leaching properties of the borosilicate glass matrix;
· use of unreacted glass-forming chemicals versus premelted
glass frit;
foami ng i n Jou [e-heated melters;
precipitation of noble metals and crystalline phases in Joule-
heated melters;
limitations of Joule-heated melters in achieving higher process-
ing temperatures; and
· alternative immobilization processes to Joule-heated melting.
Immobilization Issues
The issues listed above are described in the following section,
along with research activities that could contribute to their resolution.
The overall objective of the long-term basic research recommended
for immobilization is to provide the scientific basis to develop
robust, high-loading3 immobilization methods and materials that
could provide enhancements or alternatives to the current immobi-
lization strategy.
[imitations of BorosiIicate Glasses as
Immobilization Medium
Under the current waste acceptance guidelines for the future geo-
logical repository (see Sidebar 5.3) the borosilicate glass waste form
must meet certain performance requirements, developed on a case-
by-case basis. Borosilicate glasses are appropriate immobilization
media for many DOE HEW streams at the present level of waste load-
ing (currently approximately 28 weight percent at DWPF on a dry cal-
cine basis including Na2O and SiO2~. However, it may not be possi-
ble to achieve substantial increases in waste loading using borosili-
cate glasses. Furthermore, these glasses may not be the optimum
media for some problematic wastes such as the INEEL calcines, as
3Waste loading is the fraction of waste contained in a glass log or other waste
form.
H ~ G H - L E V E E W A S T E
52
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SIDEBAR 5.3 WASTE ACCEPTANCE CRITERIA
The Waste Acceptance System Requirements Document~determines the conditions necessary to be
met by spent nuclear fuel and HLW, in order for DOE to be able to accept it for disposal" in a geological
repository (DOE-OCRWM, 1 999).The Yucca Mountain site in Nevada is currently under consideration
for that repository.The DOE-OCRWM,which is responsible for the development of the HLW repository
project, while the USNRC regulates the repository site, is currently developing the waste acceptance
criteria.
Before 1977, the United States expected to reprocess all spent fuel from commercial reactors. It was
intended that all HLW from reprocessing of commercial fuel would be immobilized by incorporation in
a borosilicate glass matrix, prior to disposal in a geological repository. Glass-technology programs
were initiated to identify suitable waste-glass compositions that would be resistant to leaching under
many repository conditions.The glass waste form would thus constitute a primary barrier against
release of radionuclides to the environment. A similar search for a practical glass waste form was
underway in France and the United Kingdom, two countries that intended to reprocess their spent
reactor fuel. When the United States decided, at the beginning of 1977, to forego reprocessing of
commercial reactor fuel, the primary form of HLW became spent fuel, and the plans and regulations
governing repository disposal were changed accordingly. Later, the DOE was authorized to dispose of
HLW from the defense program sites in the first HLW repository.The defense wastes are expected to
constitute only about 10 percent of the HLW in the first repository, with the balance of about 90 per-
cent being spent fuel from commercial nuclear reactors.The initial defense HLW designated for the
first repository will be borosilicate glass from the SRS and from the WVDP.
At the present time, the DOE wastes are scheduled for disposal along with spent commercial fuel in a
common repository, where the performance acceptance criteria are based on spent fuel characteristics.
The underlying objective for the DOE wastes is that disposal performance is predictable and at least as
good as spent fuel. As part of the current SRS criteria for waste form acceptance, the borosilicate glass
waste form must meet the following requirements (Janzten, 1 993a; 1 993b; 1999):
1. The glass must have a leach resistance greater than the EA glass, the benchmark waste glass
identified in the DWPF EA.
2. The glass must exhibit no evident glass-in-glass phase separation.
3. The glass must be essentially free of crystal content.
These criteria are SRS guidelines for their current glass waste form, and do not apply to waste forms to
be produced at the other DOE sites. The other sites might consider alternative waste forms, and would
have to establish their own criteria to achieve comparable performance and acceptance.
m m 0 b i I i z a t i 0 n
53
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stated by a previous N RC committee (N RC, 1 999b). Alternative waste
form materials, such as other glasses, glass-ceramics, and polyphase
ceramics, may be better suited for these types of waste.
Off-gas particulates4 from slurry volatilization represent another
potential problem with the use of borosilicate glasses because the
particulate compositions will likely differ significantly from that of the
original waste feed. Thus, they may have to be blended into the waste
feed at relatively low concentrations because they would be highly
enriched in volatile species, such as technetium, mercury, iodine,
ruthenium, cesium, boron, sodium, and possibly molybdenum. In
particular, the high volatility of boron from borosilicate melts is
known to induce significant losses of cesium, volatized as a cesium
borate (Vance et al., 1 988~. Although cesi u m volati I ization does not
seem to create problems for the DWPF,5 it may become a significant
factor with future cesium-rich wastes at the Hanford Site or at the
SRS, particularly if an improved method for cesium removal from the
salt fraction is adopted (NRC, 2000b). Another potential problem
caused by off-gas emissions in noted in Sidebar 5.4.
Precipitated effluents in the form of fine powders or nanocrystalline conden-
sates.
Measurements in the initial waste and in the glass indicated that greater than
90 percent of the cesium-137 is incorporated in the product glass (Bibler et al.,
2000).
SIDEBAR 5.4 VOLATILE EMISSIONS FROM THE DWPF MELTER LINKED TO EVAPORATOR
SHUTDOWN?
One example of a problem potentially exacerbated by the carryover of off-gas emissions is the severe
sodium aluminosilicate fouling of the 2H evaporators at the SRS,where at least 1,100 kilograms of alu-
minosilicate have been deposited in the evaporator, to depths of over a meter in some locations.
Evaporators are used to concentrate supernatant liquid waste, thus conserving storage space.The car-
ryover of condensate from the DWPF evaporator melter is also recycled back to the tank farm 2H evap-
orators. It is possible that entrainment or volatilization of sodium, silica, and aluminum species from
the DWPF melter off-gas was in part responsible for 2H evaporator fouling. In fact, the 2H evaporator
has historically processed streams high in aluminum, and in the past, small quantities of aluminosili-
cate buildup have been observed. Since the DWPF began operating and recycling condensate to the
evaporator, the concentration of silicon in the latter has increased dramatically. However, a recent
study has shown that only half of the aluminosilicate in the 2H evaporator originated from the frit car-
ryover from the DWPF melter; the remaining half originated from a different evaporator and from lab-
oratory analysis (Jantzen and Laurinat, in press). The EMSP is collaborating with the TFA, the Savannah
River Technology Center, and the Oak Ridge National Laboratory to study the formation of aluminosili-
cates under conditions similar to those in the 2H evaporator.
H ~ G H - L E V E L VV A 5 T E
~ A ~
54
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A further issue relates to possible pre-blending of wastes from vari-
ous tanks before feeding them to the melter. This method has been
used at the SRS to increase the glass waste loading by smoothing the
concentration of critical waste components. However, the proposed
strategy for Phase I at Hanford is to empty and vitrify the wastes on a
tank-by-tank basis, with little or no inter-tank blending of the wastes.
Furthermore, the Hanford tank-to-tank composition variability is gen-
erally much greater than that at SRS. Thus, this strategy will require
separate composition windows for waste plus glass-forming materials
to be developed for each tank and may ultimately increase the overall
number of glass logs.
For low-sodium wastes, the maximum waste loading is usually
dictated by the concentrationts) in the waste feed of species with lim-
ited solubility in borosilicate glass, such as halides, sulfates, phos-
phates, chromium, and bismuth. Glass-in-glass phase separation
and/or crystallization of possibly undesirable phases will occur if the
solubility limits of these species are exceeded, producing a waste
form that may not meet current acceptance criteria.
A good example of this limitation is the sulfate content in the
waste feeds, which is determined by the various pretreatment stages
(see Sidebar 5.51. The solubility of sulfate in borosilicate glass is low,
so that a high sulfate content in either the HLW or the LLW streams
will lead to separation of a molten sulfur-rich phase within the melter
causing foaming problems. Many waste ions, including cesium, are
known to partition preferentially into the sulfate phase in coexisting
sulfate-silicate melts.6 Furthermore, if the solubility limit of sulfate in
6p.J. Hayward, unpubl ished work, 2000.
SIDEBAR 5.5 IMMOBILIZATION OF HANFORD LLW
Consideration of LLW immobilization is not part of the task of this committee. Nevertheless, the quan-
tity and composition of LLW will be dictated by pretreatment of the HLW feeds. In 1994, an amendment
of the TPA between the State of Washington, the EPA, and DOE established that the LLW at Hanford will
be immobilized by vitrification in borosilicate glass, using essentially the same technology proposed
for HLW immobilization (TPA, 1 998).The Hanford LLW stream will consist predominantly of soluble
salts (nitrates, sulfates, phosphates, and carbonates) of sodium, aluminum, and potassium, together
with traces of fission products and transuranic elements.The limiting factor in determining waste load-
ing in the LLW glass will likely be the sulfate content.This limit will produce an estimated 20 percent
increase in the required glass volume for LLW immobilization, compared to the volume that would be
required if the LLW sodium content were the limiting factor (Pegg, 2000).
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55
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H LW or LLW glass is exceeded i Inadvertently, separation and accumu-
lation of a molten sulfate or sulfide phase at the melt surface could
also cause enhanced corrosion of the upper electrodets) and refracto-
ries. The alloy used for electrodes, Inconel-690@, is known to be sus-
ceptible to attack from sulfur compounds, particularly under reducing
conditions.
tong-Term Research Needs
Basic long-term research in material sciences is needed to seek
alternative waste form materials, such as glass-ceramics and
polyphase crystalline ceramics, for producing acceptable immobilized
waste with higher concentrations of HLW from variable-composition
feeds. Descriptions of many previously developed alternative waste
form materials for HLW can be found in the relevant literature
(Hayward, 1988; Lutze and Ewing, 1988; Donald et al., 1997~.
Further development of some of these materials could allow this goal
to be achieved.
Further research is needed to identify more economic alternatives
to borosilicate glass for immobilizing LLW in Hanford. One example
could be to investigate alternative waste form materials (e.g., cement-
based materials) that can incorporate higher sulfate concentrations
than are possible with borosilicate glass.7
In the event of future immobilization of Hanford LLW in Joule-
melted borosilicate glass, research will be needed to study the corro-
sion mechanisms) of Inconel-690'~' in glass melts containing high sul-
fur concentrations under various redox conditions, with the goal of
minimizing corrosion and/or identifying more corrosion-resistant
al lays.
Crystal Content of the BorosiIicate Glass Matrix
The current stipulations for the SRS borosilicate glass waste form
state that the waste glass should not contain any significant degree of
crystallinity or glass-in-glass phase separation Oantzen et al., 1999~.
Crystallization can occur during cooling of molten HLW glass if there
is sufficient overlap between the temperature ranges for substantial
crystal nucleation and crystal growth. Potential crystalline phases
appearing in SRS and Hanford wastes glasses are spinels (A2+LB3+1204,
em.. NiFe-OA). clinoovroxenes (orincioallv acmite NaFeSi-O ). alkali
L) ' ~ of'' ~ ~ ', , , ~ O',
aluminosilicates (principally nepheline NaAISiO4, albite NaAISi3O~,
7The committee is aware of the difficulties of reversing the TPA decision to vit-
rify LLW (TPA, 1998). However, the EMSP should investigate alternative waste
forms as part of a contingency approach to the current baseline program.
H ~ G H - L E V E E W A S T E
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which can occur in partial solid solution with nepheline, and
eucryptite LiAISi3O~), lithium silicates (e.g., Li2SiO3), cristobalite
(SiO2), hematite (Fe203), and zircon (ZrSiO41. Some projected precipi-
tations in INEEL glass compositions are nepheline, fluorapatite
(Ca~0PPO416F2), lithium phosphate (LiPO4), baddeleyite (ZrO2) and
alkali aluminosilicate sulfides. Above the glass transition temperature
of HEW glasses, canister centerline cooling rates of approximately
0.05°C per second permit rapid-nucleating and rapid-growing phases,
such as spinels and nepheline, to precipitate but disfavor difficult-to-
nucleate or slow-growing phases like acmite or zircon. Spinels have
been identified as major crystalline phases in glass after heat treat-
ment at temperatures between 500°C and 900°C in WVDP glass com-
positions Gain et al., 1993) and, as indicated earlier, may accumulate
in Joule melters and be entrained in glass carried over from the
melter.
Uncontrolled crystallization or phase separation of certain of these
crystalline phases within the glass log during cooling has the potential
for reducing the durability of the final waste form. The glass durability
is determined by employing the product consistency test (PCT), which
measures the release rates for sodium, lithium, silicon, and boron dur-
ing water leaching. The deleterious influence on glass durability of
certain crystalline phases derives from both chemical and mechanical
effects on the surrounding glass: the residual glass composition is
altered, and the glass matrix is stressed by the volume mismatch of
the crystal and the glass space it replaces. Survey studies (Bailey and
Hrma, 1995) have suggested that the residual glass composition is the
major factor that controls the PCT response of glasses with durable
crystal I i ne phases.
Spinels are general Iy conceded (Jantzen and Bickford, 1 985) to
have little effect on glass durability, because they do not substantially
alter the chemistry of the remaining glass-forming and modifying ele-
ments (silicon, aluminum, boron, and sodium). Moreover, spinels are
characterized by their cubic crystal symmetry, which leads to near-
isotropic~ interface energies and strain distributions. Therefore, it is
likely that spinels lead to equiaxed crystalline precipitate with mini-
~lsotropy refers to having equal physical properties (such as refractive index,
thermal expansivity, elastic constants) in different crystallographic directions.
Cubic crystals are usually more isotropic than other crystal systems with lower
symmetry (tetragonal, hexagonal, orthorhombic, monoclinic, or triclinic).
However, in some cases, cubic crystals can exhibit properties that are far from
isotropic.
m m 0 b i I i z a t i 0 n
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mal tetragonal distortion of the surrounding glass matrix. The reported
effects of acmite and related clinopyroxenes, which are non-cubic
and non-isotropic and comprise one of the most likely crystalline
fractions, are controversial Oantzen and Bickford, 1985), but the most
recent Pacific Northwest National Laboratories (PNNL) studies con-
clude that acmite has virtually no impact on PCT release rates (Riley
et al., 2001~. Nepheline precipitation appears to have the most detri-
mental impact on dissolution of Hanford and Savannah River HLW
glasses and can decrease chemical durability by several orders of
magnitude (Ri ley et al., 2001 ). Precipitation of cristobal ite (which is
nonetheless cubic) and baddeleyite (which is nearly so) also impact
durability negatively. Substantial crystallization of a number of other
phases has been shown to have little or even a positive impact on
glass durability.
Titanium (derived from monosodium titanate or crystalline silicoti-
tanate ion exchangers, see Chapter 4), zirconium, phosphorous, and
fluorine can all function as effective nucleating agents for crystalliza-
tion and are commonly used for nucleating commercial glass-ceram-
ics. Thus, it may be necessary to control the concentrations of these
potential nucleating agents in the HLW feed to the melter to avoid
crystallization of phases that could adversely affect waste form dura-
bility. The fact that some crystalline precipitations appear to have little
or no adverse impact on chemical durability of HLW glasses suggests
that it may be possible to relax the restriction on crystal content in the
glass to accommodate a small content of crystalline phase. In turn,
this could allow the waste loading in the glass log to be increased.
The committee is aware of the research efforts to this end being
pursued at the SRS and at PNNL. DOE investigators are currently
exploring the potential to increase waste loadings for Hanford,
Savannah River, and INEEL by allowing crystalline precipitation upon
cool i ng with i n the can ister (Pittman et al ., 2001 ). Other research
efforts related to DOE's programs are under way at universities such
as The Catholic University of America (Kot and Pegg, 2001 ) and the
University of Missouri (Marasinghe et al., 1999, Ray et al., 19991.
These research efforts could be effectively complemented by a long-
term basic research program within the EMSP to obtain innovative
approaches on the effect of crystal precipitation in borosilicate glass.
tong-Term Research Need
Long-term basic research is needed to broaden the envelope of
acceptable borosilicate glass compositions to include a level of crys-
tal content that does not adversely affect product durability.
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[ong-Term [caching Properties of
BorosiIicate Glass Matrix
There has been a considerable amount of experimental research
on the chemical durability (leach resistance) of various glass waste
forms under a variety of hypothetical repository conditions and on
glass corrosion mechanisms, alteration products, and long-term radia-
tion-induced degradation.9 However, as noted elsewhere (NRC,
1 996a), the continued development of phenomenological models4°
that would allow glass-leaching data to be extrapolated over long
time intervals would be advantageous.
tong-Term Research Needs
Long-term basic research is needed to further develop and verify
phenomenological models to predict long-term leachabi I ity of
borosilicate glass and other waste forms. Such a model should be
developed and verified as a contingency against future waste form
disposal issues, such as performance. A similar predictive model
would be needed to support the possible future use of alternative
waste form materials with higher waste loading, including glasses,
glass-ceramics, and polyphase ceramics. The models should also
i ncl ude consideration of the i nfl uence on waste form du rabi I ity of
such factors as groundwater radiolysis and internal radiation damage
(Weber et al., 1997; 19981.
Use of Unreacted GIass-Forming Chemicals Versus
PremeIted Glass Frit
Plans to immobilize HLW in borosilicate glass at Hanford include
the option of using a melter feed containing "raw" chemicals (mainly
oxides and carbonates), rather than premelted frit. Given the great
variability of waste streams in Hanford, this option would avoid the
tailoring of premelted frit to the different waste compositions to fall
within the target composition window.
The rationale for using premelted frits in the melter feeds at DWPF
was that residence time in the melter would be reduced because
many of the glass-forming reactions would have been performed
9Much of this research is documented in journals, symposium proceedings,
(e.g., the annual Materials Research Society "Scientific Basis for Nuclear Waste
Management" proceedings) and DOE workshops.
'PA phenomenological model is a multi-component model with predictive
power that combines a series of mathematical descriptions of the individual phe-
nomena involved. See the glossary in Appendix G.
m m 0 b i I i z a t i 0 n
59
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ahead of time during frit manufacture. However, this would not be
true if the overall rate of glass production were governed by the rate
of waste dissolution in the melt pool or by the rate of heat transfer
from the melt pool to the cold cap. There appears to be some uncer-
tainty about this issue that should be resolved. The use of unreacted
glass farmers at Hanford could conceivably produce unanticipated
melter problems (e.g., corrosion, foaming, and precipitation) that have
not previously been encountered at the WVDP and at the SRS. For
instance, the volume and complexity of the off-gas emissions could
be greatly increased from volatilization of the slurry water content
and from chemical breakdown of nitrates, nitrites, oxalates, and other
organic molecules. These emissions can cause potential entrainment
of other volati le species, incl uding technetium, mercury, iodine,
ruthenium, cesium, boron, and sodium.
tong-Term Research Need
Long-term basic research is needed to evaluate the controlling
parameters of reaction rates and heat transfer processes in the melter.
Results will strengthen the scientific basis for a rational choice
between using un reacted glass farmers and using premelted frit in
waste feeds for future melter designs (including Hanford melters).
Foaming in JouIe-Heated Melters
Foaming is the result of redox reactions within the melt and also
the breakdown of anions, such as nitrates and carbonates, that gener-
ate gas during melting. Excessive foaming can form a physical and
thermal barrier between the cold cap and the melt pool and can ulti-
mately lead to melter shutdown. Foam formation is generally associat-
ed with highly oxidizing conditions in the melter Oain and Pan,
20001. The foam acts as an insulating layer of bubbles between the
melt pool and the newly introduced waste slurry feed, eventually
forming a cold-cap "bridge" and preventing further waste feed from
dissolving in the melt. Foaming also introduces the possibility of
enhanced corrosion of the upper electrodeks) and refractories.
The problem of foaming in melters has been encountered at differ-
ent DOE sites. At the WVDP this phenomenon has been 'accommo-
dated' by adding a reducing agent (usually sugar) to the melter feed.
The mechanism by which sugar reduces the formation of micro-bub-
bles during melting is not well understood. Foaming continues to be
an issue at the SRS Oain and Pan, 2000) and it is one of the anticipat-
ed problems at the Hanford Site, because of the wider waste compo-
sition range. Careful redox and rheology control is required to prevent
foaming during the water boil-off stage. A further factor that would
probably exacerbate any foaming tendency in Hanford is the pro-
H ~ G H - L E V E E VV A 5 T E
~ A ~
OCR for page 61
posed use of oxides and carbonates as glass farmers, rather than frit.
Any foaming tendency within the cold cap would be exacerbated by
CO2 generation from thermal decomposition of carbonates (Li2CO3
and Na2CO3) in the glass batch. Similar foaming problems were also
encountered with the Joule-heated melter used for vitrifying mixed
wastes at the Fernald site in Ohio. These problems were attributed to
poor red ox control, and also by foaming within immiscible sulfate
layers that formed on the melt surface. Further details on foaming and
red ox control in melters are described by Jain and Pan (20001.
tong-Term Research Needs
Long-term basic research is recommended to characterize the
behavior of the cold cap formed on the melt surface. Specifically, the
sequence of reactions occurring in the cold cap and their influence
on foaming tendency do not seem to be well characterized. The items
to be eval uated i ncl ude (1 ~ the rates of water removal and breakdown
of salts (e.g., nitrates, carbonates, and formates) and of organic addi-
tives (e.g., sugar, urea) used to control melt redox conditions; (2) the
influence of feed chemistry, including sulfate content; and (3) possible
oxygen evolution from red ox reactions occurring within the melt.
Thus, it may be possible to minimize or eliminate the potential for
foaming using modifications to pretreatment and/or to the physical or
chemical properties of the waste stream, (e.g., by pH or red ox adjust-
ment, change in the solids-liquid content, or particle size adjustment).
Precipitation of Noble Metals and Crystalline Phases in
JouIe-Heated Melters
Future melting campaigns at the SRS and at the Hanford Site will
involve tank wastes with higher concentrations of noble metals (palla-
dium, rhodium, and ruthenium), derived from the fission of uranium-
235. Ruthenium is the most abundant noble metal in the Hanford
HEW (Jain and Pan, 20001. Noble-metal precipitation within the
melter could cause plating-out, short-circuiting, and downward
drilling of accumulated metal into the refractory floor, all of which
would reduce melter life. Metallic precipitates also have the potential
to cause alloying reactions with the Inconel-690@ electrodes. Some or
all of these problems are reported to have occurred elsewhere, such
"This phenomenon involves an enhanced refractory attack in a glass melter at
contact sites between metallic inclusions and refractories.Typically, the attack
involves gravity-assisted "drilling" of vertical holes or cavities in the refractories
that constitute the melter floor.
m m 0 b i I i z a t i 0 n
61
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as in the first melter used at the Pamela vitrification planter (Demonic,
1996).
Many of the Hanford wastes, and future wastes to be immobilized
at the SRS,43 will also have relatively high iron, aluminum, nickel,
manganese, and chromium contents, which, together with chromium
oxide sludge from refractory corrosion, will likely cause precipitation
of crystalline A2+B3+204-type spinel compounds. These dense insolu-
ble phases may accumulate on the melter floor and could conceiv-
ably cause throat and/or riser blockage. Furthermore, many iron and
chromium-rich spinel compounds exhibit relatively high electrical
conductivities at glass-melting temperatures. Thus, their precipitation
could lead to possible disruption of the electrical current distribution
within the molten glass pool.
tong-Term Research Need
Modeling efforts, possibly combined with reduced-scale experi-
ments, are recommended to study the consequences of precipitation
and accumulation of noble metals and spinels on the melter floor. The
ultimate goal of this long-term basic research is to increase glass pro-
duction rates and prolong the operating life of the melter.
[imitations of JouIe-Heated Melters in Achieving Higher
Processing Temperatures
As noted previously, it may be advantageous to develop alternative
glass or glass-ceramic waste form materials to the present generation
of borosilicate waste glasses in order to achieve higher waste load-
ings, or to immobilize problematic wastes with unusual compositions.
For example, this could be the case if the vitrification route is chosen
for immobilizing the INEEL high-aluminum and high-zirconium cal-
cines, tank heels, and other secondary waste streams from pretreat-
ment and vitrification. These alternative waste form materials will like-
ly require higher melting temperatures and, if fabricated using Joule-
heated melters, may also require new electrode alloys and glass-con-
tact refractories with improved corrosion resistance.
'2The Pamela vitrification plant consists of two Joule-heated melters operated in
Mol. Belgium, from 1985 to 1991. The first melter was shut down after three years
as a result of electrical failure from buildup of noble metal sludge on the floor.
'3The SRS staff has tested a wide range of glass compositions simulating the dif-
ferent types of waste streams projected for the next 25 years (Postles and Brown,
1991). Based on this study, no problems arising from future waste-composition
variations are anticipated at the SRS.
H ~ G H - L E V E E W A S T E
62
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tong-Term Research Needs
There is a need to study higher-temperature Joule-melting tech-
niques as a step toward developing alternative glass or glass-ceramic
waste forms with higher waste loadings and as a contingency against
difficulties with future problematic wastes. Long-term basic research is
required to identify (1 ) improved electrode materials, such as new
alloys, ceramics, or cermets; (2) advanced refractories; and (3) alterna-
tive electrode-refractory configurations. In all cases, the primary goal
is to minimize hi~h-temneratr~re corrosion in the presence of high
.. O.. ..., . .. .. . .. ... .. , . .. . ... O..
concentrations of simulated waste under appropriate red ox condi-
tions.
Alternative Immobilization Processes to
JouIe-Heated Melting
Alternative immobilization processes may be advantageous as a
contingency against unforeseen problems with continuous (Joule-
melter) vitrification during the Hanford Phase I and 11 programs. A
recent survey of waste immobilization technologies gives examples of
some batch-processed alternatives to the continuous melters in cur-
rent use at DOE sites, including processes such as induction melting,
"cold-crucible" melting, or microwave melting (Jain, 2001 ). In many
cases, these alternative processes can avoid some of the inherent
problems with a continuous melter, including refractory corrosion and
precipitation of noble metals and crystalline phases, although they
would likely introduce other technical issues.
In general, the use of batch melting would allow greater flexibility
in the range of compositions and temperatures for vitrification. This
flexibility could be important if a glass or glass-ceramic waste form
with a higher melting temperature were selected to immobilize future
problematic waste streams or to increase the waste loading.
Furthermore, the eventual task of decommissioning and disposal may
be simpler with a smaller batch-type melter than with a continuous
melter.
Some of these alternate immobilization technologies may require
drying or pre-calcining of the HEW slurry before it could be blended
with other glass or glass-ceramic precursor chemicals and processed.
The committee notes that pre-calcination is used at the La Hague,
Marcoule (]ouan et al., 1996) and Sellafield (Fairhall and Scales,
1996) vitrification facilities, albeit with an acidic waste stream (i.e.,
with little or no NaNO3 content) and has also been performed on
much of the I N EEL waste.
Pre-calcination may offer a number of technical advantages,
including prior removal of process off-gases, such as water and nitro-
m m 0 b i I i z a t i 0 n
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gen oxides (NOx), and possible elimination of foaming and associated
problems. However, it would add a further step to the overall immo-
bi I ization process and wou Id i evolve hand I i ng fi ne powders. Other
issues could include exothermic nitrate-organic reactions and forma-
tion of viscous sodium nitrate melts. While pre-calcining of the HLW
feed does not, in itself, give higher waste loading, it may be a neces-
sary step i n any i n novative process to ach ieve th is goal . The I after
could include batch-type processes where initial dry blending of the
HLW feed with processing additives (e.g., frit or glass-forming chemi-
cals) is required, or where it is important to minimize the initial vol-
ume of HLW feed.
tong-Term Research Needs
Long-term basic research is needed to identify and develop alter-
native melting techniques, including batch-type processes using con-
cepts other than the continuous melters in current use at DOE sites
for preparing waste forms with higher waste loadings. This research
could include the study of drying or pre-calcining options for the
waste feed.
H ~ G H - L E V E E W A S T E
64
Representative terms from entire chapter:
borosilicate glass
