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OCR for page 175
7
Waste Form Performance
in Disposal Systems
T
he third charge of the statement of task for this study (see Box 2.1.
in Chapter 2) calls for the identification and description of “state-
of-the-art tests and models of waste forms used to predict their
performance for time periods appropriate to their disposal system.” This
chapter provides a discussion of the modeling portion of this charge, includ-
ing waste form performance in disposal systems and models for evaluating
waste form performance. Waste form testing is discussed in Chapter 5.
For the purposes of the discussion in this chapter it is important to
distinguish between a disposal facility and a disposal system. The term
disposal facility (see Chapter 6, Figure 6.1) refers to physical infrastruc-
ture; it typically includes tunnels (in the case of deep disposal) or surface
excavations (in the case of shallow disposal), the surrounding host rock,
and engineered barriers, including the waste form if present. A disposal sys-
tem, on the other hand, refers to both physical infrastructure and how the
natural and engineered barriers in that infrastructure function to sequester
radioactive and hazardous constituents. The ability of a disposal system to
sequester these constituents is referred to as disposal system performance.
The performance of a disposal system can be evaluated using performance
assessment (see Section 7.2).
7.1 WASTE MANAGEMENT SYSTEMS
As discussed in Chapter 2, waste processing and waste form produc-
tion are key activities in the Department of Energy, Office of Environmental
175
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176 WASTE FORMS TECHNOLOGY AND PERFORMANCE
Management’s (DOE-EM’s) cleanup program and, indeed, in any integrated
waste management system. A conceptual diagram showing the important
steps in DOE-EM’s waste-management system is provided in Figure 7.1.
There are interdependencies among the steps in this system; moreover,
decisions at each step can affect waste management options and activities
at subsequent steps.
The International Atomic Energy Agency (IAEA, 1995) addresses these
interdependencies explicitly in Principle 8. Radioactive Waste Generation
and Management Interdependencies, which states:1
Since the steps of radioactive waste management occur at different times,
there are, in practice, many situations where decisions must be made
before all radioactive waste management activities are established. As far
as reasonably practicable, the effects of future radioactive waste manage-
ment activities, particularly disposal [emphasis added], should be taken
into account when any one radioactive waste management activity is being
considered.
In the context of the present report, this principle suggests the need to
consider waste form development and selection in the context of disposal
system performance. The principle also explicitly recognizes that, although
there are many steps and activities that can optimize the safety, capacity,
schedule, and cost of a waste management system, all paths eventually lead
to final disposal.
7.2 DISPOSAL SYSTEM PERFORMANCE
The National Research Council (NRC) has published numerous reports
bearing on the performance of disposal systems for spent nuclear fuel (SNF)
and high-level radioactive waste (HLW) (e.g., NRC, 1995), transuranic
(TRU) waste (NRC, 1996), and low-level radioactive waste (LLW) (e.g.,
NRC, 2005), as well as specifically on the performance of disposal systems
(e.g., NRC, 1983, 1990, 2003). The NRC Committee on Technical Bases
for Yucca Mountain Standards (NRC, 1995, p. 70) provided the following
definition for disposal system performance, which, as noted previously, is
usually referred to as performance assessment (PA):
The only way to evaluate the risks of adverse health effects and to compare
them with the [Environmental Protection Agency] standard is to assess the
1 This same principle to consider interdependencies in waste management operations and
disposal is part of the Chapter 2, Article 4.iii of the Joint Convention on the Safety of Spent
Fuel Management and on the Safety of Radioactive Waste Management, of which the United
States is a signatory.
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
Disposal
EM Waste Pretreatment Waste Form
Waste Form Waste Form
Systems
Streams & Conditioning Selection Processing
FIGURE 7.1 Schematic illustration of DOE-EM’s waste management system.
NOTE: WAC = waste acceptance criteria (see Chapter 8).
Figure 7.1.eps
bitmap
estimated potential future behavior of the entire repository system . . . this
procedure, involving modeling of processes and events that might lead to
releases and exposures, is called performance assessment.
PA modeling has several useful applications in the design and licensing
of disposal facilities, such as a repository for SNF/HLW; for example, PA
can be used to:
• Demonstrate compliance with regulatory requirements, typically
health-risk metrics such as dose rate to a critical group. Abstracted
and simplified versions of PA can also be used to communicate with
concerned stakeholders about disposal system performance. These
applications are considered to be the conventional roles of PA.
• Identify system components and processes that strongly affect the
isolation of radionuclides within disposal systems to coordinate
and guide repository design, site characterization, and safety assess-
ment activities.
• Evaluate the radiological safety of disposal systems in the larger
context of costs, schedules, alternative options, and optimization
of overall waste management policies.
This chapter focuses primarily on the first type of PA modeling: i.e.,
assessment and demonstration of compliance with regulatory guidelines. It
describes a logical and systematic approach for carrying out PA modeling—
an approach that is not always used in real-world applications.
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178 WASTE FORMS TECHNOLOGY AND PERFORMANCE
When selecting a PA model, it is important to consider its fitness-
for-purpose—that is, its suitability for the intended application. In many
instances a “best estimate” analysis is warranted, especially with respect to
establishing regulatory compliance. For exploratory purposes, conservative
bounding analyses may be appropriate, although it must be cautioned that
there is always a danger of compounding so many conservative assumptions
and data values together that the resulting analysis becomes unrealistic and
potentially misleading.
There are two important elements in a PA. First, the PA must consider
the entire disposal system composed of multiple barriers and multiple con-
tainment processes. Modeling radionuclide releases and exposures requires
consideration of the potential pathways by which radioactive constituents
in the waste form could migrate through the disposal facility and eventu-
ally lead to future radiological exposures (health risks). Second, PA must
consider the future behavior of these barriers (including the waste form)
with respect to safety standards, such as those developed by the Environ-
mental Protection Agency (see Chapter 8). The NRC (1995) recommended
that a risk-based health standard should be applied as the appropriate
metric for assessing the long-term safety of geological disposal for radio-
active waste.
Repository programs typically employ a hierarchy of PA models to
assess long-term safety, barrier design, and regulatory compliance of dis-
posal systems containing radioactive waste. This hierarchy, referred to here
as the PA Pyramid, is illustrated in Figure 7.2. The models of fundamen-
tal physicochemical processes affecting repository performance form the
base of this pyramid. These models focus on processes such as chemical
reactions among the waste form, other engineered barriers systems (EBS)
in the disposal facility, and groundwater. Successively higher levels of PA
models represent abstractions (with computational simplifications) of these
underlying models. At intermediate levels in the PA Pyramid, the design
and layout of EBS in the disposal facility are incorporated into the models.
Models at the top of the pyramid represent abstractions with computational
simplifications of the underlying models into a total system performance
assessment, or TSPA (DOE, 2008; Whipple, 2006).
Uncertainties in assumptions, alternative conceptual models, and data
are passed upward through the PA levels to ensure that all identified uncer-
tainties are maintained at each level (e.g., DOE, 2008; Nagra, 2002). Like-
wise, sensitivity analyses made at an upper PA level can be used to identify
specific risk-important factors or processes for which more detailed model-
ing and analysis at a lower PA level may be desirable. In other words, the
PA Pyramid represents an iterative process for assessing the performance
the waste form and its intended disposal system.
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
FIGURE 7.2 The PA pyramid showing the hierachical structure of PA models to
evaluate the safety of disposal for radioactive waste. Uncertainties in assumptions,
alternative conceptual models, and data 7.2.eps upward through the PA levels
Figure are passed
(upward arrow). Sensitivity analyses bitmap to identify specific risk-important
can be used
factors or processes for which more detailed modeling and analysis may be desir-
able (downward arrow).
7.3 MODELS FOR WASTE FORM PERFORMANCE
IN DISPOSAL SYSTEMS
As noted in Chapter 6, the dominant potential pathway for radio-
nuclide release from a disposal facility to the biosphere is via groundwater
transport. Radioactive constituents may be released either as dissolved
species or as suspended, radionuclide-bearing colloids. There is also the
possibility of gaseous release of radionuclides that are volatile or form
volatile compounds.2 Because groundwater flow and radionuclide transport
2 A repository constructed in salt would likely have no aqueous, colloidal, or gaseous path-
ways for radionuclide release, unless there is some disruptive future human-based or natural
event (NRC, 1996).
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180 WASTE FORMS TECHNOLOGY AND PERFORMANCE
are primary concerns with respect to long-term performance of disposal
systems for radioactive waste, the following conclusions from NRC (1990,
pp. 14-15) are pertinent:
Appropriate and successful models of groundwater flow and transport
can range from simple analytical solutions for 1-dimensional flow in a
homogeneous aquifer to highly complicated numerical codes designed
to simulate multi-phase transport of reactive species in heterogeneous,
3-dimensional porous media. A useful model need not simulate all of
the physical, chemical, and biological processes that are acting in the
subsurface. The model that is appropriate for analyzing a particular prob-
lem should be determined primarily by determining the objective of the
study. . . . Efforts should be made to avoid using models that are more
complicated than necessary. Overly complicated models require informa-
tion that cannot be obtained reliably from . . . measurements, which
introduces unnecessary uncertainty into the modeling output.
Transport processes link waste form dissolution (Chapter 5) to disposal
system performance and safety. Transport of dissolved and colloidal species
released by the dissolution of a waste form may be controlled by either
advective flow of the groundwater, or, if the engineered or natural barriers
surrounding the waste form have sufficiently low permeability (Chapter 6),
by diffusion.
The following three subsections provide more detailed descriptions of
the PA model hierarchy illustrated in Figure 7.2 and the key physical and
chemical processes that affect disposal system performance.
!
!
7.3.1 PA Models
!
The NRC’s Waste Isolation System Panel (WISP) report (NRC, 1983)
was the first multi-disciplinary study to integrate waste form dissolution
and transport of dissolved radionuclides into a PA model for disposal
systems. The WISP report applied well-understood, mass-transfer analyti-
cal models to elucidate the linkages between dissolution and transport in
disposal systems. Independently, similar analytical mass-transfer models
were being developed for international repository programs (e.g., KBS,
1983; Nagra, 1985; Neretnieks, 1978). The application of mass-transfer
models to disposal system performance assessment is now routine for all
types of radioactive waste (e.g., Andra, 2005; JNC, 2000; Nagra, 2002;
SKB, 2006). Such simple mass-transfer models have, over time, been sup-
ported by more detailed and data-intensive physicochemical modeling (e.g.,
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
Steefel et al., 2005) in a manner consistent with the hierarchy of models
shown in Figure 7.2.
7.3.1.1 Basic Release and Transport Processes
that Control System Performance
To illustrate the linkage between waste-form dissolution and radionu-
clide transport, a simple but rigorous mass-transfer analytical model (NRC,
1983; Zavoshy et al., 1985) is cited here. The model is for a simplified
geometry of a dissolving waste form surrounded by a diffusion barrier3 (i.e.,
buffer/backfill) and is based on well-validated, mass-transfer principles. A
so-called flux ratio R is defined as (Zavoshy et al., 1985):
waste form dissolution rate j0 r0
R= =
ε DeC SAT
steady-state diffusive flux (7.1)
where
j0 is the dissolution rate (i.e., chemical durability) of the waste form, nor-
malized for the mass fraction of radionuclide i in the waste form4
r0 is the radius of the waste form
e is the connected porosity of the buffer
De is the effective diffusion coefficient of dissolved radionuclide i in the
buffer
CSAT is the solubility limit for a solid phase incorporating radionuclide i.
For conditions where the flux ratio R is much greater than 1 (fast disso-
lution rate relative to diffusive transport flux), the long-term concentration
of radionuclide i increases at the waste-form surface until the point that
a solid phase containing radionuclide i precipitates; this precipitate sets a
solubility-limited concentration (CSAT) for radionuclide i at the waste form
3 In all concepts for the disposal of SNF/HLW in saturated rock, a low-permeability buffer
or backfill engineered barrier is placed around the SNF/HLW containers (e.g., NWTRB, 2009;
Witherspoon and Bodvarsson, 2006). Such a buffer has several important safety functions,
including promoting diffusive transport of all radionuclides released from the dissolution of
waste forms. Low-permeability buffers also promote the filtration of any radionuclide-bearing
colloids that might form from dissolution of the waste form (Nagra, 1994; SKB, 2006). For
a repository located in unsaturated rock, buffer/backfills (so-called Richards barriers) based
on the principle of capillary-breaking have been tested and built. These barriers are designed
to perform the twin safety functions of assuring diffusive transport and colloid filtration (e.g.,
EPRI, 1996; Gee et al., 2002).
4 Mass fraction is the mass of radionuclide i (i.e., waste loading of radionuclide i) divided
by the mass of the entire waste form.
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182 WASTE FORMS TECHNOLOGY AND PERFORMANCE
surface. This is called transport control, and the same basic process has
long been recognized and applied to the interpretation of diagenetic mineral
dissolution in nature (e.g., Berner, 1979). Conversely, when the flux ratio
R is much lower than 1, the long-term concentration of radionuclide i at
the waste form surface is basically controlled by the dissolution rate of the
waste form. This is called surface-reaction control and also is well known
from studies of natural diagenetic systems.
Note that this simple model does not depend on any specific waste
form, dissolution rate mechanism, or model; it is general to any waste form
for which a long-term dissolution rate (or release rate of radionuclides) can
be defined. Similar mass-transfer expressions that link waste-form dissolu-
tion rate and advective transport have also been developed (Pigford and
Chambré, 1987). These mass-transfer analytical models provide the critical
linkage between waste form fabrication and geological disposal (Figure 7.1).
The important characteristics of waste forms with respect to long-term
(103-106 years) performance and safety of a disposal system depend on a
number of factors:5
• Type of waste form (see Chapter 3)
• Radionuclide inventory and waste loading of the waste form
• Environmental conditions in the near field of the disposal facility
(see Chapter 6)
• Long-term dissolution rate of the waste form under those environ-
mental conditions (see Chapter 5)
• Solubility limits of dose-contributing radioelements
• Rate of diffusive or advective aqueous transport of dissolved and
colloidal radionuclides
• Presence of engineered barriers (e.g., clay buffer, Richards barrier)
For SNF/HLW repositories that include a low-permeability buffer sur-
rounding the waste form, it is the solubility limits of the solid phases that
incorporate the radionuclides that (i.e., CSAT in Equation 7.1) are the domi-
nant factors in limiting the long-term release rates of most radionuclides
(e.g., Andra, 2005; DOE, 2008; JNC, 2000; Nagra, 2002). Such solubility
limits are also considered to be controlling factors for potential releases
via human intrusion from the WIPP site for disposal of defense TRU
waste (DOE, 1995; NRC, 1996). Performance analyses of LLW disposal
systems also typically apply these solubility limits as controls on radionu-
5 The timeframe for regulatory compliance and the half-lives of key radionuclides present in
the waste are also considerations; the dissolution rate of a waste form may limit radionuclide
releases from disposal systems for an initial period before the onset of solubility limits imposed
by precipitation of radionuclide-bearing solids.
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
clide releases (e.g., Andra, 2005; Nagra, 2002; NRC, 2005). Waste-form
dissolution rates can, however, provide an important constraint on the
release of highly soluble radionuclides, such as carbon-14, chlorine-36, and
iodine-129, if such radionuclides are present in the waste form.
Application of PA models for disposal systems can place diverse factors
such as waste-form dissolution rate, waste loading, and solubility limits
of the solid phases containing radionuclides into a common system-level
context for evaluation and optimization. Furthermore, such PA models can
also provide guidance to future decisions on whether there is a safety-based
reason for further development of advanced waste forms. The most notable
application of system-level PA models to advanced waste form development
relates to thresholds at which extremely low waste form dissolution rates
would constrain (and simplify the calculation of) the performance of a given
disposal concept.
NRC (1983, pp. 279-280) made a detailed analysis of the necessary
fractional dissolution rate for waste forms performance to control the per-
formance of disposal systems:
The effect of low-solubility waste forms on radionuclide release rates is to
decrease the number of radionuclides that may dissolve more slowly than
the host, until, in the limit, all waste products will be released congruently
or diffuse out and dissolve faster than the host. This limiting condition
probably occurs at waste-form dissolution rates around 10–9 or 10–10.
NRC (1983) further identified potential advantages of a low-solubility
waste form with such exceptionally low fractional dissolution rates, includ-
ing the following:
• Verification of the safety performance of the entire disposal system
would depend largely on the laboratory measurements made under
appropriate site-specific conditions (i.e., risk-based testing of waste
forms),
• Release rates of an increasing number of radionuclides would
become proportional to decreasing fractional dissolution rate,
• The need for estimating separate solubility limits would be greatly
attenuated if not eliminated, and
• The number of sites that could serve as suitable repositories might
increase.
A more recent analysis (SKB, 2006, Figure 10-44) of a deep geologi-
cal repository for the disposal of spent nuclear fuel in granitic crystalline
rock suggests the fractional dissolution-rate threshold for a waste form (in
this case UO2) might be as low as 10–6/year for certain key radionuclides.
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184 WASTE FORMS TECHNOLOGY AND PERFORMANCE
The exact threshold at which the waste-form dissolution rate controls
the release performance of a disposal system (i.e., when the flux ratio R
in Equation 7.1 becomes much less than 1) depends on a number of fac-
tors. In particular, waste loading and solubility limits, which typically vary
among different radioelements over many orders of magnitude, influence
this threshold value.
The sensitivity of factors affecting the threshold value at which waste-
form fractional dissolution rate controls the release performance of a hypo-
thetical disposal is illustrated in Table 7.1. This table applies Equation 7.1
to calculate flux ratios R for a several key, long-lived dose-contributing
radioelements present in a standard HLW glass (Nagra, 2002) for refer-
ence waste loadings, solubility limits (CSAT in Equation 7.1), and fractional
dissolution rates.
For the reference fractional dissolution rate of 10–5 parts per year,
the releases of selenium-79, technetium-99, and neptunium-237 from the
disposal system would be constrained by their respective solubility limits,
TABLE 7.1 Sensitivity of Calculated Flux Ratios Using Equation 7.1 for
Radioelements with a Key Long-lived Radionuclide Present in a Reference
HLW Borosilicate Glass
Waste Loading Solubility, Fractional
CSAT
Radioelement/ (kg of radionuclide/ Dissolution Rate Flux
(kg/m3)b
kg glass)a (j0, in parts per year)c
Key Radionuclide Ratio, R
1.63 × 10–4 4.0 × 10–7 10–5
Selenium-79 360
10–7 3.6
2.79 × 10–3 4.0 × 10–7 10–5
Technetium-99 6100
10–7 61
5.52 × 10–7 10–5 3.7 × 10–6
Iodine-129 130
10–7 3.7 × 10–8
1.36 × 10–3 1.2 × 10–6 10–5
Neptunium-237 990
10–7 9.9
NOTES: Diffusional transport from the waste-form surface is assumed, with ε = 0.01, De =
3.15 × 10–2 m2/year, and r0 = 0.4 m (Zavoshy et al., 1985). R values much greater than 1
indicate release performance of the disposal system would be constrained by radioelement
solubility, whereas R values much lower than 1 indicate release performance of the disposal
system for that radionuclide would be constrained by waste form dissolution rate.
a Waste loading for a Reference HLW Borosilicate Glass (McGinnes, 2002, Tables A.1-1 to
A.1-4).
b Reference Case radioelement solubilities for reducing disposal conditions (Nagra, 2002,
Table A2.4).
c The reported long-term dissolution rate of 5.5 × 10–4 kg/m2 year for the Reference HLW
Borosilicate Glass is stated to correspond to a fractional dissolution rate of 10–5 parts per year
(Nagra, 2002, p. 144).
SOURCE: Nagra (2002).
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
whereas the release of highly soluble iodine-129 would be limited by the
dissolution rate of the HLW glass. Even for a postulated fractional dissolu-
tion rate of 10–7 parts per year, the releases of selenium-79, technetium-99,
and neptnium-237 from the disposal system would still be constrained by
their solubility limits. It would require a speculative fractional dissolution
rate on the order of 10–9 parts per year (i.e., the waste form would take
1 billion years to completely dissolve) for a waste form to control, and
thereby lower, the release rates of these key radionuclides from the disposal
system. This value is in basic agreement with the previous NRC (1983)
estimate. The important point is that such sensitivity analyses provide a
defensible basis by which to determine “how much better” an advanced
waste form would have to perform to significantly enhance the safety of
disposal systems compared to current HLW borosilicate glass, for example.
7.3.1.2 Integrated PA Models
The analytical models discussed above link the release and transport
boxes shown in Figure 7.2. However, there are additional processes and
barriers that affect the overall safety of disposal systems, including con-
tainment (i.e., barriers designed to delay contact between groundwater
and waste forms), transport through the natural barrier (host rock) of the
disposal facility, and finally the various pathways in which released radio-
nuclides might migrate through the biosphere and lead to doses to humans.
A system-level analysis is needed that incorporates all of the design aspects
and properties of natural and engineered barriers that affect overall safety.
Numerical codes have been developed to allow more complete linkage
among the models for the process boxes shown in the upper part of the
PA Pyramid in Figure 7.2, such as the GoldSim code used in the recent
license application for a SNF/HLW repository at Yucca Mountain (DOE,
2008) and the IMARC code (EPRI, 2009), which was also applied to the
Yucca Mountain Site. Such codes provide great flexibility for evaluating
uncertainties and sensitivities in model parameters; inclusion of alterna-
tive conceptual models for certain processes; detailed spatial expansion of
important regions (compartments) of disposal facilities (especially the EBS);
and relatively easy use of alternative data sets from pre-configured libraries.
Such top-level PA codes are now being used widely across DOE sites to
provide a more accesible means for communicating about and addressing
uncertainties and sensitivity about disposal system performance with non-
technical stakeholders. With the detailed models discussed in Sections 7.3.2
and 7.3.3, scientific and engineering understanding about the disposal
system can be established. The top-level PA model is established with such
fundamental understanding, while stylization for unverifiable assumptions,
such as biosphere radionuclide pathway models, is introduced.
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186 WASTE FORMS TECHNOLOGY AND PERFORMANCE
!
!
!
7.3.2 Intermediate Level Models
Significant insights on the performance of waste forms in disposal sys-
tems can be gained from the application of relatively simple performance
assessment models described in the previous section. However, the abstrac-
tions in these models may not account for all of the important variables in
the disposal system or changes in system conditions over time, especially
during the initial period following facility closure. Consequently, there can
be a need to develop engineering-type models that more fully incorporate
the features of the facility design (e.g., EBS configuration, dimensions, and
layout) and system conditions. Such models occupy the intermediate layer
of the PA Pyramid in Figure 7.2 and bridge the fundamental process models
at the bottom of the pyramid to the abstracted models at the top.
A facility for disposal of SNF/HLW will contain thousands of waste
packages, usually in a two-dimensional array, each of which is surrounded
by multiple engineered barriers. Some internal structure and heterogeneous
radionuclide distribution will be present within each waste package. Conse-
quently, the repository will display heterogeneity at different spatial scales.
In conventional PA, radionuclide transport is modeled by reducing this
heterogeneity to some extent (i.e., heterogeneity is homogenized). Packages
are represented by several end-member types, and radionuclide transport in
the repository is modeled without considering interferences from adjacent
packages or the effects of the two-dimensional package-array configuration
(Ahn et al., 2002).
The homogenization of spatial heterogeneity can obscure important
processes that govern the performance of the disposal system, for example,
the existence of advection-dominant flow paths. Radionuclide release from
the near-field region to the far-field region is strongly influenced by the
existence of these fast paths. The existence of fast paths can affect degrada-
tion of the engineered barriers, which in turn can affect fast-path geometry
(Murakami and Ahn, 2008; Steefel et al., 2005).
Taking into account heterogeneity at all spatial scales requires a tre-
mendous amount of computation. For instance, a relatively small-size simu-
lated repository, containing fewer than 100 packages and millions of rock
fractures in the near field, was the maximum size that could be simulated
by the Earth Simulator supercomputer (Tsujimoto and Ahn, 2008). Com-
partmentalization is a logical modeling approach to improve computational
efficiency because a repository contains thousands of packages surrounded
by similar combinations of barriers. In a compartmentalization approach,
modeling can be made at two levels: one at a local scale within a compart-
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
ment and the other at a repository scale connecting compartments. For
waste form performance, local-scale modeling is more urgently needed.
!
!
!
7.3.3 Fundamental Process Models
Fundamental process models, which occupy the base of the PA Pyramid
shown in Figure 7.2, integrate the knowledge obtained in the experiments
and tests described in Chapter 5. These process models can be used to
estimate performance at “local” (i.e., sub-facility) scales. Such models can
be used to obtain best-estimates of waste form performance in particu-
lar disposal environments; to select suitable combinations of waste forms
and engineered-barrier configurations; and to evaluate system performance
using metrics other than dose, which can aid in optimizing facility designs.
More complex models for waste form durability need to account for
waste form material properties (Chapter 3), disposal environment (Chap-
ter 6), and interactions with other engineered and natural barriers in the
disposal system (Chapter 6). The importance of such interactions is high-
lighted in a recent summary of the GLAMOR6 project (Van Iseghem et al.,
2007, 2009). A specific focus of this project was to understand the long-
term decrease in the rate of dissolution of glass waste forms (Figure 7.3),
with two competing hypotheses considered:
1. The effect of silica concentrations in solution on the depression of
the rate (the so-called “chemical affinity effect”7).
2. The role of surface layers that develop during the corrosion/disso-
lution process in limiting transport of reactive constituents to and
from the primary glass phase, assuming that such layers do not
spall off.
The rate of glass dissolution could be accelerated by placing it in
proximity to a bentonite (clay) buffer or steel and iron canister corrosion
products. Accelerated dissolution seems to be caused by the sorption and
removal of glass reaction-products from solution, which if present would
6 A Critical Evaluation of the Dissolution Mechanisms of High Level Nuclear Waste
Glasses in Conditions of Relevance for Geological Disposal; see ftp://ftp.cordis.europa.eu/pub/
fp6-euratom/docs/euradwaste04pro_pos9-van-iseghem_en.pdf.
7 Chemical affinity is defined as the log (Q/K ), where Q is the ion activity product of dis-
sp
solved species in solution and Ksp is equilibrium constant for the waste form (Lasaga, 1979).
Borosilicate glass is thermodynamically unstable and cannot be re-precipitated from solution,
so a proxy Ksp is derived for it.
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188 WASTE FORMS TECHNOLOGY AND PERFORMANCE
FIGURE 7.3 Schematic representation of predominant mechanisms and resulting
Figure 7.3.eps
kinetics affecting the concentration of glass alteration elements silicon (Si), boron
bitmap
(B), and sodium (Na).
SOURCE: Van Iseghem et al. (2009).
slow the rate of dissolution due to the chemical affinity effect. To under-
stand the underlying mechanism it was necessary to conduct a series of
experimental studies supplemented with detailed microscopic characteriza-
tions of the evolving glass surface layers. Similar studies will be necessary
for any candidate waste forms considered by DOE-EM because a mecha-
nistic understanding of the controls on waste form dissolution provides the
basis for understanding waste form performance at long time scales.
Evaluation of the performance of waste forms in disposal systems may
be required for periods ranging up to 1 million years, depending on the
pertinent regulations (see Chapter 8). Figure 7.4 provides an illustration of
important processes that can occur in the near-field environment of a dis-
posal facility for SNF/HLW over these time scales. The durability of a waste
form depends, in addition to its own properties, on several environmental
factors:
• solution composition and pH
• flow rate
• temperature
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
FIGURE 7.4 Simplified scheme of chemical processes in the near-field environment
Figure 7.4.eps
of a disposal facility.
bitmap
SOURCE: Grambow et al. (2000), as modified by Horst Geckeis.
• redox conditions
• speciation in solution
• radiolysis
• interactions with corroded canisters and near-field geology
• formation and mobility of colloids
These individual factors often interact and are coupled in a repository
environment. The dissolution of waste forms containing radioactive waste
can be complex, particularly following the closure of a disposal facility
when thermal, radiological, mechanical, hydrological, and chemical pertur-
bations to the disposal system are highest. (This is a primary reason that
radioactive wastes are typically placed in canisters with containment life-
times of several thousands of years or more, which prevents groundwater
contacting waste forms until these initial perturbations dissipate.)
Evaluating the complexity of disposal system performance can be
accomplished using models that explicitly couple thermal, hydrological
(transport), mechanical, and chemical processes in 3-dimensional repre-
sentations of the barriers and spatial variation in properties. All of these
processes occur against a backdrop of changing environmental conditions
that may be externally imposed, including water infiltration rates, water
chemistry, and decay of the thermal field. Local-scale mechanistic modeling
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190 WASTE FORMS TECHNOLOGY AND PERFORMANCE
based on understanding of near-field processes can be made by coupling
thermal (T), hydrological (H), chemical (C), radiological (R), and mechani-
cal (M) processes for evolution and transport of materials in the near-field
environment, including the waste form and its radioactive constituents.
Estimates are typically restricted to pair-wise analyses such as T-H, T-C,
T-M, or R-T, under the assumption that such sub-set analyses capture the
full system behavior. Attempts to develop such models have already been
made (e.g., Steefel et al., 2005) but much more work is needed. The devel-
opment of such models could provide the scientific basis for best-estimates
of waste form performance.
The challenge for using coupled models to evaluate waste form per-
formance in disposal systems is the identification of key processes in the
near-field environment, including:
• Rate-limiting steps. These could include the dissolution mechanism
of the waste form; the formation and decomposition of radiolytically
produced species in solution; initial surface sorption/desorption reac-
tions; or the nucleation and precipitation of secondary phases.
• Reaction mechanisms: Even when rate-limiting steps have been
identified, reaction mechanisms can change in response to changing
environmental conditions. The full range of possible mechanisms
and the conditions under which these mechanisms control waste
form reactions must be evaluated.
• Radiolysis effects: Radiolysis at the waste form/solution interface
can have an important effect on dissolution processes, particularly
for redox-sensitive elements such as uranium and plutonium (see
Chapter 6). The long-term effects of radiolysis, for example the
formation of H2O2, have barely been explored, although in nature
U(VI) phases that contain peroxyl groups have recently been dis-
covered (Kubatko et al., 2003).
For coupled models to be realistic and useful, relevant chemical and
physical processes must be represented. Relevant chemical processes include
the following:
• Kinetically controlled dissolution of the waste form
• Nucleation and kinetic controls on the growth and sequence of
metastable phases
• Solid-solution (co-precipitation, see Figure 7.4) models for substitu-
tion of minor components
• Kinetic and equilibrium sorption via different mechanisms (ion
exchange, surface complexation)
• Aqueous complexation
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
• Oxidation-reduction state of the waste form as influenced by trans-
port, local reactions, and radiolysis
• Composition and chemical concentration of pore water chemistry
caused by evaporation
The relevant physical processes include the following:
• Heat transport as a result of convection, conduction, and radio-
active decay, which provides the time-dependent temperature field
affecting relative humidity, reaction kinetics, and thermodynamics
• Water flow under variably saturated conditions
• Diffusive and advective transport of solutes in the aqueous phase
• Gas phase transport via advection and diffusion, especially for the
reactive gases O2 and CO2
The application of advanced reactive transport modeling of the near-
field environment (Steefel et al., 2005) can aid in the overall safety assess-
ment of disposal systems by reducing unwarranted conservatisms in more
abstracted PA models and by enhancing the comprehensiveness and confi-
dence in the PA.
7.4 DISCUSSION
The PA of waste forms containing radioactive waste can only be mean-
ingfully accomplished within the context of disposal system PA, in which
health-risk consequences are the appropriate basis for evaluations. As
shown in Figure 7.2, there is typically a hierarchy of PA models employed
in assessing any waste form/disposal system, with each level of PA models
having the appropriate fitness for purpose, for example, design optimiza-
tion, identification of risk-informed R&D needs, or regulatory compliance.
The development of new or improved waste forms by DOE-EM could
offer two potential benefits: (1) more efficient waste processing and immo-
bilization; and (2) enhanced performance of the disposal systems into
which the waste forms will eventually be emplaced. With respect to the
first benefit, increasing waste loading and/or processing rates could lower
production costs and accelerate cleanup schedules (see Chapters 2 and 4).
With respect to the second benefit, enhanced performance of the waste
form, established under relevant disposal system conditions, could eliminate
unwarranted conservatisms and provide greater confidence in the overall
performance of the disposal system.
It is important to recognize that repository performance is an optimiza-
tion problem, and the waste form is one of several elements in the optimi-
zation. Other elements include the physical and chemical characteristics of
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192 WASTE FORMS TECHNOLOGY AND PERFORMANCE
the rocks hosting the repository as well as the design and characteristics of
other engineered barriers. In other words, there is no single figure of merit
for waste form performance.
PA can provide safety-based insights to guide future decisions on fur-
ther development of waste forms. The most notable application relates to
estimating thresholds at which extremely low waste-form dissolution rates
would control (and simplify the calculation of) the release of all radio-
nuclides for a given disposal environment and design concept. As noted
elsewhere in this chapter, the necessary threshold value for waste form dura-
bility (expressed as a fractional dissolution rate) to control the performance
of a disposal system might be on the order of 10–6 to as low as 10–10 per
year, depending on factors such as disposal system design, environmental
conditions, applicable radioelement solubility limits, and radionuclide load-
ings in the waste form.
It must be stressed, however, that such exceedingly low dissolution
rates for waste forms under disposal conditions are not requirements. Cur-
rent safety assessments of disposal systems (e.g., Andra, 2005; DOE, 2008;
JNC, 2000; Nagra, 2002; SKB, 2006) for a wide variety of HLW and LLW
waste forms in various geological formations (e.g., salt, granite, tuff, clay)
generally show wide margins of compliance with applicable regulatory
safety standards.
As presented in NRC (2003) and illustrated in Figure 7.1, the develop-
ment of a multiple-barrier repository concept is initially based on (1) identi-
fication of waste forms meeting specific waste acceptance criteria (WAC, see
Chapter 8); (2) establishment of national laws and regulations (Chapter 8);
and (3) selection of a specific disposal site (Chapter 6). Based on this design
and collection of field and laboratory data, a series of PAs can be made and
the results used to adapt the design of the disposal facility and the iterative
collection of additional data, as necessary. From such analyses, the disposal
system concept can be adapted and new data collected in iterative stages
as necessary. Flexible, staged-adaptation in disposal system development
leading to eventual licensing is being implemented in numerous national
programs worldwide (NRC, 2003).
This inherent adaptability would also apply to a situation in which a
new waste form might be proposed. The waste form would need to pass
WAC regarding its physical form, dimensions, and potential impacts on the
site environment (e.g., no introduction of major chemical components that
might compromise the safety functions of other barriers). As discussed in
Chapter 8, there are no specific long-term waste form performance require-
ments in the WAC; short-term (seven-day) leach rates are used instead as
a measure of quality assurance and product consistency. The absence of
such performance requirements on waste forms means that an adaptive
repository program should readily accommodate new waste forms through
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WASTE FORM PERFORMANCE IN DISPOSAL SYSTEMS
the iterative process of modifying the repository design and updating per-
formance assessment, as illustrated in Figure 7.1. In the case where the
calculated releases from a disposal system meet safety criteria because of
radioelement solubility limits, then the motivation for developing advanced
waste forms would be based more on factors such as waste loading, favor-
able chemical environment during and after waste form dissolution/altera-
tion that assures such a low solubility, and ease of fabrication, rather than
durability.
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