TITLE: Radioactive Wastes Dispersed in Stabilized Ash Cements

AUTHOR(S): James B. Rubin CST-12 LANL

Craig M. V. Taylor CST-12 LANL

Loren D. Sivils CST-12 LANL

James W. Carey EES-1 LANL

SUBMITTED TO: 1997 International Ash Utilization Symposium

October 20-22, 1997

Lexington, KY, USA

**Radioactive Wastes Dispersed in Stabilized Ash Cements**

James B. Rubin, Craig M. V. Taylor, and Loren D. Sivils, Organic
Chemistry Group, Chemical Science and Technology Division, Los Alamos
National Laboratory, Los Alamos, New Mexico 87545;

James W. Carey, Geology and Geochemistry Group, Earth & Environmental
Science Division, Los Alamos National Laboratory, Los Alamos, New Mexico
87545

**INTRODUCTION**

One of the most widely-used methods for the solidification/stabilization
(S/S) of low-level radwaste is by incorporation into Type-I/II ordinary
portland cement (OPC). OPC, as an immobilization matrix, has many
desirable properties, which are listed in Table 1. Also listed in the
table are several disadvantages associated with cement as a wasteform.
Several of these shortcomings have been successfully addressed, however,
with the help of a recent innovation in OPC processing. Treating of OPC
with supercritical fluid carbon dioxide (SCCO~2~) has been shown to
significantly increase the density, while simultaneously decreasing
porosity. In addition, the process significantly reduces the hydrogenous
content, reducing the likelihood of radiolytic decomposition reactions.
This, in turn, permits increased actinide loadings with a concomitant
reduction in disposable waste volume.

Table 1. Advantages and disadvantages to the use of OPC as a radwaste
immobilization matrix

In this article, we will discuss the combined use of fly-ash-modified
OPC and its treatment with SCCO~2~ to further enhance immobilization
properties. We begin with a brief summary of current cement
immobilization technology in order to delineate the areas of concern.
Next, supercritical fluids are described, as they relate to these areas
of concern. In the subsequent section, we present an outline of our
results on the application of SCCO~2~ to OPC, and its effectiveness in
addressing these problem areas. Lastly, in the final section, we proffer
our thoughts on why we believe, based on the OPC results, that the
incorporation of fly ash into OPC, followed by supercritical fluid
treatment, can produce highly efficient wasteforms.

**STATE OF THE CURRENT TECHNOLOGY**

The current practice at LANL for stabilizing low-level radwaste involves
casting of OPC inside 55-gallon drums. Such cemented wasteforms are
intended for transfer to the Waste Isolation Pilot Plant (WIPP) for
permanent underground storage. The cemented wasteforms are transported
to WIPP using the TRUPAC II container, which can accommodate up to
fourteen individual drums. In order to meet Department of Energy (DOE)
transportation requirements,^1,2,3^ however, the individual cemented
wasteforms must satisfy several regulatory requirements. These
requirements prevent the occurrence of potentially flammable
concentrations of gases within the container as well as the generation
of unacceptably large internal pressures of non-flammable gases. These
controls take the form of restrictions on the packaging, as well as the
types of materials which can be present within individual wasteforms and
maximum limits on the decay heat. (Decay heat is defined as the heat
produced by radioactive emissions that is absorbed by the surrounding
materials.)

During transportation of cemented wasteforms, the primary mechanism for
flammable and nonflammable gas generation is radiolytic decay of
hydrogenous (hydrogen-bearing) materials, including plastics, paper,
fabrics, rubber, and water. Under conditions of radiolysis such
materials undergo decay to simpler products, namely CO, CO~2~, free
carbon and hydrogen gas. The imposed decay heat limit is intended to
minimize the level of flammable gas generation by specifying upper
limits on the amount of radioactive material in an individual wasteform.
The current limits are based, in part, on the overall hydrogen content.
If the hydrogenous content (which would be primarily water) of a Type I
cemented wasteform can be reduced to less than 30 percent by weight,
then the maximum allowable decay heat for this modified wasteform
increases fourfold. Clearly, if the hydrogenous content can be reduced,
a greater loading of radioactive material can be achieved, thereby
reducing the volume of disposable waste while still meeting
transportation requirements.

The hydrogenous content of OPC, largely water, is dictated by processing
constraints. A minimum water/cement ratio is necessary to achieve a
cement paste sufficiently fluid to permit thorough mixing. There have
been extensive studies made on the use of additives to reduce the water
content. Various organic plasticizers and inorganic set-retarders have
been examined. The addition of organic compounds, however, exacerbates
the gas generation problem, while inorganic set retarders, primarily
sulfates, result in swelling of the wasteform and occasional rupturing
of the drums. Non-portland cements, such as those based on gypsum, have
also been evaluated. These wasteforms, however, are prone to incomplete
setting in the presence of organics. The final consequence of these
investigations is that unmodified OPC remains the matrix of choice for
low-level waste immobilization.

**SUPERCRITICAL FLUIDS**

Supercritical CO~2~ occurs naturally in the headspace above oil
reservoirs, and high-pressure CO~2~ has been used for many years by oil
producers in secondary and tertiary recovery processes. In addition,
supercritical fluids are used as solvents in many commercial
applications, including the extraction of caffeine from coffee, fats
from foods, and essential oils from plants for use in perfumes. The
attractiveness of supercritical fluids as solvents stems from their
unique combination of liquid-like and gas-like properties. In this
regard, supercritical fluids can be thought of as occupying an
intermediate position between gases and liquids. The following,
phenomenological description will help to illustrate the concept of a
supercritical fluid.

The temperature at which the vapor pressure above a pure liquid reaches
one atmosphere is known as the normal boiling point. For water, the
normal boiling point at one atmosphere is 100°C. In an open container,
the temperature of liquid water cannot be raised above 100°C since this
would cause the vapor pressure of water to rise above one atmosphere,
which would exceed the ambient pressure. If we place a quantity of
liquid water in a sealed container, however, we can heat to higher
temperatures, since there is no longer any limit on the vapor pressure
we can attain (assuming that the container does not burst). As we
uniformly heat the sealed container, the density of the liquid water
decreases through thermal expansion. Simultaneously, the density of the
water vapor increases as more molecules leave the liquid and enter the
vapor phase. We can continue this heating process until the density of
the liquid has been so reduced, and the density of the vapor phase has
been so increased, that the two densities become equal. The temperature
at which the liquid and vapor densities become equal is called the
critical temperature. Since the temperature and density inside our
sealed container is equal throughout, the laws of thermodynamics dictate
that the pressure inside the container be equal throughout. This
pressure is called the critical pressure. A liquid (or gas) which has
been brought to conditions above its critical temperature and pressure
is known as a supercritical fluid.

Figure 1 (above) shows the pressure-temperature-density surface for pure
CO~2~.^4^ The critical condition of temperature and pressure (31°C and
1072 psi) is shown in the figure by the solid circle. It can be seen
that relatively small changes in temperature or pressure, above the
critical values, can result in large changes in density. It is this
tunability of density which is one of the most attractive attributes of
supercritical fluids. The reason is that, to a first approximation, the
ability of a fluid to solubilize other fluids is related to density:
higher density results in higher solubilities. Also, the gas-like
properties of low viscosity and high diffusivity provides for efficient
mass transport into and out of granular and micro-porous matrices, such
as cast cements. Finally, the absence of surface tension provides for
excellent substrate wetting.

Supercritical CO~2~, as a candidate for scale-up to a production
treatment process, has several safety and economic advantages. CO~2~ is
non-toxic, non-flammable, and inexpensive (1040 cents per pound). Its
critical conditions of temperature and pressure are easily achievable
using readily available process equipment. Also, there is already a
transportation infrastructure (virtually all restaurants serve
carbonated drinks, which require the use of compressed CO~2~ cylinders).
It is the goal of our work to use this unique combination of
physico-chemical and economic properties of supercritical fluid SCCO~2~
to enhance the performance of cemented wasteforms.

**SUPERCRITICAL FLUID CO2 TREATMENT OF STANDARD PORTLAND CEMENTS**

The natural curing reactions which occur in OPC involve the formation of
calcium hydroxide (portlandite), as well as calcium silicate hydrates
(CSH). Over time, the cement will abstract CO~2~ from the air,
converting the calcium hydroxide and CSH to anhydrous calcium carbonate.
It turns out, however, that this secondary conversion results in the
closure and/or blockage of pores, impeding the ingress of reactants
(CO~2~) and the egress of reaction products (H~2~O), drastically slowing
the reaction rate with time. It is known that even the oldest cements,
thousands of years in age, are still undergoing this natural carbonation
reaction. By exposing an OPC to SCCO~2~ it is found that the carbonation
reaction can be greatly accelerated. This acceleration is due to the
ease of penetration of the supercritical fluid into the micro-pores of
the cement, providing continuous availability of fresh,
hyper-stoichiometric reactant, as well as the solubility of the reaction
product in the supercritical fluid, facilitating its removal.

Figure 2 (below) shows a schematic pressure-temperature phase diagram
for CO~2~, onto which is superimposed a process flow diagram for a
closed-loop cement-treatment system. The extraction cycle begins with a
liquid- CO~2~ storage reservoir. The liquid is brought to above its
critical pressure during a pumping operation, which sends the
pressurized liquid to a heating unit. The heating unit brings the
temperature of the pressurized CO~2~ above its critical temperature. The
supercritical fluid enters the extraction vessel and is brought into
contact with the cement wasteform to be treated. There is a constant
flow of CO~2~ through the treatment vessel so that clean, dry CO~2~ is
continuously made available to the cement. In the supercritical state,
SCCO~2~ is a non-polar \"organic\" solvent. Although water is a polar
substance, the high SCCO~2~ density allows for water solubilities of
0.2-0.5 mole percent, so that a flowing system can remove large amounts
of water in relatively short times. On exiting the extraction vessel,
the supercritical fluid CO~2~ is sent to a separation vessel, where the
fluid is de-pressurized to below its critical pressure, reducing the
CO~2~ density to a gas-like value. The solubility of the entrained
materials (water and organics) is greatly reduced in the low-density gas
phase and are deposited in the bottom of the separator. The clean CO~2~
gas exits the top of the separator, where it is chilled to liquefaction
by a cooling unit before re-entering the storage vessel.

This type of closed-loop treatment, applied to cemented wasteforms,
means that there are no waste streams exiting the treatment system. All
of the extracted materials are retained in the separation vessel for
subsequent analysis, treatment and/or disposal. Further, the extracted
materials are concentrated in the separator, reducing the volume of
waste. It is important to note that SCCO~2~, while solubilizing organic
compounds and water, is not expected to leach inorganic compounds, as
these will be bound up in the crystal structure of the inorganic phases.
The projected advantage of this type of SCCO~2~ treatment scheme, in
terms of operations at a radwaste production-facility, is that the
cemented wasteforms can continue to be cast using established methods,
with the treatment process involving only an additional, post-casting
step.

By treating straight OPC, using a closed-loop treatment system like the
one shown in Figure 2, we can extract the majority of the hydrogenous
material. Our work with 1\" x 3\" cylinders of OPC shows that nearly
complete carbonation (with simultaneous dehydration) can be achieved by
a SCCO~2~ treatment at 4000 psi and 40°C for 2-hours. Measured
permeability values for the treated material are reduced by an order of
magnitude over untreated values. The porosity values are similarly
improved for the treated material. Specific results will be reported in
the verbal presentation.

We are currently performing treatment tests on 6\" x 12\" OPC cylinders.
We are measuring the total water and CO~2~ content, using a measurement
system similar to ASTM standard method D 3178. These studies will
quantitatively determine the degree of chemical conversion of the OPC,
as a function of the treatment parameters listed below.

Table 2. Relevant issues for the evaluation of supercritical fluid CO~2~
to S/S using OPC and fly ash-modified OPC.

**ANTICIPATED USE OF FLY ASH-MODIFIED CEMENTS**

The incorporation of fly ash into OPC has been identified as one of the
treatment parameters of cement composition to be evaluated. There is
already an extensive \"experience database\" on the performance of fly
ash-modified OPC for radwaste immobilization. The United Kingdoms and
the United States^6^ have used these materials, in the form of cement
grouts, for the S/S of low and intermediate-level radioactive wastes. In
this section, we will review the known benefits of fly ash-modified OPC
over straight OPC, along with the anticipated improvements expected by
the treatment of modified OPC with SCCO~2~.

1.  Fly ash increases the density, decreases the permeability, and
    increases the leaching resistance of OPC. It is a truism that \"The
    leach resistance of solidified cement-waste systems can be improved
    by any process which accelerates curing, limits porosity, or
    chemically bonds fission product or actinide elements.\"^7^ SCCO~2~
    treatment of a modified OPC is expected to further increase the
    density over the untreated material, so that a reduced porosity, and
    improved leachability should result. In addition, the high silica
    content of fly ash, with its well-known sorbent properties towards
    plutonium and certain other radionuclides, should also enhance the
    immobilization characteristics.

2.  The incorporation of fly ash into OPC helps to partially compensate
    for the set-retarding effect of heavy and non-noble metals. Although
    the reduction hydrogenous material content resulting from SCCO~2~
    treatment of an OPC will allow for increased actinide loadings, the
    higher metals content may significantly interfere with proper
    setting. Fly ash-modified cements perform somewhat better in this
    regard, and a SCCO~2~ treatment of a modified cement should be even
    better.

3.  Fly ash incorporation increases OPC mix fluidity, which improves
    workability. For S/S applications, the increased fluidity may allow
    a reduced water/cement ratio to be used in the casting operation.
    Since the casting is done in a glovebox environment, the need to
    maintain process simplicity is paramount.

4.  The incorporation of fly ash lowers the initial heat evolution
    during setting, reducing the incidence of cracking and spelling. It
    is desirable to maintain the wasteform in monolithic form for
    optimum leach resistance.

5.  Fly ash, as it is typically generated in a high free-carbon
    environment, contains iron in a reduced state, which helps to lower
    the redox potential in the cement. As is the case for regulated
    heavy metals, such as Cr, maintaining the actinides in lower
    oxidation states should result in lower solubilities. We hasten to
    point out that one of the major unanswered questions regarding the
    use of SCCO~2~ for the treatment of cemented wasteforms is the
    effect of the altered pH on actinide solubility. In OPC, the highly
    alkaline environment precipitates the actinides as hydroxides,
    rendering them immobile. Since the SCCO~2~ treatment converts the
    alkaline phases into neutral pH phases, a straight or fly
    ash-modified OPC which has been treated will have a much reduced pH.
    An thorough investigation of this solubility issue is essential.

6.  While portland cement is considered to be an inexpensive
    immobilization matrix, relative to other candidate materials, its
    cost is expected to rise in light of current and future projected
    shortages. Fly ash, as it is a large-volume industrial waste, is
    both cheap and abundant, so that there is an economic incentive to
    use fly ash-modified cements. In addition, CO~2~ is also produced as
    a waste by-product of industrial processes (power generation, cement
    manufacture, etc,), and its permanent sequestration into cement is
    an added environmental benefit.

**CONCLUSIONS**

Although cementation is a standard S/S treatment process, many cemented
wasteforms are failing to qualify for underground disposition because of
transport and/or disposal requirements. We have been evaluating
supercritical CO~2~ carbonation of OPC to alter the bulk phase
chemistry, which accelerates the natural aging (carbonation) reactions.
This treatment process produces a chemically stable form having
substantially-reduced levels of free liquids and organics, as well as
reduced porosity, permeability and pH. The promising results obtained
with SCCO~2~ treatment of OPC leads us to speculate that fly
ash-modified OPC may produce a highly efficient immobilization matrix.
The structural and chemical changes produced by the treatment process
should allow for a reduction on waste volume and a reduced mobility of
anions, cations, and radionuclides in aboveground and underground
repositories.

**REFERENCES**

1\. U. S. DOE, "TRUPAC-II User Requirements Document", Rev. February 2,
1992.

2\. U. S. DOE, "TRUPAC-II Content Codes (TRUCON)", Rev. 10, December
1996, DOE/WIPP 89-004.

3\. DOE WlPP Project Office, SOP 9.4, "Waste Acceptance Criteria
Certification Committee (WACCC)", December 7, 1988.

4\. F.-H., M.-H. Li, L. L. Lee, K. E. Starling, and F. T. H. Chung,
1985, An Accurate Equation of State for Carbon Dioxide, J. Chem. Eng.
Japan V. 18 p.490.

5\. Wilding C. R., The Performance of Cement Based Systems, Cement and
Concrete Research 22 (1992) 299.

6\. Huang, F. H., D. E. Mitchell, and J. M. Conner, 1994, Low-Level
Radioactive Hanford Wastes Immobilized by Cement-Based Grouts, Nuclear
Technology, V. 107 p. 254-269.

7\. Jantzen C. M., F. P. Glasser, and E. E. Lachowski, 1984, Radioactive
Waste-Portland Cement Systems: I, Radionuclide Distribution, J. Am.
Chem. Soc., V. 67 p. 668-673.