Evaluation of the Use of Synthetic Zeolite as a Backfill Material in Radioactive Waste Disposal Facility

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Evaluation of the Use of Synthetic Zeolite as a
Backfill Material in Radioactive Waste Disposal
Facility

• 
  

Presented by Dr Ahmed Mohamed
El-Kamash Hot Lab. Waste Management
Center AEAE, Egypt
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AIM OF WORK
Evaluate the feasibility of using synthetic
zeolite NaA-X prepared from fly ash (FA) as
backfill material in the proposed radioactive
waste disposal facility in Egypt. Also, the
migration behavior of cesium and strontium ions,
as two of the most important radionuclides
commonly encountered in Egyptian waste streams
through the proposed backfill material is studied
using mathematical models
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Radioactive disposal system

• The principle objectives of radioactive
  waste management are to assure that workers and
  public are not harmed now or in future by the
  effects of radiation from the wastes and that the
  environment is not adversely affected.
• The fundamental safety concept for the
  disposal of radioactive wastes is to isolate the
  waste from the accessible environment for a
  period sufficiently long to allow substantial
  decay of the radionuclides and to limit release
  of residual radionuclides into the accessible
  environment.

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A disposal system is intended to

• isolate the waste from the accessible
  environment for certain amount of time until
  waste activity reduced to acceptable hazardous
  level.
• control the radionuclides that reach the
  accessible environment
• limit the consequences of any unacceptable
  release to accessible environment

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Major Types of Radioactive Waste Disposal
facilities

• Near surface disposal facility means a land
  disposal facility in which radioactive waste is
  disposed of in or within the upper 30 meters of
  the earths surface.
• Deep Geological Disposal for high level waste
  such as spent nuclear fuel, gt400 meters
  underground

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Repository design components

• The engineering barrier system
• Engineered barriers can be used as physical and
  /chemical obstruction to prevent or delay
  migration of radionuclides.
• The natural barrier system
• Consists of the geological media hosting the
  repository and any other geological formations
  contributing to waste isolation.

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Multiple barrier concept

• The long term safety of a repository relies on a
  series of barriers The Engineered Barrier and
  The natural Barrier
• Multiple barrier concept is employed in which the
  waste form, the engineered barriers and the site
  itself all contribute to the isolation of the
  radionuclides.
• The failure of one or more of these barriers
  will be compensated by the rest of them

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Function of barriers

• Barriers can either provide
• absolute containment for a period of time, such
  as the metal wall of a container, or
• may retard the release of radioactive materials
  to the environment, such as a backfill or host
  rock with high sorption capability.

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Elements of engineered barriers
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Backfill materials

• Backfills are used for a number of purposes
  void filling to avoid excessive settlement,
  limitation of water infiltration, sorption of
  radionuclides, precipitation of radionuclides.
  Typical materials used, either singly or as
  admixtures, include clays, cement grout, rock,
  and soil.
• It is important to select the appropriate
  backfill. Selections of backfill materials for
  radioactive waste disposal have been derived from
  a much data on adsorption behaviour of
  radionuclides on several natural and synthetic
  materials.
• For long-term performance assessment of
  radioactive repositories, knowledge concerning
  the migration of radionuclides in the backfill
  materials is required .
• Sorption reactions are expected to retard the
  migration of radionuclides thereby reducing the
  potential radiological hazard to humans resulting
  from disposal of radioactive waste.

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In respect to fly ash

• Fly ash is an inorganic spherical residue
  obtained at coal power plants.
• The spherical microscopic structure of fine fly
  ash is related to the equilibrium between the
  operating forces on the molten inorganic
• The past applications of fly ash were restricted
  to its application in industry as an additive or
  as an adsorbent.

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Synthesis of zeolites from fly ash

• Zeolite synthesis is one of a number of potential
  applications for obtaining high value industrial
  products from fly ash for environmental
  technology.
• The composition similarity of fly ash to some
  volcanic materials, precursor of natural zeolites
  promoted the synthesis of zeolite from this waste
  material.

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Results in the present work are divided into
three main parts

• Synthesis and characterization of pure zeolites
• Sorption studies
• Long term behavior of zeolite NaA-X blend as
  proposed backfill

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Synthesis and characterization of pure
zeolites
PART 1
Physical properties of fly ash
XRF technique
SiO2 Al2O3 Na2O MgO P2O5 SO3 Cl K2O CaO TiO2 Fe2O3 Oxide
43.81 23.18 0.87 0.80 0.49 15.68 4.01 2.72 6.10 2.31 0.01 Wt
Intermediate glass content of about 66.99
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XRD technique for Fly Ash
mullite (3Al2O3.2SiO2) and a-quartz
(SiO2)
exits as crystalline substances, as identified by
sharp peaks, while the presence of amorphous
phases were identified by broad peaks (near 24
angle)
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Silica-Aumina extraction by fusion

• - The available silica in fly ash was extracted
  by the alkali fusion method using sodium
  hydroxide.
• The amount of extracted silica was131.43g/kg fly
  ash.
• The amount of extracted alumina was about 41.72
  g/kg.
• Synthesis of pure A-X ZEOLITE blend

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Synthesis of pure NaA-X zeolite
The synthesis of NaA-X zeolite blend was carried
out using the molar oxide ratios
of SiO2/Al2O3 2.1 Na2O/SiO2 1.4
H2O/Na2O 39.0 Sodium aluminate solution
was used externally to adjust the SiO2/Al2O3
ratio to the desired value
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Flow sheet diagram for the synthesis of NaA-X
zeolite blend from fly ash using extraction method
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XRF technique for synthesized Zeolite NaA-X
Na Al Si Ca Ti Mg Fe S K P other elements Element
27.79 33.41 38.34 0.067 0.081 0.062 lt0.01 0.002 0.056 0.004 lt0.1 Wt.
It clear that Si/Al ratio equals 1.15 which lied
in the region of zeolite-A and X as reported in
Breck ternary diagram
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XRD technique For Synthesized Zeolite NaA-X
I
zeolite X and zeolite A
The spectrum exhibits fingerprint lines of both
zeolite X at 2? 6.10 and zeolite A at 2?
7.20 and 9.93.
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Scanning electron microscopy for raw FA and
fused FA at different intervals
SEM

• Untreated FA Smooth and spherical particle
  interspersed in aggregates of crystalline
  compounds which may correspond to a-quartz and
  mullite.
• After 15 min fusion with Na OH (The amorphous
  aluminosilicates in fly ash were dissolved -Small
  surface cracks appeared - The particle surface
  changed, like unevenness
• After 30 min (The surface of FA became rough and
  burst - Larger cracks were appeared librating
  small aggregates
• After 60 min ( Small cenosphere were appeared
  -Several crystalline materials were precipitated
  onto the surface of FA particle

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Scanning electron microscopy for Synthesized
Zeolite NaA-X
SEM picture of the synthesized zeolite blend
providing an evidence for cubic crystal
characteristic for Na-A zeolite and the
pyramidal octahedral crystal of Na-X zeolite
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(No Transcript)
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sorption studies
PART 2

• Effect of pH
• The effect of pH on the sorption of Cs ions from
  aqueous chloride solutions using prepared zeolite
  NaA-X material was investigated over the pH
  range from 2.0 to 8.0.
• It was observed that the acidic medium has an
  inhibitory effect on the sorption process. This
  may be due to the competition behavior between
  hydrogen ions and studied ions for sorption onto
  the synthesized powder.
• The uptake was continuously increased from 18.6
  to 62.6 with the increase in pH value and the
  maximum uptake was found to be 64.1 and it was
  observed at pH range from 6.0 to 8.0.

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Sorption kinetics

• Effect of time

A higher initial removal rate within the first
30 minutes followed by slower rate till reaching
plateau. The amount sorbed for both ions was
increased with time and attained equilibrium
within 90-120min The amount sorbed of Sr2 gt
Cs
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Kinetic models

• Pseudo first order

(Lagergren)

• Straight line obtained suggest the applicability
  of the pseudo first order model to fit the
  experimental data over the initial stage of the
  sorption process up to 40 min.

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• Pseudo second order

(Ho and Mckay )
It was shown that the sorption process of each
ion follows pseudo second order model
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Pseudo first and second-order rate constants for
the sorption of cesium and strontium ions onto
synthetic A-X zeolite blend at 298 K and 50 mg/l
concentration.
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Estimation of diffusion coefficient
(Boyed et al)
Diffusion effective diffusion coefficient Di coefficient De Metal ions
6.9910-12 4.19410-12 6.2610-12 3.72 10-12 Cs Sr2
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Sorption thermodynamics

• Sorption can be described using an empirical
  relationship that defines the distribution of
  radionuclides between solid and liquid
• Many isotherm models can describe sorption
  process such as Langmuir , Freundlch, and D-R.
• The parameters of the isotherm equations express
  the surface properties and affinity of the
  sorbent, at fixed temperature and pH.

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Sorption of Cs and Sr2 ions on zeolite NaA-X
at different temperatures (Langmuir)
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Sorption of Cs and Sr2 ions on zeolite NaA-X
at different temperatures (Freundlich)
The metal concentration retained in the solid
phase (mg/g) was calculated using the following
equation
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Sorption of Cs and Sr2 ions on zeolite NaA-X
at different temperatures (D-R)
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Isotherm models

• Langmuir Isotherm model

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Langmuir model parameters
The value of saturation capacity Q0 corresponds
to the monolayer capacity Q0 and b increased with
temperature showing that the sorption capacity
and intensity of sorption are enhanced at higher
temperatures.
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Isotherm models

• Freundlich isotherm model

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Freundlich model parameters
1/n value is dependent on the nature and
strength of sorption process. Kf represent
sorption capacity of both ions on zeolite NaA-X.
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Isotherm models

• D-R isotherm model

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D-R model parameters
qm The maximum sorption capacity , the values of
the mean free energy ,E, of sorption in all cases
is in the range of 8-16 k J/mol, which are within
the energy ranges of ion exchange reaction
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Effect of Temperature

• In order to gain insight into
• the thermodynamic nature of
• the sorption process, several thermodynamic
  parameters
• for the present systems were calculated.

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Thermodynamic Parameters
-The -ve values of ?Go confirm the spontaneous
nature of the sorption processes with preference
towards Sr2 than Cs ions. - The ve values of
?Ho for both studied ions confirms the
endothermic nature of the sorption processes. -
The entropy change was ve and was greater in
Sr2gtCs
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Column investigations

• Fixed bed column sorption experiments were
  carried out to study the sorption dynamics. The
  fixed bed column operation allows more efficient
  utilization of the sorptive capacity than batch
  process.
• The breakthrough curves measured are useful to
  determine the main transport parameters under
  dynamic conditions.

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Breakthrough curves for Cs and Sr2 ions sorbed
onto zeolite NaA-X
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Fixed Bed Data
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Estimation of dispersion coefficient
The dispersion coefficient may then be calculated
from the breakthrough curve using the following
equation
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Long term behavior of the proposed backfill
material (Zeolite NaA-X) in disposal facility.
PART 3

• Transport mechanisms and governing equations
• Diffusion
  (Ficks law)
• Advection-Dispersion
• Radioactive decay
• Sorption qe Kd
  Ce

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Modeling migration of radionuclides in the waste
disposal facility
System description

Development of conceptual
model
Selection of mathematical
models
Selection of numerical
technique
Carry out simulation
Performance
assessment steps
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Conceptual model
Simplified diagram
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Modeling migration through waste form

• 
  
• Where
• ? decay constant, s-1
• x spatial coordinate in x direction
• x spatial coordinate in y direction
• t time, s
• C contaminant concentration in the waste, Bq/ml
• D diffusivity of contaminant in the waste.
• Rd retardation coefficient in the waste
• where A area of the interface

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Numerical solution and computer simulation
C u
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Alternating Direction Implicit method (ADI)

• First step

Second step
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Equations in Matrix form


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Computer program flow chart for waste model
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Modeling migration through backfill
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Equations in Matrix form


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Computer program flow chart for backfill model
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Model validation
(Ogata, 1970)
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Results of the long term studies
Concentration profile of Cs in zeolite backfill
after 300 y
C,Bq/m3
,m
,m
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Concentration profile of Sr in zeolite backfill
after 300 y
C,Bq/m3
,m
,m
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Release rate of Cs and Sr radionuclide from the
proposed zeolite backfill
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Release rate for the Cs radionuclides from the
waste form ,the proposed and commonly applied
backfill
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Conclusions

• The results obtained in this work show the
  following
• The synthetic zeolite NaA-X proposed as backfill
  material was successfully prepared and completely
  characterized using XRD, XRF, and SEM
  techniques.
• The sorption studies indicated the feasibility of
  using the prepared zeolite NaA-X as backfill
  material compared to bentonite because of its
  high capacity and selectivity for the concerned
  radionuclides (Cs and Sr) these characteristics
  are fundamental to the performance of such
  zeolite in radioactive waste interactions.

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conclusions

• Column investigation yield a realistic picture
  of the sorptrion of Cs and Sr on zeolite NaA-X
  and lead to determination of dispersion
  coefficient which in turn used in migration
  modeling.
• Transport properties of zeolite NaA-X packed
  column have been determined. The classical
  advection-dispersion model described successfully
  Cs and Sr breakthrough curves under saturated
  flow conditions. Based on this experimental data
  the dispersion coefficient needed for long-term
  migration study was determined.
• The mathematical simulation performed in the
  long-term studies show the capability of the
  prepared zeolite NaA-X to prevent the migration
  of Cs and Sr from the repository to the
  environment.

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Thanks