Title: Evaluation of the Use of Synthetic Zeolite as a Backfill Material in Radioactive Waste Disposal Facility
1Evaluation 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
2AIM 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
3Radioactive 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.
4A 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
5Major 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
6Repository 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. -
7Multiple 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 -
8Function 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.
9Elements of engineered barriers
10Backfill 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.
11In 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.
12Synthesis 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.
13Results 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
14 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
15XRD 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)
16Silica-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
-
17Synthesis 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
18Flow sheet diagram for the synthesis of NaA-X
zeolite blend from fly ash using extraction method
19XRF 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
20XRD 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.
21Scanning 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
22Scanning 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
23(No Transcript)
24sorption 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.
25Sorption kinetics
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
26Kinetic models
(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. -
27(Ho and Mckay )
It was shown that the sorption process of each
ion follows pseudo second order model
28Pseudo 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.
29Estimation 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
30Sorption 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.
31Sorption of Cs and Sr2 ions on zeolite NaA-X
at different temperatures (Langmuir)
32Sorption 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
33Sorption of Cs and Sr2 ions on zeolite NaA-X
at different temperatures (D-R)
34Isotherm models
35Langmuir 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.
36Isotherm models
- Freundlich isotherm model
37Freundlich 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.
38Isotherm models
39D-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
40Effect of Temperature
- In order to gain insight into
- the thermodynamic nature of
- the sorption process, several thermodynamic
parameters - for the present systems were calculated.
41Thermodynamic 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
42Column 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.
43Breakthrough curves for Cs and Sr2 ions sorbed
onto zeolite NaA-X
44Fixed Bed Data
45Estimation of dispersion coefficient
The dispersion coefficient may then be calculated
from the breakthrough curve using the following
equation
46Long 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 -
47Modeling 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
48 Conceptual model
Simplified diagram
49Modeling 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
50Numerical solution and computer simulation
C u
51Alternating Direction Implicit method (ADI)
Second step
52Equations in Matrix form
53Computer program flow chart for waste model
54Modeling migration through backfill
55Equations in Matrix form
56Computer program flow chart for backfill model
57Model validation
(Ogata, 1970)
58Results of the long term studies
Concentration profile of Cs in zeolite backfill
after 300 y
C,Bq/m3
,m
,m
59Concentration profile of Sr in zeolite backfill
after 300 y
C,Bq/m3
,m
,m
60Release rate of Cs and Sr radionuclide from the
proposed zeolite backfill
61Release rate for the Cs radionuclides from the
waste form ,the proposed and commonly applied
backfill
62Conclusions
- 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.
63conclusions
- 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. -
64Thanks