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

Presented by

Dr Ahmed Mohamed El-KamashHot Lab. & Waste Management CenterAEAE, Egypt

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

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.

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

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 earth’s surface.

– Deep Geological Disposal for high level waste such as spent nuclear fuel, >400 meters underground

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.

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

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.

Elements of engineered barriers

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.

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.

In respect to fly ash

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.

Synthesis and characterization of pure zeolites

Sorption studies

Long term behavior of zeolite NaA-X blend as proposed backfill

OxideSiO2 Al2O3 Na2O MgO P2O5 SO3 Cl K2O CaO TiO2 Fe2O3

Wt %43.81 23.18 0.87 0.80 0.49 15.68 4.01 2.72 6.10 2.31 0.01

Intermediate glass content of about 66.99%

Synthesis and characterization of pure zeolites

:mullite (3Al2O3.2SiO2) and : α-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)

0.0 10 20 30 40 50 60 70

2θ angle

I

- 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•

Silica-Aumina extraction by fusion

The synthesis of NaA-X zeolite blend was carried out using the molar oxide ratios of:

SiO2/Al2O3 = 2.1Na2O/SiO2 = 1.4 H2O/Na2O = 39.0

Sodium aluminate solution was used externally to adjust the SiO2/Al2O3 ratio to the desired value

Synthesis of pure NaA-X zeolite

Flow sheet diagram for the synthesis of NaA-X zeolite blend from fly ash using extraction method

Element Na Al Si Ca Ti Mg Fe S K P other elements

Wt.%27.79 33.41 38.34 0.067 0.081 0.062 <0.01 0.002 0.056 0.004 <0.1

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

2θ angle0.0 10 30 4020

The spectrum exhibits fingerprint lines of both zeolite X at 2θ = 6.10 and zeolite A at 2θ = 7.20 and 9.93.

I

:zeolite X and : zeolite A

(a) (b)

(c) (d)

)a( Untreated FA Smooth and spherical particle interspersed in aggregates of crystalline compounds which may correspond to α-quartz and mullite.

)b( 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

)c( After 30 min )The surface of FA became rough and burst - Larger cracks were appeared librating small aggregates

)d( After 60 min ) Small cenosphere were appeared -Several crystalline materials were precipitated onto the surface of FA particle

SEM

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

Examination of Proposed backfill material: Synthetic zeolite Na A-X as backfill material in radioactive disposal facility

Efficiency of the material(Capacity )

Kinetics Studies Equilibrium Studies

Estimation of Sorption mechanism

(Pseudo first-second order)

Effect of temperature,Thermodynamic Parameters, ∆H, ∆G, ∆S

Thermodynamic Models ( Capacity )

Long term behavior of zeolite NaA-X as backfill material in disposal facility

Mechanical stability

Experimental Investigations

Column Studies

Distr. Coeff.,KdChemisorption

Diffusion, Di

Test

Dispersion coefficient, DL

• 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.

sorption studies

Sorption kinetics

• Effect of time

0 20 40 60 80 100 1200

20

40

60

80

qt,m

g/g

Time,min

Cs+

Sr2+

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+ > Cs+

Kinetic models

• Pseudo first order

0 10 20 30 40

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(a)

Y2 =1.54671-0.02787 X

Y1 =1.5085-0.02336 X

log

(qe-

q t)

Time,min

Cs+

Sr2+

• 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.

t.k

eqtqeq 30321log)log(

)Lagergren(

0 20 40 60 80 100 1200.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

(b)

Y2 =0.12195+0.02078 X

Y1 =0.2174+0.02388 X

t/qt,m

in g

mg-1

Time,min

Cs+

Sr2+

• Pseudo second order

tqhq

t

et

11

It was shown that the sorption process of each ion follows pseudo second order model

)Ho and Mckay (

Metal ions

 

First order Rate constant,k1(min-1)

Second order Rate constant,k2(min-1)

Cs+

Sr2+

0.0537  

0.0640

0.0031

0.0039

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.

Estimation of diffusion coefficient

• In order to identify the step governing the removal rate of sorption process

0 10 20 30 400.0

0.5

1.0

1.5

2.0

2.5

2- Y =-0.00771+0.0587 X1- Y =-0.13186+0.05815 X

Bt

Time,min

Cs+

Sr2+

)2exp(1 2

12

61)( Btn

n ntF

2

2

or

iD

B

Metal ions Diffusion effective diffusion coefficient Di coefficient De

Cs+ Sr2+

6.99*10-12 4.194*10-12

6.26*10-12 3.72 *10-12

)Boyed et al(

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.

0 1000 2000 3000 4000 5000 6000 7000

200

400

600

800

1000

1200

1400

1600

Cs+

q e(m

mol

/kg)

Ce(mmol/m3)

298 K 313 K 333 K

0 2000 4000 6000 80000

500

1000

1500

2000

2500

3000

3500 Sr2+

q e(m

mol

/kg)

Ce(mmol/m3)

298 K 313 K 333 K

Sorption of Cs+ and Sr2+ ions on zeolite NaA-X at different temperatures )Langmuir(

Sorption of Cs+ and Sr2+ ions on zeolite NaA-X at different temperatures )Freundlich(

0 1000 2000 3000 4000 5000 6000 70000

200

400

600

800

1000

1200

1400

1600

Cs+

q e(m

mol

/kg)

Ce(mmol/m3)

298 K 313 K 333 K

0 2000 4000 6000 8000

1000

1500

2000

2500

3000

3500

4000

Sr2+

q e(m

mol

/kg)

Ce(mmol/m3)

298 K 313 K 333 K

The metal concentration retained in the solid phase )mg/g( was calculated using the following equation :

M

Vccq e

e

)( 0

Sorption of Cs+ and Sr2+ ions on zeolite NaA-X at different temperatures )D-R(

0 1000 2000 3000 4000 5000 6000 70000

200

400

600

800

1000

1200

1400

1600

Cs+

q e(m

mol

/kg)

Ce (mmol/m3)

298 K 313 K 333 K

0 2000 4000 6000 8000

1000

1500

2000

2500

3000

3500

4000

Sr2+

q e(m

mol

/kg)

Ce(mmol/m3)

298 K 313 K 333 K

Isotherm models

• Langmuir Isotherm model

0 1000 2000 3000 4000 5000 6000 7000

0

1

2

3

4

5

Langmuir cs-zeolite

3- Y =0.41304+6.26764E-4 X2- Y =0.52506+6.42354E-4 X1- Y =0.68176+6.46796E-4 X

Ce/

q e(m

3 /kg)

Ce(mmol/m3)

298 K 313 K 333 K

-2000 0 2000 4000 6000 8000 10000-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Langmuir Sr-zeolite

Y3 =0.07485+2.83973E-4 XY2 =0.10257+2.86104E-4 XY1 =0.13334+2.889E-4 X

Ce/

q e,

kg/

m3

Ce, mmol/m3

298 K 313 K 333 K

eCe

qe

C oo QbQ )/1)/1 (()/(

Langmuir model parameters

Table: Langmuir isotherm parameters for Cs+ and Sr2+ sorbed onto Zeolite NaA-X

Q0)mmol/kg( b)L/mmol( R2 RL Temperature

)K( Metal ion

1546.0 0.948 0.995 0.123 1556.7 1.223 0.995 0.098 1595.6 1.517 0.996 0.087 3461.4 2.166 0.997 0.042 3495.2 2.789 0.998 0.031 3521.5 3.793 0.997 0.024

298 313 333

298 313 333

Cs+

Sr2+

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.

Isotherm models

• Freundlich isotherm model

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

2.7

2.8

2.9

3.0

3.1

3.2

Freundlich Cs-zeolite

3- Y =2.26438+0.23861 X

2- Y =2.0962+0.27791 X1- Y =1.91241+0.32219 X

log

q e(q e,

mm

ol/k

g)

log Ce(C

e,mmol/m3)

298 K 313 K 333 K

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

3.0

3.1

3.2

3.3

3.4

3.5

3.6

Sr-zeolite

Y3 =2.7094+0.22861 XY2 =2.54197+0.27235 X

Y1 =2.41705+0.30261 X

log

q e(q e,

mm

ol/k

g)

log Ce(C

e,mmol/m3)

298 K 313 K 333 K

efe CnKq log)/1(loglog

Freundlich model parameters

Table: Freundlich isotherm parameters for Cs+ and Sr2+ sorbed onto Zeolite NaA-X

1/n Kf )mmol/kg( R2

Temperature )K( Metal ion

0.3222 81.730 0.985 0.2779 124.79 0.985 0.2386 183.81 0.982

0.2681 342.45 0.964 0.2428 416.19 0.965 0.2082 573.90 0.974

298 313 333

298 313 333

Cs+

Sr2+

1/n :value is dependent on the nature and strength of sorption process.

Kf represent sorption capacity of both ions on zeolite NaA-X.

Isotherm models

• D-R isotherm model

0 200 400 600 800 1000-8.0

-7.8

-7.6

-7.4

-7.2

-7.0

-6.8

-6.6

-6.4

D-R isotherm plots for sorption of Cs+ions onto Zeolite NaA-X at different temperatures

3- Y =-5.97132-0.00275 X

2- Y =-6.08442-0.00293 X

1- Y =-6.01334-0.00389 X

ln q

e(q e,

(mo

l/g))

2(,kJ/mol)

298 K 313 K 333 K

200 300 400 500 600 700 800 900-7.0

-6.8

-6.6

-6.4

-6.2

-6.0

-5.8

-5.6

Sr2+

Y3 =-5.27976-0.00187 X

Y2 =-5.19247-0.00265 XY1 =-5.14828-0.00334 X

ln q

e(q e,

mo

l/g)

2(kJ2/mol2)

298 K 313 K 333 K

2lnln m

qqe

D-R model parameters

Table )6( D-R isotherm parameters for Cs+ and Sr2+ sorbed onto Zeolite NaA-X

qm)mmol/kg( R2 E)kJ/mol(

Temperature )K(

Metal ion

-0.00389 2445.9 0.966 11.337 -0.00293 2278.0 0.963 13.063 -0.00275 2550.9 0.958 13.480

-0.0032 5508 0.986 12.50 -0.0025 5318 0.988 14.00 -0.0018 4934 0.988 16.62

298 313 333

298 313 333

Cs+

Sr2+

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

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.

co KRTG ln

0.0030 0.0031 0.0032 0.0033 0.0034

0.5

1.0

1.5

2.0

2.5

Y =7.51968-1640.95029 X

Y =5.17944-1426.91259 X

ln K

c

1/T K-1

Cs+

Sr2+

RT

H

R

S Kln c

Thermodynamic Parameters

Table )7(: Values of thermodynamic parameters for sorption of Cs+ and Sr2+ions

onto Zeolite NaA-X

Metal

ion

Temperature

K

Kc ΔGo

)kJ/mol(

ΔHo

)kJ/mol(

ΔSo

)J/mol(.K

Cs+

Sr2+

298

313

333

298

313

333

1.466

1.904

2.420

7.49

9.74

13.36

-0.947

-1.670

-2.446

-4.988

-5.921

-7.176

11.86

13.64

43.058

62.35

-The -ve values of ΔGo confirm the spontaneous nature of the sorption processes with preference towards Sr2+ than Cs+ ions.- The +ve values of ΔHo for both studied ions confirms the endothermic nature of the sorption processes.- The entropy change was +ve and was greater in Sr2+>Cs+

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.

0 200 400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

1.2

Sr2+ Flow rate=3 mL/minBed depth=3 cm

150 mg/L 100 mg/L 50 mg/L

Bre

akth

rou

gh

, Ct/C

o

Effluent Volume, mL

0 200 400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

1.2

Cs+ Flow rate=3 mL/minBed depth=3 cm

150 mg/L 100 mg/L 50 mg/L

Bre

akth

rou

gh

, Ct/C

o

Effluent Volume, mL

Breakthrough curves for Cs+ and Sr2+ ions sorbed onto zeolite NaA-X

Fixed Bed Data

Table) 8(: fixed bed data of Cs+ and Sr2+ions onto Zeolite NaA-X at

different metal ions feed concentrations

metal ions

C0)mg/L(

X)mg(

qtot)mg( Column performance%

Bed capacity)mg/g(

Cs+

50 100 150

39.0 47 58.5

23.35 28.5 30.5

59 55 52

23.35 28.5 30.5

Sr2+

50 100 150

57.5 75 78.75

37.5 48.75 45

65 60 57

37.5 48.75 53.5

Estimation of dispersion coefficient

21

0 )/(2

1

2

1

LvUD

Uerfc

c

c

fL

dx

dCv

dx

CdD

dt

dCfL

2

2

tD

tvLerf

D

Lv

tD

tvLerf

C

C

L

f

L

f

L

f

2exp

22

1

0

The dispersion coefficient may then be calculated from the breakthrough curve using the following equation

Transport mechanisms and governing equations

• Diffusion (Fick`s law)

• Advection-Dispersion

• Radioactive decay • Sorption qe = Kd Ce

dx

dCDF

z

CnDnvCf

Ct

C

Long term behavior of the proposed backfill material )Zeolite NaA-X( in

disposal facility.

Modeling migration of radionuclides in the waste disposal facility

System description

Development of conceptual model

Selection of numerical technique

Carry out simulation

Selection of mathematical models

Performance assessment steps

Conceptual model

Host rock

Groundwater table

Cover

Waste packages

Concrete vault

Backfill

Simplified diagram

Modeling migration through waste form

Where: decay constant, s-1 x : spatial coordinate in x direction x: spatial coordinate in y directiont: time, sC: contaminant concentration in the waste, Bq/mlD: diffusivity of contaminant in the waste.Rd: retardation coefficient in the waste

where A: area of the interface

Cy

C

x

C

Rd

D

t

C

2

2

2

2

dAx

CDRate w

A

Numerical solution and computer simulation

Cy

C

x

C

Rd

D

t

C

2

2

2

2

nji

Cy

nji

Cy

nji

Cy

x

nji

Cx

nji

Cx

Rd

D

t

nji

Cnji

C

,2)(2

,21

,2

2)(2

,21

,2

,1

,

yx 2Rd

tDR

C = u

nj,i

2y

1nj,i

2y

nj,i

2x

1nj,i

2x

nj,i

1nj,i uuuuR

2

1uu

nj,1i

nj,i

nj,1i

nj,i

2x uu2uu

n1j,i

nj,i

n1j,i

nj,i

2y uu2uu

Alternating Direction Implicit method (ADI)

First step

n1j,i

nj,i

n1j,i

nj,i

2/1nj,1i

2/1nj,1i

2/1nj,i vv2vv

R

1uuu2

R

2/1n1j,i

2/1nj,i

2/1nj,i

1nj,1i

1nj,1i

1nj,i vv2v

R

1uuu2

R

Second step

Equations in Matrix form

2100

0021

0012

R

R

R

nu

u

u

u

u

4

3

2

1

1

21

)2()(

)2()1(

nn vvR

nBCND

vvR

BCND

=

2100

0021

0012

R

R

R

nv

v

v

v

v

4

3

2

1

=

1

21

)2()(

)2()1(

nn uuR

nBCND

uuR

BCND

Computer program flow chart for waste model

Start

Input Data

Calculate R

Setup the coeffs. Matrices

Perform L.U. decomposition on v and u Coeffs.

Get values in the B.C .

Vector for u

Get values in the B. C .

Vector for v

A

Time = 0.0

Get values in the u vector top & bottom

Get values in the u vector other rows

M<3

M>3

Time>Tmax No

Yes

C

Get values for the second row

Back substitution for the u vector

Get values in v vector top &bottom

M<3

M>3

Get values in the second row

Get values in the v vector for other rows

C

Time=time *t

Output the concentration profile

Calculate the concentration profile

Calculate the rate

Output the rate

End

No

Yes

Yes

No

No

Stop

Back substitution for the v vector

BA

B

Yes

No

Yes

Modeling migration through backfill

nji

nji

nji

yynji

nji

nji

xxji

nnji CCC

DCCC

D

t

CC1,,1,2

2/1,1

2/1,

2/1,12

,2/1

, 222

2/1nj,i

n1j,i

n1j,iy

2/1nj,1i

2/1nj,1ix CCCvCCv

x

vxx

DB

2

2

22 xxD

tA

yv

yyD

C2

2

22

yyD

tD

yv

yyD

E2

y

vyy

DF

2

Cy

Cv

x

Cv

y

CD

x

CD

t

Cyx2

2

yy2

2

xx

Equations in Matrix form

AC

B

BAC

BAC

BA

00000

000

00

000

nu

u

u

u

u

4

3

2

1

=

11

243

132

021

nnn FvEvvD

FvEvvD

FvEvvD

FvEvvD

AC

B

BAC

BAC

BA

00000

000

00

000

nv

v

v

v

v

4

3

2

1

=

11

243

132

021

nnn FuEuuD

FuEuuD

FuEuuD

FuEuuD

Computer program flow chart for backfill model

Initialization For the B.C. and initial condition

Set up the coefficient matrices for both waste and backfill by overwriting on the diagonal and certain off diagnonal elements

Get values into B.C. vector for u matrix in the waste and backfill

Get values into B.C. vector for v matrix in the waste and backfill

Perform L.U. decomposition on the diagonal terms for the waste and backfill

A

A

Forward substitution for the waste equations

Backward substitution for the waste equations

Differentiate to find gradient

Integrate and multiply by Dw*Z w to find the release rate from the waste

Forward substitution for the backfill equations

Backward substitution for the backfill to find the concentration profile due to instantaneous unit release

B

Perform convolution integral

Differentiate to find concentration gradient

Integrate and multiply by Db*Zb to find the release rate from the backfill

If Time> Tmax

Output

END

B

Model validation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.1 0.15 0.2 0.25 0.3 0.35 0.4

Distance, m

C/C

o,

Numeriacl

Analytical

tRD

VtxRerfc

D

Vx

tRD

VtxRerfc

CtxC

LLL 2

.exp

2

.

2),( 0

(Ogata, 1970)

Results of the long term studies

0.05 0.1

0.15 0.2

0.25 0.3

0.35

0.05

0.2

0.4

0

100

200

300

400

500

600

C

X(Cs)

Y

C,Bq/m3

,m

,m

Concentration profile of Cs in zeolite backfill after 300 y

0.05 0.1 0.15 0.2 0.25 0.30.05

0.15

0.3

0.4

0

10

20

30

40

50

60

70

80

C

X(Sr)

Y

C,Bq/m3

,m

,m

Concentration profile of Sr in zeolite backfill after 300 y

Release rate of Cs and Sr radionuclide from the proposed zeolite backfill

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

0 50 100 150 200 250 300

Time )y(

Rel

ease

Rate

)G

Bq

/Y(

Sr

Cs

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

0 50 100 150 200 250 300

Time )y(

Rele

ase

Ra

te

) GB

q

/ Y (Waste Form

Zeolite

Bentonite&crushed rock

Release rate for the Cs radionuclides from the waste form ,the proposed and commonly applied backfill

ConclusionsThe 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.

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.