Presented by Dr Ahmed Mohamed El-Kamash Hot Lab. & Waste Management Center AEAE, Egypt

1. 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

2. 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

3. 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.

4. 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

5. 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

6. 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.

7. 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

8. 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.

9. Elements of engineered barriers

10. 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 backfillmaterials 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.

11. 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.

12. 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.

13. 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

14. Synthesis and characterization of pure zeolites PART 1: Physical properties of fly ash XRF technique • Intermediate glass content of about 66.99%

15. I 0.0 10 20 30 40 60 70 50 2θ angle XRD technique for Fly Ash :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)

16. 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

17. 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

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

19. XRF technique for synthesized Zeolite NaA-X 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

20. 20 10 0.0 40 30 2θ angle XRD technique For Synthesized Zeolite NaA-X I :zeolite X and : zeolite A The spectrum exhibits fingerprint lines of both zeolite X at 2θ = 6.10 and zeolite A at 2θ = 7.20 and 9.93.

21. (a) (b) (c) (d) 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 toα-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

22. 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

23. Examination of Proposed backfill material: Synthetic zeolite Na A-X as backfill material in radioactive disposal facility Mechanical stability Efficiency of the material (Capacity) Test Experimental Investigations Column Studies Kinetics Studies Equilibrium Studies Estimation of Sorption mechanism (Pseudo first-secondorder) Thermodynamic Models (Capacity) Dispersion coefficient, DL Distr. Coeff.,Kd Chemisorption Diffusion, Di Effect of temperature, Thermodynamic Parameters, ∆H, ∆G, ∆S Long term behavior of zeolite NaA-X as backfill material in disposal facility

24. 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.

25. 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+ > Cs+

26. Pseudo first order 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. Kinetic models (Lagergren)

27. Pseudo second order (Ho and Mckay ) It was shown that the sorption process of each ion follows pseudo second order model

28. 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.

29. Estimation of diffusion coefficient • In order to identify the step governing the removal rate of sorption process (Boyed et al)

30. 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.

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

32. 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 :

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

34. Isotherm models • Langmuir Isotherm model

35. Langmuir model parameters The value of saturation capacity Q0 corresponds to the monolayer capacity Q0and b increased with temperature showing that the sorption capacity and intensity of sorption are enhanced at higher temperatures.

36. Isotherm models • Freundlich isotherm model

37. 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.

38. Isotherm models • D-R isotherm model

39. 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

40. 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.

41. Thermodynamic Parameters -The -ve values ofΔGo 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+

42. 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.

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

44. Fixed Bed Data

45. Estimation of dispersion coefficient The dispersion coefficient may then be calculated from the breakthrough curve using the following equation

46. Transport mechanisms and governing equations Diffusion (Fick`s law) Advection-Dispersion Radioactive decay Sorption qe = Kd Ce Long term behavior of the proposed backfill material (Zeolite NaA-X) in disposal facility. PART 3:

47. 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

48. Host rock Groundwater table Cover Waste packages Backfill Concrete vault Conceptual model Simplified diagram

49. 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

50. Numerical solution and computer simulation C = u