الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 144 |  |  |
| | | from | 144 | | |
Abstract
Natural bentonite clay can be used as shelters from nuclear waste because of its large availability and low cost. Bentonite clay was used in two shapes natural as it is from its ores and in ground and pressed phase. Natural bentonite was cut into cylindrical pellets at different thicknesses, also the other form pressed into cylindrical pellets with different thicknesses and different pressing pressures (50,100 and 150 bar). The different samples are coated with polyvinyl alcohol polymer to prevent nuclear waste leakage through porosity of clay. Chemical analysis and density are measured for all samples. Bentonite clay was found naturally in nanometer scale because it is formed from volcanic ash deposits. The nano particle size was determined by dynamic light scattering (DLS) and Williamson-Hall size analysis using XRD patterns and the help of x-powder program. The particle size of natural bentonite was found to be 59.79 nm. The microstructure was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The linear and mass attenuation coefficients of nano-structured bentonite clay (natural and pressed) were determined at 662 keV energy of 137Cs; at 1173 and 1332 keV energies of 60Co gamma-ray sources by using NaI (Tl) scintillation detector. The experimental results showed that the ground bentonite pressed at 150 bar gave the highest linear and mass attenuation coefficients than other samples. The experimental data and calculated mass attenuation coefficient (by XCOM program) were found to be in a good agreement. Shielding characteristics parameters (linear attenuation coefficient, mass attenuation coefficient, half value layer, Tenth value layer and mean free path) are calculated for natural bentonite nano-particles coated poly vinyl alcohol polymer using MCNPX code. Data are determined at 662, 1173 and 1332 KeV emitted from Cs-137 and Co-60 sources and compared with the experimental data. There is an agreement between theoretical and experimental
ii
mass attenuation coefficients while a little difference between simulated and both experimental and theoretical data were recorded. The deviation between experimental and theoretical mass attenuation coefficient is equal to 4.3 %, 2.5 % and 1.25 % at 662, 1173 and 1332 (KeV) while deviation between experimental and simulated mass attenuation coefficient is equal to -3.1 %, -10.96 % and -10.35 % at the same energies. The deviation between experimental shielding factors (HVL, TVL and Mean free path) and simulated data are equal to 2.5 %, 13.4 % and 13.8 % at 662, 1173 and 1332 (KeV). The data obtained from MCNPX program is a reliable data for bentonite/PVA sample within ± 14 % in comparison with experimental data and ± 5% in comparison with theoretical data. The difference percentage in comparison with different techniques is due to different database and different utilized extended nuclear cross section libraries.
Calcination process is a method used for gamma rays mass attenuation coefficient enhancement of natural bentonite clay nanoparticles as it eliminates water and organic matter from bentonite clay structure which has low mass attenuation coefficient values, thus declared two opposite effects on mass attenuation coefficient values: first, oxides content rise which enhance mass attenuation coefficient values; second, particle size of calcinated bentonite increase which decreases mass attenuation coefficient values. In order to enhance mass attenuation coefficient value than natural bentonite, a physical ball milling must introduced after Calcination process that decreases particle size. Calcination process is done at 700 ̊C for two hours because dehydration is completed above 500 ˚C while dehydroxylation observed at 700 ˚C. Mass attenuation coefficient measured for calcinated and ball milled bentonite clay at different energies (662, 1173 and 1332 kev) and different pressing pressures (50, 100 and 150 bar). Narrow beam transmission technique and two different sources (Cs-137 and Co-60) are
ii
used for mass attenuation coefficient measurements, also particle size are measured by two different methods dynamic light scattering and Williamson-Hall size analyses using XRD patterns. The all samples are coated by polyvinyl alcohol polymer.
The particle size of calcinated bentonite decreased from 406.27 nm to 26.05 nm. Due to that decrease in particle size, mass attenuation increased. Ball milled calcinated bentonite clay has the highest mass attenuation coefficient (0.0836cm2/gat 662 kev) than natural and ground bentonite. By comparing the value of mass attenuation coefficient of ball milled calcinated bentonite pressed at 150 bar and lead, it represents 75.4 % from lead value at photon energy 662 keV. The value increases from 69 % for natural bentonite to 75.4 % for ball milled calcinated bentonite. The result of this raising proves the efficiency of calcination process in shielding properties enhancement but it must accompanied with ball mailing process for particle size reduction.
Finally natural bentonite was ground and pressed into cylindrical pellets at 100 bar and at different thicknesses. The pressed pellets were heated to 1000 °C and 1200 °C to produce bentonite ceramic samples. The ceramic samples had investigated against gamma rays to calculated shielding parameters. The microstructure has been investigated using Scaning electron microscope. It showed a large increase in the grain size of ceramic sample structure thus decreases attenuation coefficient. Mass attenuation coefficients of ceramic samples heated to 1200 °C is higher than that heated to 1000 C as it has higher density. The highest value of mass attenuation coefficient for all ceramic samples is equal to 0.074 cm2/g at 662 KeV, this value is less than mass attenuation for natural bentonite (0.076 cm2/g) so ceramic process is not effective for gamma rays shielding parameters enhancement.