الفهرس 
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Abstract
The safety of radioactive wastes in the repository depends on its capacity to confine radionuclides from being released to the biosphere. The most likely process that can lead to the release of radionuclides from a repository is the transport by groundwater. The release rate of radionuclides from the solidified blocks (usually cemented blocks) is controlled by the solubility limits and leaching rate of the radionuclide. The solubility and leaching processes and their effect on the safety assessment of a radioactive waste in the repository are investigated for different concentrations of three elements (Sr, Co and Fe). The cement blocks should have proper mechanical and chemical properties. Moreover, the effect of radiation emitted from the radionuclides of these elements on the blocks should be studied to assure the retention of radionuclides by the cement matrix under radiation fields. To carry out the study, cement blocks that confine the elements were irradiated by Gamma radiation at doses 40, 120 and 240 KGy. The morphology of the blocks was studied before and after irradiation using X-ray Diffraction Patterns (XRD), and Scanning Electron Microscopy (SEM). The results of non-irradiated and irradiated blocks are compared. The most important results from the study can be summarized as follows:
The chemical analysis of the most important elements that may be released from the cement blocks in both cases (non-irradiated and irradiated) illustrates that Ca release increases with radiation, but Al and Na release decreases under radiation.
The effect of gamma radiation on pH of the leachate from the cement blocks was studied under 40, 120, and 240 KGy, and without radiation. All the cement blocks resulted in a significant decrease in pH values due to the radiolysis-produced H2. The non irradiated cement blocks resulted in an increase in the pH, with a value that can reach 14.This may be due to the release of Ca(OH)2.
The Leaching rate calculation of the elements under radiation and without radiation in both cases of single and mixture of salts showed a decrease under radiation in all cases except for Co single salt.
The solubility values were calculated for elements under radiation and without irradiation in cases of single and mixture of salts. The solubility calculated values decrease under radiation for most of concentrations in single and mixture of salts, except for Co single salt.
The relation between the leaching rate and solubility calculated value is developed in the form of an equation represented by a linear line.
The presence ratio of Calcium Hydroxide(Ca(OH)2) and Calcite (CaCO3) are directly affected by irradiation. In the environment of a repository for radioactive waste, the carbonation reaction can be influenced by the presence of bicarbonate ions in the groundwater, or due to the presence of other factors (as presence of sulfate and chloride, temperature difference, pressure, etc.).
The results of the chemical analysis of leachate indicate that there is a decrease in the release of the key elements except Calcium under gamma radiation.
from the results of the SEM it is clear that the effect of radiation on the pure cement structure, crystals and pores change. This may be due to carbonation.
The XRD of pure cement illustrates that decrease in the number and size of anhydrated cement particles. Carbonation of cement blocks results from the reaction of C3S and C2Swith CO2.
In case of Sr and Fe additives, the results of gamma radiation show an increase in the intensity of some peaks and a decrease in others and the appearance of new peaks. In case of Co and mixture there is a decrease in the intensity and number of peaks.