|Ph.D Student||Bar-Nes Gabriela|
|Subject||Immobilization of Low Level Waste Containing Strontium and|
Cesium in Cement Based Systems
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Amnon Katz|
Cement-based waste forms are used worldwide for disposal of radioactive and hazardous waste. The ability of cementitious systems to immobilize Cs and Sr ions was studied in this work. A series of cementitious pastes differing in their composition was prepared and the effect of these compositional changes on the waste ion immobilization was studied. The irradiation effect was studied using gamma irradiation coming from an external source (60Co).
The pozzolanic additives studied in this research were Blast Furnace Slag, Metakaolin and Silica Fume. The transport properties of the ions were studied from through-diffusion and leaching experiments, leading to the evaluation of the diffusion coefficients in the paste, Di and Da, respectively.
Mercury Intrusion Porosimetry (MIP) was performed to study the porous structure of the matrices, Scanning Electron Microscopy (SEM) together with Energy Dispersive Spectroscopy (EDS) gave morphological and local chemical information, Single Differential Thermal Analysis (SDTA) was used to measure the extent of the pozzolanic reaction and batch adsorption experiments were performed to obtain the distribution coefficient (Kd) of the ions in the pastes.
The immobilization of Sr in unblended Portland cement pastes was found to be quite efficient and it improved as water/cement ratio decreased or when cement was replaced with pozzolans. Sr ions exhibited a large decrease in leachability due to irradiation due to enhanced carbonation of the paste, leading to precipitation of Sr as the highly insoluble carbonate salt (SrCO3).
Cs ions are very difficult to immobilize in unblended pastes due to the high solubility of its salts. Introduction of pozzolanic additives and lowering the w/c ratio, resulted an improved, although not satisfactory, immobilization.
It was found however, that the addition of Densified Silica Fume (DSF) reduced the apparent diffusion coefficient of Cs ions by approximately two orders of magnitude. Scanning Electron Microscopy (SEM) together with Energy Dispersive Spectroscopy (EDS) results showed that Cs ions accumulate inside large Silica-Fume agglomerates (diameter of ~100 μm) found in the DSF paste. Our results showed that the formation of a hydrated rim around the agglomerated clusters of the DSF, created a barrier for the Cs ions chemically bounded to the silica-fume, thus leading to a significant improvement in the immobilization of this ion. When high concentrations of Na ions were added to 20% DSF blended pastes, we have found evidence of the occurrence of the destructive Alkali-Silica Reaction (ASR).