|Ph.D Student||Ofer Rozovsky Elina|
|Subject||Solidification in Geopolymers of Low-Level Nuclear|
Waste Containing Cesium
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Amnon Katz|
|Dr. Michal Arbal Haddad|
|Full Thesis text - in Hebrew|
In recent years, there is a world-wide interest in finding a solution for the huge volumes of accumulating nuclear wastes. Geopolymers are promising candidates to immobilize low-level wastes containing cesium.
The aim of the present work was to determine the sequence of phase formation at moderate temperatures (35-42°C) in low-silica geopolymers in presence of sodium nitrate and to correlate between the crystalline phases present and the immobilization efficiency of cesium.
In order to achieve this goal, geopolymerization was examined using metakaolin (MK) and fly-ash (FA) from different sources. Impact of nitrate and cesium on the reactions and the resulting geopolymers, was comprehensively studied under various alkaline environments. The products were characterized by XRD, FTIR, SEM-EDS, MIP, absorbency in water and compressive strength measurements. Leaching tests were performed according to the American Nuclear Society procedure ANS-16.1. Chemical analysis of the leaching solutions and pore water was performed by ICP-OES, Ion Chromatography and by Automated Ion Analyzer. Quantitative analysis of the phases in several geopolymeric matrices was conducted on the XRD patterns using Rietveld method.
The results presented in this thesis indicate that several parameters, acting simultaneously in geopolymeric systems, control the dissolution, condensation into amorphous phase and reorganization towards crystalline phases. The most important parameters identified are the alkalinity of the activation solution, transient SiO2:Al2O3 ratio, Na2O:Al2O3 ratio, the nature of the activating cation and the presence of nitrate ions. Moreover, geopolymers produced from MK and FA had different reactivity rates and consequently different crystalline products. In systems without nitrate zeolites A, X, P, chabazite and pollucite were formed depending on the different formulations. A Cs-bearing analogue of zeolite K-F, (Cs,Na)AlSiO4*nH2O, was formed from MK and FA in presence of Cs. In geopolymers generated in presence of sufficient alkalinity and concentration of nitrate, two nitrate-bearing feldspathoids, nitrate sodalite and nitrate cancrinite, were formed. The sequence of phase evolution in these geopolymeric systems was elucidated.
The cumulative leaching curves were interpreted as resulting from two concurrent diffusion processes. Leaching of non-bound ions from the pore solution within the amorphous fraction of the geopolymer dominates the initial 24 hours, while leaching of bound Cs and Na ions dominates the leaching at later ages, after depletion of the non-bound ions. The apparent diffusion constants, Da, for the later leaching process were found to depend upon the nature of the crystalline phases formed. The lowest leaching rates in systems without nitrate were measured in MK-based matrices with 7%Cs and were related to the formation of zeolite F, (Cs,Na)AlSiO4*nH2O. Zeolites A and X exhibited poor immobilization properties for Cs, whereas Na has preferred binding to zeolite A compared to zeolite F. In systems with nitrate, both nitrate sodalite and nitrate cancrinite were shown to have high retention of Cs. The leaching of Cs from FA-based systems was higher due to the lower content of crystalline phase. Nevertheless, both in FA-based and MK-based geopolymers leaching indices higher than 6 were obtained for Cs, suggesting that low-Si geopolymers may serve as waste forms for Cs-bearing radioactive waste streams.