Thermochemistry of phosphate glasses for immobilization of dangerous waste

Abstract

Vitrification is currently considered to be an effective method for immobilization of radioactive waste. It is based on the enclosing of harmful elements in the structure of the glass. This work presents the results of studies on the thermal properties of glasses from P2O5–Al2O3–Na2O and P2O5–Al2O3–Fe2O3–Na2O systems for rendering nuclear waste in the form of salts such as sulfates, halides, and phosphates with high sodium content. These substances are not accepted by borosilicate glass, commonly used up to now for nuclear waste immobilization. Formation of sinters of glass-waste mixtures was selected as the method for immobilization, and the thermal chemistry of this process was studied. CaCl2 was used as the model chloride waste substance. The process of immobilization consists of its sintering with Na, Al, Fe-phosphate glasses containing more than 50 wt% P2O5 as the amorphous matrix. Thermal analysis showed that all glasses exhibit an ability for crystallization, with that the intensiveness of this process is determined by the chemical composition of these glasses. The addition of Fe2O3 to the glass intensified crystallization process. Leaching of components of sinters tests established that glass containing Fe2O3 in its composition most effectively binds waste in comparison to Al2O3 containing phosphate glass. The test results allow for the statement that the waste substance in the form of chloride salts such as CaCl2 is stable bound in the glass–crystalline sinters, which ensures its effective immobilization.