PNNL

Abstract

Predicting the long-term fate of low-level radioactive waste forms requires understanding how the radionuclides interact with the waste form. Concrete encasement is one method being considered for containment of low-level radioactive wastes. The necessary data to conduct an accurate performance assessment of such a waste form requires understanding the behavior and interactions of the radionuclides with the concrete matrix. The formation of uranium mineral phases has been investigated in simulated concrete pore fluids and Ordinary Portland Cement/Pulverized Fuel Ash (fly ash) concrete waste forms. X-Ray diffraction analyses of uranium precipitates from concrete pore fluids suggest diuranate salts, uranium-oxyhydroxides, and –silicates as solubility limiting phases. Scanning electron microscopy – energy dispersive spectroscopic analyses of uranium-spiked concrete suggests that under conditions both under-saturated and over-saturated with respect to the formation of uranium mineral phases, uranyl-oxyhydroxide phases precipitate within the initial two weeks. Subsequently, uranyl-silicate phases form after approximately one month and uranyl-phosphate phases provide a significant contribution to the long-term control over uranium in concrete waste forms after two months. This investigation demonstrates the importance of investigating the solubility of complex contaminants such as uranium in the complete matrix (i.e. concrete matrix versus pore fluids) and suggests the importance of secondary uranium mineral phases in the long-term retention within concrete waste forms.