Effects of plutonium dioxide encapsulation on the physico-chemical development of Portland cement blended grouts

The effects of alpha radiation on cementitious systems used for nuclear waste encapsulation, and the subsequent physico-chemical properties, have been subject to limited investigation comparative to the effects of gamma and neutron irradiations. This paper outlines an assessment of the impact of PuO2 incorporation on the bulk characteristics of BFS and PFA blended Portland cements, with specific focus on the microstructure, phase assemblage and the radiolysis of pore water. Cellulose was also added to the cements to investigate the effects of organics on these systems. Characterisation of the bulk phase assemblage and microstructure were completed using optical and scanning electron microscopy (SEM), x-ray diffraction (XRD), and thermogravimetric analysis (TGA). Gas evolution was measured to determine the radiolytic breakdown of pore solution. In all samples the PuO2 appeared well encapsulated, with good physical contact to the cement grout and no large scale defects observed. Pu-containing hydrates were not observed, but PuO2 containing BFS based systems showed variations in the ratio of sulfate-containing phases, with increased ettringite observed. Gas evolution results were consistent with expectations based on likely radiation deposition, and increased G(H2) values were observed for cellulose containing samples. The findings of this study suggest the investigated cements are suitable encapsulants matrices for wastes containing PuO2.