Nanoscopic structure of borosilicate glass with additives for nuclear waste vitrification

We investigated the nanoscopic structure of borosilicate glasses as a host for high-level radioactive liquid waste (HLLW) in the presence of the additives Na ₂O and CaO/ZnO with and without Li ₂ O. These additives have been used to lower the glass melting point, suppress the macroscopic phase separation, and increase the chemical durability of glasses. Small-angle neutron scattering was used to elucidate the effect of the additives on the nanoscopic structure along with macro- and atomic-scale observations obtained using neutron imaging and X-ray diffraction, respectively. The borosilicate glass made without Na ₂O formed a microphase-separated structure of SiO ₂- and B ₂O ₃-rich domains approximately 100 nm in size, and the material was homogeneous at length scales up to millimeters. With the addition of increasing amounts of Na ₂ O, the domain spacing decreased. Introducing CaO/ZnO additives induced inhomogeneities in the glass, such as void structures several nanometers in size, although the inhomogeneity was suppressed by the coaddition of Li ₂O. These results provide insights into the performance of host glasses for HLLW vitrification. Incorporating HLLW into glasses is likely to cause changes in the nanoscopic structure of host glasses similar to those revealed here.