Nanoscopic structure of borosilicate glass with additives for nuclear waste vitrification

Motokawa, Ryuhei   ; Kaneko, Koji   ; Oba, Yojiro  ; Nagai, Takayuki  ; Okamoto, Yoshihiro  ; Kobayashi, Taishi*; Kumada, Takayuki   ; Heller, W. T.*

We investigated the nanoscopic structure of borosilicate glasses as a host for high-level radioactive liquid waste (HLLW) in the presence of the additives NaO and CaO/ZnO with and without LiO. These additives have been practically 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 formed a microphase-separated structure of SiO- and BO-rich domains approximately 100 nm in size in the absence of NaO, even in a structure homogeneous on a millimeter scale, and the domain spacing decreased with increasing NaO content rate. The CaO/ZnO additives induced inhomogeneities, such as void structures several nanometers in size, although the inhomogeneity was suppressed by the coaddition of LiO. 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.