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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 Na$$_{2}$$O and CaO/ZnO with and without Li$$_{2}$$O. 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$$_{2}$$- and B$$_{2}$$O$$_{3}$$-rich domains approximately 100 nm in size in the absence of Na$$_{2}$$O, even in a structure homogeneous on a millimeter scale, and the domain spacing decreased with increasing Na$$_{2}$$O 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 Li$$_{2}$$O. 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.

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