Glass - Back to the Future!

Presenting Author:
Toshiaki Ohira

article posted 14 April 2016

Toshiaki Ohira has worked as a technician at Akita University, Japan, since 2006. In 2010, he received a Ph.D. in Engineering from the same university. He has been in Associate Professor Sugawara’s work group since 2012, and joined in the research project about vitrification of high-level waste (HLW) arising from reprocessing of spent nuclear fuel. His research interest is mainly the phase equilibrium relationship in simulated HLW glass.

MoO3 solubility in alkali-borosilicate melts : Implications to yellow phase separation in high-level waste glass

Toshiaki Ohira*1, Toru Sugawara Satoshi Komamine2, Eiji Ochi2
1 Center for Engineering Science, Akita University, 1-1 Tegata Gakuenmachi, Akita 010-8502, Japan
2 Research and Development Department, Reprocessing Business Division, Japan Nuclear Fuel Limited, 4-108 Okitsuke, Obuchi, Rokkasho-mura, Aomori 039-3212, Japan

Molybdenum-rich crystalline phase (Yellow phase) is generally known to be separated in the high-level waste (HLW) glass because of the low solubility of molybdate compound in alkali-borosilicate melt. This phase separation results in decreasing the chemical durability of HLW glass. We performed phase equilibrium experiments in the system SiO2-B2O3-Al2O3-CaO-Na2O with excess MoO3 amount of 5 or 10 mol% at 1273K and 1473K. The run products were investigated by electron microprobe analysis. All experimental samples were separated into SiO2-rich opalescent glass and MoO3-rich crystalline precipitate. Small particles of Na2MoO4 and CaMoO4 (~1µm) were observed in the former glass and opalescence becomes more pronounced with increasing MoO3 content in the glass. It is suggested that the separation of MoO3-rich precipitate is related to the occurrence of the yellow phase in HLW glass, while the small molybdate crystals in the SiO2-rich glass may be associated with recrystallization of molybdate compound resulting from slow cooling during canister filling.
Figure 1 shows the relationship between MoO3 solubility and B2O3 content in borosilicate melts. The MoO3 solubility increased with increasing temperature and by replacement of SiO2 by B2O3 or CaO by Na2O, whereas decreased by replacement of B2O3 with SiO2 or Al2O3 in SiO2-B2O3-Al2O3-CaO-Na2O system. These results suggest that the occurrence of yellow phase can be suppressed by lowering the degree of polymerization in the network structure and the Na2O activity in HLW glass.