Observation of nepheline crystallization in the Na2O-B2O3-Al2O3-SiO2 system for the vitrification of zeolites absorbing Cs

Abstract

ABSTRACT For the vitrification of zeolites used for absorbing radioactive cesium, the composition of the glass in which nepheline forms at the cooling rates of 0.5, 1, 2.5°C min−1 were clarified for the Na2O-B2O3-Al2O3-SiO2 quaternary system to avoid crystallization during cooling of the melt. The trend of the nepheline formation composition was similar to that suggested in previous studies, i.e. the nepheline discriminator (N.D.) < 0.62 and the optical basicity (O.B.) >0.575, especially at the higher B2O3 content, while the criteria was eased at a lower B2O3 content at the cooling rate of 2.5℃ min−1. The increase in the B2O3 content of the glasses seemed to reduce the effect of the cooling rate on the crystallization. The results obtained by adding 1 wt% Cs2O in the quaternary system shows that the effect of Cs2O would be negligible as long as its content in the formed glass would not exceed 1 wt%. In contrast to the quaternary system, the Cs2O-Na2O-Al2O3-SiO2 system showed the different formation of the crystals. The fine granulous crystals were dispersed in the glass matrix and not in the region contacting the crucible as observed in the Na2O-B2O3-Al2O3-SiO2 system. Moreover, the number of non-crystal samples was greater than that predicted by the criteria, N.D. <0.62 and O.B. >0.575.