Crystallization of molybdenum oxide phase from simulated high-level waste glass under slow cooling

Abstract

In the vitrification of high-level radioactive waste, it is important to understand the crystallization behavior because precipitation of molybdenum (Mo) phase reduces the chemical durability of the waste form. In this study, the crystallization of Mo phase under slow cooling conditions was investigated with glasses of SiO 2 B 2 O 3 Al 2 O 3 CaONa 2 OMoO 3 systems and simulated high-level waste (HLW) compositions. When the B 2 O 3 in the glass decreases and the Al 2 O 3 increases, the phase separation of Na 2 MoO 4 and the crystallization of powellite (CaMoO 4 ) were promoted. The waste components included in the HLW glass has the effect of increasing the MoO 3 solubility and suppressing crystallization. Furthermore, it was found that the suppressing effect was further enhanced by addition of rare earth element oxides. In contrast, there was no suppressing effect of addition of V 2 O 5 on the crystallization of powellite. The precipitation of the Na 2 MoO 4 is mainly related to liquidliquid immiscibility, while the crystallization of powellite depends on the multiple factors, an equilibrium MoO 3 solubility at high temperature, nucleation and growth rates under slow-cooling and crystallization of Ca-silicate phase. The compositional effects on the crystallization can be described using the melt viscosity.

MoO3 Solubility and Chemical Durability of V2O5-Bearing Borosilicate Glass

“…In this case, because MoO 3 content is less than equilibrium solubility at temperature (T 1 ), phase separation and/or crystallization may occur during cooling, even if it is a single liquid at T 1 . The opposite effect of vanadium oxide on the powellite crystallization described above may have been caused by the different cooling rate, quench in [26] and slow cooling in [29].…”

“…In previous studies [19][20][21][22][23][24]26,27,29], solubility and crystallization of molybdenum phases have been examined by using glasses with a relatively smaller amount of MoO 3 (<10 wt% or 5 mol%, for example, composition B in Figure 1). In this case, because MoO 3 content is less than equilibrium solubility at temperature (T 1 ), phase separation and/or crystallization may occur during cooling, even if it is a single liquid at T 1 .…”

“…In contrast, phase equilibrium experiments for the samples with an excess amount of MoO 3 provide equilibrium solubility. Based on those experiments, we have determined the phase separated compositions D and E in the borosilicate systems and used them for the thermodynamic analysis of liquid-liquid immiscibility [25,28]. In this study, we determined equilibrium MoO 3 solubility by phase equilibrium experiments at 1200 • C or 1000 • C for a V 2 O 5 -bearing borosilicate system.…”

“…Therefore, the separation of YP significantly reduces the chemical durability of the vitrified waste [16,17]. The crystallization of the molybdenum phase has been investigated for glasses with various compositions [18][19][20][21][22][23][24][25][26][27][28][29]. It has been indicated that the solubility of MoO 3 in borosilicate glasses increased with increasing B 2 O 3 /SiO 2 ratio [19,25,28,29] and decreasing Al 2 O 3 content [25,28,29].…”

“…Moreover, chemical durability test (PCT) results showed that the normalized leaching rate of V 2 O 5 -bearing glass remained at a much lower level than that of standard borosilicate glass. Sugawara et al [29] indicated that the addition of V 2 O 5 accelerates the crystallization of powellite when the glass is slowly cooled (1 • C/min). This is the opposite result mentioned by Lian et al [26], suggesting that cooling rate affects the crystallization tendency of powellite.…”