In vitrification of high-level radioactive waste, molybdenum (Mo) contained as a fission product tends to separate into a low-viscosity fluid called the yellow phase when the waste loading is high. The occurrence of yellow phase is attributed to phase separation of the Mo phase in the borosilicate melt. The yellow phase can incorporate various radioactive elements and is highly water-soluble. It is necessary to understand the behavior of molybdenum in borosilicate melts in order to develop glass matrix with sufficient chemical durability and high waste loading.
We carried out the phase equilibrium experiments for molybdenum-bearing borosilicate systems. The thermodynamic database for calculating phase separation between silicate liquid and molybdate liquid were developed by means of the CALculation of PHAse Diagram (CALPHAD) technique.
Phase equilibrium experiments were performed with 94 different chemical compositions in the system SiO2-B2O3-Al2O3-ZnO-CaO-Na2O-Li2O-MoO3. An excess amount (13-15mol%) of MoO3 was added to ensure phase separation. The reagent mixture was placed in a platinum crucible and held at 1200°C or 1000°C for 24 hours and then rapidly cooled with water. All experimental samples were phase-separated, yielding a silicate glass and an underlying molybdenum-rich layer. Both phases were analyzed by EPMA, XRF and ICP-AES.
Figure 1 shows MoO3 solubility as a function of melt viscosity. We found that the MoO3 solubility increases with decreasing viscosity and Na2O activity at constant temperature. The experimental results of phase separation were analyzed by combining literature data on the phase equilibria of binary and ternary systems including MoO3 and a thermochemical data of oxides. The enthalpy of mixing was approximated by the Redlish-Kister polynomial. The database in the system SiO2-B2O3-Al2O3-ZnO-CaO-Na2O-Li2O developed by GTT Technologies Inc. (GTOX) was used as the basic oxide database. Then, Na2MoO4, CaMoO4, Li2MoO4 and ZnMoO4 were set as associate species. In the CALPHAD analysis, the newly measured enthalpy and heat capacity for Na2MoO4 and the recently modified phase equilibria for SiO2-Na2O-MoO3 system were considered. The calculation was performed with thermochemical software, FactSage 8.2.
Table 1 indicates an example of comparison between the calculated and the experimental results of phase separation. The multicomponent phase separation between borosilicate and molybdate melts was well reproduced by calculation, including not only the major elements but also trace components such as SiO2 and B2O3 in the molybdate melt. The standard deviation between the experimental and calculated values of MoO3 content in the silicate melt is ±2.3% in the range of 3–22 wt%.
As an example application of thermodynamic database developed in this work, we will demonstrate the following:

(1) Calculation of compositional dependence of MoO3 solubility in borosilicate glass and search for glass matrix with high waste loading.
(2) Consideration of glass compositions that combines high MoO3 solubility and durability.
(3) Calculation of phase separation and crystallization of molybdenum phase in various temperature ranges in a glass melter.
This work was carried out as a part of the basic research programs of vitrification technology for waste volume reduction（JPJ010599）supported by the Ministry of Economy, Trade and Industry, Japan.