Subliquidus Phase Relations in the Low-Alumina Section of the Na2O-Al2O3-P2O5 System and the Radioactive Waste Vitrification

The formation of crystalline phases during cooling of the Na2O-Al2O3-P2O5 melts obtained in corundum crucibles at a temperature of 1100 degrees C and at atmospheric pressure from a pre-prepared glass with the composition close to that used for immobilization of highly active radioactive waste (RW) is studied. It is shown that under slow cooling, the crystallization process is accompanied by changes in the liquidus compositions of the melts leading to their transformations into glasses with compositions that, in some cases, are very different from the initial ones. A new topology of the melting diagram of a low-alumina section of three-component system is proposed, the melt's compositions are determined, and the temperatures of phase equilibria, which play a crucial role in industrial technologies, are estimated. The formation of crystalline phases under the slow cooling of the Na2O-Al2O3-P2O5 melts should be taken into account in the vitrification of real RW.