The densities of two simulated radioactive waste glasses and melts were determined between 298 and 1278K by the Archimedean immersion method for annealed glasses at room temperature, dialtometric measurements for glasses in the temperature range from 373K to the glass transition region and Archimedean densitometry using a molten chloride salt for liquids above 825K. At temperatures above 1079K, the density of a stable liquid can be measured with a precision of 0.3%. The molar volume of the waste glass melt between 1079-1278K is consistent with both the high temperature extrapolation of the molar volume at the glass transition temperature and the value estimated using partial molar volumes reported previously, which has been obtained using double-bob Archimedean densitometry. The temperature dependence of the molar volume dV/dT) between the glass transition temperature and 1278K is significantly larger than that calculated using partial molar thermal expansivities in the previous model. This suggests that the thermal expansivity of borosilicate melts is temperature dependent.
