A thermodynamic model of glass durability based on the hydration of "structural units" has been applied to natural glass, medieval window glasses, and glasses containing nuclear waste. The relative durability, predicted from thermodynamic calculations, correlates directly with the experimentally observed release of silicon to the leaching solution in short term lab tests. By choosing natural glasses and ancient glasses, whose long term performance is know and which bracket the durability of waste glasses, the long term stability of nuclear waste glasses can be interpolated. The current Savannah River defence waste glass formulation is a durable as natural basalt from the Hanford Reservation. The thermodynamic hydration energy is shown to be related to the bond energetics of the glass.
