Temperature dependence for hydrothermal reactions of waste glasses and ceramics

Samples of simulated high-level waste glasses, supercalcine, SYNROC-B and the natural glass were leached three days in deionized water. The test temperatures were 250, 500, 1000, 1500, 2500 and 3500C, except that the SYNROC was not tested at below 1000C. Each sample was placed in a Hastelloy autoclave with 100 ml of deionized water, giving solution-volume-to-sample-surface-area ratios of between 12 and 17 cm. The autoclaves were pressurized with nitrogen so that the operating pressure at temperature was 13.8 MPa (2000 psi) - except at 3500C, when the pressure was 16.9 MPa (2450 psi). After the tests, the solutions were chemically analyzed using the inductive-coupled plasma spectrometer and the atomic adsorption spectrometer. Each waste form sample was sectioned and examined optically and with the scanning electron microscope with an energy-dispersive x-ray attachment. This study shows that simulated high level waste glasses, supercalcine and SYNROC exhibit similar leaching behaviors in hydrothermal environments. The ceramics show a smaller temperature dependence than do the glasses. At temperatures above 2500C, the ceramics show lower releases; but below 2500C, where we expect actual repository temperatures to lie, releases from the waste glasses are lowest. The actual temperature relationship observed depends on the elements studied. The chemistry of the element, changes in the solution properties and secondary reactions all influence the release of elements from the waste forms