Melter Feed Reactions at T700°C for Nuclear Waste Vitrification

To understand feed-to-glass conversion for the vitrification of nuclear waste, we investigated batch reactions and phase transitions in a simulated nuclear waste glass melter feed heated at 5K/min up to 700 degrees C using optical microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray diffraction. To determine the content and composition of leachable phases, we performed leaching tests; the leachates were analyzed by inductively coupled plasma atomic emission spectroscopy. By 400 degrees C, gibbsite and sodium borates lost water and converted to amorphous phase, whereas other metallic hydroxides dehydrated to oxides. Between 400 degrees C and 700 degrees C, carbonates decomposed before 500 degrees C; amorphous aluminum oxide and calcium oxide reacted with the sodium borate and formed the more durable amorphous borate phase along with intermediate crystalline products; above 500 degrees C, quartz began to dissolve, and hematite started to convert to trevorite.