MICROSTRUCTURES AND LEACH RATES OF GLASS-CERAMIC NUCLEAR WASTE FORMS DEVELOPED BY PARTIAL VITRIFICATION IN A HOT ISOSTATIC PRESS

A high level nuclear waste calcine simulant is transformed to a dense and durable glass-ceramic waste form by addition of glass and crystal forming components, and hot isostatic pressing at 1000 degrees C and 1 38 MPa. The waste forms are abundantly composed of zircon, beddeyelite , apatite, fluorite, greenockite and boroaluminosilicate glass. The cr ystal nucleating, glass forming and volatilizing components of the cal cine are partitioned into crystalline and glass phases such that 95 wt % of the waste components, including actinide surrogates, stoichiometr ically reside in the crystalline phases. This results in a high waste loading of 60-80 wt% calcine in the total glass-ceramic. The partition ing follows the natural association of elements, as a result, species like P avoid the glass phase. Instead glass accommodates the incompati ble solutes like Cs. It minimizes porosity and bonds the polyphase cer amic microstructure, which resembles rhyolite or basalt volcanic rocks . Both glass and crystals contribute to high chemical durability, whic h is degraded when glass devitrifies with lowering of partial liquid v iscosity by higher MgO additions. The devitrified phases are layered m ica, dendritic nepheline and fibrous alkaline-earth borate. These phas es are enriched in the mobile elements Cs, Na and B, respectively. (C) 1998 Chapman & Hall.