, Volume 33, Issue 7, pp 1887-1895

Microstructures and leach rates of glass–ceramic nuclear waste forms developed by partial vitrification in a hot isostatic press

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Abstract

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 °C and 138 MPa. The waste forms are abundantly composed of zircon, beddeyelite, apatite, fluorite, greenockite and boroaluminosilicate glass. The crystal nucleating, glass forming and volatilizing components of the calcine are partitioned into crystalline and glass phases such that 95 wt% of the waste components, including actinide surrogates, stoichiometrically reside in the crystalline phases. This results in a high waste loading of 60–80 wt% calcine in the total glass–ceramic. The partitioning follows the natural association of elements, as a result, species like P avoid the glass phase. Instead glass accommodates the incompatible solutes like Cs. It minimizes porosity and bonds the polyphase ceramic microstructure, which resembles rhyolite or basalt volcanic rocks. Both glass and crystals contribute to high chemical durability, which is degraded when glass devitrifies with lowering of partial liquid viscosity by higher MgO additions. The devitrified phases are layered mica, dendritic nepheline and fibrous alkaline-earth borate. These phases are enriched in the mobile elements of Cs, Na and B, respectively. © 1998 Chapman & Hall