Development of phosphate glass and multi-phase titanate ceramic compositions for thermal treatment of irradiated nuclear fuel residues

Abstract

The highly heterogeneous nature of UK legacy damaged and degraded spent nuclear fuels and so called, ‘orphan fuels’, prohibits the use of standard conditioning methods. An inventory of UK residual fuels yielded an account for three main fuel types: Magnox, AGR (advanced gas-cooled reactor) and MOx (mixed oxides). A series of glass and ceramic type host systems have been investigated for potential conditioning of these high uranium content spent fuel materials. Electron microscopy and powder X-ray diffraction techniques were used to characterise the prototypical wasteforms. Two sets of low-melt temperature phosphate glass compositions were trialled with additions of CeO2 to simulate the fluorite structure and large ionic radius of U in oxide fuels. Evolution of monazite-type phases at simulant oxide fuel loadings above 15 wt.% highlighted a potential development into a glass-ceramic hybrid assemblage. Investigation into the use of an alkoxide nitrate synthesis route for SYNROC-F type ceramic precursors has allowed for the demonstration of a sintered host pyrochlore phase containing up to ∼40 wt.% fuel simulant CeO2. Gas evolution has led to increased porosity at higher temperatures and longer sintering times, this may be mitigated by higher pre-calcination temperatures.