Ion-specific media (ISM) have played an integral role in the clean-up and remediation efforts at the Fukushima Dai-ichi disaster site, through the processing of contaminated wastewaters. The use of these materials generates a secondary nuclear waste stream, presenting its own series of engineering problems arising from stringent handling and long-term storage requirements. A reactive spark plasma sintering (SPS) method was investigated for conditioning of the spent cesium exchanged zeolite, chabazite. A natural form of the zeolite was used as an analogue to the engineered ISM used at the Fukushima NPP site. Simulant wasteforms were sintered using different temperature and pressure parameters followed by analysis of phase assemblage, density, and durability (using the product consistency test (PCT)). The results indicated that zeolite structure had collapsed completely, with the exchanged cesium partitioned primarily into a durable feldspar to assure stability of the sintered material for passively safe storage or geological disposal.

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.

Multi-phase material systems make up a significant proportion of the currently proposed and researched wasteforms for sequestration of heterogeneous nuclear material feeds. Quantification of the components for such multi-phase assemblages is typically performed using diffraction-based Rietveld methods, many of which necessitate long measurement times of several hours. Furthermore, careful additions of an internal standard are typically required, to facilitate inclusion of amorphous phases in the quantification. The application of an image analysis method has been investigated, using the z-contrast greyscale of back-scattered electron micrographs to determine the relative quantities of component phases in a suite of monolithic phosphate glass ceramic wasteforms. This work demonstrates an alternate methodology for accelerated quantification which could be applied to other heterogeneous wasteforms and multi-phase materials. Graphical abstract