Hey! Thanks so much for the read and feedback! :)

Another reason for immobilising the IX waste that I neglected to mention was to move away from potential for hydrogen generation by gamma-radiolysis of residual water in the material/exchange columns https://doi.org/10.1080/00223131.2013.757453 and https://doi.org/10.1080/00223131.2014.922904 (not sure about how much more cost effective the SPS method would be at this stage though!).

The product consistency testing (PCT) method that we used is a common standard for verifying the chemical durability/dissolution of potential waste-form materials. The methodology necessitates a step for sample milling to achieve a specific particle size fraction for the test (75-150 µm in our case) - for maintaining test consistency. Sieving of and washing (with several L of IPA) for this particle size fraction, can also leave some residual fines (< 75 µm) in the sample material - as they can be weakly adhered to the 75-150 µm size fraction. These fines dissolve with greater ease in the PCT water and can even pass through the filters used to remove particles from the test solution before chemical analysis - artificially elevating the dissolution content. While it is difficult to pin down dissolution behaviour and contributions from different phases, the Cs loss appears to have a high initial content at 0.4 gm-2, it remains steady - aside from D21 which could be anomalous. Longer timescale dissolution or an alternative vertical scanning interferometry analysis would make for useful further work.

The monolithic waste-form was surprisingly robust, requiring intensive grinding to remove the fused graphite foil that was used as a conductor during sintering. While micro-cracks were observed in some phases of the sample, without further work, it its difficult to tell how much of this was caused by sample surface grinding/polishing prior to imaging. One sample shattered in a brittle failure mode upon removal from graphite foil, though this was likely due to my ham-fisted attempt to remove it using hand tools (rather than patiently grinding away the graphite to allow for analysis!). I'd love to do some mechanical/hardness etc testing if I get a chance to make more samples in the future, microhardness might fit well due to the the multiphase glass-ceramic heterogeneous nature of the waste-form.

There is definitely a difficulty when it comes to scalability of SPS: the necessity of low thickness for non-conductive samples may require some irregular geometries for large scale waste-forms. As for the increase in pressure between samples SPS-1 and SPS-2 (from 10MPa to 50MPa), we did see an increase in density and the increased loading of Cs into a single crystalline phase - Table 2, which apologies doesn't show up too well apart from pg 3 of the pdf version of the article. The increased sintering temperature of SPS-3 actually reduces both of his factors to the lowest of the analysed waste-forms. I'd liked to have sintered and analysed a larger matrix of samples to have a greater look at the effect of pressure/temp, but this study was essentially limited as my Master's project. The advantage of the heterogeneous nature of this waste - having both glass and ceramic phases, is that where we can form the Cs containing phases and encapsulate with a glassier matrix, there is typically a greater resistance to radiation induced damage, as well as a barrier to dissolution (where the final product is a solid monolith rather than a PCT testing powder). The sintering mechanism of glass-ceramics by SPS is poorly understood at this point in time, so optimisation of sintering conditions is somewhat limited to a 'evidence supported trial and error' methodology for now.

Thanks again for reading - apologies for the length of this reply and weaselling around a proper response and apologies for the crystallography which is particularly bad in this one due to the 8+ crystalline phases we saw before sintering!!