Several issues arose that were common, or were applicable, to all the samples investigated and are discussed here.
5.7.1 Partitioning of plutonium simulants
The sequestration of the actinide simulants (Ce/La) into a single phase, or in this case partial sequestration, could pose an issue from a criticality point of view. Whilst the two elements were in both the glass and the CeLa silicate, the increased amount of these elements in the silicate phase could theoretically result in a criticality incident, an extremely undesirable event. However, the shape and size of the CeLa silicate should mitigate this extreme, most fissionable materials critical mass is determined based on the material having the shape of a sphere (the most efficient shape for this process); clearly this is not the case here. The small particle size, and spacing between particles should be enough to prevent any sort of criticality incident. Consistent with current safety measures in the field of nuclear and nuclear waste, a further precaution would be necessary, as a failsafe in the unlikely event that crystallisation did produces a phase that was both big enough and dense enough to produce a criticality event. It is suggested that the addition of a neutron poison be added to the GCM recipe, in particular, boron. The behaviour of boron as a neuron posion, and glass additive is already well known [248], with most HLW glasses containing boron for this reason. An additional neutron poison such as gadolinium could also be added, aimed at partitioning into the CeLa silicate
197 itself, further reducing a criticality event, however it is unclear if gadolinium would partition into this phase and should this course of action be taken, further research would be required.
With the addition of just boron to the recipe, coupled with the unfavourable microstructure of the CeLa silicate, this should reduce if not eliminate any criticality issues.
5.7.2 Anomalous pH at 14 weeks
At 14 weeks’ corrosion time an increase in the pH was observed across all samples where pH was analysed, of approximately 0.5. It is unclear whether this was a statistical anomaly, or the beginning of a renewed corrosive attack on the samples. SEM images of both the cross sections and surface showed no evidence that a secondary corrosion mechanism was occurring; the images indicated that corrosion was reduced, in line with the relatively unchanged size of the leached layer, where one was present. Indeed, ICP-OES results confirm that a reduction in Si concentrations (a good indicator of network dissolution) occurred in all samples except PF-Asb. Further, only the PF-Asb sample showed any noticeable increase in other elements that could be responsible for an increased pH. Indeed, all other samples showed a decrease in the element concentration. Overall, the evidence would suggest that the increased pH at 14 weeks was either due to contamination, or due to the solution analysed being the last of solution in the digestion vessel; for all other aliquots, only a portion of the solution was removed (see section 3.4). There is evidence that this is conclusion may not apply to sample PF-Asb, where other analysis techniques suggested an increase in corrosion, more easily seen in the normalised leach rates for magnesium, and to some extent sodium. Further corrosion time experiments are necessary to establish the nature of the Ph increase, to assess whether it is a secondary corrosion mechanism, or an artefact of the experimentation setup.
The results from pH analysis revealed a sudden increase of approximately 0.5 at 14 weeks’ corrosion time. Corresponding ICP-OES results also revealed an increase in the concentration of Si, Mg, and Na good indicators that network attack was occurring at an increased rate relative to previous times. This is clearly observed in the normalised leach rate results for these elements at that time. However, no corresponding change in either leach depth or SEM surface/cross sectional images were observed.
5.7.3 Normalised leach rate comparisons
Normalised leach rates for elements of importance from the materials analysed here are displayed in Table 54. In addition, comparative normalised leach rates for other materials are displayed in Table 55 note the comparative NLR are often averaged over the experiment time and where this is the case the experiment duration is noted. Little literature exists on leach rates of cemented nuclear waste, as during experimentation on cement as a HLW encapsulate it was found to have an
unacceptably high leach rate, relegating it to LLW/ILW encapsulation [249]. This is reflected in the release rate for sodium shown in Table 54, it has been reported that a generic (no elements quoted) intrinsic leach rate of cementitious wastes to be 100x10-6 g cm-2 day-1 [250].
198 Table 54 results from a generic ILW cement [251] (1 year), HLW borosilicate glass [252] (28 days), and yellow phase contaminated glass composite material (28 days), along with the noted normalised released rates for the elements shown.
Table 54: Comparison of cement, borosilicate glass, and yellow phase GCM normalised leach rates for Cs, Na, Sr, and combined actinides.
NRR (10-6 g/cm2 day) Elements 137Cs Na Sr Actinides Cement1 4.8 7300 - - Borosilicate glass2 0.3 0.9 0.3 - Yellow phase gcm2 1-10 1-10 1-0.1 0.001
Table 55 summarises the important NLR for the materials analysed here, and shows the highest NLr found for that element over the course of the experiment. It is clear from these results that the GCMs analysed here outperform cement encapsulation by several orders of magnitude, not an unexpected result considering the general performance of cement versus glasses and ceramics for leach rates. Unexpectedly, they had lower NLr for Na compared to typical HLW borosilicate glass. Considering the dual and sometimes multi-component facet to GCMs this value was expected to be much higher, clearly this is not the case, and is a promising indicator towards the durability of the wasteforms. However, these results cannot be taken as an absolute indicator of how well the GCMs will fair in the long term, as has already been discussed.
Table 55: Summarised leach rates for Na, Mg, Si, and Ca (where available) for all the wasteforms investigated here, with the exception of PF-HMS (due to ICP-OES data surpassing the maximum limit) for comparison with table 54. It is clear these wasteforms are on par with borosilicate glasses, and significantly better than cementation. NRr (10-6 g/cm2 day) Elements Na Mg Si Ca JH-PCM 0.68 0.02 0.10 0.14 JH-HMS 0.16 0.08 0.09 JH-SIXEP 0.58 0.02 0.09 0.47 PF-Asb 2.93 0.04 0.10 0.23 PF-HM2 0.03 0.04 0.08
199