Low Temperature Immobilisation of Spent Adsorbents from Fukushima
Name: Dimitri Pletser
Sponsor: Hitachi Ltd.
The clean-up of the Fukushima Daiichi site continues to generate large volumes of spent adsorbents which need to be disposed of permanently. One of the candidates is a low temperature immobilisation process to avoid volatilising radioactive Cs and Sr species. Detailed radionuclide volatilisation experiments were performed at Hitachi Research Laboratories in Japan, to investigate possible Cs volatilisation behaviour from commercial zeolitic adsorbents. Cs volatilisation was shown to be negligible, in most cases below detection limits, at temperatures below 600ºC, with volatilisation starting at 700 ºC and strongly increasing above 800ºC.
To minimise Cs loss in the final waste treatment an immobilisation process with a maximum temperature of 600ºC was developed by sintering model waste with glass frit to form a dense Glass Composite Material (GCM) wasteform. A zeolitic model wasteform, chabazite, was loaded with between 8-12 wt.% Cs waste loading and was sintered with two lead-based glass compositions, lead borosilicate (PBS) and a lead borate (PB) glass composition, at a maximum temperature of 600ºC.
Full encapsulation of the zeolitic model wasteform was achieved for waste loadings up to 50 wt.% in PBS and 40 wt.% in PB in all cases, with both GCM systems showing dense microstructures. PBS GCM formation was successful at a maximum of 500ºC, while the PB GCM formation was shown to be possible at a maximum temperature of 400ºC. Sintering optimisation undertaken has shown that the microstructure of the final wasteform can be finely tuned by controlling composition, sintering profiles and thermal treatment regimes. Sintering behaviour was shown to be independent of Cs waste loading of the adsorbent, but was shown to be strongly influenced by thermal treatment of the wasteform during sintering. Figure 1. Cross-sectional SEM images of PBS wasteforms. Show the progressive densification of three generations of wasteforms with increasingly optimised sintering profiles