Interaction of simulant thermally treated intermediate level wastes with a high-pH cementitious backfill

Thermally treated wasteforms are being considered for the disposal of intermediate-level waste (ILW) within a geological disposal facility (GDF). This requires, in part, assessment of the long-term interactions of these potentially new wasteforms with the backfill employed in a GDF. This paper presents results of an experimental study undertaken to assess the interactions of two simulant thermally treated ILWs (glasses) with a high-pH cementitious backfill, the Nirex Reference Vault Backfill (NRVB). The experiments consisted of NRVB monoliths with glass coupons embedded within them. These were submerged in water at 50 °C (under anoxic conditions) and were cut open periodically over four years to examine the glass coupon/NRVB interface and reveal any changes resulting from the interactions. A complementary experimental setup was also utilised that included powdered glass and powdered NRVB samples to accelerate the interactions between them. The results confirmed that high pH was maintained across the duration of the experiment, that glass dissolution was limited (likely via a Ca-rich passivating layer), and that all the systems evolved to form abundant calcium silicate hydrates. Overall, the study supports the idea that a range of UK thermally treated ILW wasteforms are likely to be compatible with the existing ILW disposal concept using a high-pH cement backfill.