Engineering considerations for multiple barrier structures have been the subject of extensive research, with design considerations varying between countries and according to waste type (see Table 1). For example, the latest Swedish design concept for spent fuel which has also been adopted by Finland (KBS-3 concept; see Table 1) favours waste encapsulation in corrosion-resistant copper canis­ters, with waste packages emplaced in isolated boreholes lined with bentonite, and with clay or bentonite blocks and crushed host rock used a backfill (see Table 1).7,8 In contrast, the Belgian Spent fuel/HLW concept favours waste encapsulation in steel supercontainers, which are axially emplaced in disposal tunnels, with Ordinary Portland Cement (OPC)-based concrete used as a buffer and cement-based materials used as backfill (see Table 1).9,10 Generally, international plans for ILW favour co-disposal alongside spent fuel and HLW, albeit in separate areas of the same repository (see Figure 2 and Table 1). Furthermore, ILW waste conditioning is typically in concrete/grout with encapsulation in steel and emplacement in boreholes or caverns that are backfilled with concrete, grout, clay and/or crushed host rock (see Table 1; review by Hicks 2008; ref. 11).

It is also worth mentioning that the USA currently hosts the world’s only operational GDF, the Waste Isolation Pilot Plant (WIPP). This facility receives transuranic (TRU) wastes containing uranium, plutonium and other actinides, which are broadly similar to UK ILW. The site selection process for this facility began in the 1950s and waste emplacement commenced in 1999. WIPP is housed in stable salt rock formations, and design features include waste packaging in steel containers, waste emplacement in horizontal bore­holes or shafts, and the use of MgO as a backfill material.11 However, whilst WIPP has commenced geodisposal of US defence related wastes, the long term management pathway for US civil wastes is uncertain (and subject to current review) due to the US Department of Energy (DOE) recently with­drawing their license application for a HLW repository at Yucca Mountain, Nevada.12