Closure of the concrete supercontainer in hot cell under thermal load

► We model the behaviour of the supercontainer for the disposal of high-level waste and spent fuel assemblies during fabrication at ground surface. ► We study the early-age cracking behaviour of the buffer and evaluate the crack creating mechanisms. ► In case accurate measures are taken, cracking of the buffer can be avoided. For the final disposal of long-lived, heat-emitting vitrified high-level waste (HLW) in a clayey host rock, an intensive study is conducted to investigate the early-age behaviour of concrete supercontainers. Self-compacting concrete (SCC) is taken as the reference concrete type as it facilitates the casting process in combination with an improved homogeneity compared to the traditional concrete compositions. A laboratory characterization program is conducted to obtain the relevant thermal, mechanical and maturity-related properties of the SCC. These obtained data are implemented into the material database of the finite element tool HEAT to study the behaviour of the concrete layers during the different construction stages of the supercontainer: (i) Stage 1: Fabrication of the concrete buffer inside a stainless steel envelope. No early-age cracking is expected in case accurate measures are taken to reduce the thermal gradient between the outer surface and the middle of the buffer, e.g. by providing insulation and excluding wind. (ii) Stages 2–4: Emplacement of the carbon steel overpack containing the HLW canisters, filling the remaining annular gap with cementitious filler and closure by fitting the lid under thermal load. The construction stages (2–4) for the closure of the supercontainer are executed in hot cell. In this study, the crack creating mechanism and the behaviour of the concrete supercontainer during these construction stages in hot cell are investigated. In case precautionary measures are taken, such as reducing the coefficient of thermal expansion (CTE) of the overpack, prolonging the preceding cooling period of the HLW or reducing the modulus of elasticity of the filler material, the formation of superficial tangential macro cracks can be prevented. In addition, the effect of elevated temperatures, due to the heat emitted by the HLW, on the strength of the hardened SCC is investigated by means of compressive strength tests and fluorescence microscopy analysis.