Early age behaviour of concrete supercontainers for radioactive waste disposal

Various types of radioactive waste were and are produced in Belgium. This waste originates from different producers: nuclear power plants, medical applications, industry, research centre, etc. During the past 25 years several preliminary repository designs were proposed. Today, the cylindrical supercontainer is considered to be the most promising Belgian design on the matter of enclosing the vitrified high level radioactive waste (HLW) and the spent fuel assemblies and is based on the use of an integrated waste package composed of a carbon steel overpack surrounded by an Ordinary Portland Cement buffer. For the choice of this cementious buffer two compositions, a self-compacting concrete (SCC) and a traditional vibrated concrete (TVC), are being considered, tested and compared by means of an intensive laboratory characterization program. Through-going cracks in the concrete buffer should, at all times, be avoided because they will considerably ease the transport mechanisms inside the supercontainer. Therefore, finite element simulations are performed, using a 2.5-D thermal and crack modelling program, to predict the mechanical and thermal behaviour of the concrete buffer at any time during hardening. Looking at the finite element simulation results of the first stage of manufacturing of the supercontainer (cast in one), and the emplacement of the heat-emitting waste canister (second stage), we experience no early age cracking of the concrete buffer. The impact of environmental conditions and shrinkage and creep behaviour on the simulation results are noticeable