Keywords: concrete, supercontainer, high level waste, thermal cracking.

Worldwide thousands of cubic metres of highly active radioactive waste is being produced each year originating from the most diverse sources. The Belgian reference disposal concept is founded on the use of a concrete supercontainer disposed inside geological clay layers (Boom clay). The Supercontainer concept is based on a cylindrical multiple barrier system where the carbon steel heat-emitting waste canister is enclosed in a non-reinforced massive concrete buffer and an outer stainless steel liner [1]. The main purpose is to prevent contact of the waste with the water coming from the host formation during the thermal phase i.e. the first 500 years for vitrified waste and 2000 years for the spent fuels assemblies. For corrosion protection purposes, the overpack is enveloped by a high pH concrete buffer (high alkaline concrete). This buffer, with a thickness of about 70 cm, also performs as a well-defined radiological protection buffer for the workers and simplifies underground waste transportation operations.

For the realization of the buffer two types of concrete have been tested and compared. One of the possible solutions is the use of self-compacting concrete (SCC) because it will considerably ease the precast process, needs no additional vibration energy and complies with all other requirements regarding compressive strength, long term durability and chemical interactions. Another possibility is the use of a traditional vibrated concrete composition (TVC). The thermal, mechanical and maturity related properties of the two types of concrete considered for the buffer are obtained using a laboratory characterization program and large scale tests and are implemented into the material database of the simulation program [2]. For the numerical simulation during the manufacturing of the supercontainer, the finite element programme HEAT has been used. HEAT calculates the stresses (due to temperature) and the strength in the concrete structure using a state parameter approach linked by a material database. This engineering tool yields the evolving temperature fields and the resulting stresses at any time during hardening.

Concrete is characterized by a low tensile strength. This is the main reason why cracks in concrete occur. Stresses that lead to cracks in concrete can be caused by different actions such as direct loading, prevented shrinkage and swelling, differential setting, etc. Especially young concrete is very sensitive to crack formation [3]. In massive hardening concrete, the hydration heat has a considerable effect on the stress development inside the concrete structure due to the created thermal gradients. On the other hand, through-going cracks inside the buffer, which will considerably diminish the durability and safety of the Belgian disposal concept for HLW, are not expected for both types of concrete.

1

J. Bel, A. Van Cotthem, C. De Bock, "Construction, operation and closure of the Belgian repository for long-lived radioactive waste", Proceedings of the 10th International Conference on Environmental Remediation and Radioactive Waste Management, Glasgow, Scotland, September 2005.

2

B. Craeye, G. De Schutter, H. Van Humbeeck, A. Van Cotthem, "Early age behaviour of concrete supercontainers for radioactive waste disposal", Nuclear Engineering and Design, 239, 23-35, 2009. doi:10.1016/j.nucengdes.2008.10.006

3

G. De Schutter, "Finite elements simulation of thermal cracking in massive hardening concrete elements using degree of hydration based material laws", Computer & Structures, 80, 2035-2042, 2002. doi:10.1016/S0045-7949(02)00270-5

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