A numerical approach to assess damage effects on concrete shields for nuclear facilities under cyclic radiation profiles

It is common practice in nuclear engineering to use concrete as shielding material for nuclear facilities, thanks to its versatility, cheapness but also its capacity to absorb radiation, due to its intrinsic water content, namely hydrogen. The presence of this light-atomic weight element in the mixture of concrete is responsible for slowing down the produced neutrons, which is the reason why, in the nuclear field, concrete is referred to as a “biological” barrier. For gamma-rays shielding purposes, heavy elements are conceived of having good shielding properties, instead; therefore ordinary concrete mixtures are often enriched with heavy aggregates to ensure a good absorption of secondary radiation. Hence, the knowledge of the moisture content evolution with the work history of a nuclear facility is fundamental to ensure the maintenance of its biological shielding, which justifies the suggested thermo-hygro-mechanical approach for the durability assessment of a concrete radiation shielding, through a FEM research code developed in agreement with the theory of porous media, already implemented in its fundamental lines and upgraded to account for radiation damage effects on exposed concrete. The required damage law has been defined based on the enveloping curve of several empirical data describing the behaviour of the Young modulus of exposed specimens, with respect to that proper of blank samples, in function of the neutron fluence. The damage parameter is supposed to follow the isotropic damage theory. The main physical quantities, included realistic neutron fluence values for the design condition of a facility under study, have been defined through a Monte Carlo tool developed by CERN and INFN of Milan, able to handle 3D radiation transport calculations in matter. The combined use of the Monte Carlo technique and the FEM code, upgraded to take into account the radiation exposure effects on concrete, has allowed us to identify in the thermal aspect, i.e. the temperature rise in the shielding due to radiation energy deposition, the most severe factor for prescribing a work scenario, for the nuclear facility under study, consistent with concrete durability, when no other protective device is present, such as outer metallic liners working as coats for the biological shield or the presence of a cooling systems of the walls.