Repairing structures for nuclear facilities: A numerical approach by means of FEM and Monte Carlo techniques

Concrete is commonly used as a biological shield against nuclear radiation. As long as, in the design of nuclear facilities, its load carrying capacity is required together with its shielding properties, changes in the mechanical properties due to nuclear radiation are of particular significance and they have to be taken into account in such circumstances. The study presented here allows for reaching first evidence on the behavior of concrete when exposed to nuclear radiation, in order to evaluate the consequent effect on the mechanical field, by means of a proper definition of the radiation damage, strictly connected with the strength properties of the building material. This is expected to be of help to forecast a possible repair for concrete shielding within reasonable time spans, according to the serviceability of the nuclear structure. Experimental evidence on the decay of the mechanical properties of concrete have allowed for the implementation of the required damage law within a 3D F.E. research code which accounts for the coupling between moisture, heat transfer and the mechanical field in concrete treated as a fully coupled porous medium. The upgrade of the numerical model allows for assessing the durability of concrete under the effects of a radioactive environment; considerations on the ultimate strength resource of concrete cannot neglect the temperature rise due to the heat produced by radiation, which, in fact, is proved to represent the most serious source of damage for concrete. The case study is represented by a next generation nuclear facility under design at the National Institute of Nuclear Physics (INFN), National Laboratories of Legnaro (LNL) in Padua (Italy): the SPES Project. The research structure is expected to produce neutron-rich unstable nuclei, called "exotic beams", by fission reactions of a primary radioactive proton beam on an uranium-carbonium target, in a dedicated underground bunker. Contour diagrams and time evolution graphs of the most significant variables (temperature, humidity, displacements, damage parameter) are reported, for the practical scenario of the SPES Project, under an irradiation profile lasting approximately six months, i.e., a working rate of the facility for the production of the exotic species of 5000 hours per year, in order to limit the admissible exercise temperatures for concrete. © 2012 Taylor & Francis Group.