Concrete as a multiphase material in biological shields against nuclear radiation

Concrete is commonly used as a biological shield against nuclear radiation. As long as, in the design of nuclear facility, 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 may have to be taken into account in such circumstances. The study presented here allows for reaching first evidences 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. Experimental evidences on the decay of the mechanical modulus of concrete have allowed for implementing the required damage law within a 3D F.E. research code which accounts for the coupling among 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 can not neglect the temperature rise due to heat produced by radiation which, in fact, is proved to represent the most serious source of damage for concrete. The study case is represented by a next generation nuclear facility (currently under design) for the National Institute of Nuclear Physics (INFN) at the 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 scenario of the SPES Project, under an irradiation profile lasting six months, i.e. a working rate of the facility for the production of the exotic species of approximately 4500 hours per year, in order to limit temperatures to admissible exercise ranges for concrete.