ANALYSIS OF THE DYNAMIC BEHAVIOUR OF A NUCLEAR CONTAINMENT STRUCTURE UNDER MISSILE IMPACT

The evaluation of the integrity of the NPP against a missile impact requires considerable attention because of the complexity of the accident scenario and consequences it may cause. To assess whether such type of thread may determine the failure of the safeguard system, the effects of both the impact force and the resistance force must be determined. This study deals with the currently used general procedures and criteria. Particularly, it focuses on the evaluation of global and local effects caused by an impacting missile on the reinforced concrete structural elements, like the containment system. To the aim, a numerical approach based on the use of the finite element (FE) code was adopted. As the analysis of reinforced concrete is made complex by the several arising dominant energy absorbing and failure modes mechanisms, a methodology which was validated with reference to experimental data available in the open literature from large-scale impact tests was employed. Therefore, a 3D FE model of the RCS was implemented with structural 3-D Solid element to represent the dynamic interaction occurring at the impact surface. The strain-rate effects are considered through the dynamic increase factor (DIF) to account for strain-rate hardening effects on both deformation and failure; for the steel behavior the Cowper–Symonds’model was assumed. Results showed that at the instant of the impact, the intense shock wave that characterize the instant of the projectile-target impact is responsible of the cracking, and penetration of the RCS wall. For a striking velocity of about 150 m/s the wall penetration extends over about 80 mm.