FINITE ELEMENT ANALYSIS OF AIRCRAFT CRASH ON NUCLEAR CONTAINMENT STRUCTURE

Abstract

In view of the previous missile and aircraft attacks on important structures and the associated very high risk of failure the present study has been designed to evaluate response of nuclear containment structure against aircraft crash. The numerical simulations have been carried out using the implicit and explicit integration schemes of ABAQUS finite element code. The Concrete Damaged Plasticity model has been employed to predict the behavior of concrete. The model is based upon the concept of isotropic damaged elasticity in conjunction with isotropic tensile and compressive plasticity to represent the post elastic behaviour of concrete. The material behavior of the steel reinforcement as well as the aircraft was incorporated using the Johnson-Cook elasto-viscoplastic material model that is capable to predict the flow and fracture behavior of the ductile materials. The outer containment of the BWR Mark III nuclear power plant has been impacted by commercial and fighter planes in order to identify the most vulnerable location and the most damaging aircraft. The preliminary studies led to the conclusion that the reaction-time curve of aircraft can be efficiently employed to accurately predict the response of the containment. However, the present approach for estimating the reaction-time response of aircraft against flat and rigid target is unrealistic and hence requires careful investigation. Thus the geometric models of Boeing 707-320 and Boeing 747-400 aircrafts have been developed and the corresponding reaction-time curves were obtained assuming deformable and non-deformable targets of varying curvature. The curvature and deformability of the target has been found to have significant influence on the reaction-time response of the aircraft. A decrement in the peak reaction force has been noticed with increase in target curvature. Further, the arrival of the peak has also been found to have delayed with an increase in the curvature radius. The magnitude of the reaction offered by the deformable target was lower than that of the rigid target of equivalent curvature radius. The reaction-time curves thus obtained have been compared with those obtained through the available analytical expressions. The area of containment in contact with that of the aircraft has also been found to vary with respect to time and the striking velocity. The effect of contact area on the response of containment has been studied employing two different approaches for the application of reaction-time curve. In the first approach an average of the total contact area of aircraft was considered iii for the application of curve. In the second approach the contact area was trifurcated representing fuselage, first set of engines and second set of engines and the reaction was assigned with respect to location and the time of the respective component. The response of containment obtained through the average and trifurcated surface areas was then compared with that obtained against geometric models of aircraft. It has been observed that the average area approach for the application of loading overestimated the local deformation while underestimated the global deformation of the containment. However, the magnitude of deformations predicted by the area trifurcation approach and the geometric model of the aircraft are comparatively low and in close agreement. The trifurcation approach is therefore more accurate and hence represents a more realistic estimate of the contact area of the aircraft. The reaction-time curves and the corresponding contact area thus obtained have been idealized and employed to seek the response of the containment. The geometric models of the aircrafts, developed numerically, have also been employed to hit the containment structure at the most vulnerable location. The response of the containment has been obtained in terms of local and global deformation, stresses induced in the concrete and reinforcement and the corresponding material degradation. The results thus obtained corresponding to the geometric model and the reaction-time curve have been compared and discussed. The effect of fire induced due to aircraft crash has also been studied against Boeing 707-320 and Boeing 747-400 aircrafts. The reaction-time response curve obtained against curved flexible target was applied on the surface of containment based on the average area approach. The impact of aircraft was considered to occur up to the point the engines came in contact. Thereafter the fire was assumed to have spread since the majority of the aircraft fuel is stored in and around the wings. The fire effect was considered to be most severe at the bottom of containment up to 10 m height from the base. This is due to the fact that most of the fuel will immediately flow down to the bottom of containment. In the impact region, moderate fire effect has been considered since some of the fuel will also burn in this region immediately after the breach of the fuel tank. The heat transfer analysis has been carried out to obtain the nodal temperature in the concrete as well as reinforcement elements. The fire duration has been considered to be 2hrs for Boeing 707-320 and 3hrs for Boeing 747-400. Thereafter, thermal stress analysis has been performed considering the deformed geometry of the containment due to aircraft impact as the initial state. The stresses due the induced fire have been found to cause the iv local scabbing of concrete leading to exposure of reinforcement. However, the induced fire has not been found to affect the global behavior of the containment structure.