The accidental or intentional crash upon structures of Nuclear Power Plant (NPP) by an aircraft cause the fuel spreading followed by a fireball formation. This fireball is large enough to engulf the entire NPP and radiates a large amount of heat. This engulfment may lead to a local rise in temperature, which causes the spallation of concrete structure and fatalities to the human being. This may affect the integrity of NPP structures and has safety implications. The building structures influence the spatial evolution of fireball due to the generation of large turbulent structures, which further increases the local temperature. The evolution of fireball following fuel dispersion due to the aircraft crash upon an NPP is presented in this article. Numerical simulation to study the effect of fireball on the target NPP structure and its surrounding has been performed using a three-dimensional computational fluid dynamics (CFD) code. This analysis is used to study the evolution of fireball and thermal hazard associated with the radiated heat. It is found that some parts of fireball energy go as heat input to containment and the remaining portion is dissipated to the atmosphere by convection and radiation.