Quantitative risk assessment on fire and explosion impacts for nuclear power plants

Abstract

The International Atomic Energy Agency (IAEA) requires all nuclear power plant operators to identify, assess and evaluate potential hazards either internal or external, including the potential of human-induced events that can directly or indirectly affect the safety, security, and safeguard of the nuclear power plant (NPP). One of the external hazard that the operator of a licensed nuclear reactors has to consider is that of external explosion with potential for consequential damage to the site. In this study, effects of jet fuel (dedocane and butane) and hydrogen gas induced external explosion from aircraft impact on nuclear plants were investigated and analyzed A turbulence model based on Reynolds-averaged Navier-Stokes in the computational fluid dynamic (CFD) solver called Flame Acceleration Simulator (FLACS) and empirical correlations were used to determine the explosion parameters within the plant vicinity. The influence of obstacle separation distance on explosion severity was investigated with the aim of obtaining the minimum safety distance between buildings. The results of the FLACS simulation and empirical data were analysed and evaluated in order to demonstrate the safety assessment based on two generic plants (Fukushima and Horizon nuclear plants). The simulation results of key explosion parameters for hydrogen show a deflagrative overpressure, Pmax of 0.37 bar, and impulse load of 0.022 bar • s at the exterior walls of building structures. The findings showed that the local temperature of about 1523 K and flame speed of 266 m • s"1 from the hydrogen-air explosion. Butane/air explosion causes an overpressure, Pmax of 0.27 bar, with a maximum positive pressure impulse of 0.015 bar • s. An elevated local temperature of 2030 K and a flame speed of 44 m • s"1 are recorded for this fuel. It was found that for a safety consideration regarding the explosion of these fuel gases, a physical distance of 150 m between the explosion source and the target structure should be sufficient to provide protection against their potential hazards. The computed overpressure and impulsive loadings observed are capable of causing substantial structural damages and vulnerabilities. A significantly elevated flame temperature recorded would have a harmful effect on the safety function of structures, systems and components that are needed to execute reactor shutdown. The analysis also showed that consequential damage of explosion overpressure is strongly dependant on the global load of flammable gas volume and plant layout. In this case, 5000 m3 of hydrogen/butane explosion is sufficient to produce a blast load wave for total plant destruction. The findings of this study may be used to evaluate the safety improvement needed at NPP site with regards to risks and consequences associated with external explosion due to aircraft impact. It is also useful in designing the layout of the NPP and placement of relevant items important to safety.