Integrated use of modeling and simulation in high winds PRA

It is generally recognized that high winds pose a significant externally induced hazard to nuclear power plant (NPP) sites. In general, the high wind hazard occurs in three distinct forms: (1) tornados, (2) tropical storms (i.e. hurricanes / typhoons), and (3) straight-line winds (e.g. from thunderstorms and extratropical cyclones). In response to the accident at the Fukushima Dai-ichi NPP, many regulatory authorities and NPP owner / operators are reevaluating NPP vulnerabilities to high wind hazards. Additionally, plant operating experience has shown that high wind events have been responsible for extended losses of offsite AC power (ELAP) events at several NPP sites while simultaneously impacting the plant's ability to cope with these events. An important conclusion obtained from such reassessments is the need to develop methods and tools that can support the assessment of the high wind hazard (including the effects of wind borne missiles) and its effects on NPP risk and safety in a manner that is both efficient and cost effective. For application to high wind hazards, a number of physical phenomena are relevant to assessing the hazard and determining its impact at a particular NPP. These physics can be categorized into the following: (1) aerodynamics of the local windfield experienced at the NPP site, (2) structural loading and response of plant SSCs to the effects of the wind forces, (3) dynamics of wind generated missiles (lift, flight trajectory, etc.) and whether or not they impact particular plant SSCs, (4) damage imparted to plant SSCs due to impact with a wind-borne missile (SSC fragility). Thus, high winds represent a set of phenomena that has the characteristics of being multi-physics and multi-scale. This is particularly true when the direct effects of the windfield are combined with other correlated hazards such as locally intense precipitation and wind generated missiles. The phenomena are also highly complex in both the spatial and temporal realms. In this paper we provide a summary of relevant high wind events that have had significant impact on operational NPPs. We also describe the need to develop an integrated approach to addressing the issue of risk assessment of high winds on NPP risk and safety. Finally, we provide the results of preliminary research on application of smooth particle hydrodynamics (SPH) as a framework to conduct such assessment.