NuScale Plant Safety in Response to Extreme Events

The extreme events that led to the prolonged electrical power outage and finally to sever damage of four units of the Fukushima nuclear plant have highlighted the importance of ensuring a technical means for stable, long-term cooling of the nuclear fuel and the containment following a complete station blackout. This paper presents an overview of the advanced passive safety systems designed for the NuScale nuclear power plant and their role in addressing extreme events. The NuScale plant may include up to 12 power modules, and each module incorporates a reactor pressure vessel (core, steam generator, and pressurizer) and a containment vessel that surrounds the reactor vessel. During normal operation, each containment vessel is fully immersed in a water-filled, stainless steel-lined concrete pool that resides underground. The pool, housed in a Seismic Category I building, is large enough to provide 30 days of core and containment cooling without adding water. After 30 days, the core decay heat generation is so small that the natural convection heat transfer to air at the outside surface of the containment, coupled with thermal radiation heat transfer, are completely sufficient to remove the core decay heat for an unlimited period. These passive safety systems can perform their function without requiring an external supply of water or electric power. Computational and experimental assessments of the NuScale passive safety systems are being performed at several institutions, including the one-third scale NuScale integral system test facility at Oregon State University.