Seven Layers of Defense
NuScale’s design incorporates seven layers of defense to prevent the release of contamination into the environment, including: 1) oxide fuel pellet and cladding; 2) reactor vessel; 3) containment vessel; 4) reactor pool; 5) underground stainless steel lined concrete pool walls and floor; 6) biological shield; and 7) an HVAC filtered Seismic Category 1 reactor building. The reactor building and reactor pool are described below.
The reactor building houses the systems and components required for plant operation and shutdown. The reactor building is a Seismic Category 1 reinforced concrete structure designed to withstand the effects of aircraft impact, environmental conditions, natural phenomena, postulated design basis accidents, and design basis threats. The reactor building also provides radiation protection for plant operation and maintenance personnel.
Portions of the reactor building are located above and below grade. The NuScale Power Modules, reactor pool, and the spent fuel pool are located at or below nominal plant grade level, while the hoisting and handling equipment is located above grade. Also located below grade are the Class 1E batteries for post-accident monitoring, main control room, most primary systems, and some radioactive waste equipment.
The surface of the reactor pool water is located at approximately ground level. At a 12-module facility, the NuScale Power Modules are installed in a vertical position and are arranged into two rows of 6 modules along the reactor pool walls. Concrete walls separate the modules in individual reactor bays. An extra reactor bay is located adjacent to the units either for module maintenance or storage of a possible spare module. A central channel is provided between the rows of modules to allow for moving of the modules between the reactor pool and the connected refueling pool.
Pipes interfacing with the NuScale Power Module (i.e., feed water piping, steam piping, the chemical and volume control system, containment evacuation system, instrumentation, and power connections) are located above the water level. Pipe fittings are provided in this area to permit manual connection and disconnection during module installation, refueling outages, and during replacement or removal of modules.
To the maximum extent practical, equipment rooms or vaults within the reactor building are partitioned to provide separation between primary and supporting systems for each NuScale Power Module.
The reactor pool consists of a large, below-grade concrete pool with a stainless steel liner that provides stable cooling for the NuScale Power Modules for an unlimited period of time following any actuation of the Emergency Core Cooling System (ECCS). During normal plant operations, heat is removed from the pool through a closed loop cooling system and ultimately rejected into the atmosphere through a cooling tower or other external heat sink. In an accident where AC power is lost, core decay heat is transferred to the reactor pool by passive safety systems and, over time, the pool will gradually heat up and begin to boil. Water inventory in the reactor pool is large enough to remove core decay heat from all the NuScale Power Modules for an unlimited period of time without adding water. After an extended period the pool inventory will have been reduced and the remaining heat in each module will be less than 200KW (the same as 200 common hair dryers), which transitions to air-cooling by the natural circulatory flow in the operating bays. In the evolution of a design-basis accident scenario the reactor core NEVER becomes uncovered with water.