Simplified Plant Arrangement
The AP1000 has a smaller footprint than an existing nuclear power plant with the same generating capacity. The plant arrangement provides separation between safety-related and non-safety-related systems to preclude adverse interaction between safety-related and non-safety-related equipment.
Separation between redundant, safety-related equipment trains and systems provides confidence that the safety design functions of the AP1000 can be performed. In general, this separation is achieved by partitioning an area with concrete walls.
The AP1000 plant is arranged with the following principal building structures, each on its own base mat:
Nuclear Island
The nuclear island is designed to meet Seismic Category I structural requirements. The volume of this building is much smaller than the buildings in previous nuclear power plant designs. This provides a large capital cost savings as seismic structures cost roughly three times as much as non-seismic structures. The nuclear island consists of the steel containment vessel, the concrete shield building and the auxiliary building. The nuclear island is designed to withstand the effects of postulated internal events such as fires and flooding without loss of capability to perform safety functions.
The containment vessel is a high integrity, freestanding steel structure with a wall thickness of 1.75 inches (4.44 cm). The containment is 130 feet (39.6 m) in diameter. The ring sections and vessel heads are constructed of steel plates pre-formed in an off-site fabrication facility and shipped to the site for assembly and installation using a large-capacity crane.
The primary containment prevents the uncontrolled release of radioactivity to the environment.
It has a design leakage rate of 0.10 weight percent per day of the containment air during a design basis accident and the resulting containment isolation.
The AP1000 containment contains a 16-foot (4.9m) diameter main equipment hatch and a personnel airlock at the operating-deck level, and a 16-foot (4.9m) diameter maintenance hatch and a personnel airlock at grade level. These large hatches significantly improve accessibility to the containment during outages and, consequently, reduce the potential for congestion at the containment entrances. These containment hatches, located at the two different levels, allow activities occurring above the operating deck to be unaffected by activities occurring below the operating deck.
The containment arrangement provides significantly larger laydown areas than most conventional plants at both the operating deck level and the maintenance floor level. Ample laydown space is provided for staging of equipment and personnel, equipment removal paths, and space to accommodate remotely operated service equipment and mobile units. Access platforms and lifting devices are provided at key locations, as are service provisions such as electrical power, demineralized water, breathing and service air, ventilation and lighting.
Concrete Shield Building -- The AP1000 containment design incorporates a shield building that surrounds the containment vessel and forms the natural convection annulus for containment cooling. This building is a cylindrical, reinforced concrete structure with a conical roof that supports the water storage tank and air diffuser (or chimney) of the passive containment cooling system (PCS). It shares a common base mat with the primary containment and auxiliary building, and is designed as a seismic Category 1 structure. It has an inner diameter of about 140 feet (43m), a height of 73.25 ft (22 m), and a wall thickness of 3 ft (0.9 m) in the cylindrical section.
The two primary functions of the shield building during normal operation are 1) to provide an additional radiological barrier for radioactive systems and components inside the containment vessel and 2) to protect the containment vessel from external events, such as tornados and tornado-driven objects that might impinge on it. As described earlier, under design-basis accident conditions, the shield building serves as a key component of the PCS by aiding in the natural convective cooling of the containment.
Auxiliary Building -- The auxiliary building is designed to provide protection and separation for the Seismic Category 1 mechanical and electrical equipment located outside the containment building. The building also provides protection for safety-related equipment against the consequences of internal and external events. Specifically, the auxiliary building houses the main control room, Class 1E I&C systems, Class 1E electrical systems, fuel handling and spent-fuel handling area, mechanical equipment areas, liquid and gas radwaste areas, containment penetration areas, and main steam and feedwater isolation valve compartments. Large staging and laydown areas are provided outside the two equipment hatches.
Non-seismic Buildings
The following buildings are non-seismic Category 1 structures, and contain no safety-related equipment.
They are designed for wind and seismic loads in accordance with the Uniform Building Code. The foundation of each building is a reinforced concrete mat on grade.
