Westinghouse AP1000 Pressurized Water Reactor (PWR)
The AP1000, which produces 1117 MWe electric, is a two-loop pressurized water reactor with the following main building structures: a nuclear island turbine, an annex, a diesel generator and the radwaste area. The nuclear island is the only Seismic Category 1 building, and consists of a steel containment vessel, a concrete shield and an auxiliary building. The containment vessel is a free-standing steel structure, and in case of an accident would prevent the uncontrolled release of radioactivity into the environment. The reinforced concrete cylinder-shaped shield surrounds the vessel and provides natural convection cooling of the containment. It also supports the water storage tank above, is passive gravity-fed, and provides a chimney for protection from the outside environment and a barrier against radiation release.
AP1000 Safety Systems
The guts to the Generation III+ nuclear power plant are the safety systems and this is where most of the largest design changes occurred. Requiring no operator actions to mitigate design-basis accidents, passive, gravity-fed, natural flow systems are designed into the AP1000. Fukushima needed back up diesels to initiate and sustain emergency pumps, whereas the AP1000 does not, and Fukushima had no long term gravity fed cooling system while the AP1000 does.
Another glaring difference between this model and previous nuclear power designs is the number of penetrations into the containment. This design has 50 percent fewer penetrations, which translates into fewer potential sources for leakage. Couple this with the penetrations failure mode being closed rather than open, and it means that no power needed to close the penetrations. In addition, the containment is steel rather than concrete. There is a definite increase in safety where lessons learned from the Fukushima and Three Mile Island accidents have been applied.
Future Nuclear Power Plant Design Modifications
During the Fukushima event a spent fuel pool fire occurred. Analysis of the maximum heat load of a full core off loading immediately following a refuelling was used in the design criteria of the AP1000 for the minimum water level required to keep the fuel assemblies covered. For the long term, in case of a station blackout, cooling can be provided through safety-related makeup connections to the passive containment cooling system and gravity fed. These connections must be implemented by station personnel. Maybe an issue for future design modification considerations? Maybe not.
New Nuclear Power Reactors Approved
On February 9, 2010, the NRC approved the construction of two new reactors at the Southern Company Vogtle site. These reactors will be the first in the United States in over 30 years, and the first of the new design, Generation III+ Nuclear Reactor. GE Nuclear is also designing Generation III+ Reactors, but Westinghouse AP1000 was the first to obtain a license in the U.S.
Lessons learned from the Three Mile Island, Chernobyl and Fukushima accidents drive the new designs towards natural, passive safety systems, requiring no power nor operator action and a simplified design. Only time will tell if more is needed. But one thing is for sure; the NRC, watchdog of nuclear power within the U.S., will keep a sharp eye on the construction of the plants and the safety system upgrades.
Nuclear Regulatory Commission. NRC Auxiliary Systems AP1000 Design Control Document CHAPTER 9. (2010). Accessed February 21, 2012.
Nuclear Street. Nuclear Power Plants. (2012). Accessed February 21, 2012.
Llanos, M. US licenses first nuclear reactors. (2012). MSNBC. Accessed February 21, 2012.
Cummins, W.E., et al. Westinghouse AP1000 Advanced Passive Plant. (2003) Westinghouse Electric Company, LLC. Accessed February 21, 2012.
US. Department of Energy. The Westinghouse AP1000 Advanced Nuclear Plant. (2003). Westinghouse Electric Company, LLC. Accessed February 21, 2012.
Southern Company. Nuclear Energy The Next Generation. (2012). Accessed February 21, 2012.
Vartabedian, R., Duncan, I. First new U.S. Nuclear Reactors in decades approved. (2012). Los Angeles Times. Accessed February 21, 2012.
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