A nuclear power plant (NPP) is a thermal power station in which the heat source is one or more nuclear reactors.
Nuclear power plants are base load stations, which work best when the power output is constant (although boiling water reactors can come down to half power at night).
Their units range in power from about 40 MWe to over 1200 MWe.
New units under construction in 2005 are typically in the range 600-1200 MWe.
As of 2006 there are 442 licensed nuclear power reactors in operation in the world, operating in 31 different countries.
Together they produce about 17% of the world's electric power.