Nuclear reactor. Fuel rods being loaded into a nuclear power station's reactor core. The reactor itself is immersed beneath water (blue) and this circular area is used to access the core to remove and add fuel rods. Some of the 205 rods used here are seen forming an array inside the core. The rods contain uranium and plutonium fuel, used to produce a controlled nuclear fission reaction. This releases large amounts of energy from a small amount of fissionable material. The water is used as both moderator (to control the reaction rate) and coolant (to absorb and transfer the energy). This is the Chooz B1 nuclear power station in the Ardennes region of France. Brought online in May 2000, this pressurized water reactor generates 1500 megawatts of power. Photographed in 2004.