A pressurized water reactor (PWR) is a power reactor so named because the primary coolant that passes through the reactor core is pressurized (about 160 bar) to prevent it from boiling. The uranium-235 fuel is loaded into the reactor in pellets contained by the fuel rods. To prevent an uncontrolled chain reaction, the fuel rods are separated by control rods of graphite. All the rods are loaded into the reactor from above. The primary coolant is heated by the fission reaction in the fuel rods and circulates into a steam generator where it superheats the secondary coolant. The secondary coolant leaves the protective containment vessel and drives turbines, which produce electricity through a generator. A third coolant loop cools the secondary coolant, transferring the heat to a sea, river, or lake. Reducing the temperature of the secondary coolant increases the efficiency of the transfer from the primary to the secondary coolant.