<P> Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator . About 1 / 3 are boiling water reactors where the primary coolant undergoes phase transition to steam inside the reactor . About 2 / 3 are pressurized water reactors at even higher pressure . Current reactors stay under the critical point at around 374 ° C and 218 bar where the distinction between liquid and gas disappears, which limits thermal efficiency, but the proposed supercritical water reactor would operate above this point . </P> <P> Heavy water reactors (CANDU) use deuterium oxide which has identical properties to ordinary water but much lower neutron capture, allowing more thorough moderation . </P> <P> As the hydrogen atoms in water coolants are bombarded with neutrons, some absorb a neutron to become deuterium, and then some become radioactive tritium . Water contaminated with tritium sometimes leaks to groundwater by accident or by official approval </P> <P> The fuel rods create high temperatures which boil water then turn water to steam . During a disaster, when a power outage happens and diesel power generators which provide emergency power to the water pump are damaged by a tsunami or an earthquake, if no fresh water is being pumped to cool the fuel rods then the fuel rods continue to heat up . Once the fuel rods reach more than 1200 degrees Celsius, the zirconium tubes that contain the nuclear fuel will interact with the steam and split the hydrogen from the water . That hydrogen can then be released from the reactor core and containment vessel . If that hydrogen accumulates in sufficient quantities - concentrations of 4 percent or more in the air, then that hydrogen can explode, as has apparently occurred at Fukushima Daiichi reactors No. 1, 3, 4 but reactor No. 2 opened its vent to let out radioactive hydrogen gas, decreasing the pressure of the hydrogen, but it contaminated the environment, so reactor No. 2 did not explode </P>

What type of coolant usually is used to remove heat from a nuclear reactor core