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Concept
Nuclear reactor coolant
Summary
A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment.
Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-term radioactivity from the reactor.
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.
Heavy water reactors use deuterium oxide which has identical properties to ordinary water but much lower neutron capture, allowing more thorough moderation.
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.
Fuel rods create high temperatures which boil water into steam. During a power outage, diesel power generators which provide emergency power to water pumps may be damaged by a tsunami, earthquake or both; 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°C, the zirconium tubes that contain the nuclear fuel will interact with the steam and split hydrogen from water molecules. This hydrogen may leak from breaches in the reactor core and containment vessel. If hydrogen accumulates in sufficient quantities - concentrations of 4% or more in the air - then it can explode, as has apparently occurred at Fukushima Daiichi reactors No. 1, 3, and 4.
Official source
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