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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.
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The three primary objectives of nuclear reactor safety systems as defined by the U.S. Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material. A reactor protection system is designed to immediately terminate the nuclear reaction. By breaking the nuclear chain reaction, the source of heat is eliminated. Other systems can then be used to remove decay heat from the core. All nuclear plants have some form of reactor protection system.
Steam generator (nuclear power)
A steam generator (aka nuclear steam raising plant ('NSRP')) is a heat exchanger used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactor between the primary and secondary coolant loops. In typical PWR designs, the primary coolant is high-purity water, kept under high pressure so it cannot boil. This primary coolant is pumped through the reactor core where it absorbs heat from the fuel rods.
Liquid metal cooled reactor
A liquid metal cooled nuclear reactor, or LMR is a type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for breeder reactor power generation. They have also been used to power nuclear submarines. Due to their high thermal conductivity, metal coolants remove heat effectively, enabling high power density. This makes them attractive in situations where size and weight are at a premium, like on ships and submarines.