Nuclear reactor coolant
| Coolant | Melting point | Boiling point |
|---|---|---|
| Heavy water at 155 bar | 345 °C | |
| NaK eutectic | -11 °C | 785 °C |
| Sodium | 97.72 °C | 883 °C |
| FLiNaK | 454 °C | 1570 °C |
| FLiBe | 459 °C | 1430 °C |
| Lead | 327.46 °C | 1749 °C |
| Lead-bismuth eutectic | 123.5 °C | 1670 °C |
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.
Water
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 (CANDU) use deuterium oxide which has identical properties to ordinary water but much lower neutron capture, allowing more thorough moderation.
Disadvantages
Tritium leak
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[1]
Hydrogen explosion during power outage
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[2]
Molten metal
Fast reactors have a high power density and do not need neutron moderation. Most have been liquid metal cooled reactors using molten sodium. Lead, lead-bismuth eutectic, and other metals have also been proposed and occasionally used. Mercury was used in the first fast reactor.
Molten salt
Molten salts share with metals the advantage of low vapor pressure even at high temperatures, and are less chemically reactive than sodium. Salts containing light elements like FLiBe can also provide moderation. In the Molten-Salt Reactor Experiment it even served as a solvent carrying the nuclear fuel.
Gas
Gases have also been used as coolant. Helium is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity,
Hydrocarbons
Organically moderated and cooled reactors was an early power reactor concept studied. They were not successful.
References
- Sodium as a Fast Reactor Coolant, Thomas Fanning, ANL Compares sodium favorably to lead and helium.
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