Nuclear reactor coolants

Coolant	Melting point Melting point The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...	Boiling point Boiling point The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
Light water at 155 bar		345 °C
NaK NaK NaK, or sodium-potassium alloy, an alloy, of potassium , and sodium , is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water and may catch fire when exposed to air, so must be handled with special precautions...  eutectic
785 °C
Sodium Sodium Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...	97.72 °C	883 °C
FLiNaK FLiNaK FLiNaK is the name of the ternary eutectic alkaline metal fluoride salt mixture LiF-NaF-KF . It has a melting point of 454 °C and a boiling point of 1570 °C. It is used as electrolyte for the electroplating of refractory metals and compounds like titanium, tantalum, hafnium, zirconium and their...	454 °C	1570 °C
FLiBe FLiBe FLiBe is a mixture of lithium fluoride and beryllium fluoride . As a molten salt it is proposed as a nuclear reactor coolant, and two different mixtures were used in the Molten-Salt Reactor Experiment....	459 °C	1430 °C
Lead Lead Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...	327.46 °C	1749 °C
Lead-bismuth eutectic Lead-bismuth eutectic Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead and bismuth used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative....	123.5 °C	1670 °C

A nuclear reactor coolant is a coolant

Coolant

A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

 in a nuclear reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 used to remove heat from the nuclear reactor core

Nuclear reactor core

A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.- Description :...

 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 plant

Nuclear power plant

A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

s are light water reactor

Light water reactor

The light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...

s using ordinary water under high pressure as coolant and neutron moderator

Neutron moderator

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

.
About 1/3 are boiling water reactor

Boiling water reactor

The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...

s where the primary coolant undergoes phase change to steam

Steam

Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

 inside the reactor.
About 2/3 are pressurized water reactor

Pressurized water reactor

Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

s at even higher pressure.
Current reactors stay under the critical point

Critical point (thermodynamics)

In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...

 at around 374 °C and 218 bar

Bar (unit)

The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 where the distinction between liquid and gas disappears, which limits thermal efficiency

Thermal efficiency

In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...

, but the proposed supercritical water reactor

Supercritical water reactor

The supercritical water reactor is a Generation IV reactor concept that uses supercritical water as the working fluid...

 would operate above this point.

Heavy water reactor

Heavy water reactor

A pressurised heavy water reactor is a nuclear power reactor, commonly using unenriched natural uranium as its fuel, that uses heavy water as its coolant and moderator. The heavy water coolant is kept under pressure in order to raise its boiling point, allowing it to be heated to higher...

s use deuterium

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 oxide which has similar properties to ordinary water but much lower neutron capture

Neutron capture

Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...

, allowing more thorough moderation.

Molten metal

Fast reactors have a high power density

Power density

Power density is the amount of power per unit volume....

 and do not need neutron moderation. Most have been liquid metal cooled reactor

Liquid metal cooled reactor

A liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation...

s using molten sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

. Lead, lead-bismuth eutectic

Lead-bismuth eutectic

Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead and bismuth used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative....

, and other metals have also been proposed and occasionally used. Mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 was used in the first fast reactor

Clementine (nuclear reactor)

Clementine was the code name for the world's first fast neutron nuclear reactor. It was an experimental scale reactor. The maximum output was 25 kW and was fueled by plutonium and cooled by liquid mercury. Clementine was located at Los Alamos National Laboratory in Los Alamos, New Mexico...

.

Molten salt

Molten salt

Molten salt

Molten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt which is normally liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...

s share with metals the advantage of low vapor pressure

Vapor pressure

Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

 even at high temperatures, and are less chemically reactive than sodium. Salts containing light elements like FLiBe

FLiBe

FLiBe is a mixture of lithium fluoride and beryllium fluoride . As a molten salt it is proposed as a nuclear reactor coolant, and two different mixtures were used in the Molten-Salt Reactor Experiment....

 can also provide moderation. In the Molten-Salt Reactor Experiment

Molten-Salt Reactor Experiment

The Molten-Salt Reactor Experiment was an experimental molten-salt reactor at the Oak Ridge National Laboratory researching this technology through the 1960s; constructed by 1964, it went critical in 1965 and was operated until 1969....

 it even served as a solvent carrying the nuclear fuel.

Gas

Gases have also been used as coolant. Helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity

Heat capacity

Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...

, necessitating rapid circulation. Carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 has also been used in Magnox

Magnox

Magnox is a now obsolete type of nuclear power reactor which was designed and is still in use in the United Kingdom, and was exported to other countries, both as a power plant, and, when operated accordingly, as a producer of plutonium for nuclear weapons...

 and AGR

Advanced gas-cooled reactor

An advanced gas-cooled reactor is a type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant...

reactors. Gases of course need to be under pressure for sufficient density at high temperature.