Nuclear reactor coolant

Nuclear reactor coolants

Coolant	Melting point	Boiling point
Heavy waterat 154 bar		345 °C
NaKeutectic	-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

Anuclear reactor coolantis acoolantin anuclear reactorused to remove heat from thenuclear reactor coreand transfer it toelectrical generatorsand theenvironment. Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-termradioactivityfrom the reactor.

Water

Almost all currently operatingnuclear power plantsarelight water reactorsusing ordinary water under high pressure as coolant andneutron moderator. About 1/3 areboiling water reactorswhere the primary coolant undergoesphase transitiontosteaminside the reactor. About 2/3 arepressurized water reactorsat even higher pressure. Current reactors stay under thecritical pointat around 374 °C and 218barwhere the distinction between liquid and gas disappears, which limitsthermal efficiency,but the proposedsupercritical water reactorwould operate above this point.

Heavy water reactorsusedeuteriumoxide which has identical properties to ordinary water but much lowerneutron capture,allowing more thorough moderation.

Disadvantages

Tritium leak

As the hydrogen atoms in water coolants are bombarded with neutrons, some absorb a neutron to becomedeuterium,and then some become radioactivetritium.Water contaminated with tritium sometimes leaks to groundwater by accident or by official approval.

Hydrogen explosion

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 atFukushima Daiichi reactors No. 1, 3, and 4.

Such an explosion was avoided at Reactor No. 2,which opened its vent to let out hydrogen, decreasing pressure by releasing radioactive hydrogen gas.

Borated water

Borated water is used as a coolant during normal operation ofpressurized water reactors(PWRs) as well as inEmergency Core Cooling Systems(ECCS) of both PWRs andboiling water reactors(BWRs).

Advantages

Boron,often in the form ofboric acidor sodium borate, is combined with water — a cheap and plentiful resource — where it acts as a coolant to remove heat from the reactor core and transfers the heat to a secondary circuit.Part of the secondary circuit is thesteam generatorthat is used to turn turbines and generate electricity. Borated water also provides the additional benefits of acting as aneutron poisondue to its large neutron absorption cross-section, where it absorbs excess neutrons to help control the fission rate of the reactor. Thus, the reactivity of thenuclear reactorcan be easily adjusted by changing the boron concentration in the coolant. That is, when the boron concentration is increased (boration) by dissolving more boric acid into the coolant, the reactivity of the reactor is decreased. Conversely, when the boron concentration is decreased (dilution) by adding more water, the reactivity of the reactor is increased.

Disadvantages

Approximately 90% of thetritiumin PWR coolants is produced by reactions of boron-10 with neutrons. Since tritium itself is a radioactive isotope of hydrogen, the coolant becomes contaminated withradioactive isotopesand must be kept from leaking into the environment. Additionally, this effect must be taken into account for longer cycles of nuclear reactor operation and thus requires higher initial concentration of boron in the coolant.

Molten metal

Fast reactorshave a highpower densityand do not need, and must avoid, neutron moderation. Most have beenliquid metal cooled reactorsusing moltensodium.Lead,lead-bismuth eutectic,and other metals have also been proposed andoccasionally used.Mercurywas used in thefirst fast reactor.

Molten salt

Molten saltsshare with metals the advantage of lowvapor pressureeven at high temperatures, and are less chemically reactive thansodium.Salts containing light elements likeFLiBecan also provide moderation. In theMolten-Salt Reactor Experimentit even served as a solvent carrying the nuclear fuel.

Gas

See also:Gas-cooled reactor

Gases have also been used as coolant.Heliumis extremely inert both chemically and with respect to nuclear reactions but has a lowheat capacity,

Hydrocarbons

Organically moderated and cooled reactorswere an early concept studied, using hydrocarbons as coolant. They were not successful.

References

• Sodium as a Fast Reactor Coolant, Thomas Fanning, ANLCompares sodium favorably to lead and helium.

External links

• Media related toNuclear reactor coolantsat Wikimedia Commons