| Coolant | Melting point | Boiling point |
|---|---|---|
| Heavy water at 154 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 |
Anuclear reactor coolant is acoolant in anuclear reactor used to remove heat from thenuclear reactor core and transfer it toelectrical generators and theenvironment.Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-termradioactivity from the reactor.
Almost all currently operatingnuclear power plants arelight water reactors using ordinary water under high pressure as coolant andneutron moderator.About 1/3 areboiling water reactors where the primary coolant undergoesphase transition tosteam inside the reactor.About 2/3 arepressurized water reactors at even higher pressure.Current reactors stay under thecritical point at around 374 °C and 218bar where the distinction between liquid and gas disappears, which limitsthermal efficiency, but the proposedsupercritical water reactor would operate above this point.
Heavy water reactors usedeuterium oxide which has identical properties to ordinary water but much lowerneutron capture, allowing more thorough moderation.
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.[1]
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 atReactor No. 2, which opened its vent to let out hydrogen, decreasing pressure by releasing radioactive hydrogen gas.[2]
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).[3][4][5]
Boron, often in the form ofboric acid or 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.[6] Part of the secondary circuit is thesteam generator that is used to turn turbines and generate electricity. Borated water also provides the additional benefits of acting as aneutron poison due 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 reactor can 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.[7]
Approximately 90% of thetritium in 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 isotopes and 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.[7]
Fast reactors have a highpower density and do not need, and must avoid, neutron moderation. Most have beenliquid metal cooled reactors using moltensodium. Lead,lead-bismuth eutectic, and other metals have also been proposed andoccasionally used.Mercury was used in thefirst fast reactor.
Molten salts share with metals the advantage of lowvapor pressure even at high temperatures, and are less chemically reactive thansodium. Salts containing light elements likeFLiBe can also provide moderation. In theMolten-Salt Reactor Experiment it even served as a solvent carrying the nuclear fuel.
Gases have also been used as coolant.Helium is extremely inert both chemically and with respect to nuclear reactions but has a lowheat capacity,
Organically moderated and cooled reactors were an early concept studied, using hydrocarbons as coolant. They were not successful.
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Media related toNuclear reactor coolants at Wikimedia Commons
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