In the double-circuit power reactors, the reactor coolant enters the steam generator, which produces steam that drives the turbines, and in the single-circuit reactors the coolant (steam or gas) can serve as the working fluid of the cycle of the turbine. In research (for example, materials science) and special reactors (for example, in reactors for the accumulation of radioactive isotopes) the coolant only cools the reactor, the resulting heat is not used.
The following requirements are imposed on heat coolants:
- Weak absorption of neutrons (in thermal reactors) or weakening of them (in fast reactors).
- Chemical resistance in conditions of intense exposure to radiation.
- Low corrosivity with respect to the structural materials with which the coolant is in contact.
- High coefficient of heat transfer.
- High specific heat.
- Low working pressure at high temperatures.
Thermal neutron reactors use water (normal and heavy), water vapor, organic liquids, carbon dioxide as a coolant. In fast nuclear reactors: liquid metals (mainly sodium) and also gases (for example, water vapor, helium). Often, the liquid serves as a coolant, which is also a moderator.
Characteristics of coolants
One of the most common heat carriers is water. Natural water contains a small amount of heavy water (0.017%), various impurities and dissolved gases. The presence of impurities and gases makes the water chemically active with metals. Therefore, before using it as a heat carrier, the water is purified from the impurities by distillation and deaerated, that is, the gases are removed from the water.
In the first circuit, radioactive water circulates. The main source of radioactivity in water are impurities, whose appearance in the water is due to the corrosion of the nodes of the primary circuit and the technological contamination by fissile substances on the external surface of the fuel elements. The concentration of radioactive impurities in the water is reduced by filtration.
The disadvantages of water as a coolant are the low boiling point (100 ° C at a pressure of 1 atm) and the absorption of thermal neutrons. The first drawback is eliminated by increasing the pressure in the primary circuit. The absorption of thermal neutrons by water is compensated by the use of nuclear fuel based on enriched uranium.
Heavy water in its chemical and thermal properties is little different from ordinary water. It practically does not absorb neutrons, which makes it possible to use natural uranium as a nuclear fuel in nuclear reactors with a heavy water moderator.
However, heavy water is still little used in the construction of the reactor due to its high cost.
Of the metallic coolants, sodium is the most developed. This coolant is chemically active with most metals at a relatively low temperature, and this sodium activity is due to the mixture of sodium oxides. Therefore, the sodium is completely cleaned of the oxides, after which it does not react with many metals (Mo, Zr, stainless steel, etc.) at 600-900 ° C.
Of the tested organic liquids, some of the polyphenyls, including diphenyl and triphenyl, were found to be the most stable under conditions of high temperatures and exposure to radiation. However, despite the advantages, these coolants turned out to be too unstable to neutron irradiation, so these reactors were not used industrially.
The main coolant of the gas is carbon dioxide. It is economical, characterized by a higher density and volumetric heat capacity compared to other gases. The corrosive effect of carbon dioxide on metals depends on the oxygen content.