In the cases when coolant does absorb neutrons, however, the resulting radioactivity should have a short lifetime. Lastly, cost-effectiveness is a relevant consideration for reactors. Note also the coolant affects significant aspects of the reactor itself, such as the operating temperature and pressure, the size of the core, and methods of fuel handling.
Since no coolant qualifies as perfect for all, various substances are used in industry. Below I will cover two common coolants: water and liquid sodium.
Both use light normal water, but with slightly differing cooling mechanisms. In a BWR, the water turns into steam in the reactor core and is then pumped directly to the turbines that power electrical generators. In a PWR, the primary loop of coolant flowing through the core is at very high pressure psi so it will remain a liquid.
This latter method ensures that any radioactivity activated in the coolant remains within the reactor. Because the heat of vaporization that is required for the phase change from liquid to steam limits thermal efficiency, there is currently research being done on a Generation IV supercritical reactor. After a few minutes, the reactor achieves passive shut-down. An even more effective coolant and moderator is heavy water, or deuterium liquid D 2 O , because its absorption cross section is three orders of magnitude smaller than that of hydrogen.
When it comes to fast breeder reactors, molten sodium is the coolant of choice because it causes negligible moderation. The sodium becomes intensely radioactive from contact with the fuel, but it stays contained within the reactor and has a short half-life of approximately 15 hours. However, liquid sodium has significant disadvantages as well: it ignites spontaneous upon contact with the air, and reacts violently with water.
Besides burning, sodium exposed to the air produces aerosols that are highly toxic and can cause equipment damage to the surfaces onto which they are deposited. An alternative to liquid metal is molten salt. This coolant can run at high temperatures for better thermodynamic efficiency, but remains at a low vapor pressure, which reduces the effects of mechanical stress and increases the intrinsic safety of the reactor.
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