Nuclear Reactor Coolant

   The nuclear reactor coolant is a substance used to remove heat from the nuclear reactor core. As nuclear reactions take place, the core heats up to enormous temperatures and to avoid damage it needs to be cooled down. Most frequently 2 coolant loops are used. This is so because the primary coolant loop takes up a bit of short-term radiation from the core.

    Almost all nuclear reactors are water cooled reactors. They use simple, plain water as coolant under high pressure. This is so mainly because it is cost effective (cheap :P). Other coolants are: heavy water (instead of one hydrogen molecule it contains a deuterium molecule), mercury, sodium, FLiBe, Lead. 

    Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator. About 1/3 are boiling water reactors where the primary coolant undergoes phase change to steam inside the reactor. About 2/3 are pressurized water reactors at even higher pressure. Current reactors stay under the critical point at around 374 °C and 218 bar where the distinction between liquid and gas disappears, which limits thermal efficiency, but the proposed supercritical water reactor would operate above this point.

    Fast reactors have a high power density and do not need neutron moderation. Most have been liquid metal cooled reactors using molten sodium. Lead and other metals have also been proposed and occasionally used.

    Gases have also been used as coolant. Helium is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity, necessitating rapid circulation. Carbon dioxide has also been used. Gases of course need to be under pressure for sufficient density at high temperature.