During normal operation, reactor coolant is not high radioactive. Otherwise it could not be transformed into steam that goes directly to turbines in Boiling Water Reactors.
The pressure vessel of a nuclear reactor contains the reactor core and helps to maintain a high pressure environment to prevent coolant water from boiling. It also serves as a barrier to contain radioactive materials and provide structural support for the reactor.
Helium is used as a coolant in nuclear gas reactors because it is chemically inert, meaning it does not react with other materials in the reactor. It has high thermal conductivity, which helps in transferring heat away from the reactor core efficiently. Helium also has low neutron capture cross-section, making it less likely to absorb neutrons and become radioactive.
A nuclear reactor has two coolant loops to prevent radioactive contamination. The primary loop cools the reactor core to generate power, while the secondary loop transfers heat to produce steam for electricity generation. This dual-loop system ensures that radioactive material from the core does not mix with the water used to generate electricity.
The three main barriers are the fuel cladding, the reactor coolant system, and the containment building. The fuel cladding encases the nuclear fuel to prevent the release of radioactive materials, while the reactor coolant system circulates water to cool the reactor and remove heat. The containment building is the final barrier that surrounds the reactor to prevent the release of radioactive materials in case of an accident.
For water reactors the danger would be mainly from the high temperature, escaping water would flash to steam and scald anyone nearby. CO2 cooled reactors would also present high temperature and risk of asphyxiation. There might be some radioactivity, but not a huge amount as failed fuel would have been removed before it became high enough to be dangerous. Of course if a Loss of Coolant (LOCA) occurred resulting in fuel failure, there might be more activity released, but design is aimed at preventing a LOCA, even in extreme cases such as an earthquake.
In a properly operating nuclear reactor, water used to cool the reactor is not contaminated. This water, called primary coolant, is quite pure. And after shutdown and cooldown, the water has little radiation in it. But if the reactor has some malfunction that overheats the fuel, fuel elements can rupture or melt (a meltdown) and fission products, which are hightly radioactive, can be released into the coolant (the water). The water is then contaminated.
The pressure vessel contains the reactor core with its fuel, coolant, moderator, control elements, and emergency systems. The turbine is the device that on receiving the steam it spins and turns the electric generators to produce electricity.
Heat is eliminated through use of reactor coolant passing over the nuclear fuel in the primary coolant loops.
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The amount of coolant used in a nuclear power reactor can vary depending on the design of the reactor. Generally, a nuclear power reactor may use thousands to millions of gallons of water or a different coolant material to remove heat generated during the nuclear fission process. The coolant circulates through the reactor core to transfer heat and help regulate the temperature of the reactor.
In my understanding, this is because a fusion reactor reacts deuterium to produce helium, which is not radioactive, whereas a fission uses uranium or plutonium, for example, which may react to form various radioactive isotopes. A fusion reactor may contain small quantities of tritium, in which case a radioactive isotope of hydrogen may be produced, but given that the majority of reactions occurring involve solely the deuterium, there is less radioactive waste produced.
At Chernobyl there was a steam explosion which blew off the top of the reactor followed by a fire due to the graphite moderator burning in air, and a huge amount of radioactive material was discharged, including fission products from the fuel. I'm not sure which fast reactor you are referring to, but certainly the incident did not involve massive catastrophic failure of the reactor vessel, or it would have become a world incident as Chernobyl did.