Magnox coolant used

Although helium gas was used in the previous illustration, similar arguments can be made for CO2 as a coolant. Carbon dioxide has been the preferred coolant for the magnox and AGR reactors helium is required for high-temperature reactors using graphite as a moderator and either helium or CO2 might be used in a fast-spectrum reactor. The choice of a gas coolant for a particular reactor can depend on heat-transfer considerations, chemical and radiation stabUity, and interaction effects, or perhaps other factors. 

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