The early days of nuclear power involved reactors cooled by a gas – air! Relatively quickly, the unhelpful ramifications of sending air through a reactor and back into the environment became apparent, so reactors were designed with closed-systems that circulated a cooling gas such as carbon dioxide or inert helium. However, ultimately, these designs were unable to compete in the marketplace and water cooled reactors eventually predominated.
A gas, typically helium or carbon dioxide, is heated up by uranium atoms surrounded by graphite. The hot gas produces steam in a boiler with the cooled gas sent back to the core by a blower or fan. Outside air, directly or indirectly, is used to passively remove decay heat from the reactor. Typically, a confinement building is used around the reactor. In the event of pipe breaks or vessel failures, some radioactive material is released to the environment.