The primary coolant enters the pressure vessel through two 16-in.

The primary coolant in the reactors is carbon dioxide gas.

By contrast, in a boiling water reactor the primary coolant is designed to boil.

The cooled primary coolant is then returned to the reactor vessel to be heated again.

All of the primary coolant is in the liquid phase during normal operation.

That gets transferred to the primary coolant, then to the secondary system.

Energy absorbed from the fast neutrons is extracted and passed into the primary coolant.

Alternative designs use lead instead of sodium as the primary coolant.

In fact, it is practical to route the primary coolant directly to power generation turbines.

The following year, however, "the K-64 suffered a major reactor problem when the liquid metal in the primary coolant hardened".