EBR II is cooled by liquid sodium, no less than 66,000 gallons of it. There is a primary cooling system whose sodium entering the core is subject to intense gamma ray and neutron bombardment. It transfers its heat to a secondary system whose sodium boils water to generate steam and the electric power. The blanket too must be cooled by liquid sodium. The reactor and first heat exchanger are immersed in the huge tank of liquid sodium.
Control is achieved by lowering or raising rods containing boron carbide.
In an experiment the reactor was going full blast and the sodium pumps were deliberately stopped. The reactor has a negative temperature coefficient of reactivity, and soon shut itself off. This is because the reactor is barely critical, and expansion of the metallic uranium fuel lowers the neutron flux, the opposite of the effect of compression on implosion. There is another fast reactor installation in the U.S., namely the Enrico Fermi Atomic Power Plant, located in Michigan. The hexagonal steel assemblies of pins, now deadly radioactive, are pulled from the top of the core into lead shielding weighing 30 tons.