GEM stands for Gas Expansion Module which is a perfect name for these revolutionary assemblies. Six assemblies containing these pins are distributed symmetrically around the core and at the same radial distance. These pins consist of a hollow pressure tube that is capped at the top and filled with Helium. Sodium coolant is allowed to enter at the bottom of the pin and fill the pressure tube when the coolant pumps are operating.
The GEM's are designed to lower the power level in the core if the main coolant EM pumps malfunction or stop. In this event, the Helium pressure will cause the Sodium level in the pins to drop and thus allow more neutrons to leak out of the core. Since the neutron flux of this core is very sensitive, the lower number of neutrons causes fewer fissions to occur and the power level drops.
