S7G reactorThe S7G was a design of United States Naval reactor that did not use control rods. It was tested in the early 1980s at the Modifications and Additions Reactor Facility (MARF). It consisted of a unique reactor core installed in a modified S5W reactor plant.
Instead of moveable hafnium-based control rods, which are used in all other United States Navy reactors, reactivity in the S7G core was controlled by stationary hafnium-clad tubes partially filled with water. Water could be pumped from the portion of the tube inside the core up to a reservoir above the core, or allowed to flow back into the tube. A higher water level in the tube within the core slowed more neutrons, allowing them to be captured by the hafnium, thus lowering the power level. The pumping system was configured so that the pump needed to run continually to keep the level pumped down; on loss of power, all the water would flow back into the tube, shutting down the reactor. The design had the advantage of negative feedback: an increase in reactor power caused the water to expand, thermalizing more neutrons, lowering the power.
The S7G was never used on a ship, and the prototype was fitted with rods in the late 1980s.