S7G reactor

The S7G reactor was a prototype naval reactor designed for the United States Navy to provide electricity generation and propulsion on warships. The S7G designation stands for:

* S = Submarine platform
* 7 = Seventh generation core designed by the contractor
* G = General Electric was the contracted designer

This prototype design was a land-based nuclear reactor that did not use control rods. It was tested in the late 1970s and early 1980s at the Knolls Atomic Power Laboratory's Modifications and Additions to a Reactor Facility (MARF) in Ballston Spa, New York. It consisted of an experimental reactor core installed in a modified S5W reactor plant.

Design and operation

Instead of the movable hafnium-based control rods used in all other United States Naval reactors, reactivity in the S7G core was controlled by stationary gadolinium-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 down into the tube. A higher water level in the tube within the core slowed more neutrons, allowing them to be captured by the gadolinium tube cladding rather than the uranium fuel, 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 also had the advantage of negative feedback: an increase in reactor power caused the water to expand, thermalizing more neutrons in a region where they would not be absorbed by the fuel, lowering the power. Thus, the water level in the tubes controlled average coolant temperature, not reactor power, while an increase in steam demand (caused by, "e.g.", opening the main engines' throttles) would automatically increase reactor power without action by the Reactor Operator.

The S7G reactor was never used on a ship, and the prototype was fitted with rods in the late 1980s when the reactor was refueled.