The S7G reactor was a

prototype A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...

naval reactor Naval Reactors (NR), also known as the Naval Nuclear Propulsion Program, is an umbrella term for the U.S. government office that has comprehensive responsibility for the safe and reliable operation of the United States Navy's nuclear propulsion p ...

designed for the

United States Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

to provide

electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delivery (Electric power transmi ...

and

propulsion Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

warship A warship or combatant ship is a naval ship that is built and primarily intended for naval warfare. Usually they belong to the armed forces of a state. As well as being armed, warships are designed to withstand damage and are usually faster a ...

s. The S7G designation stands for: * S =

Submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

platform * 7 = Seventh reactor designed by the contractor * G =

General Electric General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...

was the contracted designer This prototype design was a land-based

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...

that did not use

control rods Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing ...

. It was tested in the late 1970s and early 1980s at the Modifications and Additions to a Reactor Facility (MARF) plant located at the Knolls Atomic Power Laboratory's Kesselring Site in

Ballston Spa, New York Ballston Spa is a village and the county seat of Saratoga County, New York, United States, located southwest of Saratoga Springs. The population of the village, named after Rev. Eliphalet Ball, a Congregationalist clergyman and an early settler, wa ...

. It consisted of an experimental reactor core installed in a modified

S5W reactor The S5W reactor is a nuclear reactor used by the United States Navy to provide electricity generation and propulsion on warships. The S5W designation stands for: * S = Submarine platform * 5 = Fifth generation core designed by the contractor * ...

plant.

Design and operation

Instead of the movable

hafnium Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...

-based

control rod Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing ...

s used in all of the other United States Naval reactors, reactivity in the S7G core was controlled by stationary

gadolinium Gadolinium is a chemical element with the symbol Gd and atomic number 64. Gadolinium is a silvery-white metal when oxidation is removed. It is only slightly malleable and is a ductile rare-earth element. Gadolinium reacts with atmospheric oxygen ...

-clad tubes partially filled with water. Water could be

pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...

ed 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 slowed more

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

s in the core, causing more neutron capture by the gadolinium tube cladding rather than by the

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

fuel, thus lowering the power level. The system was configured with the pump running continually to keep the water level low; on loss of electrical power, all of the water would flow back into the tube, shutting down the reactor. As with all small

pressurized water reactor A pressurized water reactor (PWR) is a type of light-water reactor, light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary ...

s, the design also had the advantage of

negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...

: an increase in reactor power caused the water to expand, leading to reduced

thermalization In physics, thermalisation is the process of physical bodies reaching thermal equilibrium through mutual interaction. In general the natural tendency of a system is towards a state of equipartition of energy and uniform temperature that maximizes ...

of neutrons and lowering absorption by the fuel, therefore lowering the power. Thus, changes in the average coolant temperature, notably from the steam demand of engine throttles, naturally maintains reactor power without intervention from a reactor operator. The S7G reactor was never used on a ship. In the late 1980s the S7G core was replaced with the experimental DMC (Developmental Materials Core)

References

{{DEFAULTSORT:S7g Reactor United States naval reactors