Nuclear Power Generation

Pressurized Water Reactor

A simplified diagram of a modern pressurized-water nuclear reactor. The energy released during nuclear fission heats pressurized water in the primary coolant system. Pipes carry the super-heated liquid through the steam generator in the secondary coolant loop, where water in the secondary system vaporizes to steam. The steam spins a series of turbines, thereby generating electricity. SM-1 pioneered this type of design but on a much smaller scale than today’s commercial nuclear power plants.

How Nuclear Fission Generates Power

Uranium, the fuel in a nuclear reactor, is naturally unstable. When a foreign neutron strikes an atom of uranium, the atom captures it, splits (fissions) into two lighter atoms, expels new neutrons, and releases energy. The newly-created neutrons hit other uranium atoms, creating a self-sustaining chain reaction. The energy released from the chain reaction heats water into steam, which powers a turbine and generates electricity. Controlling the rate of fission is done with a moderator, like graphite or water, to absorb or slow down neutrons to prevent rampant chain reactions.

Reactor Designs

There are a number of nuclear reactor designs, all of which have advantages and disadvantages. The Army concentrated on three types:

Pressurized Water Reactor

The Navy’s nuclear warships and all but two Army nuclear reactors were of this design, the most common type of nuclear power reactor today. In this design, cooling water (under high pressure to prevent it from boiling) passes through the reactor core and is heated to roughly 600 degrees Fahrenheit. Pipes then carry this super-heated water through a secondary water system to create steam which then powers turbines to generate electricity. This design’s advantage is that the radioactive cooling water is confined to the primary system and does not contaminate other parts of the structure. Its disadvantages include the costs of the secondary water system, the loss of efficiency (compared to a boiling water reactor), and the substantial maintenance requirements caused by the high heat, high pressure, and exposure to radiation, all of which slowly degrade the reactor vessel.

Boiling Water Reactor

Like the Pressurized Water Reactor, ordinary water cools and moderates the reactor. But it is this water, not water in a secondary system, that boils, turns to steam, and feeds the turbine that spins the generator. With just one water system, this reactor is simpler and more efficient, but the water is radioactive and contaminates the turbine and condensing equipment. The Army-operated SL-1 reactor in Idaho used this design.

Gas Turbine Reactor

In this design, used for the Army’s mobile ML-1 reactor, pressurized nitrogen gas cools the reactor and operates the turbine to generate electricity while pressurized hot water serves as the reactor moderator in a separate closed system. The design was innovative, highly efficient, and small—two flatbed trucks could move it—but it was also complex and difficult to operate and maintain. Although the ML-1 functioned from 1960 to 1965, its technical challenges proved insurmountable.

Apr. 2017


Visit the other pages of the Army Nuclear Power Program Virtual Exhibit

SM-1 Booties

Exhibit Home

SM-1 Dedication

Origins of the ANPP

SM-1 Control Panel

SM-1 Ft. Belvoir


MH-1A Sturgis


Experimental Reactors


Shutdown & Legacy