NASA's program for designing nuclear reactors for use in space is currently studying three basic concepts, recently discussed at a meeting of the Intersociety Energy Conversion Engineering Conference:
* A lithium-cooled fast reactor with thermoelectric conversion. This design was selected by General Electric and Westinghouse. The reactor core is about 12 inches in diameter and 16 inches deep and is filled with a latticework of pins containing uranium fuel. When operating, the core is filled with liquid lithium, a lightweight metal. Pumped through the core, it removes the heat.
Control of the nuclear chain reaction comes from control rods in the core and drums around the outside. The rods are made from material that dampens the reaction. When fully inserted, they prevent the reaction from taking place. The drums serve the same function. On one side, they are coated with the same material as the rods. On the other side, they are coated with material that increases the reaction rate. By rotating the drums to different positions, the power level in the reactor can be altered and controlled.
The miniature reactor sits at the tip of what looks like a large metal umbrella, about a third open. The umbrella is the radiator required to get rid of the waste heat. It also contains cells of a special material that converts some of the heat to electricity.
* A thermionic reactor system. Being pursued by GA Technologies and Martin Marietta Aerospace, the system consist of flashlight-battery-sized cells. Inside each cell is a core of enriched uranium encased in tungsten, the metal used for light-bulb filaments. This is supported inside the cell, so that there is a gap between the tungsten and the wall of the outer casing. When the tungsten is heated it gives off a stream of electrons, which are absorbed by the casing wall , creating an electric current.
The cells are stacked in tubes and the tubes arranged in a lattice in the reactor core. Electrical leads coming from the top of each tube draw off the electrical output, while coolant flows between the tubes, carrying waste heat to an umbrella-shaped radiator. Like the fast reactor, the nuclear reaction is regulated by control rods in the core and control drums around its periphery.
* A lithium-cooled fast reactor with a Stirling engine. Under study by Rockwell International, the reactor itself is nearly identical to that being developed by General Electric. The difference is how the heat is converted into electricity. The Stirling engine was invented more than 150 years ago. Unlike the more familiar gasoline or steam engine, which take in air and exhaust it, the working fluid in the Stirling (typically air or helium) is sealed in. Heating one portion of the engine and cooling another causes pressure fluctuations that can drive a piston, turn a crank, and do mechanical work. It can do this with an impressive 40 to 50 percent efficiency, far higher than other methods of energy conversion.