Nuclear electric rocket
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In a nuclear electric rocket, nuclear thermal energy is changed into electrical energy that is used to power one of the electrical propulsion technologies. So technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes have been proposed.
One of the more practical schemes is a variant of a pebble bed reactor. It would use a high mass-flow nitrogen coolant near normal atmospheric pressures. This leverages highly-developed conventional gas turbine technologies. The fuel for this reactor would be highly enriched, and encapsulated in low-boron graphite balls probably 5-10 cm in diameter. The graphite serves to slow, or moderate, the neutrons.
This style of reactor can be designed to be inherently safe. As it heats, the graphite expands, separating the fuel and reducing the reactor's criticality. This property can simplify the operating controls to a single valve throttling the turbine. When closed, the reactor heats, but produces less power. When open, the reactor cools, but becomes more critical and produces more power.
The graphite encapsulation simplifies refueling and waste handling. Graphite is mechanically strong, and resists high temperatures. This reduces the risk of an unplanned release of radioactives.
Since this style of reactor produces high power without heavy castings to contain high pressures, it is well suited to spacecraft.
See also: spacecraft propulsion, nuclear reactor