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 radioactive elements, including fission products. Since this style of reactor produces high power without heavy castings to contain high pressures, it is well suited to power spacecraft. Novel electric propulsion concepts A variety of electric propulsion technologies have been proposed for use with high power nuclear electrical generation systems, including VASIMR, DS4G, and pulsed inductive thruster (PIT). PIT and VASIMR are unique in their ability to trade between power usage, specific impulse (a measure of efficiency, see specific impulse) and thrust in-flight. PIT has the additional advantage of not needing conditioned power. Electrical generation A number of heat-to-electricity schemes have been proposed. In the near term, Rankine cycle, Brayton cycle, and Stirling cycle generators go through an intermediate mechanical phase, with attendant energy losses. More exotic technologies have also been proposed: thermoelectric (including graphene-based thermal power conversion), pyroelectric, thermophotovoltaic, thermionic and magnetohydrodynamic type thermoelectric materials. Other types of nuclear power concepts in space Radioisotope thermoelectric generators, radioisotope heater units, radioisotope piezoelectric generators, and the radioisotope rocket all use the heat from a static radioactive source (usually Plutonium-238) for a low level of electric or direct propulsion power. Other concepts include the nuclear thermal rocket, the fission fragment rocket, nuclear pulse propulsion, and the possibility of a fusion rocket, assuming that nuclear fusion technology is developed at some point in the near future. See also Electrically powered spacecraft propulsion Ion thruster Magnetic sail Nuclear pulse propulsion Nuclear thermal rocket Nuclear reactor Polywell Radioisotope thermoelectric generator Spacecraft propulsion References Nuclear spacecraft propulsion Nuclear technology