Nuclear Electric Propulsion
Nuclear Electric Propulsion (NEP) uses a nuclear reactor to generate electricity to power an Ion thruster. It requires some kind of converter from heat to electricity and is subject to thermodynamic limitations. The reactor usually needs to be cooled using radiators. The exhaust velocity is generally higher than for Nuclear Thermal Propulsion so the engine requires less propellant. However, the mass of the required radiators and energy conversion systems reduces the overall effectiveness.
NEP is in direct competition with Solar Electric Propulsion (SEP). As technologies change and evolve either SEP or NEP are advantageous. For missions past the asteroid belt and Jupiter, SEP is no longer viable du to the reduced solar constant and NEP is the most likely solution for larger scale missions that require stopping and maneuvering at the target planet. Near Earth and towards the inner solar system SEP is usually less expensive in terms of mass and is more advantageous in terms of development costs, as there are currently no nuclear reactor available for space propulsion, while photovoltaic solar is a mature technology, readily available.
Fissile materials are nature's batteries, charged up at their time of creation in supernovae explosions. Although it may not be possible to design reactors and engines that extract all of the potential energy from these materials, what we can get out of them is enough for the needs of the Martina transfer vehicles. For a 1 MW power source, lasting 10 years, a minimum of 10 kg of radioactive material is required. In practice, extracting 10% of the energy (known as the burn up fraction) is currently feasible with breeder type reactors, so about 100 kg of uranium or thorium should suffice for the needs of a ship or settlement for a decade of operations. Higher burn fraction are probably possible with traveling wave reactor designs.
Available energies in radioactive materials
- Uranium: 80 000 GJt/kg
- Thorium: 79 400 GJt/kg
- Plutonium: 2 239 GJt/kg
- Tritium decay: 583 GJt/kg
The efficiency of the reactor might be about 40% or 0,4, and burn up fraction 10%.
So for a 1 MWe reactor powered by uranium:
1 MW x 10 yrs x 8760 hrs/ year x 3600 seconds/hour = 3,13e8 MJ ÷ 8e5 GJ/kg ÷ 0,4 ÷ 0,1 ≈ 100 kg of nuclear fuel
- TerraPower, L. L. C. (2010). Traveling-wave reactors: a truly sustainable and full-scale resource for global energy needs. In Proceeding of ICAPP (Vol. 2010).
- Wikipedia, Energy density