Nuclear Power is a method of energy generation. It uses nuclear fuel to produce heat, which is usually transformed into electricity. Nuclear power is considered the preferred energy source for most plans for medium to long term human expeditions to Mars.
The Cost of nuclear energy will vary with time and the colony development. There may be periods when solar is more cost effective. The long delays for the development of nuclear power sources may limit their availability when the first settlements are started. The availability of nuclear fuel on Mars may be limited.
Nuclear reactors produce heat, that can be used by thermal engines to produce electricity, of by chemical reactions to produce hydrogen. Leftover heat, that can range in proportion to 90% for RTG type devices down to 60% for more efficient thermal cycles, can be stored in molten salt thermal storage, or used directly to heat greenhouses, the habitat itself or industrial processes such as ice melting with Rodwells or evaporation processes.
Nuclear reactor designs for Mars
There are a large number of types of nuclear reactor. The development of nuclear reactors has also known a number of design generations, usually classified from generation 1 through generation 2 (mature designs), generation III (optimisation of generation II) to generation 4 (currently under development) and generation 5 (future) reactors. Most reactors considered for Mars are generation 4, as these have a high emphasis on fail safe design, simple maintenance and durability, often with decade long periods between refuelings.
In a generation 2 Light Water Reactor, heat from the radioactive core boils water to create steam. Turbines are driven by the steam's pressure, spinning turbo generators to generate electrical energy. In a generation 4 Molten Salt Reactor, the heat generated from the core is transferred by a molten salt to a heat exchanger, that also boils water or heats an inert gas that turns a turbogenerator. The molten salt provides opportunities of shutting down the reactor passively that do not exist for generation 2 reactors, that depend on actively moving control rods into the core of the reactor to dampen the nuclear reaction.
Although steam powered turbogenerators operating on the Rankine cycle are by far the most common type of thermal systems used to produce electricity from nuclear reactors, inert gas Brayton cycles, Stirling engines and supercritical CO2 turbines have also been identified as possible heat to electricity conversion systems. Thermocouples have are another interesting energy conversion system, as they have no moving parts, but their low efficiency limits their use. Heatpipes are also heat transfer elements that might be used in future nuclear reactors.
The following types of reactors have been identified as possible models for a Mars settlement, of for vehicles on Mars.
Radioisotope thermoelectric generators (abbr.: RTG) are simple devices. They produce a heat difference, transformed by a thermocouple to electrical energy. The maintenance requirements are practically non existent. However, RTGs do not provide enough power for a base, and even less for a settlement. They also have low efficiency, in the order of 10%. Furthermore, RTG have a nuclear core made of plutonium, and the supply for this nuclear material is extremely low, in the order of a few kg for the entire Earth. RTGs do have the distinction of already operating on Mars, providing power to the Curiosity rover and heating some elements of the other Mars Rovers.
Kilopower heat pipe reactors
The Kilopower project designs are Heat-Pipe Reactors  that offer stable, safe power that requires no outside support system or personnel and is immune to meltdown. Their operating efficiency as electrical generators is about 23%. They can be scaled from .5 kW to 50 MW for remote bases, small cities, and mining sites on Earth. On Mars and in space, these reactors could power a Settlement or a Spaceship for between 5 to 40 years with no maintenance except replacement of the Sterling engines and with no in-situ resources needed. Heatpipe Reactors are inherently safe. If no energy is removed, the system stabilizes to a constant temperature that is below the melting temperature of the components, so it will not melt down or change state.   The heatpipes are used to transfer heat from the core to the gas running through the Sterling engines. The fluid in the heat pipes moves by capillarity action, so these have no moving parts, increasing reliability.
However, these reactors do have secondary systems, such as Sterling engines, connected to generators to produce electric power, and radiators to dissipate excess heat. These add to the mass of the reactor, making the system somewhat less advantageous. The kilopower reactors use enriched uranium235 that is cast into a solid steel core. This nuclear fuel is unlikely to be available on Mars and will need to be imported from Earth. As they can produce power unaided they are good candidates for backup power systems, this includes as a 'bootstrap power system' providing the initial energy to operate the mechanics and energize the alternators of a larger main power reactor.
Molten Salt Reactors
Molten Salt Reactors use a liquid salt coolant that extracts energy from a nuclear fuel, that can be a liquid or a solid. The cooling salt is usually at atmospheric pressure, as opposed to a conventional reactor that uses high pressure water as a cooling agent. The high pressure water represents an explosion risk if it overheats beyond the capacity of the pipes that hold it. In contrast, if the molten salt temperature rises too much, it will expand and separate the suspended nuclear fuel particles, slowing the nuclear reaction. As an added safety feature, some designs have, at the lowest point in the core, an opening that is kept shut by an externally cooled plug of frozen salt powered by the reactor itself. If for any reason the external source of cooling is lost, the plug melts and the liquid salt contents of the core are dumped into a storage tank, where the salt is separated by a graphite moderator into a number of compartments. The salt becomes sub-critical and cools down. In the event of a leak, the Salt solidifies quickly allowing for far easier cleanup vs liquid water seeping into the environment. Since no pressurized steam is involved there is no need for a large domed pressure building around the core. For a more detailed explanation from Dr. Kirk Sorensen please see   . Most molten salt reactors use enriched 235U or Th as fuel.
See the page on LFTR for more specifics on Thorium powered molten salt reactors, and why they are especially suited for a Martian colony.
Nuclear reactors require fuel that eventually needs to be replaced. Generation 4 designs for small nuclear reactor often include fuel for ten to twenty years of operation, operating as a form of nuclear battery. Nuclear reactors often operate at high temperatures and have secondary systems that are subject to wear. Most existing designs require well trained operators and considerable support staff for maintenance. Reactors designed for Space and Mars will need to have mostly hands off operations.
Reactors produce heat, that is converted at an efficiency of 25 to 40% depending on the design. The rest of the heat needs to be removed and dissipated into the environment using radiators. These add mass to the system as well as potential leak points.
The nuclear fuel is a very small part of the entire reactor power producing system. Most of the mass is in the containment, piping, generators and radiators.
The transportation of nuclear fuel to Mars should be simple and safe. However, there is significant opposition to nuclear energy in civil society that needs to be taken into account when a nuclear system is chosen.
Available energies in radioactive materials
The following list shows the energy available from various nuclear 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 5%. So for a 10 MWe reactor powered by uranium and operating for 10 years:
10 MWe x 10 yrs x 8760 hrs/ year x 3600 seconds/hour = 3,13e9 MJ ÷ 8e5 GJ/kg ÷ 0,4 ÷ 0,05 ≈ 2000 kg of nuclear fuel
Burn up fraction is the amount of nuclear fuel that is actually consumed in a nuclear reactor. Most nuclear fuels have low burn up fraction, meaning much of the nuclear energy remains in the fuel after it is used. Breeder reactors can modify the existing fuel to make more of it available.
Usage On Mars
Nuclear reactors might be required during long dust storms, when the power from solar arrays could dip low enough to endanger a solar powered colony.
The generation of electricity from nuclear fuel does not depend on weather conditions so would be useful for maintaining a reliable source of power on Mars. Thorium is available on Mars in large, low concentration, deposits at Mid latitudes. This is the preferred fuel in Molten Salt Reactors, particularly Liquid Fuel Thorium Reactors LFTRs.
Most nuclear reactors designs have a thermal efficiency of about 35%. This means 35% of the heat from the core can be turned into electricity, the rest needs to be dissipated as low grade heat into the environment. This makes nuclear reactors useful for combined heat and power (CHP) plants, where electricity and plant heat can be used by a settlement. Given that mars has no natural bodies of water and only a thin atmosphere, having a source of ice as a heat sink becomes critical for large scale production of electricity. Radiators are inefficient at low temperatures and can easily be covered by dust, and at high temperature the efficiency of electrical production is effected, see .
Excess electrical power can also be stored in a cryogenic energy storage system or a "cryobattery". This is useful on a martian colony as the plant machinery required to maintain cryogenic temperatures is already required to store liquid oxygen/methane/etc. As the boiling point of liquid nitrogen is below that of liquid oxygen, etc. it can be used as sacrificial blanket, ensuring that as the cryogenic tanks heat up, the nitrogen boils off first. Waste heat can also be used as the heat source to boil the liquid nitrogen to produce the high pressure nitrogen to drive the turbines.
Supercritical CO2 Turbines
Supercritical CO2 Turbines use high pressure CO2 to drive a small turbine and compressor system at high efficency to generate power. Combined with a Liquid Fuel Thorium Reactor, a unit the size of a desk might generate 10 MW of power. 
ISRU turbines are turbines produced through local materials and so are generally less efficient and operate in a smaller envelope than high precision turbines produced on earth. The major upside of ISRU turbines is that they don't need to be transported from earth, which is complex as turbines are heavy and precision components. The lowered efficiency is also less of a concern on mars than for a terrestrial reactor as martian reactors are used in combined heat and power system, so the waste heat is used for industrial processes or district heating.
Liquid metal magnetohydrodynamic generators produce electricity through the passage of a metal gas through a strong magnetic field. These generators are generally compact, with compact heat exchangers due to the high thermal conductivity of liquid metals and with small superconducting electromagnets producing the magnetic field. Single phase generators use a single metal, say mercury, which is boiled and recondensed using two heat exchangers providing a steady metal gas loop which flows through the magnet. DC current is extracted from the metal gas through a set of electrodes. These generators are reasonably inefficient, but are very simple with no moving parts. Two phase generators contains a mix of an electrodynamic fluid (sodium, etc.), and a thermodynamic fluid (helium, etc.), and are significantly more efficient at the cost of being significantly larger and more complex.
Heating greenhouses and other buildings may be done indirectly by the waste heat of the nuclear fission. The heat can be transported in pipes from the reactor to the buildings. Heat exchangers avoid radiation pollution of the buildings.
Rodriguez Well (RODWELL)
RODWELLS Are a form of well melted into Antarctic Ice to provide a constant source of water for use on a base, this lowers a heat source deep into the ice melting an area of ice that is partially pumped out as the ice cave grows  Dr Chris Zacny has a brief on drilling RODWELLS for the Mars Society from the 21st annual Mars Society Convention 
Waste heat from nuclear reactor cooling could be used to melt ice in this type of wells.
Molten Salt Energy Storage
Molten Salt Energy Storage  is a process used in Concentrated Solar Thermal  that allows storing large amounts of heat energy in the form of high temperature molten salt. This reserve can be tapped for direct use in colony heating, Evaporative water purification, and Rodriguez Wells. However, the mass of the molten salts is high and the amount required might be prohibitive. It may prove more economical to use the waste heat from an operating reactor directly, as nuclear reactors are the most efficient when they operate continuously.
Nuclear Fuel Sources on Mars
Thorium Deposits on Mars
JPL has identified areas of high Thorium concentration on Mars. This is the preferred fuel in a number of Molten Salt Reactor designs. 
However, it is important to note that high concentration in this case means 1ppm, and that common soil on Earth has a Thorium concentration of 6ppm. There are granitic deposits on Earth with Thorium concentrations of 56ppm, that are considered very low grade resources. There is no evidence yet of thorium ore deposits that might be mined in an economical way.
In general, Thorium maps made from Mars Odyssey data suggest that the martian crust is poor in Thorium or uranium. It is possible that this reflects a formation model for Mars that would be much poorer in heavy metals than for Earth, and that Mars might have formed with more volatiles. This might be an explanation for Mars' lower density compared to Earth, 3.95 tonnes per m3 (g/cm3) vs 5,51 tonnes per m3 (g/cm3) for Earth.
The production of enriched nuclear fuel required for most designs complicates the case for in-situ production of nuclear fuel.
Usage in space
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- Megawatt Level Heat-Pipe Reactors, Mcclure, Patrick Ray Poston, David Irvin Dasari, Venkateswara Rao Reid, Robert Stowers DESIGN OF MEGAWATT POWER LEVEL HEAT PIPE REACTORS , https://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-15-28840, Nov 2015.
- Solid-Core Heat-Pipe Nuclear Battery Type Reactor, Ehud Greenspan Solid-Core Heat-Pipe Nuclear Battery Type Reactor , https://www.osti.gov/servlets/purl/940911
- Idaho National Labs, Dr. K.P Annath, Dr. Michael Kellar, Mr. James Werner, Dr James Sterbentz Portable Special Purpose Nuclear Reactor (2 MW) for Remote Operating Bases and Microgrids , https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2017/power/Ananth19349.pdf, May 2017.
- NASA Kilopower Project, Dr. David Poston Small Nuclear Reactors for Mars - 21st Annual Mars Society Convention , https://www.youtube.com/watch?v=NLE5YFuCmhw, Sep 2018.
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- How Molten Salt Reactors Might Spell a Nuclear Energy Revolution, Stephen Williams How Molten Salt Reactors Might Spell a Nuclear Energy Revolution , https://www.zmescience.com/ecology/what-is-molten-salt-reactor-424343/, Feb 2019.
- Supercritical CO2: The Path to Less-Expensive, “Greener” Energy, Machine Design Supercritical CO2: The Path to Less-Expensive, “Greener” Energy , https://www.machinedesign.com/mechanical/supercritical-co2-path-less-expensive-greener-energy
- Rodwell, Raul Rodriguez South Pole Station - Rodwells , https://www.southpolestation.com/trivia/rodwell/rodwell.html
- Molten Salt Energy Storage, Solarreserve MOLTEN SALT ENERGY STORAGE , https://www.solarreserve.com/en/technology/molten-salt-energy-storage
- Concentrated Solar Thermal, BY ROBERT DIETERICH Concentrated Solar Thermal , https://apnews.com/Business%20Wire/832487eb77e04612af8bde5f7642f0f7
- Map of Martian Thorium at Mid-Latitudes, JPL Map of Martian Thorium at Mid-Latitudes , https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA04257, March 2003.