Nuclear power

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.

Nuclear reactor

In a Light Water Reactor, heat caused by the radioactivity boils water to steam. Turbines are driven by the steam's pressure, spinning a generator to generate electric energy. In a Molten Salt Reactor the heat generated from the core is redirected for use in Molten Salt Thermal Storage, structural heating, Stirling Generators to provide electricity, Evaporative Water Purification, and to melt water in RODWELLS. In a Heatpipe reactor a solid core generates heat that is transferred via "Heatpipe" technology to Stirling Generators with the option to transfer the heat to Molten Salt Storage for further direct heat energy use or Stirling Generators or Supercritical CO2 Turbines at separate locations in the colony.

RTG

Radioisotope thermoelectric generators (abbr.: RTG) are simple devices. They produce a heat difference, transformed by a thermocouple to electrical energy. The maintenance effort is low. However, RTGs do not provide enough power for a base. They also have low efficiency, in the order of 10%. Furthermore RTG depend on plutonium as a power source, and the supply is extremely low, in the order of a few kg. Most RTGs use plutonium as fuel.

Nuclear Heatpipe Reactor

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 bellow the melting temperature of the components. Heat-Pipe Reactors are inherently "Walk-Away Safe" meaning if an emergency happens and the reactor is left alone it will not melt down or change state.

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.

Molten Salt Reactor

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 the salt temperature rises it will separate the suspended nuclear fuel and slow the reaction. As a safety feature, at the lowest point in the core a pipe is cooled forcing the salt in that section of pipe to freeze and create a plug of salt, if that external cooling is lost the plug melts and the liquid salt contents of the core dump into a storage tank, when the salt is separated from the graphite moderator. 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 https://youtu.be/D3rL08J7fDA . Most molten salt reactors use enriched uranium²³⁵ or Thorium as fuel.

Operations

Nuclear reactors require fuel that eventually needs to be replaced. Modern 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.

Usage On Mars

Electrical power

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 services, where electricity and heat can be used by a settlement.

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.

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.

Heat generation

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 https://youtu.be/NFDOrpljNAY

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 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.

External links

• None