Difference between revisions of "Nuclear power"

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[[Image:nuclear_warning_sign.png|right|Nuclear Danger Icon]]
 
[[Image:nuclear_warning_sign.png|right|Nuclear Danger Icon]]
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[[File:Reactor tile.JPG|right|frameless|50x50px|link=Create a settlement]]
  
'''Nuclear Power''' is a method of [[energy]] generation. It uses nuclear fuel to produce heat, which is usually transformed into [[electricity]].
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'''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]].
  
Nuclear power has been considered as the preferred energy source for most plans for medium- to long-term human expeditions to [[Mars]]. It does not depend on [[environmental conditions|weather conditions]].
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The [[Cost of energy on Mars|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.
  
The availability of radioactive resources on Mars is unclear. Due to the vast effort of the nuclear enrichment process the nuclear fuel must be brought from [[Earth]], preventing the [[settlement]] from being [[independence from Earth|independent from Earth]].
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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.
  
The maintenance effort of a legacy nuclear power station requires a huge staff. However, due to Russian plans to build fully self-contained device on Mars the required maintenance staff comprises only 6 engineers.
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In a generation 2 Light Water Reactor, heat from the radioactive core boils [[water]] to create steam. [[turbine|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.
  
Heatpipe Reactors are self contained reactors that require no maintenance or maintenance personnel and can last from 5 to 40 years. <ref name=":0" /> <ref name=":1" />
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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<ref name=":5">Inspired Heat-Pipe Technology,  Los Alamos '' Inspired Heat-Pipe Technology '', https://www.lanl.gov/science/NSS/issue1_2011/story6full.shtml</ref> are also heat transfer elements that might be used in future nuclear reactors.
  
==RTG==
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The following types of reactors have been identified as possible models for a Mars settlement, of for vehicles on Mars.
[[Radioisotope thermoelectric generator]]s (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.
 
  
==Nuclear reactor==
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===RTG===
In a nuclear reactor the heat boils [[water]] to steam. [[turbine|Turbines]] are driven by the steam's pressure, spinning a dynamo to generate electric energy.
+
[[Radioisotope thermoelectric generator]]s (abbr.: RTG) are relatively simple devices, with no moving parts. 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<ref>https://spacenews.com/plutonium-supply-for-nasa-missions-faces-long-term-challenges/</ref>, in the order of a few kg for the entire Earth, and extremely expensive. RTGs have the distinction of already operating on Mars, providing power to the Curiosity rover and heating some elements of the other Mars Rovers. Miniature RTGs can be used to heat specific components in rovers.
==Nuclear Heatpipe Reactor==
 
Small to Medium Heatpipe Reactors offer stable safe Power that requires no outside support system or Personnel and is immune to meltdown, it can be scaled from .5kw to 50mw for remote bases, small cities, and forward operating bases here on Earth, these reactors could power a Colony or a Spaceship for between 5 and 40 years with no maintenance, with no in-situ resources needed. <ref name=":0">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.</ref> <ref name=":1">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.</ref>
 
==Nuclear heating==
 
Heating [[greenhouse]]s and other [[building]]s may be done indirectly by the heat of the nuclear fission. The heat can be transported in pipes from the reactor to the buildings. Heat exchangers avoid nuclear pollution of the buildings.
 
  
==Open issues==
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===Kilopower heat pipe reactors===
 +
The Kilopower project designs<ref>https://www.youtube.com/watch?v=NLE5YFuCmhw</ref> are Heat-Pipe Reactors <ref name=":6">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.</ref>  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.<ref name=":7">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</ref> <ref name=":8">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.</ref> <ref name=":9">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.</ref>  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.
  
*What sort of nuclear fuel is needed?
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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 uranium<sup>235</sup> 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.
*How long can the described nuclear power stations work without replenishment of nuclear fuel?
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*What is known about nuclear resources on Mars?
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===Traditional reactors===
 +
Traditional generation 1 and 2 light water reactors were designed in the 1950's and are far too large, expensive, and inefficient for Martian settlements.  Basically, no one expects a traditional light water reactor to be used on Mars.  Modern generation 4 reactors are smaller, and more efficient.  Additionally several companies are designing small modular reactors which can fit in a shipping container.  If such a reactor (non-radioactive until it is turned on and fission starts) were to be created, it could be shipped to Mars complete.
 +
 
 +
===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 into a number of sub-critical compartments (which may have neutron absorbers between them to even more quickly kill any reaction).  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 <ref>https://youtu.be/D3rL08J7fDA</ref> <ref name=":10">Safety assessment of molten salt reactors in comparison with light water reactors, Badawy M.Elsheikh '' Safety assessment of molten salt reactors in comparison with light water reactors '', https://www.sciencedirect.com/science/article/pii/S1687850713000101, Oct, 2013.</ref> <ref name=":11">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.</ref>.  Most molten salt reactors use enriched <sup>235</sup>U 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.
 +
 
 +
==Operations==
 +
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.
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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.
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 +
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.
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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''' ===
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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
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* Tritium decay: 583 GJt/kg
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 +
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
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 +
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. Note that reactors using liquid fuels do not have to worry about cracking fuel rods, so the fuel can remain in the reactor for much longer period, and will therefore have a far better burn up fraction.
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==Usage On Mars==
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===Electrical power===
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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. (Mars gets 48% of the sunlight as Earth since it is farther from the sun.  Further, dense dust storms have reduced the energy from the Opportunity Rover by 95%.  Finally, to keep solar panels clean, EVA's will be required to dust them, and every EVA increases the chance of a human dying.)
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 +
The generation of electricity from nuclear fuel does not depend on [[environmental conditions|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.  (Tho hopefully we will find richer ores.)  This is the preferred fuel in Molten Salt Reactors, particularly Liquid Fuel Thorium Reactors LFTRs.
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 +
Most nuclear reactors designs have a thermal efficiency of about 35 to 40%.  This means 35-40% 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 <ref>https://en.wikipedia.org/wiki/Carnot's_theorem_(thermodynamics)</ref><ref>https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law</ref>.
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 +
Excess electrical power can also be stored in a [https://en.wikipedia.org/wiki/Cryogenic_energy_storage 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.
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 +
====Supercritical CO2 Turbines====
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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. <ref name=":4">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</ref>
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 +
====ISRU turbines====
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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.
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 +
====LMMHD generators====
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Liquid metal [https://en.wikipedia.org/wiki/Magnetohydrodynamic_generator 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<ref>https://scholarsmine.mst.edu/masters_theses/7522/</ref>. 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<ref>http://www.scielo.org.za/pdf/jesa/v20n2/03.pdf</ref>.
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===Heat generation===
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Heating [[greenhouse]]s and other [[building]]s 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.  Note that if you wish to heat a colony with solar cells, the cells convert sunlight to electricity at ~20% efficiency.  Where as heat from nuclear reactions does not experience this loss.  Further, if high process heat is needed for chemical reactions, or industrial uses, nuclear heat can be used without the inefficiency of converting it to electricity and then converting it back to heat.  This reference discusses using nuclear heat to create carbon neutral Portland cement. <ref>http://nucleargreen.blogspot.com/2010/04/nuclear-power-in-future-of-cement.html</ref>
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====Rodriguez Well (RODWELL)====
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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 <ref name=":3">Rodwell, Raul Rodriguez '' South Pole Station - Rodwells '', https://www.southpolestation.com/trivia/rodwell/rodwell.html</ref> Dr Chris Zacny has a brief on drilling RODWELLS for the Mars Society from the 21st annual Mars Society Convention <ref>https://youtu.be/NFDOrpljNAY</ref>
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 +
Waste heat from nuclear reactor cooling could be used to melt ice in this type of wells.
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 +
====Molten Salt Energy Storage====
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Molten Salt Energy Storage <ref name=":1">Molten Salt Energy Storage, Solarreserve '' MOLTEN SALT ENERGY STORAGE '', https://www.solarreserve.com/en/technology/molten-salt-energy-storage</ref>  is a process used in Concentrated Solar Thermal <ref name=":2">Concentrated Solar Thermal, BY ROBERT DIETERICH '' Concentrated Solar Thermal '', https://apnews.com/Business%20Wire/832487eb77e04612af8bde5f7642f0f7</ref> 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<ref name=":3" />.  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==
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 +
===Thorium Deposits on Mars===
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JPL has identified areas of high Thorium concentration on Mars.  This is the preferred fuel in a number of Molten Salt Reactor designs. <ref name=":12">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.</ref>
 +
 
 +
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.  (That said, there is no evidence that Thorium ores do not exist.  Igneous rocks are richer in Thorium than average stones, and Mars has huge areas of volcanic rocks.  Unless the Thorium ore is in the top 1 meter of the soil, it would not be detected by the Mars Odyssey orbiter.)
 +
 
 +
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.
 +
 
 +
==Problems with nuclear power on Mars==
 +
Mars has a unique problem for nuclear power which is getting rid of waste heat.  To provide useful power, a nuclear power plant is a heat engine, which moves heat from a source of high temperature to a sink of low temperature.  The larger the temperature difference, the more efficient the power plant. 
 +
 
 +
On the surface this seems like good news.  Mars is very cold, so a nuclear power plant will be more efficient.
 +
 
 +
But on Earth, the heat is dumped via convection (usually) into water cooling towers which transfer that heat into the air.  However, on Mars there are no easy sources of water, and the air is so thin that moving heat into it will be very difficult.  Nuclear reactors on Mars will need careful engineering to allow them to dump waste heat (likely using radiators).  This is less of a problem for small power plants and harder to solve for large ones.  For example, the Kilopower plants that NASA is making (mentioned above), have a large radiator at the top of the reactor to remove waste heat. 
 +
 
 +
If radiating heat to space is used on Mars, this suggests that power plants would have highest power output at night.  Electricity dependent industry might go into high gear when the sun sets.  Conversely, such power plants would dump less waste heat during dust storms which warm the atmosphere, and slow radiators.
 +
 
 +
A possible way to dump waste heat is with pipes carrying hot fluid underground.  This heat would transfer into a large field of soil which would gradually radiate the heat into the air.
 +
 
 +
In any case, dumping waste heat will be a trickier job on Mars than on Earth for nuclear power.  It is no means impossible, but will require thought and careful engineering.
 +
 
 +
==Usage in space==
 +
Nuclear power can serve as an energy source for the propulsion of space vehicles.  [[Nuclear Electric Propulsion]] or [[Nuclear Thermal Propulsion]] are the two main possibilities.
 +
 
 +
==References==
 +
{{reflist}}
  
 
==External links==
 
==External links==
  
*[http://news.bbc.co.uk/2/hi/europe/3162129.stm BBC: Russia plans Mars nuclear station]
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*None
  
[[Category:Hi-tech]]
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[[Category:Sources]]
 
[[Category:Energy]]
 
[[Category:Energy]]
<references />
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[[Category:Power Systems]]

Latest revision as of 08:47, 24 June 2022

Nuclear Danger Icon
Reactor tile.JPG

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[1] 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.

RTG

Radioisotope thermoelectric generators (abbr.: RTG) are relatively simple devices, with no moving parts. 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[2], in the order of a few kg for the entire Earth, and extremely expensive. RTGs have the distinction of already operating on Mars, providing power to the Curiosity rover and heating some elements of the other Mars Rovers. Miniature RTGs can be used to heat specific components in rovers.

Kilopower heat pipe reactors

The Kilopower project designs[3] are Heat-Pipe Reactors [4] 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.[5] [6] [7] 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.

Traditional reactors

Traditional generation 1 and 2 light water reactors were designed in the 1950's and are far too large, expensive, and inefficient for Martian settlements. Basically, no one expects a traditional light water reactor to be used on Mars. Modern generation 4 reactors are smaller, and more efficient. Additionally several companies are designing small modular reactors which can fit in a shipping container. If such a reactor (non-radioactive until it is turned on and fission starts) were to be created, it could be shipped to Mars complete.

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 into a number of sub-critical compartments (which may have neutron absorbers between them to even more quickly kill any reaction). 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 [8] [9] [10]. 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.

Operations

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. Note that reactors using liquid fuels do not have to worry about cracking fuel rods, so the fuel can remain in the reactor for much longer period, and will therefore have a far better burn up fraction.

Usage On Mars

Electrical power

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. (Mars gets 48% of the sunlight as Earth since it is farther from the sun. Further, dense dust storms have reduced the energy from the Opportunity Rover by 95%. Finally, to keep solar panels clean, EVA's will be required to dust them, and every EVA increases the chance of a human dying.)

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. (Tho hopefully we will find richer ores.) 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 to 40%. This means 35-40% 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 [11][12].

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. [13]

ISRU turbines

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.

LMMHD generators

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[14]. 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[15].

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. Note that if you wish to heat a colony with solar cells, the cells convert sunlight to electricity at ~20% efficiency. Where as heat from nuclear reactions does not experience this loss. Further, if high process heat is needed for chemical reactions, or industrial uses, nuclear heat can be used without the inefficiency of converting it to electricity and then converting it back to heat. This reference discusses using nuclear heat to create carbon neutral Portland cement. [16]

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 [17] Dr Chris Zacny has a brief on drilling RODWELLS for the Mars Society from the 21st annual Mars Society Convention [18]

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 [19] is a process used in Concentrated Solar Thermal [20] 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[17]. 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. [21]

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. (That said, there is no evidence that Thorium ores do not exist. Igneous rocks are richer in Thorium than average stones, and Mars has huge areas of volcanic rocks. Unless the Thorium ore is in the top 1 meter of the soil, it would not be detected by the Mars Odyssey orbiter.)

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.

Problems with nuclear power on Mars

Mars has a unique problem for nuclear power which is getting rid of waste heat. To provide useful power, a nuclear power plant is a heat engine, which moves heat from a source of high temperature to a sink of low temperature. The larger the temperature difference, the more efficient the power plant.

On the surface this seems like good news. Mars is very cold, so a nuclear power plant will be more efficient.

But on Earth, the heat is dumped via convection (usually) into water cooling towers which transfer that heat into the air. However, on Mars there are no easy sources of water, and the air is so thin that moving heat into it will be very difficult. Nuclear reactors on Mars will need careful engineering to allow them to dump waste heat (likely using radiators). This is less of a problem for small power plants and harder to solve for large ones. For example, the Kilopower plants that NASA is making (mentioned above), have a large radiator at the top of the reactor to remove waste heat.

If radiating heat to space is used on Mars, this suggests that power plants would have highest power output at night. Electricity dependent industry might go into high gear when the sun sets. Conversely, such power plants would dump less waste heat during dust storms which warm the atmosphere, and slow radiators.

A possible way to dump waste heat is with pipes carrying hot fluid underground. This heat would transfer into a large field of soil which would gradually radiate the heat into the air.

In any case, dumping waste heat will be a trickier job on Mars than on Earth for nuclear power. It is no means impossible, but will require thought and careful engineering.

Usage in space

Nuclear power can serve as an energy source for the propulsion of space vehicles. Nuclear Electric Propulsion or Nuclear Thermal Propulsion are the two main possibilities.

References

  1. Inspired Heat-Pipe Technology, Los Alamos Inspired Heat-Pipe Technology , https://www.lanl.gov/science/NSS/issue1_2011/story6full.shtml
  2. https://spacenews.com/plutonium-supply-for-nasa-missions-faces-long-term-challenges/
  3. https://www.youtube.com/watch?v=NLE5YFuCmhw
  4. 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.
  5. 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
  6. 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.
  7. 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.
  8. https://youtu.be/D3rL08J7fDA
  9. Safety assessment of molten salt reactors in comparison with light water reactors, Badawy M.Elsheikh Safety assessment of molten salt reactors in comparison with light water reactors , https://www.sciencedirect.com/science/article/pii/S1687850713000101, Oct, 2013.
  10. 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.
  11. https://en.wikipedia.org/wiki/Carnot's_theorem_(thermodynamics)
  12. https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law
  13. 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
  14. https://scholarsmine.mst.edu/masters_theses/7522/
  15. http://www.scielo.org.za/pdf/jesa/v20n2/03.pdf
  16. http://nucleargreen.blogspot.com/2010/04/nuclear-power-in-future-of-cement.html
  17. 17.0 17.1 Rodwell, Raul Rodriguez South Pole Station - Rodwells , https://www.southpolestation.com/trivia/rodwell/rodwell.html
  18. https://youtu.be/NFDOrpljNAY
  19. Molten Salt Energy Storage, Solarreserve MOLTEN SALT ENERGY STORAGE , https://www.solarreserve.com/en/technology/molten-salt-energy-storage
  20. Concentrated Solar Thermal, BY ROBERT DIETERICH Concentrated Solar Thermal , https://apnews.com/Business%20Wire/832487eb77e04612af8bde5f7642f0f7
  21. 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.

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