Kilopower: NASA’s Offworld Nuclear Reactor

Here on Earth, the ability to generate electricity is something we take for granted. We can count on the sun to illuminate solar panels, and the movement of air and water to spin turbines. Fossil fuels, for all their downsides, have provided cheap and reliable power for centuries. No matter where you may find yourself on this planet, there’s a way to convert its many natural resources into electrical power.

But what happens when humans first land on Mars, a world that doesn’t offer these incredible gifts? Solar panels will work for a time, but the sunlight that reaches the surface is only a fraction of what the Earth receives, and the constant accumulation of dust makes them a liability. In the wispy atmosphere, the only time the wind could potentially be harnessed would be during one of the planet’s intense storms. Put simply, Mars can’t provide the energy required for a human settlement of any appreciable size.

The situation on the Moon isn’t much better. Sunlight during the lunar day is just as plentiful as it is on Earth, but night on the Moon stretches for two dark and cold weeks. An outpost at the Moon’s South Pole would receive more light than if it were built in the equatorial areas explored during the Apollo missions, but some periods of darkness are unavoidable. With the lunar surface temperature plummeting to -173 °C (-280 °F) when the Sun goes down, a constant supply of energy is an absolute necessity for long-duration human missions to the Moon.

Since 2015, NASA and the United States Department of Energy have been working on the Kilopower project, which aims to develop a small, lightweight, and extremely reliable nuclear reactor that they believe will fulfill this critical role in future off-world exploration. Following a series of highly successful test runs on the prototype hardware in 2017 and 2018, the team believes the miniaturized power plant could be ready for a test flight as early as 2022. Once fully operational, this nearly complete re-imagining of the classic thermal reactor could usher in a whole new era of space exploration.

A Revolutionary Reactor

Any humans looking to spend more than a few days on the surface of the Moon or Mars will need to bring along a power source that checks an unreasonable number of boxes. It needs to be small and light enough to put into a spacecraft, while at the same time robust enough to survive the rigors of space travel. The lives of the crew will depend on it being infallible, but it will also need to be so simple and safe that it can operate autonomously for years. Most of these are traits not commonly associated with nuclear reactors.

The prototype Kilopower reactor

Typically, the incredible energy released by nuclear fission inside the reactor’s core is used to heat water, which generates high pressure steam that powers turbines connected to electrical generators. It’s a relatively low-tech method of harnessing fission energy, but it has the advantage of being a simple and well understood process. Unfortunately there are far too many moving parts, figuratively or otherwise, to make such a system practical and safe on a small scale.

Which explains why the Kilopower bears little resemblance to traditional nuclear reactors. In fact, it’s more like an evolved version of the radioisotope thermoelectric generators (RTGs) which NASA has used to power everything from the Voyager missions to the Curiosity rover. There’s no dangerous high pressure steam, finicky turbines to spin, or coolant pumps to fail. Thermal energy is passively carried away from the reactor core using sodium-filled heat pipes, which lead to the “hot” side of a Stirling engine array. With a large deployable radiator on the other side, the Stirling engines would use the temperature differential to produce reciprocal motion that can drive a small generator.

The Kilopower has been designed as a self-regulating system where everything happens automatically and without the need for external control. There would naturally be sensors for basic diagnostics, for example checking temperatures at key points in the system, the RPMs of the Stirling engines, and the output of the generators. But outside of monitoring for these possible signs of trouble, the human crew could largely ignore the Kilopower and go about their mission.

Smaller and Safer

Kilopower core bonded to end of the heat pipe assembly.

The use of passive heat pipes and Stirling engines rather than steam-driven turbines results in an incredible reduction of overall system complexity and size. But those improvements are only half the equation. An equally important aspect of the Kilopower design is the vastly simplified reactor core: a cylinder of uranium-235 that’s about the size of a paper towel roll and weighs just 28 kilograms (62 pounds). Despite its diminutive proportions, the core is designed to run at 850 °C (1,560 °F) for as long as fifteen years.

A single boron carbide control rod in the center of the core is used to control the rate of fission, which in turn can be used to adjust its heat output. When launched, the core would be in a “cold” state, where the control rod is fully inserted and no fission is taking place. Launching in an inert state means that the nuclear fuel won’t be consumed until the reactor is actually ready to be used at the destination location, further extending its useful lifespan. Once the Kilopower touches down on the Moon or Mars, the control rod will be removed and the nuclear reaction will begin.

The fact that the core is not going through active fission while here on the Earth also means it’s far less radioactive than one might expect. According to an interview that lead Kilopower engineer Marc Gibson gave to, it was riskier to launch the old-style RTGs:

What a lot of people don’t know is that when you look at how many curies of radioactivity the fuel has at launch, the fission reactor is several orders of magnitude lower than the radioisotope systems that were launched with the Curiosity mission. We can easily prove that launching a fission reactor is going to be several orders of magnitude safer than the radioisotope systems that have already launched.

Ready for the Future

While the Kilopower has yet to leave the lab, much less slip the surly bonds of Earth, the team is already looking at scaling the system up for the sort of output that would be required for a large Mars base. The prototype reactor uses eight Stirling engines which can output approximately 125 watts each. That 1000 watts of total output is, incidentally, where the Kilopower gets its name. But with larger Stirling engines and the use of a 43.7 kg (96 lb) core, NASA believes the output could be increased to 10,000 watts. Cluster a few of those together, and you’ve got enough power for the first Martian neighborhood.

But increased energy output isn’t the only reason NASA is excited about the Kilopower. The U-235 fuel it consumes is far cheaper and easier to obtain than the Pu-238 used in radioisotope thermoelectric generators. The limited amount of Pu-238 the agency can get their hands on each year has made the logistics of planning deep space missions even more difficult than they already are, so a power source that uses a more common nuclear fuel would enable robotic missions that simply weren’t practical in the past.

At this point, NASA hasn’t announced when they intend to launch the first Kilopower reactor. But with the push to return to the Moon by 2024, it’s not unreasonable to think a small space-rated nuclear power plant might end up on our nearest celestial neighbor in the relatively near future.

107 thoughts on “Kilopower: NASA’s Offworld Nuclear Reactor

  1. Fascinating post, thanks
    FWIW. We have heaps of Thorium on Australian beaches and a UK bomb test site contaminated region almost central to most high population centres, worth considering putting a sizable LFTR there with appropriate management. I’d prefer that than shipping more coal as we are pretty high in top 3 of global coal exporters :-(
    I like the idea of fission units for Mars as suggested here, heck we might even speed up evolution with what little radiation release there might be, that is if we also deal with bacteria already there or added by us, cheers

    1. Haha, funny thing. I suggested something like this over at Hacker News. There they helpfully explained in detail to me why it’s actually impossible. Then I stumble on this NASA project on Hackaday! :-D

    2. And with the coal you export also tons of U and Th which land in the ashes. In normal operation a coal fired power plant emits more radioactivity then a nuclear one. Of course nuclear accidents are another, very bad, thing.

      1. Oh no, a whole kilo of P238! Surely the worst thing that humans have ever put into the environment.

        Seriously, while control of radioactive materials is very important, any spacecraft contains a whole heap of stuff (propellants, coolants) that you wouldn’t want to particularly have dispersed into the ecosystem. The uranium is largely incidental.

        1. Ok my bad, as long as you’re putting worse things into the environment just go ahead and dump more sh!t into it.

          If you don’t keep a check on it you’ll end up poisoning your kids with lead, oh wait you’ve already done that in Flint and Washington. Never mind just keep going.

  2. That’s neat.

    Must admit this made me laugh though…
    “We can count on the sun to illuminate solar panels, and the movement of air and water to spin turbines.”
    We can’t count on the sun in the UK, and the wind is often too weak or too strong to (safely) operate wind turbines 😭
    Makes me think this might be useful in many other situations even in earth.

    But it must kick out a lot of heat? A Stirling engine has a typical efficiency of say 20%? So a 1kW generator would kick out 5kW of heat. Seems like that’d be quite a lot to shift in thin Martian air?

    1. 20% maybe on earth at 1 bar and NTP but, on Mars higher temp range since ambient temp heatsink to Mars atmosphere lot lower, maybe their stirling more efficient in their design as intended Mars configuration, details of their setup mechanically would be of interest as Carnot suggests less than 50% asymptotic so 20% seems far too low :-/

    2. Sure, but most of Mars is very cold so the temperature differential would help make up for that. Gounod diffusion might also work, though I’m not sure whether martian soil would be better or worse than earth soil for thermal conduction.

      1. Well not that much cooler depending where as Mars reaches a balmy 25 to 30C+ in shade in summer due to Radiative Transfer of CO2 despite low insolation and one 100th of Earth’s atmosphere, if same co2 conditions on earth as in added mass our equatorial day temps would be at 75C, getting there already at 52 to 57C in shade in some places Africa and India killed few people last few years = uninhabitable still climbing & even before more ocean warming will hit despite its much higher specific heat :-(

        1. I don’t see those numbers in your link, but there are some specific to Mars below, that show much cooler averages, even in summer:

          Spirit did record a maximum 35C air temperature, but that was over 2208 sols and still near the equator (14.5deg south). Average highs at Gale Crater (5.4deg south) range from -23C in March to 2C in July, with nighttime lows between -88C and -68C.
          Even if daytime highs were nearer 25-30C, that doesn’t make up for such a huge temperature swing.

          Also worth mentioning is that Spirit had survival heaters, and NASA’s explanation for its failure is due to insufficient power to run them.

          1. On the other hand, the cold martian atmosphere is 100 times thinner than ours, so it’s ability to freeze habitats (and spacesuits, and radiators) is also 1% of that of Earth’s air at the same temperature. Radiative coooling is the main thing to consider, besides the connection to the ground.

          2. Apologies Tõnu Tamm – was planning on clicking reply not report. Need more coffee…

            So the point – That beats the temp range I’ve had in Finland.. +37 °C in summer to -45 °C in winter.

            So it seems sometimes you have to wrap up warm on mars and sometimes throw in the towel for the day and drink a cold beverage too.

            So if on balance it’s a bit nippy – but countered by Tõnu Tamm’s point about there being 100 x less convection, how does that change the wrap up warm/cold beverage ratio?

    3. Stirling engines on earth usually run on low quality heat (low peak temperature)which lead to their low efficiency. But that LOW efficiency is actually close to thermodynamic max possible one. NASA plain to use whopping 800+ degree on the hot side, so the efficiency is pretty optimistic.

      1. The ASE project tried to use a high quality heat source (burning gasoline), but they too failed to get much better than 25% efficiency.

        Trick is that stirling engines are pretty difficult to throttle. They really only run well at one power setting, which is a matter of optimizing the pressure differential, the pressure, and the amount of working fluid in the loop. When you have a generator, or an engine, that needs to respond to varying demand, the efficiency goes to the dumps.

    4. As the article says, it has a “large deployable radiator” to reject its heat. They’re using radiator in the technical sense, i.e. it can reject all its waste heat via thermal radiation alone. That’s how it can work on the moon or in the vacuum of space. An atmosphere of any sort for convection cooling is probably just a small efficiency boost.

        1. Interesting question. The radiators would emit at a much different wavelength (infrared) than sunlight. It may penetrate dust better.

          The radiator may also be an optional accessory for airless places like orbit or the moon.

          On Mars, I would put the ‘waste’ heat to a bunch of other uses before radiating it away. Boiling native ice to get drinkable water would be a neat use, and the sterling engine might like the extra temperature differential.

          1. I don’t believe the radiator is an optional accessory, as without it that would mean that it would be dependent on active cooling (such as replenishing the ice to be melted) and it would then lose it’s passively cooled failsafe selling point. I’m sure the waste heat could be channeled to something useful primarily, but the radiator would have to remain as a failsafe.

            As for the dust, there’s not a whole lot of that floating around in the vacuum of space, and what is floating around is probably quite spread out. On the moon there is obviously dust, but it can’t float around without an atmosphere. It can be kicked up, but it will immediately fall straight back down since there’s no air resistance to keep it afloat. So dust would have to be actively kicked towards it by something (like rocket exhaust) for it to collect on it.

            On Mars there is an atmosphere, so dust would definitely collect on it, but the fact that there is an atmosphere to provide convection cooling in addition to the radiant cooling would likely offset any reduction in radiative capacity and then some. Unless the dust is thermally conductive it would probably hinder radiant cooling as you would expect. Even if it is an efficient blackbody radiator, if it can’t conduct the heat from the metal to be radiated it won’t help.

  3. My only question is about the effectiveness of heat pipes in low or microgravity situations. In my experience heat pipes rely on convection to move warm gas away from the heat source. I don’t know if that is necessary, but it would certainly be less effective in martian gravity and useless in interplanetary space. I assume that a bunch of rocket scientists have made allowances for this, but I wonder what they are and how much each reactor will have to be designed for it’s mission gravity.

    1. Heatpipes work in “true” zero G, maybe you are thinking of thermosiphones? Proper heatpipes have a wick structure on the inner walls which allows capillary effects to draw cold liquid into the hot spot however convection can improve efficiency. One example of a heatpipe system not using convection for circulation is the notebook computer I’m typing this on, here the height difference is a maximum of 2mm which wouldn’t be enough to cool the over 200W peak heat output.

    2. It’s fairly easy to make heat pipes gravity-independent. Moving vapor away from the heat source takes no effort at all; there’s higher pressure from boiling at the heat source and lower pressure from condensation at the heat sink.

      Getting the condensed liquid back to the heat source is the hard part, but a porous medium (usually sintered metal powder) lining the pipe allows capillary action to spread the liquid around. Gravity return is a lot faster if you have it available, but gravity-independent heat pipes are quite common. The one connecting my laptop’s CPU to its fan-cooled fins, for example, has to work no matter which way the computer is tilted. Desktop CPU heat sinks can’t always depend on the user installing them with one particular side up, so they’re also lined.

      The deployment concept for the reactors seems to be pretty consistent — the reactor core goes in a hole in the ground for shielding, waste heat gets radiated from a big above-ground sheet. That allows a gravity-based or gravity-assisted heat pipe design, and it doesn’t take much gravity to convince a liquid to eventually migrate to the bottom of a pipe. If the generators are 20% efficient, then that 1 kW electric output means 5 kW thermal. That equates to about 1.2 grams of sodium boiling and condensing per second, spread between 8 fairly large looking pipes.

          1. Not quite.
            It’s a starightforward issue of chemical activation energy in that CO2 when it collides with a native sodium atom will give up an oxygen some of the time given it’s electro negativity with sodium being positive, nothing to do with water at all. It is a lot easier the warmer the sodium Eg Sort of related is if you fire CO2 gas molecules at the right velocity into an inert metal such as gold and it breaks up into carbon and oxygen gas…

    1. U serious ?
      Take up of recent solar powered costs in few countries below that of fossil & nuclear even when including batteries. Though I am in favour of Thorium development LFTR such as in regions already contaminated eg parts of Australia where UK did nuclear bomb tests, no one is going to live there anyway and that region close enough to few population centres allowing for transmission losses though reactor would be sizable and happily not encumbered by GEC’s inflated costings and USA political manipulation of their ‘allies’…

      1. There’s always an anti-green energy troll in posts like these. They know they’re wrong, they’re just here to rile people up. Not just here either, same thing on other sites too.

        1. sorry sir, but you are the troll, when you are trolling the already proven technology of mankind in a hippy fashion, the technology what is already proved to deliver electrical energy in a high volume in a non-toxic manner, the nuclear waste is manageable, if you take your job seriously, if these green-hippies will not stop, sooner or later everybody will regret it, just like in germany, where right now they using fossil fuel, in a horrible way, because the green energy is not enough

          1. You got a point. Our reluctance to use nuclear to shift our industry over NOW may just well kill us all very soon. This is no joke. We are looking at hundreds of millions to billions of deaths in the next century or so due to ocean acidification and climate change. I’d take a few Chernobyls and Fukushimas over that any day.

            But I think we ought not completely shelf ‘alternative’ energy. We can work on that, too. No reason not to. What we should never have done is eschew nuclear power in favor of burning a bajillion tons of coal and gas while we veeeeeerrrryyyy slowly tinker and toy with alternative energy and then eeeeveeen moooore slooowly adopt its use around the world. We are currently sitting on the train tracks picking at lint between our toes while the freight train bears down on us. Dismantling carbon industry and reforming agriculture should be an endeavor that we rapidly pursue at all costs—because the cost of ignorance will literally be everything. We’re going to lose everything if we keep doing what we’re doing.

          2. Yeah, no.

            There’s already 90,000 metric tons of nuclear waste in the USA. It has to be stored at the site of the reactors because nobody wants to take it. Two main side products of it that are radioactive have insanely long halflifes, one is 220,000 years and the other is 15.7 million years. And it’s ridiculously expensive even just to store the waste on site. One reactor hasn’t even run in over 20 years but costs for storage last year were $35 million. And nuclear power is uninsurable.

            Not to mention the attempts to store the waste for long term has issues, such as the dome over the waste on the Bikini Atoll in the Marshall Islands is cracking and leaking radiation now.

            Newer technology may be safer, but the waste products from it are still going to be more radioactive than the fuel going into it. It’s a stupid expensive technology and the benefits from it will be very short lived but the negative effects last for an extremely long amount of time.

            It’s good for space, that’s it.

            Here on Earth, cheap solar, wind, and other sources of energy are the way to go. There are tons of energy storage technologies as well. And there’s tons and tons of energy available, we just have to collect it.

          3. In the near future any mix of non-nuclear “green” ( quote marks because to be honest nuclear does belong under the class of green) energy sources can’t be enough. Fission of U235 and of thorium is the only way to supply the power the world needs without wrecking our fragile environment wth CO2 emissions. In the longer term green technologies like wind, solar and especially tidal could catch up but by the time they do so we will, hopefully as its “only 40 years away” have broken-even with fusion energy.

          4. No Robert, Nuclear is not green by any stretch of the imagination. It’s not a renewable source and the waste from it will be with us thousands of times longer than the energy generated by it.

            Additionally, people don’t understand how much energy there is in solar and wind.
            The sunlight that hits Texas alone is 300x the total output of all worldwide power generation. Even if current technology can only convert 20% of that into electricity, that would still be significantly more than we currently generate.

            Building a nuclear power plant is a very expensive and time-consuming ordeal. In the time it would take to build one of those you could deploy solar or wind and storage that would produce more power in a better way and cheaper.

          5. @Doc Oct we are not stuck with a single technology there are other ways of doing nuclear, Solar is actually nuclear power from fusion.
            “They also demonstrated that existing long-term (240,000 years or more) nuclear waste can be “burned up” in the thorium reactor to become a much more manageable short-term (less than 500 years) nuclear waste.”
            The problem with solar and wind is it is not available 24/7 and we do not yet have an adequate storage technology.

            Even if we got every green energy source you want, it would still be a good idea to build a few plants to use up all that high level waste we have stored.

          6. Re battery storage, many on the fringe need to get up to speed on latest tech & costings of last few years
            Hmm, well South Australia had Tesla install a sizable load levelling battery and it already returned income in first 2 months operation, there are several battery storage plans around the world with costs of lithium the also ondecline. Heres a general link re batteries

            Solar prices also going down, it’s getting much less than new coal or nuclear plants especially given nuclear a mostly closed club once ensconced its very hard to change suppliers unlike solar or wind…
            And for Australia where we have heaps of open dry land sunny most of the days negligible clouds and lithium reserves too

            I read few years ago if you needed to power all of USA power requirements with pv solar array you could then how big would it need to be and of course spread over regions appropriately for spread of insolation re time of day and cloud attenuation issues etc with what economical storage of batteries, I recall it wasn’t that large, let’s see who can crunch the numbers ;-)
            Power stats for 2018

        2. Of course I am against green-energy-trolls or climate change hysterics. But if CO2 and a human influence on climate change is the great danger, as what some people see it, then I don’t see a mid term alternative to fission power. Over the long term we can hope to master controlled fusion. But at the moment we can use fusion power only in a very weather and time-of-the-day dependent way.

          1. It’s not an if about CO2 it’s very well proven since even as far back as 1896 and conceded by ExxonMobil circa 1982 and shortly thereafter by Shell with their climate models accurate to this very day and within tight error bars too. It’s isotopic signature of fossil fuels and GHG Radiative Transfer never refuted instead finessed for decades and well quantified by the Beer-Lambert relationship confirmed daily by thousands of instruments in use globally across many industries. Anyone who disputes this either needs to review their highschool physics or continue ignorance and embarrassment Or if they can refute it then win a Nobel prize and go on world speaking tours earning millions !
            Then explain why all the instruments from different manufacturers all calibrated wrong yet give correct practical outcomes – lol

            Sadly there are those with immature hubris, even university professors of geology, even structural engineering and Ba science graduates in software of all things who claim they can read papers in physics & chemistry who never learned that very basic foundational physics who still show their immense incompetence and mindless ignorance claiming AGW a hoax following their lord trumps low IQ facile attempts at authority :-(

      2. Gah! Solar (PV) is not green. It relies on heavy metals, the mining of which is destroying ecosystems and polluting the developing nations where they’re found. And they only have a lifespan of ~10 years, after which I’ll bet they’re not recyclable, so all that stuff has to be dumped to pollute somewhere else. And it’s only cheap because of unsustainable subsidies.
        Wind can be green, but many designs and installations interfere with bird migrations. (There are much safer options for birds, but they’re not commonly used.)
        Solar concentration to drive turbines would be green, but that’s not commonly being built.

        1. Really ? Heavy metals for pv solar panels Dan like aluminium and silicon with a bit of copper wire and tiny amount of dopants in the silicon ? Older panels had some lead, newer ones ROHS and newer ones can last 25 years
          Maybe check out this link to clarify the “heavy metals” issue, maybe you were thinking of rare earth magnets in alternators and Tesla motors – also recyclable btw

          Solar PV panels highly recyclable, easy to strip the aluminium and glass though it does depend on labour costs, I see it’s possible to and maybe feasible to amend the design to make it quicker to disassemble.

          1. Normal alternators do not contain rare earth magnets but an excitation coil – which allows better regulation. Tesla uses induction motors (no magnets) at least in the model S and before. I am not sure about the model 3.

          2. Don’t know what you mean buy ‘normal’, unless you are referring to old superseded tech of field coil driven alternators as used for internal combustion engine vehicles.

            For renewable power based alternators in wind turbines overwhelmingly permanent magnet rare earths far preferred used for highest efficiency for size and comparative wiring cost, ie why waste power exciting a field when you get it essentially free over the lifetime of the device with permanent magnets ? Heck even oldie Fisher Paykel washing machines used ferrite permanent magnets for their brushless motors except the small cheapie pumps of induction motors. I understand later tech also uses smaller better higher force permanent magnets – maybe not the highest coercive force of specific rare earth alloys at highest levels but improvement over ferrite and far better than old field coil driven such as the so called universal brush motors still used in mains powered power tools eg grinders, Is power drills etc

            Also as far as I understand the later tech Tesla DC motors use Halbach arrays of permanent magnets for higher efficiency, smaller size and lower weight, it is of course possible earlier production lines not yet updated still use coil driven but, I was under impression it’s all been superseded a while ago…

        2. Which heavy metals do you need for a silicon PV cell? Iron for the steel structure and copper for the cables. But these are part of any electrical power system. The dopants for silicon are also no heavy metals. They are in part toxic, but you need tiny amounts of arsenic, boron or phosphorus.
          If you use LiFePO4 batteries they also contain only iron as heavy metal and no cobalt, which is considered a “conflict mineral”. You could also use cheaper lead acid batteries. Although lead is a highly toxic heavy metal, the recycling of batteries is a well established industrial process which delivers a secondary “raw material” and is cost effective.
          Solar concentration requires moving parts, like a turbine itself. For me one of the best aspects of PV is it’s absence of moving parts. Put the panel on the roof (or the car in case of camping) and “forget it”.
          Taking a nuclear reactor to a festival in a car would not be possible :-) And a generator stinks and makes noise. Although it would be the by far cheapest option.

          1. I replied with in a thread that has been deleted.
            Long term nuclear waste can be converted to short term nuclear waste (less than 500 years danger) while also providing useful energy.
            I think It would be cheaper and safer to build such a reactor and convert the existing waste than trying to build confinement that will last for 250,000 years without maintenance.

    2. its not rubbish if it generates power, and it generates power. no its not going to meet global energy demands (unless its hydro, provided you build enough dams). but that’s not what they are for. if you own the renewables they can save you money. i know people who meet all their power demands with a small wind turbine, a battery bank, and a wood stove. you may call it primitive but they have internet and most appliances that people take for granted.

      1. Wood stove eh? That’s nice in a wooded region. It’s a no gone desert or deforested regions who followed such self-destructive practives.

        Worse, it only works when a handful are doing it. Try supplying millions or tens of miilions. You can’t

          1. Burning wood for heat is a closed carbon loop though. Assuming sustainable harvest practices the only net carbon added to the atmosphere comes from transportation and tool use (ie chainsaws, log splitters, etc.)

            That doesn’t make it practical for widespread use, but you have a smaller carbon footprint heating with wood than gas, oil or electric.

    3. All the usual ‘alternative’ energy sources don’t work very well on the Moon or Mars.
      Wind obviously won’t work on the moon, and the low atmospheric pressure means it’s not very useful on Mars.
      Solar does work great on the Moon during the daytime, but two-week nights would be a problem. On Mars, the extra distance from the Sun means they’re not useful for high power demands.
      Hydro-electric and tidal etc. are obviously right out.
      Geothermal *might* work on Mars, but it has a much less active core than the Earth.
      Even ‘traditional’ sources of energy, ie burning coal, oil or gas, are useless off-Earth with no oxygen to burn the fuel in.
      For any medium to large power demand (eg, over 1kW) off-Earth, some kind of nuclear source is an obvious route to take.

      1. The radius of the mars orbit is about 1,5 times that of earth’s. So solar panels should still deliver roughly 1/2 of the value in earths orbit. The thinner atmosphere of mars should also allow higher power to reach the surface.
        I would not consider that as really bad.

        1. Power to solar panels on the surface whether Mars or Earth not that different since Earth’s atmosphere and Mars both similatly rather transparent to visible light to some UV tailing off rapidly – with visible spectra by far the highest energy output from Sol across its energy emission spectra though UV will be effective for Mars solar panels if they are configured for wider spectra which I understand is feasible. So maybe suitable solar panels could be comparable power wise given tailoring to UV.
          The amount of Sols infra red if enough that gets to Mars that could be interfered with by Earth or Mars 99% CO2 negligible
          Ie It’s the surface that converts visible to IR with visible being very good at getting through the atmosphere (minor scattering) unless clouds or dust storms etc…

    1. Probably wouldn’t want these chunks of U-235 just sitting around all over the grid in small, lightly-guarded facilities. On Earth we have luxuries like maintenance and water, so we may as well use far more powerful and efficient traditional fusion reactors. They can squeeze a lot more energy out of the nuclear fuel, too, whereas this device must necessarily leave quite a bit on the table once the fuel is filled up with neutron poisons.

      But it’s a great incremental step between reactors and the old-style radioisotope generators, so it has a great spot in space. Hopefully there’s a microgravity version that they can use on deep-space probes as well, because we’re running out of the plutonium isotope the RTGs require and there aren’t plans to make much more due to nuclear nonproliferation, which is a good thing—but NASA could use an alternative.

      Now if we could just keep working on that nuclear propulsion! For orbit-to-orbit use, of course. I’m not a total psycho. Built probably in Lunar orbit to keep the likelihood that it’ll re-enter the atmosphere at zero. But we could do some serious manned exploration of the planets with even crude forms of nuclear propulsion. Obligatory plug of this site:

          1. And keep in mind that ITER will never produce a single watt of power ;-)
            It’s an experimental reactor, not a power plant. Those have yet a long way to go…

        1. About 60 years ago I was told as a child that fusion reactors were only a couple of years away. Also there were only a few problems that needed to be solved. Then electric power would be so cheap that it would be given away.

          As an old man I do not think fusion will be brought on line in this century and certainly not in my lifetime.

          It is my understanding that both lead and cadmium are used in some PV arrays. So there is a little extra work in recycling the older PV arrays. Some arrays will fail of old age after 20-30 years, but the will be arrays fail due to damage from wind blown debris, vandalism, abandonment, etc. This must also be taken account of in calculations for future energy supplies.

  4. Hmmm… I can certainly see where the *initial* condition is safer than an RTG. Even highly-enriched uranium is pretty safe to handle so long as you’re dealing with a sub-critical piece, certainly a far more pleasant material than the Pu-238 used in RTG’s

    You could even take the uranium up as sub-critical subassemblies, and load the reactor in situ once you land.

    Safe as houses.

    But… once you start it up you have an unshielded nuclear reactor running, even with a control rod to shut it down the uranium fuel is going to be full of fission products, themselves very radioactive. Ever see the pretty pictures of glowing spent-fuel pools here on earth?

    What I guess I’m saying is, has anybody calculated the weight of the extension cord required to get this thing over the next big hill?

    1. Like the picture shows, the reactors would be “planted” in the ground, with the core buried to contain the radioactivity. Not that anyone is likely to be walking around them out on the surface of Mars, of course.

    2. I once read, that about five meters of “dirt” are enough to shield you even from the direct radiation of nuclear fallout after a blast. Let’s make that 10-fold for long term safety. I could lend you my 50m extension cord, if necessary :-) And no, it’s not heavy, for 1kW (about 4,5amps) regular 1,5mm² is more than sufficient. If you really have to save weight, use aluminium.

  5. Interesting. My first thought was thermocouple “Seebeck” design when I read RTG and was pleasantly surprised to read Stirling Engine. Personally, in regards to efficiency, the external combustion designs seem to be a better closed loop system and with that design there is also the added control of the outputs for safety and health advantages.

    Still wondering about pressurizing to increase efficiency… though that would most likely create more safety issues I guess if not mitigated.

    I thought Lockheed was working on small nuclear reactor also and just looked. Yes, they’re working on the compact fusion reactor:

    So, basically we need to move forward with Fusion reactors that can consume the fission reactors waste I’m thinking concurrently. Seems like a better long term strategy that I’m guessing someone already came up with and unfortunately others had invested in the current scheme which gets really shady when operations are invested in and there are daily returns in the millions or more a day and a lot of jobs on the line.

    Would be nice if the industry would grow up however and deal with the range of opportunities for development and improvement here on earth. Then again… if the systems can survive making into space and be reliable in those conditions… I suppose that’s a good sign of reliability and feasibility on earth. Then again… that’s not what started the ball rolling on earth. Talk about lazy grandiose investors.

    1. You can’t realistically “consume” fission reactor waste with a fusion one. Heck, you can’t even generate net power output with any existing fusion reactor. Those still have a long way ahead of them. Net power output first, then start adding things…

      What you however CAN do is reprocess the spent fuel, and make new fuel using way less fresh U235 then otherwise needed and shove that back into a fission reactor. Repeat until you run out of spent reactor fuel (read = several centuries from now)
      Or finally develop a practically useful fast reactor, that a) makes it’s own fuel from an otherwise unusable source and b) “burns” the fuel a lot more, leaving less waste.

      1. I think I see your point, (I’m not opposing it).
        Stars implode (theoretically) when they no longer have (my word for it) “exothermic” fusion.
        That occurs when the core of the star starts to “fuse” iron. (IIRC, they stop producing enough force to keep the shell from collapsing gravitationally to the core).

        So, the idea of fusing an even heavier element such as uranium is right out… right?

        1. Reprocessing is obvious and so are fission reactions/reactors that process the fuel into smaller elements. Great call as those systems seem to need more R&D though appear to be already designed way more feasibly compared to fusion.

          I was being pushy with R&D and going off on the fusion tangent was a little over reaching and probably aligns better to start with the more obvious nucleogenesis/transmutation processes that can use more advances in isotope synthesis and stabilization to safer materials as well as separation processes.

          I wasn’t really aligning my thoughts well with the referenced fusion method and more a fission initiated reaction that would consume the raw materials to process the intermediates for the fusion reaction. Something way more complex than what is implemented now and thinking more like a chemical production and refining factory complexity with a wider range of synthetic products. I’d have to sit down and think about the raw materials, processes and finished product capabilities and performances in exhausting tabulated/charted detail if I was taking on the process.

          Right, iron, nickel and the theoretical others limits observed and/or theorized:

          I guess fusing an even heavier element such as uranium is right out to a certain extent, though not entirely. Think about how the elements are made as is:

          In particular the more common example is U238 neutron irradiated to form P238. Seems there is potential to utilize more of the elements also in electronics components since they’re not used as much so far. Maybe even potential to utilize in mechanical systems which I’m not as aware of the R&D. I have seen some exotic R&D work on transition elements over the years for electronics and as synthetic catalysts.

          Also, recycling the waste from the fission reactor underground in water buffered submarine modules as I envision to increase safety say with solar concentrating facilities above ground ( ) powering seems more resourceful thinking to deal with the waste and fusion processes as well as not adversely impacting natural resources in more lively environments.

          Yeah, though… the problem is the fusion loop processes from the fission reactions will be very limited for energy production from what I’m aware of where-as fission has more raw materials mass and energy range potential. Would be interesting to see what the theoretical capabilities are in a tabulated/chart format with multiple dimensions of quantitative mass and energy data also.

          1. I recall being at top floor of building 207 Mech Engineering at Western Australian Institute of Technology 1979 visiting my metallurgy lecturer Lee Scott who had a door stop of his office of a smallish lump of dark metal in vacuum packed plastic. I kicked it a bit on way in and he suggested I pick it up. It was Damn heavy for the size – weird, he told me it was a 20Kg ish sample of spent U238 used as ballast in the tail of a jumbo jet.

            Packed in plastic as it’s pyrophoric as well as leaving dust residue when handled as the metal upon decay is an Alpha emitter, you Don’t want that anywhere near you at all – so easy to enter the body and even lodge on skin !
            Ie.The small particulates of U238 light enough to be swept into air easily and can lodge in the lungs or land on food giving off radiation for many years causing any number of cumulative cancers !
            Just goes to show seemingly benign low radiation highly dense emitters have long term consequences…

          2. I forgot to add on the topic of space, performing some of the materials processing in space and ideally on other planets. I’ve thought since the late 90’s about processing nuclear bombs and waste on Mars. Seems that would be a more valuable endeavor to send waste to Mars to process, mine and bring back. Especially since there is no ROI with doing anything on Mars and there are many places on earth where colonization potential can be improved. Better than underground explosive processing seems on earth since the easier high energy work could be done in space (though harder to contain) or say on Mars.

          3. @ Mike:
            Yes packed in plastic, but otherwise innocuous enough to be used as ballast in a passenger airplane :-)
            But you mean probably depleted U238 (the residue after enrichment of U235) and not spent nuclear fuel. (which would be highly radioactive).

          4. Yes if course quite correct Martin
            I meant to write the words “.. processed from sample of spent U238..” or words to that effect, case of thinking processes not congruent with typing whilst distracted watching iirc a benign but, fascinating experiment whilst being fortified with some red wine ;-)
            Of course it doesn’t have to be from spent fuel, it could just as well be from the u238 as extracted from the oxide, though that would raise a few international security issues due to the tiny qty of other radioactive isotopes mostly U235 even at 0.7% or so…

    2. Industry is very reluctant to invest into any form of nuclear power in the U.S. because it can be shut down by Executive Order at anytime. Carter did that and even though Reagan canceled the EO. Industry knew right then and there, that it would only take some lunatic greenie in the Oval office to shut them down for good.

      We saw a similar thing when that morose leader of Germany panicked and shut down all of the country’s reactors which in turn made them dependent on Russia.

      I’d look to China and India for nuclear innovation right now.

  6. Spectacular tech. Very important point about the ability to launch it (and ofcourse much bigger reactors for NERVA style rockets some day if we are lucky) cold, if the entire radioactive decay of that 28kg u235 lump was focused on a person (in practice a nearby person only subtends a small solid angle when standing near the lump so real dose will be rather less, and in practice the u235 lump will mostly emit alpha radiation, plus some gamma, and only decays from nuclei near the lump’s surface will output the alpha particles, so real dose is even less again) then we’re talking 2.22×10^-5 sieverts per second for a 75kg human (again some approximations here, as all radiation will strike the front side of the person not the back they’ll be getting a higher dose in localised portions of the body). Stand near it for 261 seconds or so and you get approximately (infact you’ll get an awful lot less dose due to solid angle and absorption of most alpha decays within the uranium lump) the dose of a chest Ct scan, not too good an idea longterm but not something to panic much (read “at all really”) about the risk of launch accidents. Uranium has a long half life so very low activity, unlike the substances used for, much more dangerous heat generating RTGs. Interesting to see this running at so high a temperature as 850 celsius, I had though most reactors in power generation use were mroe like 400 or so degrees, being hotter will definitely help improve the carnot efficiency.

  7. Although the overall efficiency can’t be very high, fortunately, the waste heat is almost as valuable as the electricity. I’d hope any planetary installation would be able to keep the heat without exposure to the associated radioactive byproducts

  8. What’s a conjunction use of batteries they could use more than a kilowatt of power at any given time… The batteries will be charged on it whatever power that is not being used at the time… More efficient use of a nuclear reactor…..

  9. Chances are compact fusion reactors will be available for any serious attempt at a Mars colony, if you look at the timelines for their deployment. As for the Moon well there too, but first the uranium units, but don’t discount orbital power for the Moon as the energy can be maser beamed to the surface easily given there is no atmosphere to get in the way.

  10. It seems to me there’s plenty free energy out there, the problem is that it’s intermittent, eh solar, wind etc.

    So doesn’t that make the real problem finding a way to store energy?

    I know here in Scotland we use surplus energy to pump water from one dam up to a higher level one, and that is then available to produce hydro power, i.e. the higher dam is a great big liquid “battery”.

    I’m not opposed to nuclear, but the problem is as a commercial product we all know modern business managements standards are all about paring away safety margins and reserves to the minimum to produce the highest short term profit. I don’t trust the corporate types (or most politicians) to be concerned about what radioactive pollution will be doing to our descendants in 1,000 years time.

    1. Worrying what our stone age descendents are going to when they encounter the radioactive waste vaults is beyond our capability.

      First off all the scenarios put forth by those who are worried about the future of nuclear waste is based on humanity regressing to such a primitive state they’ve lost all literacy and are little better than the Yanomani tribesman.

      If that’s the case those poor buggers will have a lot more to worry about than some radioactive caskets they are worshiping as gods.

      1. More like that will be left to people who can’t afford to take care of the problem, or it’s hidden so they don’t know it is there. How many years has Flint Michigan been left with poisonous lead water now? 5 years? That’s fixable, but nobody will spend the money to do it. Now imagine someone stores radioactive waste in a state, it leaks and makes the water radioactive. All well, it’s their problem now?

  11. What is the reason against having an automated wiper system on solar panels to keep the dust off ? or even just having them tilt up periodically so it slides off ? or is that not enough ?

    1. Dust will be probably sticking electrostatically, or if _any_ humdity in the air (I’m talking Mars, not Moon) the dust will “glue” to the panels. Although the Mars rovers were not “intended” for long lives, their lives could have been longer if an electrostatic “mop” had occasionally swept their panels. But, launch weight, mission priorities an all that stuff…

  12. Does it mean that on this dead land, with almost no conditions to support our lives, with only few people on tge planet we are going to use more eco friendly power source than here? Sure equipment will be more durable and service friendly (one spanner set will fit all devices). I hope that at least international crew will bring some humanity there (imperial habbits against metric system and fundamental debate: vi vs emacs).

  13. I have not read the original article to find if this this information is available, however I presume a set of (Stirling – i.e. mechanical) engines running 24/7 over 15 years+ is not being considered to be a liability ? I understand if some fail, the others remaing running and so, but the original idea was not to rely on running/frictioning/pumping things, right? Well I guess there is not (to our mass knowledge) something that is robust enough to put 100% away these liability fears, right?

  14. They also caused the deaths of three astronauts during Apollo 1, should we have abandoned space travel forever? They analyzed the accident, figured out what went wrong, and took steps to prevent future contamination.

  15. Potassium 40. I find it not very strange to find that no one in this set of comments has mentioned the fact that all humans and all live things ever on earth have always been nuclear radioactive because live things, including especially plants, have radioactive potassium 40 in them. Humans did not know about nuclear radiation until about 1900 and most of them still do not. I knew about nuclear bombs and then nuclear reactors and then why the first operating nuclear reactor had to be a large pile of very pure carbon graphite bricks and a few of the graphite bricks had holes in them packed with uranium compounds or uranium metal to equally disperse them in the large pile. The few moments that this reactor had a running chain reaction it generated less heat than a large candle for some seconds. Soon the pile was dismantled and moved and rebuilt outside of Chicago USA. I did not know about potassium 40 until after Chernobyl. The lack of knowledge about potassium 40 has has most people believing that they must avoid any nuclear radiation and eliminate any and all possible existing, new and future dangers from it. Besides the nuclear radioactivity inside every cell of our being including all bacteria and other organisms, we get a lot more from earth and sun and stars. Everybody in any comments for or against energy from fission reactors and their dangers, should admit that they are sources of nuclear radiation from Carbon 14 and Potassium 40 as are all plants and animals and organisms dead or alive. What goes into their toilets or graves is radioactive waste.

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