I'm thinking of building a very long term heating system for a housing complex. I want it not to rely on sunlight, wind or other fuels.

And I just need the heat. The surroundings are pretty cold, so it would be problematic if it went out for any reason.

So I'm thinking:

  1. No moving parts

  2. Long lasting (1000 years)

  3. Perfectly safe

  4. Not slowing down the rate of heat emission too badly (10% over the aforementioned 1000 years is okayish)

Would a block containing a sphere of uranium be good for that purpose? (or maybe other fissile material?)

It would be contained within a bigger block of lead with copper heat ducts (just by heat conductivity through solid copper wire, about 2 inches in diameter) to spread the heat around.

When I was looking at it I found a few things there:

  1. Russia had fissile material "cans" that were hot to touch and people picked them up and got sick. Lead should prevent that, right?
  2. Uranium and its fissile products some which seem to have a long half-life of 16000 years or more, so it should be long enough for my purposes, right?

so: Is this a viable technology?

EDIT: People would have nearly no idea on how it works, but would be intelligent and literate (they can do minor repairs as long as they're not convoluted).

The houseblocks are insulated from the outside pretty well but I would still expect decent amount of heat gradient with the most inner rooms being rather hot (and used for baths and so on) with the most outlying places used as freezers with single digit negative temperatures Celsius.

Temperature "outside" is constantly in the double digit negative temperatures.

  • $\begingroup$ What are you trying to heat? Mars settlement with crew 100% trained scientists engineers have different needs and capabilities than apartment house in Moscow, for example. $\endgroup$
    – Mołot
    Commented Feb 16, 2018 at 9:51
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    $\begingroup$ Oh, and by the way "Perfectly safe" rules out each and every heating solution ever designed by humans. We never make perfect things. $\endgroup$
    – Mołot
    Commented Feb 16, 2018 at 9:55
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    $\begingroup$ 10% is not a reduction rate if you don't tell the time span.. 10% per second or 10% per millennium are not the same, aren't they? $\endgroup$
    – L.Dutch
    Commented Feb 16, 2018 at 10:06
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    $\begingroup$ Does "perfectly safe" have to cover scenarios like scrap metal merchants deciding to raid the buildings basement or idiot teenagers intentionally trying to break into it for shits and giggles? $\endgroup$
    – Murphy
    Commented Feb 16, 2018 at 11:06
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    $\begingroup$ Related Radioisotope thermoelectric generator $\endgroup$ Commented Feb 16, 2018 at 13:35

9 Answers 9


Maybe, but you really do not want to

The complete decay chain of U238 releases about 52 MeV, while U235 releases about 211 MeV. But while the energy per decay is 4 times lower and the decay rate is 6 times slower, this is offset by the fact that U238 is 100 times more common than U235 in natural uranium. So let us just assume you got hold of some depleted Uranium, it is all U238.

The decay rate of U238 is 12,300 Bq per gram. So that is approximately 640 000 MeV per second and gram.

But 1 MeV is only 1.6 * 10-13 Joules, so 1 gram of Uranium only supplies 0.1 μJ per second, or 0.1 μW. To reach 1 W you literally need 10 tons of uranium.

1 W means 8.7 kWh per year.

In a new, super-efficient building, you need 15 kWh per square meter and year for heating. So per square meter you would need approximately 20 tons of uranium, or a uranium foundation of your building that is about 1 meter thick. Double that because half of it will be lost to the ground beneath the buildings, so a 2 meter layer of uranium under your house.

This is not impossible...

...but not at all desirable.

This will produce lots of radioactive gasses, most noticeably Radon. Radon needs to be vented away from the building. But venting means more energy lost, especially if you want to make it a no-maintenance passive system. In fact, it will need to use the heat to drive the air flow.

And right now I have not even mentioned the fact that you said the outside temperature is "double digit negative"...

So in brief: that "ball of uranium" you are talking about is layer of uranium under your entire house that is between 5 and 10 meters thick.

...and leaking radioactive gasses.

And before you ask... NO, you cannot use all U235 instead, because that foundation will go...

enter image description here

...when you try to build it; making a house foundation of weapons grade uranium epitomises the concept of Very. Bad. Idea.

An alternative

A much more credible solution would be to go for something that has already been proposed, but just not demonstrated yet. Go for a Travelling Wave Reactor (TWR).

The Travelling Wave Reactor is like a cigar. You essentially have a cylinder, or a "tube" of fissile material, and then you "light" it at the one end. Then — just like with a cigar — you have the un-burnt side, the glowing zone, and the burnt out "ash" zone. And the glowing zone in a TWR will only very slowly "crawl" along the cylinder and regulate itself.

enter image description here

So for your setting, just stick a really looong winding TWR under the domicile.

You can even have it in segments, so that every 100 years or so, a new segment needs to be "lit". With minimal hand-waving, this can be made easy and simple enough so that the inhabitants can do that.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Commented Feb 23, 2018 at 4:15
  • $\begingroup$ make the TWR roughly a circle and after lighting it once, you just swap out segments when the lit zone is about halfway down the next piece and swap out the burnt out pieces. $\endgroup$
    – Trish
    Commented May 24, 2023 at 10:40
  • $\begingroup$ @Trish This violates "No moving parts" requirement. And if you take out a spent segment, you are left with the salient question of what to do with the spent segment, and how do you even approach it without becoming fatally irradiated. Instead, the segments should be configured like a long, winding sequence of segments, so that when one segment is approaching burnout, it will either automatically light up the next one, or require minimal manual intervention to make it happen, like pulling on a rope or a rod of some sort to "light the fuse". $\endgroup$
    – MichaelK
    Commented May 24, 2023 at 10:46
  • $\begingroup$ @MichaelK the idea was that the snake burns on, the segment about to burn out lighting the next. Swapping parts is not a moveable part, it's minimal maintenance - you drop out the spent tubes (pile them somewhere where you are not irradiated by them) and replace them with fresh ones. no part is moving, you can even work on it -while it burns and it might be a snap-fit connector even to make swaps fast. $\endgroup$
    – Trish
    Commented May 24, 2023 at 10:52
  • $\begingroup$ @Trish Or just leave the snake — including the burnt out parts — in the ground. $\endgroup$
    – MichaelK
    Commented May 24, 2023 at 10:55

Geothermal heating looks closer to "perfectly safe" than uranium, is achievable with no moving parts, is long lasting (in the timescale +1000 years)

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    $\begingroup$ Since much of the Earth's internal heat is driven by radioactive decay, maybe we can consider the Earth to be that "sealed ball of Uranium" (with a bit of silicate buffering). $\endgroup$
    – Spencer
    Commented Feb 16, 2018 at 12:10
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    $\begingroup$ As long as you don't leach too much of the heat, that is. Actual geothermal wells only have an estimated lifetime of ~50 years (depending on location and extraction rate, of course) after which they need to recuperate for a few hundred years. It would only take a few short-sighted guys to destroy a civilisation based on geothermal power. Could make for a good story :P $\endgroup$
    – Luaan
    Commented Feb 16, 2018 at 16:02
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    $\begingroup$ @Luaan I am not sure if you're being ironic - there is such a good story; it's Frederik Pohl's The Snowmen (Galaxy Magazine, 1959). $\endgroup$
    – LSerni
    Commented Feb 16, 2018 at 17:41
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    $\begingroup$ I thought much of earth's internal heat came from tidal forces and friction. $\endgroup$ Commented Feb 16, 2018 at 17:42
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    $\begingroup$ @WalterMitty, measurements and models indicate about half is from radioactive decay. Much of the rest will be primordial. Tidal drag is not expected to be significant driver on earth. en.wikipedia.org/wiki/Earth%27s_internal_heat_budget $\endgroup$
    – BowlOfRed
    Commented Feb 17, 2018 at 8:23

A sealed ball of Uranium will decay producing at some point Radon. Radon is a gas, which, besides being radioactive, will build up pressure in your system.

The fact that now you have to choose between two equally nasty options:

  1. Let the thing explode like a shrapnel
  2. Slowly leak radioactive gas in your building

rules out that your simple design can be

3.Perfectly safe

and thus I say it is not viable (ignoring all other risks).

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    $\begingroup$ Actually, he could slowly leak radioactive gas out of the building. Chimney is simple solution. Still, far from perfect. $\endgroup$
    – Mołot
    Commented Feb 16, 2018 at 10:12
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    $\begingroup$ @Mołot, being denser than air, Radon would hardly rise through a chimney. It would need some active blowing... $\endgroup$
    – L.Dutch
    Commented Feb 16, 2018 at 10:31
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    $\begingroup$ @L.Dutch No no no, chimney would prevent pressure increase all right. No actual need to remove radon. You should actually prefer it staying down, in lead pipe, over it being released into void / atmosphere. $\endgroup$
    – Mołot
    Commented Feb 16, 2018 at 10:33
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    $\begingroup$ Radon gas is released from the ground pretty steadily in areas with granite bedrock anyway. Venting/releasing radon is already a known issue in any building. $\endgroup$
    – Murphy
    Commented Feb 16, 2018 at 11:04
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    $\begingroup$ Wait a minute can't the original builders estimate how much Radon at steady state and make a void for holding exactly that much and keep it for more heat? (assumption: source material is powdered) $\endgroup$
    – Joshua
    Commented Feb 19, 2018 at 5:06

Russia used to have lighthouses that would run on small, lightweight nuclear reactors that could be left completely unattended for years. http://englishrussia.com/2009/01/06/abandoned-russian-polar-nuclear-lighthouses/

Somethig like that could work for heating.

They had to be fully autonomous, because they were situated hundreds and hundreds miles aways from any populated areas. After reviewing different ideas on how to make them work for a years without service and any external power supply, Soviet engineers decided to implement atomic energy to power up those structures. So, special lightweight small atomic reactors were produced in limited series to be delivered to the Polar Circle lands and to be installed on the lighthouses. Those small reactors could work in the independent mode for years and didn’t require any human interference, so it was very handy in the situation like this. It was a kind of robot-lighthouse which counted itself the time of the year and the length of the daylight, turned on its lights when it was needed and sent radio signals to near by ships to warn them on their journey. It all looks like ran out the sci-fi book pages, but so they were.

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    $\begingroup$ Radioactive thermal generators (RTGs) are far from safe, most of these lighthouses are either converted already, or abandoned & irradiated. $\endgroup$
    – Mołot
    Commented Feb 16, 2018 at 12:45
  • $\begingroup$ Nice, thank You, great amount of good info there. $\endgroup$
    – Gensys LTD
    Commented Feb 16, 2018 at 14:13
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    $\begingroup$ Welcome to WorldBuilding, joecro! I'm afraid this has ended up in the "low-quality posts" queue, partly because it's short, and partly because this is verging on a "link-only answer", which is frowned upon because links tend to die out over time. OP has indicated that the information in the link is useful, so could you bring some of it into your answer to flesh it out a bit more, please? $\endgroup$
    – F1Krazy
    Commented Feb 16, 2018 at 14:36
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    $\begingroup$ Note that the comments under that article state that the nuclear reactors were changed to RTGs, which are different animals. $\endgroup$
    – user3106
    Commented Feb 16, 2018 at 15:13
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    $\begingroup$ RTGs and classic reactors are good for tens of years, not thousands. $\endgroup$
    – Mark
    Commented Feb 17, 2018 at 0:54

Right idea, wrong stuff.

And you don't need to fission it, in fact you really don't want to, because that opens so many cans of worms, among them the government really, really, really not liking you having fissile material.

Decay heat will suffice.

Take Caesium-137. Please. No seriously, it's the wrong stuff for your 1000-year goal because it has only a 30 year half-life. We just have a lot of it.

The most ready source of useful material is spent nuclear fuel.

Given your design intent, you want a half-life of 5000 years +. There are no fission products (split atoms) with half-lives between 100 and 210,000 years. There are plenty among actinides: uranium which did not split, but absorbed neutrons, becoming heavier. Berklium-247, Pu240, stuff like that. There are plenty in the casks out back of your local nuclear reactor.

Things with 5000+ year half lives are essential. The problem is if your pile is heavily contaminated with things with shorter half-lives, like that overabundant Cs137, your pile will sharply cool off over the 1000 years. That's why just using the casks won't work.

The actinides with the 4000-20,000 year half lives would give the best performance per mass. (Not that mass is a problem). But the government may be ketchy about you having actinides -- as many are (or will decay into) things which are fissile. And things with such short half lives make an effective "dirty bomb".

So I would go a different direction. I would focus on the fission products with >210,000 year half-lives. Propagationwise they're inert. They're lethargic enough not to make a very good dirty bomb. The mass needed is very much larger, but that's not a huge problem for a building. The heat output will drop less than 1% over the 1000 years.

Do keep in mind that heat output and radiation output go hand-in-hand. That is another reason I prefer low-energy bulky material. It helps if the radiation is alpha or beta, which is easily shielded, but it may not be economical to separate isotopes which only decay that way.

Another option, though I dislike the idea of birthing more radioactive material, is to radiologically activate a common element via neutron bombardment or other means. I haven't pored through the isotope charts to see if this will create anything suitable. The risk is of the material picking up one too many neutrons, and becoming a contaminant.

Which particular fission products end up in the mix? That will be a function of the source material (which I presume to be spent nuclear fuel) and its origins, reactor type, but mostly, of the production engineering needed to isolate a workable mix of isotopes. Such engineering tends to be full of surprises (Los Alamos couldn't imagine gaseous diffusion would be as workable as it was.)

So you'll assay a sample set of spent fuel, say. Then you'll look at several hundred chemical separation processes to extract a usable blend of isotopes. Money is a factor, you're looking for the most efficient way to extract an acceptable set of isotopes.* Which processes make sense depend on your source material, obviously.

* Presumably, isotopic separation is cost prohibitive; an element is all-in or all-out. If an element has a desirable isotope, but is contaminated with a slower isotope that neither helps nor hurts, that's OK -- but if a third dominant isotope has a too-short half-life, then the entire element is out. Or, if the problem isotope has a 10-year half life, you should search for spent fuel that's at least 40 years old, as that is a very cheap way of excluding that isotope.

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    $\begingroup$ Actually naming a concrete suggestion would improve your answer. $\endgroup$
    – Yakk
    Commented Feb 16, 2018 at 20:44
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    $\begingroup$ @yakk I'm not at all sure what you are expecting to see as a suggestion. $\endgroup$ Commented Feb 16, 2018 at 21:00
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    $\begingroup$ An actual fission product would be a concrete suggestion. A class of fission products isn't. $\endgroup$
    – Yakk
    Commented Feb 16, 2018 at 21:03
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    $\begingroup$ @Yakk given likely sources, separating down to a single isotope would be foolhardy, as it would cost a staggering amount of money only to waste viable material. Which isotopes are most extractable is a production-engineering problem, and those tend to be byzantine in complexity and very, very full of surprises. OP prohibits a Santa Claus machine, but if you are willing to loan him yours, then pick any isotope of any kind, you hardly need to be constrained to fission products. $\endgroup$ Commented Feb 16, 2018 at 22:23
  • $\begingroup$ If you want to go full-sealed, you need to avoid alpha decays, though: Alpha particles are helium nuclei, after all, so you are slowly producing a gas. Beta and gamma radiators do not have that problem. On the other hand, the containing lead box does not need to be airtight since helium is completely harmless. You just don't want to leak stuff like radium into the building's atmosphere. $\endgroup$ Commented Feb 17, 2018 at 21:47

Build an Igloo

No, seriously. Igloos basically solve this problem for you, at least on Earth. The peoples native to the Artic Circle have used Igloos for thousands of years and built an extensive culture and society in an environment not entirely unlike the one you're proposing.

Temperatures in the Artic Circle can get as low as −45 °C (−49 °F) but the interior of an igloo can get as toasty as 16 °C (61 °F) when warmed only by body heat.


Now not all igloos are the stereotypical ice block structures you see in cartoons. Many are made simply of packed snow, which despite being cold, is a very good insulator. And Igloos aren't small either, though smaller igloos can be used when on excursions hunting. Some can house dozens of people and be made up of several rooms. And this is all without any "modern" technology.

Essentially your heat source is the people inside, you just need good insulation to keep it warm. And some warm coats.

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    $\begingroup$ This is useful information in that it indicates that with a good enough insulator, most any heating system that can provide a hot bath will suffice, no need to take into account the low external temperature. $\endgroup$
    – Law29
    Commented Feb 18, 2018 at 11:00

An alternative way to heat the house is using a solid state heat pump, and this can be implemented in a way that doesn't require fuel. Suppose that the interior of the house is to be kept at 20 C, the outside air temperature is -40 C and a few meters below the ground, the temperature is -15 C. Then one can exploit the temperature difference between the air and the environment under the ground to generate a voltage via the Seebeck effect, and the power obtained from that can be used to pump heat from the ground to the house via the Peltier effect. The entire system of both components then doesn't require any external work.

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    $\begingroup$ The Seebeck effect is abysmally inefficient at low temperature differences. $\endgroup$
    – Mark
    Commented Feb 17, 2018 at 1:00
  • $\begingroup$ @Mark Yes, but note that the temperature differences here can actually be very large. $\endgroup$ Commented Feb 17, 2018 at 18:39
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    $\begingroup$ The posited 25 C difference between the air and the ground is small. Even the 60 C difference between the air and the house is small (and counterproductive to exploit). A typical thermoelectric generator uses a temperature difference of hundreds of degrees. $\endgroup$
    – Mark
    Commented Feb 17, 2018 at 20:25
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    $\begingroup$ Also note that if the outside temperature always remains at -40 the subsurface temperature will also trend to the same temperature until you dig deeper and meet the rising geothermal heat flow. $\endgroup$
    – KalleMP
    Commented Feb 19, 2018 at 9:47

Use the thermocouple or Seebeck effect (kudos to Count Iblis for the root idea) with the house as the cold end and 20km down as the hot end. I'm pretty sure that a full crystalline packing on the foundation would provide more heat than required. Use the electrical power to keep the lights on and run fans to move the hot air around. The difference between 20C and 600C provides a lot of potential but Wikipedia doesn't have enough equations for me to finish the calculation.


You're between a rock and a hard place if you're trying to do this with passive nuclear decay. Most nuclei (see https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html) provide a few hundred keV to a few MeV of energy in decay.

  • The power they provide is then given by their lifetime: The shorter the lifetime, the faster the decay, the higher the initial power.
  • But faster decay also means a faster drop in the number of remaining nuclei, so the power drops off faster.

If you want a lifetime of 16,000 yr, so it doesn't drop off very much in 1,000 yr, then you're not going to get much energy from the few nuclei that decay in any given second. You'll need a lot of nuclei, hence a very large mass.

You'd be better off with some simple reactor (perhaps of a kind we don't know about yet) that can speed up the natural decay of thorium or uranium. If instead of letting the half-life law decay it, you 'burned' it until it was all gone, you could get a higher energy from it. The Thorium TWR (see earlier excellent answer by MichaelK) is a good example of this. For your world-building, you could just assert that we know how to make something like that with the parameters you need.

  • $\begingroup$ Sorry, the reply was incomplete: The natural alpha-decay of heavy elements doesn't produce as much energy as fission does. Using neutrons to induce fission both increases the rate, and increases the energy per nuclei produced. $\endgroup$ Commented Feb 18, 2018 at 0:46

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