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A radiothermal generator uses the heat produced by radioactive decay of the radioisotopes contained to generate electricity. The main difference with nuclear reactors is that there is no "reaction", chain reaction or otherwise, just the natural decay of the isotopes.

Currently RTGs are used as power plants in situations where a power plant that works for several years without maintenance is needed. This is because an RTG doesn't actually need any moving parts to work. Such RTGs use thermoelectric systems and highly radioactive isotopes selected based on the desired lifespan.

But there is nothing stopping you from using less radioactive isotopes for longer lifespan or using more normal solutions for turning the heat into electricity. Using such RTGs instead of nuclear reactors would have some benefits.

Can use any radioactive material. Uranium, thorium, radon, nuclear waste... Since the RTG does not need to support a chain reaction it does not care what elements or isotopes you throw in. This should make fuel much cheaper to get.

Much simpler fuel refining. Since the RTG does not care about the specific isotopes and can't be "poisoned" by inactive material, chemical refining methods are sufficient. No centrifuges or other methods capable of creating weapons grade uranium are needed. Refining fuel creates no risk of enabling nuclear weapons development. Much lower level of technology and investment is needed.

Less waste. Much less waste. RTG works until the fuel is no longer radioactive. Normal reactors only use a portion of the fuel since using the rest would requiring refining the used rods to make new fuel, which would enable refining the fuel for weapons grade material. So much of the fuel is simply wasted.

Safer waste. The high neutron flux of a reactor creates new radioactive elements such as plutonium. The RTG generates less flux and generates less such material. And any such material would be much harder to separate even if somebody opened the generator while it is operating. So unlike a reactor the RTG does not help in generating material for nuclear weapons.

Safer. No reaction means the reaction can't get out of control no matter how many safety systems you disable. You still need a containment vessel and it still can rupture and release steam containing radioactive material, but since the RTG does not need or generate highly radioactive isotopes and a runaway reaction can't melt the fuel the level of radiation would be much lower than with a reactor. Which would help a lot with fixing the damage.

So if it is so safe and solves all nuclear proliferation issues, why aren't any being used? Economics. Such RTGs would be much larger than a reactor generating the same wattage. The RTG could operate almost indefinitely with little maintenance without ever needing refueling. But reactor fuel and maintenance are not expensive enough to make up the higher initial cost with any realistic interest rate. Although the fact that RTG would be much simpler and cheaper per volume would help.

So with that intro, here is the actual question. Under what circumstances, if any, could using such large RTG systems for base power become economical? Science based, but not real world. Alternate reality or history is okay if it makes sense and obeys the same science.

First question, btw. And something I have been thinking for a while.

Important note.

From the first answers it seems that I did not properly explain how different these base power RTGs would be, would have to be, from the RTGs currently in use. I'll note some specific issues that were relevant to those answers. (And really necessary to understand the question and why I am unable to even suggest plausible economics.)

Current RTGs have relatively short lifespans and use "artificial" isotopes with relatively low half-lifes. The RTGs I am suggesting use natural radioisotopes of uranium or thorium. These materials have half-lifes of more than a billion years! (That is why they are still common.) Obviously the power density sucks big time. On the positive side it will take millions of years before the power runs out. On the negative side the installation would need to be huge beyond belief to generate useful amounts of power.

So these are not small (relatively) devices spread around that can be lost or abandoned. these are a small number of installations you can see clearly marked on the world map. Or with your own eyes from the Moon. This is because the only way to even approach usability would be to take advantage of cube-square law of the radiothermal energy going up with the volume (mass really) but of the mass (and thus cost) of the containment with the surface area. Power generating systems would scale with the power generated (the overhead becomes insignificant at this scale), so they would be fine.

I might be overestimating the scaling issue, though. Even without chain reactions neutrons released by fission would occasionally hit other nuclei. So the rate of fission would almost certainly be higher than the normal half-lifes would suggest.

So these RTGs would not be so much devices or even installations as they would be artificial geology in (more or less) state of equilibrium. With a small building (or several) on top to house the actual generators.

So both the RTGs and the fuel contained, essentially large chunks of solid metal, would be stable and secure for probably few centuries, possibly millennia. You can ignore toxicity.

The issue is the economics of something that large and with that long lifespan. I have no clue how it could be possible. Maybe a society that takes a very long view. Maybe it really works better than I thought it would.

I apologize for not explaining properly. I have no idea how I thought people would get the scale issue when I forget to mention it.

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  • $\begingroup$ Nice question. You may also find the rtg tag over on Space Exploration to be of interest. $\endgroup$ – a CVn May 20 '15 at 18:03
  • $\begingroup$ I'm now prying open 3 smoke detectors to retrieve the radioactive element Americium still have 450,801,920,543 more to go I must ensure you have enough power for your rtg. $\endgroup$ – user6760 May 21 '15 at 16:06
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RTGs produce little power for many years, as such they are ideal for situations where the power requirement is small and where maintenance is difficult or impossible, i.e. radio beacons in far North, satellites and so on.

They are opposite of ideal for the conventional power though. A coal power plant can have output of 2~3000 MW, and RTG, maybe 2~300 W, that's about ten million times less, and it takes a lot of coal plants to keep country going.

Considering that there have already been incidents of people being poisoned by abandoned RTGs, it isn't hard to see that having an equivalent of tens of millions of there things is neither efficient nor safe.

One of the main issues with RTGs is that they are horribly inefficient, so making larger scale ones would likely lead to engineering complexity growing faster then usefulness. Much larger device also means much larger chance of leaks or other structural problems, which at a certain size will start to negate the lack of maintenance advantage, as it will require constant monitoring and fixes in a potentially dangerous environment.

In most real life situations it would be better to use solar or wind power, as it also requires relatively little maintenance with the main drawback being space, which is also true of RTGs. So they have the same drawback and similar advantage, but solar and wind are less dangerous and can produce more power.

So to answer the question: if the planet had no wind and not enough solar energy reaching the surface, and the people would only use electricity for only absolute essentials, while also not having enough time or expertise to run a regular nuclear plant (like say a remote space colony or a prison planet) I think it might be plausible to use a large RTG.

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  • $\begingroup$ Yes, the RTGs would have to be huge, since they would be used provide base power with much lower power density than current RTGs. The lower power density comes from being designed for much longer life span, which with RTGs translates to inflated size. Since both answers assume I am talking about something like current RTGs, but with larger numbers, I guess there is something wrong with the question. I'll check where the errors comes from and edit the question. (If I find it. It could be it was just too long...) $\endgroup$ – Ville Niemi May 20 '15 at 19:58
  • $\begingroup$ @VilleNiemi I've edited the answer to try and address your question rather then just discussing current RTGs $\endgroup$ – Maxim May 20 '15 at 20:40
  • $\begingroup$ Thanks. You missed my added explanation of the specifics (edit in question). You got close enough for upvote anyway. Guess the details do not change the fundamentals. Your scenario would make the economics more feasible. $\endgroup$ – Ville Niemi May 20 '15 at 20:52
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We already do this. It's called Geothermal Energy:

http://phys.org/news/2006-03-probing-earth-core.html

and:

http://en.wikipedia.org/wiki/Geothermal_energy

Now if you wanted to (for some reason...) set up a world where you artificially exaggerate this effect, by say, filtering the various radiative isotopes out of the inner earth and moving them closer to the surface, I suppose you could - though I expect that there would be serious negative consequences.

http://en.wikipedia.org/wiki/Man_of_Steel_%28film%29#Plot

If you were colonizing a planet at the edge of the habitable zone, and judged it just a little too cold for comfort, it might make sense to bring lots of extra radio-isotopes and bury them to raise the overall temperature...

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  • $\begingroup$ Upvote for pointing similarity to geothermal. I guess it could be seen as artificial geothermal for places where geothermal is not available. Or simply needs a boost to be useful $\endgroup$ – Ville Niemi May 21 '15 at 14:20
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Even if you could generate a nominal amount of power, the inputs required just for building and maintaining such a massive facility would be truly awesome, so you are better off doing anything else (whatever fuel sources you use to construct this facility, just use that for your power generation and still save most of it).

As a rough idea, radioactive materials produce energy in inverse proportion to their half-life (longer the half-life, the less energy you get out of them). If you want a material that will keep the radioactivity for very long periods of time, the amount of energy coming out of it is negligible. If you want it to run 'almost indefinitely' without refueling, it would be producing so little energy that it would be just a large art project. We use fuel with short half-lives not because we just don't care about a long-term, but because generating meaningful amounts of power requires something highly energetic as a power source.

Just scaling up is not a solution to using low energy fuel as the engineering complexity and construction costs rapidly increase when you build on a huge scale (actually makes the project more difficult). The challenge of thermally isolating that giant mass from the rest of the planet, because the heat conduction needs to go through the thermocouples or working fluid if you want to extract power from it (otherwise you are just using geothermal and mixing in radioactive ores you've mined elsewhere at great expense for no discernible benefit), would be an engineering marvel by itself. Building bigger gets more complicated, not less, as you have much greater forces to contend with while material strength stays the same, and eventually you start getting geologic forces to contend with (subsidence issues from the mass over long timescales could cause serious issues).

RTGs slowly reduce their power output over time as the fuel loses energy and the components degrade (they are not maintenance free but just have components expected to live as long as the relatively short-lived fuel). Even if the fuel lasts for a couple centuries, the components used for electrical generation certainly won't.

By holding this fuel until it is no longer radioactive, you also ensure that the facility spends much of its time sitting idle as the fuel degrades below a practical operating threshold yet still radioactive through the long tail. The waste is still there, and still needs to be dealt with eventually. If your big concerns are producing waste and not generating any material which could be used for nuclear weapons, there are plenty of designs for fission plants which meet those criteria for a considerably lower cost and with massively higher output.

Bottom-line: these would be giant art installations, requiring massive resources to construct, for comparatively little energy generation.

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  • $\begingroup$ Yes, agreed. That is why I could not figure out any plausible reason to build one and asked for one. Your answer is really more a summary of reasons why I had to ask this question than an actual answer, but your summary is so much better than the way I explained my problem in the question I'll upvote anyway. I disagree on the scaling issue, mostly because I think you are underestimating the difference in building methods used for this compared to something the size of, say, an ordinary nuclear reactor. Agree an all the the issues you mention being issues, though. Like I said, a good summary. $\endgroup$ – Ville Niemi May 21 '15 at 21:47
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My guesstimate is "not a feasible scenario." The reasons:

  • RTGs provide relatively little power for their size. Taking just any random nuclear waste will only make less efficient.
  • Most radioactives are not just radioactives but poisons on a chemical level. RTGs would require strict supervision. A rusty forgotten RTG on a scrapyard could poison the entire city if it leaks slowly, no fireworks required.
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  • $\begingroup$ The first reason is why I am asking for a possible scenario to make the economics work. And it also means that these RTGs would be much larger than nuclear reactors they replace, they could not be forgotten and would have passive defense due to sheer size of their containment areas, Also these would be designed for long lifespans (increasing the size issue!), so the fuel would be designed for stability. Essentially blocks of radioactive metal coated with non-radioactive metal. Details are not really relevant to the question, though. Assume security works, economics sucks... $\endgroup$ – Ville Niemi May 20 '15 at 19:50
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The issue is cost. A pile has some amount of radioactive material in it. This is used to generate energy. At some point there is a term measured in kW/kg regarding how much power a kilogram of material can output. Note this is power, not energy. You mentioned they have to be big, but you did not mention that they have to be expensive. Uranium has a market price, and there will be a cost for packing it into the reactor. Now we can start talking about kW/dollar. Unfortunately, if you have to wait a billion years for a large portion of the uranium to decay, your ROI is not very desirable.

Perhaps you could do this once you start talking about being a Class I civilization, where massive earthmoving projects like this are reasonable.

Or, taking the other approach, you can look at codeMonkey's answer (which I +1'd), which is an example of how to do this at minimal cost... and by that, I mean not gathering anything at all and simply using the existing pile in the Earth to do the work for you.

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  • $\begingroup$ The question is specifically about economic feasibility and how I can't see any way for one and would like to know if anyone else can figure out one. And yes codemonkey has the correct general idea how it would work by comparing it to geothermal. It would essentially be a very expensive way to create artificial geothermal power. The question is: Would there be a scenario where that makes sense? $\endgroup$ – Ville Niemi May 21 '15 at 22:14
  • $\begingroup$ @Ville Niemi Does it have to be artificial? The beauty of code monkey's answer is that he found a pre-existing RTG and just used it, rather than expending all that effort to make one. $\endgroup$ – Cort Ammon May 22 '15 at 16:06
  • $\begingroup$ Since my problem was not being able to figure out how to make the economics of building such work, pointing out that you might not need to build one is of limited value. But while it isn't the answer, it sure did get me thinking bit wider. I am actually rethinking my approach based on the answers here. Basically, I was stuck, and while there isn't a single answer I would pick as correct, thanks to you guys I am no longer stuck. $\endgroup$ – Ville Niemi May 22 '15 at 20:22
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    $\begingroup$ @VilleNiemi The best answer is the one you didn't even know you asked the question for =) Good luck! $\endgroup$ – Cort Ammon May 22 '15 at 23:04
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If you use artificial isotopes you run into cost issues, while at the same time, RTGs generally have lower power output than mechanical nuclear power sources and less durability. We have power stations in USA that are already 50 years old and running, while RTGs cannot beat this due to degradation of the photovoltaic panels due to the bombardment they suffer from ionizing radiation. A RTG is simply a radiological source contained inside a photovoltaic sphere. The gamma rays that the source emits are turned into electricity by the photovoltaic cells, but at the same time, those cells suffer a much higher degradation than when used under sunlight. Gamma rays have much more energy per photon than solar light, this means more power per square meter of photovoltaic area, but means that the semiconductors are destroyed much more quickly. This, coupled with the higher cost of the power sources (artificial power sources that are good at generating gamma rays). Those characteristics together prevents widespread use. RTGs are used where maintenance is hard to impossible (like Siberian light houses) or satellites. Anything that provides easier accessibility will be using Diesel generators or have a energy line connected to mainstream energy generation technologies. Thats why, for example, we dont use RTGs to generate large scale power.

Even if you have accessibility problems in a certain area, if your power requirements are too big, you will probably use automatic nuclear power plants of the thermal cycle type (usually rankine, but you might do brayton cycle nuclear power plants if your power needs are bigger than the usual for RTG and smaller than the usual for Rankine cycle based ones), not of the photovoltaic type. On the moon, if you trully need power sources that powerful, you will probably use robotic nuclear power stations, as the risk of explosion is not significant. There is no population on the moon, or wind to spread the escaping nuclear material. RTG use is limited by the technology itself.

TL;DR

Major factor against RTG is power density, cost and shorter lifespans than properly managed nuclear thermal power plants.

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  • $\begingroup$ Could you provide a link to an example design where photovoltaic cells are used? I cannot wrap my head around such a thing. $\endgroup$ – pluckedkiwi May 22 '15 at 16:44
  • $\begingroup$ Sorry, photovoltaic or thermeletric converters. $\endgroup$ – Jorge Aldo May 22 '15 at 17:50

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