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In my world, all terrestrial bodies in the solar-system of over 900 km size have a significant human presence. Earth-moon Travel is as common as an interstate road trip, and Interplanetary travel is as common as going overseas, ranging from Venus to Earth is the same as a trip from the U.S. to U.K. today, and Earth to Pluto is the equivalent of U.S. to U.K. in 1700. This is the consequence of fusion power being mastered to the point where a fusion reactor can be stored in a truck, with reacting Deuterium, Tritium, Helium 3 and Boron 11 (of course protium fusion is still impossible, for that, feel free to build your own star). This begs the question (or rather, I beg my own question ... whatever)

Is there any practical justification for using fission power when fusion power (as described) exists?


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    $\begingroup$ Are there dramatic differences in cost between the two power cells? If yes, then that might be an angle. If not then a deep space probe or a geo satellite or a solar satellite could be worthy targets. No humans, no need to protect them. Assuming reasonable shielding for instruments. Assume they could be fueled in orbit to calm citizens re silly launch concerns like here on Dirt. $\endgroup$ – dcy665 Sep 4 '17 at 4:25
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    $\begingroup$ A properly-designed fission reactor can produce lots of useful isotopes. E.g. nature.com/news/… $\endgroup$ – jamesqf Sep 4 '17 at 4:29
  • $\begingroup$ fission releases about 200 MeV while fusion releases around 20 MeV per reaction, of course fusion releases much more energy on per weight basis! I think answer could only be political, e.g. China discovered a rich source of uranium deposit somewhere in the solar system and wants to dominate the nuclear energy market by inflating it's cost. $\endgroup$ – user6760 Sep 4 '17 at 6:45
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    $\begingroup$ Why do we still have fossil fuel powered vehicles? Maybe it's just because one is easier to build or more portable? $\endgroup$ – The Great Duck Sep 4 '17 at 23:33
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    $\begingroup$ Your question seems to imply that plentiful energy generation is the only hurdle barring interplanetary travel, but it's only a minor problem compared to some others. Even ignoring all the biological problems humans have in space, unless your world has reactionless drives (which are pseudoscience, or else most things we know about physics are wrong) then turning electrical or thermal energy into thrust is only possible by shooting bits of your spacecraft off behind you. That means incremental increases in range or payload weight require exponentially greater fuel capacity. $\endgroup$ – bcrist Sep 5 '17 at 7:45

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There are a couple of reasons:

  1. Fission scales down better. Some SNAP reactors are tiny, smaller than a trash can. Radioisotope thermoelectric generators (RTG's) can be made even smaller. Fission based "batteries" are even possible. Small and/or low power applications will favor fission.

  2. Fission generators are mechanically simpler and more robust. This is important when you are building things that need to last, especially in places where maintenance is impossible. For instance remote locations will favor the mechanically simpler fission.

  3. Fusion produces a buttload of heat, way more than fission. Normally this is a good thing since the heat is what we want to harvest. But, this heat needs to dissipate, in an atmosphere that's not too hard, in space that's a lot of extra radiator surface, which adds both mass and makes the ship more fragile. This is basically the scaling issue again but with added worries in vacuum. Many spacecraft will favor fission because heat dissipation is a bigger concern and output needs are generally small.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – a CVn Sep 5 '17 at 17:22
  • $\begingroup$ 4. Fission can produce a variety of different, heavy elements as waste products, many of which are difficult (maybe impossible?) to produce through fusion. $\endgroup$ – aroth Sep 6 '17 at 1:49
  • $\begingroup$ @aroth the way I understand it Iron acts as some sort of a barrier. Fusion isn't able to produce elements heavier than iron, and fission doesn't work for elements lighter than iron. en.wikipedia.org/wiki/Iron_peak physics.stackexchange.com/questions/26059/… $\endgroup$ – JAD Sep 6 '17 at 13:40
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It bears having a written answer that fusion reactors absolutely cannot synthesize heavy isotopes, if the synthesis can only happen during fission. That is to say, some heavy isotopes can be made by using a fusion reactor for neutron bombardment, like Cobalt-60, but other things like Cesium 137 can only be effectively produced by fission of uranium.

That being said, keeping large piles of heavy isotopes around is generally not a great idea. Cesium 137 is only industrially useful when you need to bathe something like a warship in enough energy to take an x-ray of its hull, but Cobalt 60 is actually better at that, and more stable. Cs-137 also reacts easily forming water soluble salts that your body can transport, making it deadly to consume, but useful in radiation therapy.

If you really have an easy time of all of this nuclear power stuff, I'll tell you where you might still see fission, and that's as a black-start power source, because it usually takes power to make power.

Black-start is the ability of a generating station to bring itself back online and place itself on the grid without that grid currently being energized. Currently not a lot of stations actually have this ability, and you may or may not have personally experienced slightly longer power outages because of this. What happens is that the stations that CAN black start have to power up to bootstrap adjacent stations until all the stations are ready to restore power to all the loads that will get turned back on, and it's a real pain when something like the Great Northeast Blackout happens.

Obviously, this point simply may not apply to your given level of technology. Maybe you have unobtainium batteries that can kick over a fusion generator; but if you don't, and if your shielding is good, it is entirely possible to build a fission reactor that can be started entirely by hand, with only a limited amount of power for instrumentation. Also, terrifyingly, reactors have been built this way in the past, with varying degrees of bad results, and the general consensus is that we should try not to do that again.

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    $\begingroup$ +1 for black starting, so few people even consider reactor shutdown and startup, but the wider the application the more this becomes an issue. $\endgroup$ – John Sep 4 '17 at 14:50
  • $\begingroup$ Cesium is a solid at RTP(25°C), melts at 28°C, and boils at over 600°C. Not sure where did you get the part about Cs-137 being a gas. $\endgroup$ – March Ho Sep 4 '17 at 15:47
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    $\begingroup$ Where can I read more about these black-start fission reactors and their problems? From your answer it seems like it’s supposed to be obvious why this is a bad idea, but I’m not seeing any inherent problem in the concept... $\endgroup$ – KRyan Sep 5 '17 at 0:34
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    $\begingroup$ @KRyan, ah, that is because I'm not allowed to disclose things that I have read from 'classified' documents, so I shy away from it. In short, if a reactor has control rods that can be withdrawn by hand, then that hand is in control of something called reactivity addition rate, which can cause a problematic power excursion a la launching control rods. There's probably an unclassified report on the prototype reactor that actually did this somewhere? Normally motors withdraw the rods at a known safe rate. $\endgroup$ – Sean Boddy Sep 5 '17 at 0:41
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    $\begingroup$ @KRyan, also, black-start is a feature of a larger generating station. For instance, normally a large coal station with this ability would have a multi megawatt diesel or gas turbine on site to start the water pumps and boiler equipment to get the main plant to a self sustaining condition before shutting down the auxiliary plant. It's like a car battery for a gigawatt scale heat engine. $\endgroup$ – Sean Boddy Sep 5 '17 at 0:48
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Destroy nuclear waste and weapons material

The biggest present real life reason to keep fission reactors around — after fusion has solved the problem of energy availability — is to transmute nuclear waste such as Plutonium and Americium to elements with shorter half lives. This makes the issue of nuclear waste a much briefer affair — 500 to 1,000 years — compared to present suggestions.

This also also lets us take nuclear weapons material and turn that into something useful, which is something we have already done.

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    $\begingroup$ When you think about it, a paper shredder is essentially the same function. Normally, you wouldn't necessarily want to turn perfectly good paper into tiny strips. But when that paper has sensitive information - Yes, please turn it into tiny strips. $\endgroup$ – Andon Sep 5 '17 at 3:11
  • $\begingroup$ OTOH, one could argue that in certain circumstances, having nuclear weapons material might be desirable. Not just in case of alien attack, either. Say you need to redirect that large asteroid you've just discovered on a collision course with your planet. Assuming you can't make your fusion reactors actually explode, what other alternatives are there? $\endgroup$ – jamesqf Sep 5 '17 at 4:53
  • $\begingroup$ @jamesqf I will let you figure out the huge fallacy in that statement on your own. Hint: at most the world had 65,000+ nuclear weapons. $\endgroup$ – MichaelK Sep 5 '17 at 7:10
  • $\begingroup$ @jamesqf if fusion power is this widely available, why would you need fission bombs, when you can have fusion bombs? $\endgroup$ – Doomed Mind Sep 5 '17 at 11:52
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    $\begingroup$ @Doomed Mind: You say in a comment above "...why would you need fission bombs, when you can have fusion bombs?" Barring some undisclosed handwavium-based future tech, the only ways to get a fusion explosion would be to make your fusion plant explode, or ignite the fusion reaction using a fission explosion, as is done with existing H-bombs. $\endgroup$ – jamesqf Sep 7 '17 at 16:32
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With aneurtonic fusion commonplace, fission doesn't make much sense

Fission is great if you need stupid poisonous materials (plutonium) or want to make fission weapons. The current preference for uranium in nuclear power plants over thorium is because uranium turns into nice bomb fuel. Thorium does not.

The fuels for fission are very heavy. The fuels for fusion are very light. To get very high fuel burn rates in fission materials requires reprocessing to remove the nuclear poisons that prevent more of the fuel from burning. Since fusion fuels are gases or liquids to begin with, reprocessing is often completely unnecessary. Thus, fusion doesn't require the exceptionally heavy reprocessing equipment that fission does.

While we don't know the composition of heavy fission fuels around the solar system, we do know that hydrogen and oxygen are all over the place.

Additional Reading

Nuclear Power

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One very likely reason that I haven't seen mentioned in other responses is the issues surrounding fuseable fuel. Deuterium, Tritium, and Helium 3 are not common in the solar system. On Earth, Deuterium is typically only found in water, and Tritium is usually made from heavy-water moderated fission reactors. (There's a good reason for fission: use it to make the fuel for your fusion reactors!) Helium 3 exists in trace amounts in Earth's natural helium wells, a slightly higher concentration in the lunar surface, and still higher concentration in the atmospheres of the gas giants. Unless you can extract it from one of these sources at a fairly high production rate, He3 really won't be very suitable for use as a general fusion fuel and would likely be reserved for applications that require its exceptional energy output.

Boron 11 is going to be the most commonly available fuel source of those you listed, but it may not be present or readily extracted on some small planetary bodies like the Moon. And honestly, if your civilization is capable of aneutronic fusion, Lithium 7 would be a more productive and abundant fuel.

Another thing to consider is longevity. Although fusion reactors can produce enormous amounts of energy, their fuel is short-lived and they must be fed regularly. Compare that to a fission reactor, which may be capable of running on the same chunk of material for 20-40 years without refueling. If a given application requires long voyages or you have to rely on outside sources for fuel and supplies, a fission reactor would reduce your logistics footprint.

One final point: engineers try to find the simplest, easiest solution to a given problem (and I can say that because I am one). If your goal is to power something small, like an outpost or space probe, the power requirements might not be high enough to justify using a complex, heavy, and hungry fusion reactor. You would really only need fusion for large-scale applications that consume huge amounts of power, where safety is critical (assuming fusion powerplants are actually safer than fission at this point), or where fusion fuel is abundant and easily harvested. Fission or solar power would probably suffice in almost all other applications, and your engineers will favor the simplest option practical.

Some links going into more detail about fusion fuels and processes: https://en.wikipedia.org/wiki/Aneutronic_fusion https://en.wikipedia.org/wiki/Tritium#Deuterium https://en.wikipedia.org/wiki/Deuterium https://en.wikipedia.org/wiki/Helium-3

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If we do a parallel with our present days, though we have plenty of optimized electric motors and tools, there are still places where humans are using coal or even cow dung as energy sources.

It can very well be that in some remote places the technology to use fusion is not available due to economic/logistic reasons (lack or abundance of resources, lack of skilled personnel, politic will to not depend on an external supplier), and therefore the much simpler fission is the only way to produce energy.

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While the fusion reactors may be cheap, it can very well be that hydrogen is rather expensive. Consider living on a planet without atmosphere or oceans. Most of the hydrogen on this planet would be located as frozen water, and since everybody needs water, any non-regenerative consume (i.e. nuclear fusion) might be shunned upon (or simply to expensive).

Of cause, shipping water by ships is a possibility in this case, but it might be that it's too expensive, and it creates a dangerous dependency.

For planets close to the sun, solar energy still might be preferred (no refueling, and very save while low cost, can easily be placed out of sight (in space)).

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  • $\begingroup$ +1 for suggesting that the most common element in the universe, by far, could be "expensive". $\endgroup$ – Odalrick Sep 4 '17 at 13:08
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    $\begingroup$ Except that non-solar fusion runs on Deuterium. You need to refine ordinary water to get it (Deuterium occurs at a 1/6000 ratio), so clean water is actually a byproduct of your Deuterium refinery. And given that shipping humans is easy, shipping water is also easy - humans are 2/3 water ! $\endgroup$ – MSalters Sep 4 '17 at 13:33
  • $\begingroup$ Hydrogen is the most abundant element in the solar system, and high up the list for all planets. Uranium is really rare. Unless you are planning for millions of years, having enough water to drink means plenty of fusion fuel. With fusion you can get more energy out of water than enriched uranium. $\endgroup$ – Donald Hobson Sep 4 '17 at 13:54
  • $\begingroup$ @MSalters, mobile bags of mostly water.... $\endgroup$ – ShadoCat Sep 5 '17 at 21:59
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On Earth, it's clear not worth having fission: too much waste to look after. In space, however, fission can be but unavoidable. Fusion reactors, either tokamaks or stellarators are going to be huge, way larger than fission reactors. Pushing all that mass up of a gravity well is a pain in the ass.

Sure, it can be avoided if you just buy the reactor in orbit, or in a low gravity body such as the Moon, but it's highly probable they don't have the heavy industry with the capabilities to build such specialized parts. In any case, a fusion reactor has a very costly start-up: until the fusion starts to be self-sustainable you have to depend on a very powerful external source of energy to turn on the reactor. On Earth that's not a problem, as the electrical grid can surely provide this energy from plenty of different sources and locations, but in a spaceship, or even a space colony, you just can't find the energy needed if the fusion reactor fails and has to be restarted. Restarting a fission reactor is just pulling the fuel bars back in its place and waiting for the water to heat enough.

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    $\begingroup$ "fusion power being mastered to the point where a fusion reactor can be stored in a truck" - doesn't sound that huge $\endgroup$ – Bergi Sep 4 '17 at 9:59
  • $\begingroup$ @Bergi True. I have glossed over that point. $\endgroup$ – Rekesoft Sep 4 '17 at 10:01
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There will always be a reason to use a cheaper alternative

Science may provide opportunities, but the universe is driven by economics. Consider our ploddingly slow adoption of solar power. In my area, the local power utility has been increasing rates for solar users almost exponentially because they're losing money to solar and the cost of the infrastructure (which solar users still depend on) remains the same. Consequence: lots of people not using solar.

The simple reality is that fission (lower tech) will always be cheaper than fusion (higher tech) and there will always be people who will, for whatever reasons, want to save that proverbial buck. Show me an advanced, fusion-driven glowing cityscape that people the universe-over drool over and I'll show you the very same city eight blocks away where everything from drugs to people are sold — and it'll all be run using cheap fission "they-can't-find-me-through-my-power-bill" reactors.

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  • $\begingroup$ The simple reality is that Vacuum tubes (lower tech) will always be cheaper than silicon chips (higher tech). In reality fusion uses abundant deuterium and fission relies on uranium. Fission is expensive and dangerous, fusion might not be, but if it is the world will run on solar. $\endgroup$ – Donald Hobson Sep 6 '17 at 12:41
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Its also a case of, cool we generated so much energy, what are we going to do with it. Secondly its also a sustainability issue, with Uranium reserves being more than 10 times that of Fusion reactants and hence Fission is more feasible going forward...at least for now.

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One reason to use fission would be not as a fuel, but as a mining technique. Since fission can atomize rock and stone, it could be useful for mining or terraforming purposes on very large planetary scale where the radioactive fallout is unimportant, or treatable. Since they would be using fission for this purpose, it would follow they would also use it to power their equipment to keep the systems simple and efficient.

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