It's my understanding that the sort of fusion that goes on in the sun takes four hydrogen (H-1, one proton and one electron) atoms and crunches them down into a single helium atom. Not knowing a great deal about fusion mechanics, this both confuses and intrigues me, since the process is taking four protons, and turning two of them into neutrons, somehow. Adding up the atomic mass of four hydrogen atoms and comparing the result to the mass of a helium atom tells me that about 0.71% of the mass of the hydrogen is being lost; which, while not particularly efficient for a nuclear reaction, would still be enough to meet Earth's annual energy demands with less than 200 tons of non-isotopic hydrogen.

This, obviously, is too good to be true. Current fusion reactor designs that use rare isotopes of hydrogen or helium already require more energy to use than they produce, and THIS concept isn't even being considered, apparently. So before I go about meeting my sci-fi civilization's insane energy requirements with fusion reactors that consume as fuel the second most abundant substance in the universe (the first is stupidity), just how difficult would it be?

TL;DR, what are the problems with creating a fusion reactor that runs off of non-isotopic hydrogen, and are those problems theoretically solvable for a sufficiently-advanced civilization?

  • $\begingroup$ What do you mean here by non-isotopic? Do you just mean H-1? $\endgroup$
    – Sol
    Commented Aug 29, 2020 at 23:53
  • $\begingroup$ @Sol Yep. One proton, one electron. I'll go ahead and clarify that in the question. $\endgroup$ Commented Aug 30, 2020 at 0:01
  • 1
    $\begingroup$ Proton-proton chain reaction. $\endgroup$
    – AlexP
    Commented Aug 30, 2020 at 1:17
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    $\begingroup$ I resent resembling the most abundant substance in the universe - I was sure I was unique. $\endgroup$
    – JBH
    Commented Aug 30, 2020 at 1:37
  • $\begingroup$ The main reason why people use deuterium/tritium/helium-3 for fusion is that protium fusion requires a metric f**kton of energy for starting the reaction. $\endgroup$
    – Alastor
    Commented May 17, 2023 at 18:14

1 Answer 1


The major issue with using H-1 for fusion is the high activation energy for the fusion reaction. In order for fusion to occur, the nuclei of the H atoms need to be pushed close enough together that the strong nuclear force comes into effect. Said force acts acts at femtometer scales. Above those distances, the electromagnetic force is stronger, and since the nuclei are both positively charged, prevents them from getting close enough for the strong nuclear force to take effect. enter image description here

The reason fusion can occur in stars is because the pressure is so immense that it can overcome that strong repulsive electromagnetic force. Since creating a star on the surface of the Earth is beyond human means, as well as being unwise, current fusion reactor designs use heavier H nuclides, because it lowers the activation energy for the reaction enough that "impossible" becomes "fiendishly difficult".

That being said, those restrictions are based on human capabilities. If your civilization has the technical capability to build a star or a similar environment, then yes, fusion is an obscenely good power source.

Summary: yes those problems are solvable, fusion is an insanely good power source, and you'll need some serious tech to do it with H-1, but it's definitely doable.

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    $\begingroup$ The future folks have a fusion catalyst! $\endgroup$
    – Willk
    Commented Aug 30, 2020 at 0:27
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    $\begingroup$ Notably, for stars of our sun's size, the power density at their core is comparable to that of a compost heap. That's only enough to keep them glowing because of square-cube scaling...they have much, much more internal volume for their surface area than a compost heap does. For a power reactor, you want something a little more compact, which means achieving fusion rates only normally seen in stars that are in the middle of exploding. $\endgroup$ Commented Aug 30, 2020 at 4:28
  • $\begingroup$ @ChristopherJamesHuff good point! $\endgroup$
    – Sol
    Commented Aug 30, 2020 at 4:33
  • $\begingroup$ And the sun only gets output from the proton-proton chain because there are an unimaginably large number of attempts made, so that even if only a few manage it the star still comes out ahead. $\endgroup$
    – Jon Custer
    Commented Jan 31, 2022 at 18:08

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