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My augmented society of humans have managed to engineer their power-hungry cyborg components to run on a hydrogen fusion reactor. Whilst they have so far managed to fuel it by drinking lots of water, they would rather a solution which would make their lives more convienient. Would they be able to add either hydrogen (at appropriate pressure) replacing the nitrogen, or even just a couple percent of hydrogen that would stay at ground level for them to breathe?

Other criteria:

They don't care about the hydrogen staying over geological time and can replenish it, but would like the planet to be at least inhabitable if their technology stopped maintaining it. (It has an ecosystem like Earth)

They have handwavium nanotech clouds that would help with fire supression, but would like to avoid any situations where a spark would blow up the planet anyway

So, is this possible?

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    $\begingroup$ You may want to compute how much water is "lots" of water if the water is used as a source of hydrogen for fusion reactors... I for one would not think that a few grams of water per year is "lots". $\endgroup$ – AlexP Mar 20 at 10:45
  • $\begingroup$ How small would a reactor using a few grams a year be, and would its power output be already quite high? Some of the power needs can be quite large. $\endgroup$ – Starsong67 Mar 20 at 10:48
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    $\begingroup$ One gram of hydrogen should be enough for about 200 MWh (thermal power). (For example, ITER plans to fuse about half a gram of deuterium for some 140 MWh thermal output.) How small the reactor would be I have no idea; at present nobody has any idea how make small nuclear fusion reactors. $\endgroup$ – AlexP Mar 20 at 10:56
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    $\begingroup$ Just as a heads-up, there's an "edit summary" field beneath the editor. Your "EDIT:" messages should go there, instead of at the bottom of your question. Good first question, though! Welcome to the site! $\endgroup$ – F1Krazy Mar 20 at 11:03
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    $\begingroup$ Using water is almost certainly far more convenient than trying to make an oxygen/hydrogem atmosphere remotely stable. $\endgroup$ – Mike Scott Mar 20 at 11:41
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They have handwavium nanotech clouds that would help with fire supression

You have essentially handwaved away the main issue with mixing oxygen and hydrogen, which has an obvious outcome otherwise.

What remains then are secondary concerns:

Will there be enough?

The oceans weight around 200 times more than the atmosphere on Earth, so only a small portions of them would have to be converted in order to supply some percentage of hydrogen in the air.

Will the hydrogen go away in other ways than burning?

Over geological timescales, Earth looses hydrogen and even some heavier gasses to space. But this is not a large drain, and over a much longer timescale than the one you care about.

The planet to be at least inhabitable if their technology stopped maintaining it?

When you fire suppression systems stop working, things will essentially go back to status quo, with the hydrogen going back into water. There are two things to consider for the cyborg here:

  • The hydrogen percentage can't be so high that it swipes away all the oxygen in case of a collapse, ideally 90% or more of the oxygen should remain in the air.
  • The nanobots should fail slowly, letting the hydrogen leak as water form the atmosphere over an extended period of time. A single flash would cause significant damage to all life that's not located underground.

One has to wonder how these miraculous nanobots are easier to construct than conventional pipelines, supply stations and containers...

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  • $\begingroup$ Out of curiosity - would the couple-percent hydrogen scenario immediately react without a fire-suppression system? $\endgroup$ – Starsong67 Mar 20 at 11:03
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    $\begingroup$ @Starsong67 you have 500 million square kilometres of exposed reaction area. Pray there isn't a spark anywhere. $\endgroup$ – SE - stop firing the good guys Mar 20 at 11:06
  • $\begingroup$ @Starsong67 hydrogen/oxygen explosions in non-stoichiometric ratio are going on the deflagration path - so yes, you won't be able to stop them reacting given any opportunity. The stoichiometric ratio however detonate at about 4km/s $\endgroup$ – Adrian Colomitchi Mar 20 at 11:55
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I am neither a chemist nor physicist, I cannot calculate the speed of the reaction, but what I can tell is that mixing gas oxygen and gas hydrogen isn't the best idea.

If there is any source of ignition, the reaction will be locally instant and on a planetary scale fast. I doubt any anti-fire system will be fast enough to handle this before it gets out of any control. Actually, it will probably get out of control within a fraction of a second. Once started, the reaction will continue as you have all you need for it (fuel, oxygen and ignition).

The initial ignition can be anything highly energetic. Since you want this to be part of your planet atmosphere you will actually have to keep more or less similar atmosphere everywhere, meaning for instance that if you have ignition anywhere in the entire planet. Now as long as you have any clouds, you will have storms and lightnings. It means a very powerful source of ignition, occurring on a very regular basis. On Earth, there is a number of storms running in parallel all the time with lots of lightning hitting every second round the clock. Shortly speaking, as soon as you manage to create the conditions you describe (which in itself is unplausible), one spark (ekhem, lightning) is going to blow everything up, should you have the nanoclouds ready or not.

To give you the idea, have a look at this video

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