Currently Tritium is produced by neutron bombardment of Lithium. I was looking into alternatives and learned that Helium3 has a rather large cross section for reacting with thermal neutrons. From what I understand, Lithium is somewhat hard to come by and Earth bound production systems often benefit from pre-concentration from Earth's water cycle. So, especially in the outer solar system, where refining Helium3 from the atmospheres of gas and ice giants is rather trivial and there might be a desire to be independent of inner system economies for the ever vital torch drive fuel, could the He3 + thermal neutron pathway be a credibile alternative? How might such a production facility look like, especially with the thermal neutron source?

For context, in the setting the solar system has been colonised. He3 is used in normal fusion reactors, but especially the pure fusion Orion drive vessels connecting the solar system use tritium. Military vessel, especially those of inner system origin use thermonuclear Orion drives and there are prototypes of amat catalised Orion drives as well. Helium3 is the new oil and and mined in mass on the gas giants.


1 Answer 1


So, especially in the outer solar system, where refining Helium3 from the atmospheres of gas and ice giants is rather trivial

If you first assume that extracting and exporting stuff from the atmospheres of gas giants is trivial, then production of tritium from helium 3 seems pretty credible.

Thing is though, they all have crushingly deep gravity wells. Neptune has an escape velocity from the datum level of 23.5 km/s, and an atmospheric scale height twice that of Earth, meaning that gravity and aerodynamic losses are going to be even higher than on Earth. Even without dealing with atmospheric density or the hazards of operating within a gas-giant atmosphere, the energy cost of raising something from Neptune's datum level to the edge of the gas giant's sphere of influence is 275 MJ/kg. Compare with Earth's 62 MJ/kg. Earth also has abundant solar power available, meaning you don't have to burn up a load of the fuel you're lifting out of the gravity well in order to get any fuel out. There's also presumably a whole load of infrastructure there already.

How might such a production facility look like, especially with the thermal neutron source?

It'll probably look like any other big industrial facility in space, eg. we've no idea having never built one so you're basically welcome to handwave it yourself. Probably boring and boxy, with an outer Whipple shield to protect against micrometeorites. There will be big heat radiator arrays to dump all the heat generated by the neutron source and the decay heat generated by the $\require{mhchem} \ce{n + ^3He -> ^3H + p}$ reaction (which is hot... 24TJ/kg!). They won't be glowing in the visible spectrum... just big slabs of stuff parallel to the Sun's rays so they don't gain additional heat (not that there'd be much of that at Neptune's orbit, but still).

The neutron source will probably be a fusion reactor, given the greater availability of fusion fuels in the outer solar system, with heavy water as one reasonable choice for moderation. D-D fusion seems like the most likely source of neutrons, with the deuterium refined from ice in the outer solar system. That saves burning up yet more of the helium-3.

Now, D+D fusion has two reaction paths:

  • $\ce{^2H + ^2H -> ^3H + p}$
  • $\ce{^2H + ^2H -> ^3He + n}$

You can't recover the tritium it produces, because D+T fusion is just too easy. It'll almost immediately fuse as $\ce{^2H + ^3H -> ^4He + n}$. This means that you're guaranteed one neutron per pair of deuterons, and interestingly you might also be able to recover that helium three nucleus "for free".

Have a look at this chart showing Lawson criterion curves:

Lawson criteria: confinement time and density vs plasma temperature

You'll see that it is possible to run a D-D reactor at a temperature too low for D-3He fusion... this isn't the most efficient way to run the reactor, but you're not interested in efficient power generation so much as efficient neutron generation. This sub-optimal plasma means that most of the generated 3He could be recovered from the core, giving you a steady additional trickle of helium-3. You'll need an efficient way to separate it from the helium-4 generated by the D+T side chain, which means you'll also have a load of big gas centrifuges attached to your facility. It'll probably be cheaper to operate those than to fly up twice as much gas from the planet below.

This means that for every 5 moles of deuterium you burn in your reactor, you can extract 1 mole of helium-3 and one mole of helium-4, and must inject one additional mole of helium-3 from some external source, eg. your gas giant mining operations. Your refinery will necessarily need big gas tanks attached to it... these will probably be on the outside, safely away from the emission pattern of the big heat radiators and they'll probably be spherical.

Weirdly for an industrial process, this will be quite net-power-generating... for every 5 moles of deuterium and one mole of helium-3 you put in, maybe as much as 11 MeV per triton. I can't tell how much energy you'd need to feed in, especially given the sub-optimal conditions in your D+D reaction, but the chances are you'll be able to run all the rest of the facility and have energy to spare to do other things like beamed power, or laser launch. Maybe there are big microwave antennae or laser turrets on the outside of the facility, too.

Whatever the eventual shape of the reactor, and the technologies you use, there'll be a jacket around the core to provide shielding and cooling. This would be where you'd run your fertile feedstocks for breeding tritium from, eg. lithium-6 or helium-3. Helium-3 has a pretty high thermal neutron absorption cross section (significantly higher than lithium-6, though the density is going to be lower) so repurposing existing tritium breeder systems shouldn't be too hard... just change the plumbing a bit.

Lithium is somewhat hard to come by

It'll be available somewhere between 2 and 5 parts per million in the asteroid belt. Practically that means that some asteroids will be much richer in it, and will therefore be sensible things to mine.

Given the likely need for other light elements for your space program (beryllium, and especially boron) you'll be wanting a significant asteroid mining program anyway, and clearly your society has sufficient spacefaring chops to accomplish this.

  • $\begingroup$ Thanks for the answer. I'm aware of the gravity wells, that's why Uranus and Neptune are the production centers while Saturn and Mercury only have few facilities. Nobody is touching Jupiter. My idea for getting the fuel up into orbit are laser-thermal rockets, which use a reusable scramjet stage. The atmospheres should work in favour here. Additionally, the faction owning these refineries wants to use the atmospheric facilities as antimatter refineries with the atmosphere a a great heatsink and frist strike protection. $\endgroup$ Mar 30, 2022 at 9:06
  • $\begingroup$ So, basically it would be a fusion reactor at the center and in the blanket there is a layer of heavy water that gets me the thermal neutrons and behind that there would be the He3? $\endgroup$ Mar 30, 2022 at 9:09
  • $\begingroup$ @TheDyingOfLight it'd be better to run antimatter facilities close to the sun, given the necessary power requirements and inefficiencies. You could run 'em in a gas giant atmosphere, but it'll be a lot slower and lower volume. You'll be outcompeted, I'll bet. $\endgroup$ Mar 30, 2022 at 9:09
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    $\begingroup$ @TheDyingOfLight but your goal is to generate tritium! What's the point of burning up as much tritium as you're creating by processing your helium-3? $\endgroup$ Mar 30, 2022 at 9:21
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    $\begingroup$ @TheDyingOfLight its not about efficiency. D+T gives you one neutron. n+3He gives you one T. Your neutron economy fails you. Unless you have a ready supply of some other non-lithium, non-tritium neutron source, D+D is the only game in town. $\endgroup$ Mar 30, 2022 at 9:25

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