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Comment by sebzim4500 | Hacker News Reader

Can someone familiar with Helion's plans explain how they are planning to make the deuterium fuse with the Helium-3 without making the deuterium fuse with itself to make tritium (and then D-T fusion etc.)?

Also, where are they getting the helium-3 from in the first place?

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They will have two types of reactors. First, large centralized reactors for DD fusion to make He3. These will be more expensive and challenging because of the neutron bombardment. The other for D-He3 fusion which will be for smaller sites and produces less neutron damage.

They will make the He-3 using D-D fusion reactors (which is not aneutronic) and waiting for collected tritium to decay into He-3 (12 years). In each shot, they have to remove the he3 and T to prevent them from reacting.

In the D-he3 reactors, they cannot fully prevent the side reactions of DD and DT. But they can minimize them by controlling the mixture of he3 and D in each shot and constantly extracting the T byproduct of D-he3. Basically, they will have high ratio of he3 to D ions so that all the D ions are likely to be used in D-he3 reactions. Removing and collecting the T in each shot removes the opportunity for D-T. It will probably work to an extent, but there will still be side reactions. The overall neutronicity will likely be in the 2-5 range in the D-He3 reactors.

By making deuterium fuse with itself.

Isn't the whole point of their design that they aren't producing neutrons? What's the point of picking an aneutronic reaction if they are going to be having D-D and D-T fusion going on anyway?

There will be barely any D-T fusion. Neutrons from D-D is a problem they will have to manage, but they have lower energy than D-T would have.

Really? Based on this[1] I was assuming that there would be more D-D fusion than D-He fusion and any resulting tritium nucleus would react almost immediately.

Unless they are planning to use an insanely high temperature (like 1B K or something)

[1] https://en.wikipedia.org/wiki/Nuclear_fusion#/media/File:Fus...

No the resulting T would not react immediately. This doesn't happen in existing D-T reactors, why would it happen here where the T/D ratio orders of magnitude lower? Most atoms in a cycle will never fuse.

It feels like even if they only give an opportunity for 0.1% of the He3 to fuse, then we would still expect to see significant amounts of D-T reactions.

My reasoning is that, at plausible temperatures, D-D fusion is say 5x more frequent than D-He3 reactions, and then D-T fusion is ~1000x more reactive than D-He3.

So if 0.1% of the He3 fuses, then ~0.5% of the sample will become tritium, most of which will fuse.

Either I'm significantly overestimating the amount of reactants that are expected to fuse in each cycle, underestimating their target temperature, or they plan on have a very helium-rich mix of fuel.

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