[AR] Re: He3 was Quiet times, so OT question
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sat, 17 Dec 2022 00:58:11 -0500 (EST)
On Fri, 16 Dec 2022, Keith Henson wrote:
Need a reaction that puts more of its energy into charged particles
and less into neutrons; D-D isn't great but it's better, and the
low-neutron reactions are even harder to ignite.
Henry, I have seen estimates on how much harder DD is compared to DT,
but I can't find an estimate for DHe3...
It depends on which yardstick you use. :-) By some measures D-He3 is
actually easier than D-D. The peak power densities, estimated crudely,
are in the same ballpark -- much below D-T! -- but the D-He3 peak occurs
at about 4x the temperature of the D-D peak, and that drives up some types
of losses badly.
It's definitely a big win that most of the D-He3 energy output is charged
particles, but D-D side reactions will still produce enough neutrons to
cause headaches, even with measures to reduce them. And because one
branch of D-D produces tritium, there are still some of those nasty 14MeV
neutrons from D-T. So a powerplant still needs the same sort of shielding
thickness as D-T, but the shielding doesn't need nearly as much cooling,
and you don't have to worry about trying to breed tritium in it.
It starts to be a guessing game of how much extra engineering for higher
temperatures etc. can you buy with the savings from no tritium handling
and far fewer neutrons.
Overall: Probably feasible. Probably not right away. Realistic economic
comparison too difficult now, depends too much on how well various parts
of the engineering work out.
I also kind of suspect that if you need to heat the top couple of meters
of regolith to 600 C to get out a few bbp of He3 it is an energy losing
business.
The "pessimistically, about 25% of the fusion energy" estimate came from
John Lewis (a number of years ago now), so it's fairly reliable. Not a
disaster but not great. Serious cleverness might improve on it.
And bear in mind that, as I mentioned earlier, it turns out that the D-T
reactors have a nasty fuel problem too. There's (essentially) no natural
tritium on Earth either, and breeding more in the D-T reactors is much
harder than people naively thought.
Henry
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