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Mining the Moon to fuel fusion reactors with helium 3 is a cockamamie scheme for the foreseeable future. First, nobody has made a fusion reactor that produces n

Mining the Moon to fuel fusion reactors with helium 3 is a cockamamie scheme for the foreseeable future. First, nobody has made a fusion reactor that produces net electricity even with the least demanding fuel (deuterium-tritium). He-3-deuterium fusion requires significantly higher temperatures. Second, if He-3 did become a valuable fuel, it can be manufactured here on Earth by irradiating lithium with neutrons, just like tritium; it's what tritium decays to. Finally, the lunar regolith concentrations of He-3 are at the parts per billion level. The latest He-3 prices I can find are around $2000/liter (~$15,000 per gram). At 18.354 MeV per D-He3 fusion and 40% thermal-to-electrical efficiency (generously), one gram of He-3 yields 1/3 * 6.02 * 10^23 * 18.354 * 0.4 * 4.4505 * 10^-20 = 65565 kilowatt hours of electricity. To produce 1 kilowatt hour of electricity, the helium 3 cost alone would be 23 cents. For comparison, the average retail price, delivered for electricity in the US is below 11 cents per kWh. Even if you had a working fusion reactor all ready to turn He-3 to electricity, He-3 would need to be an order of magnitude cheaper to have a prayer of He-3 fusion competing economically against other electricity sources. And if He-3 is an order of magnitude cheaper that makes it correspondingly harder to turn a profit mining the Moon for it. Helium 3 lunar mining is not an investment opportunity. It's the end product of motivated reasoning, where the motivation is "I want humans to be doing more things in space."

DennisP 9y ago
There's a fundamental reason why lunar He3 isn't worth mining: if you can get net power from D-He3, you can also get net power from the easier D-D reaction, and the output of that reaction is half He3 and half tritium. If your D-D reactor is profitable then the He3 is effectively free. Ycombinator-funded fusion startup Helion is working on a hybrid D-D/D-He3 reactor, and says the combined reaction would release only 6% of its energy as neutron radiation (compared to 80% with D-T).

DennisP 9y ago

There's a fundamental reason why lunar He3 isn't worth mining: if you can get net power from D-He3, you can also get net power from the easier D-D reaction, and the output of that reaction is half He3 and half tritium. If your D-D reactor is profitable then the He3 is effectively free. Ycombinator-funded fusion startup Helion is working on a hybrid D-D/D-He3 reactor, and says the combined reaction would release only 6% of its energy as neutron radiation (compared to 80% with D-T).