droppin' H-bombs.

September 23, 2005 11:32 PM

Why are people so hung up on Lunar Helium-3? What do we need it for?

In this thread people talk about Helium-3 mining as a reason to go to the moon, but I don't get it. How can it be cheaper to extract Helium-3 from the moon then it would be to extract Heavy Water from the oceans? I mean, isn't about 0.1% of the hydrogen on earth fusible?

Why go to the moon when we have a practically limitless supply of fusible fuel here on earth?

posted by delmoi to Science & Nature (3 answers total)

The reaction that is currently favoured for fusion research and possible future power plants is the Deuterium(H-2) - Tritium(H-3) reaction. It has the advantage of taking place at low temperatures but unfortunately much of the energy output comes in the form of neutrons.
This is bad. The levels of neutron flux that would be incident upon the reactor walls of a H3-H2 fusion vessel is almost 100 times what is seen in a modern fission reactor. That many neutrons creates all kinds of difficult materials engineering challenges, most materials will quickly become brittle under those conditions. A high neutron flux also results in a lot of 'secondary' radioactivity, when the reactor is decommissioned you'll have a lot of mildly radioactive material to dispose of.

Deuterium is - as you mentioned - plentiful on earth, if my memory doesn't fail me then 1 in a 1000 hydrogen molecules in water is deuterium. 0.1% as noted.
Tritium is another matter, it has a short half-life (12 years) and there's virtually none available at sea level, it only occurs in our upper atmosphere, deposited there by the solar wind. The tritium we have now is produced in fission reactors, hardly an acceptable source of fuel for a 'clean' power plant. In planned fusion power plants, the reactor vessel would be surrounded by a lithium 'blanket', lithium becomes tritium under neutron bombardment - woohoo, sustainable power.
This still doesn't solve the issues associated with the high neutron flux.

There are, however, other feasible reactions for use in fusion power plants. One of these is deuterium-deuterium, which is even more difficult than tritium-deuterium, and actually produces *more* neutrons. In that sense what you said about 0.1% of our hydrogen being fusible is true (to be pedantic, it all is - under the right conditions).

The holy grail of fusion research (beyond, I suppose, getting it to actually produce power at all) is aneutronic fusion. The reactions proposed for this are outlandish things like lead-boron reactions, not going to happen any time soon.

But, in the meantime it makes sense to look for reactions that produce fewer neutrons than D-T fusion. One of these is deuterium - helium3. As noted above, we've got plenty of deuterium, but the only helium3 we have is a byproduct of nuclear weapons research. If we were using He3-D fusion for power production worldwide, then we'd need another source of He3. The solar wind deposits easily mined He3 in the lunar regolith, and there - after my multiple digressions and background paragraphs, is the answer you were looking for.
posted by atrazine at 12:41 AM on September 24, 2005

"Scientists estimate there are about 1 million tons of helium 3 on the moon, enough to power the world for thousands of years. The equivalent of a single space shuttle load or roughly 25 tons could supply the entire United States' energy needs for a year, according to Apollo 17 astronaut and FTI researcher Harrison Schmitt." (Space.com, 30 June 2000)
posted by NotMyselfRightNow at 8:54 AM on September 24, 2005

And there are 70 million tons of gold dissolved in the Earth's oceans and just about as useful as Helium-3 on the moon.
posted by JackFlash at 10:07 AM on September 24, 2005