The Speculist

it seems that certain nations recognize the need for helium-3 harvesting on the moon ...

http://en.rian.ru/russia/20060125/43181357.html

Maybe the Energy Race will pave the way for the Helium Rush. I'd like to believe that the rush would lead to lunar settlement, and that the first cities on the moon will be San Francisco- or Seattle- style boom towns that emerge from the economic activity around prospecting and mining. But this gold rush will more likely be a race to get the best roboticized Helium 3 operation up and running, requiring little or no permanent settlement.

I wonder if anyone is working on that yet?

"The University of Wisconsin is working on a design of an automated lunar miner to rove across the surface of the Moon to extract helium-3 and life-support volatiles."

Tokamak reactors are powered by deuterium harvested from seawater.

Actually, they're fueled by deuterium and lithium. The lithium is essential, since the lithiun blanket is where most of the tritium that will be 'burned' is produced. The net reaction is D + 6Li --> 2 4He (ignoring various less important reactions).

I have considerable misgivings about tokamaks. They're big, complicated, and expensive, and they don't scale down terribly well. These are not features that lead to rapid technological advance. Unless their economics can be dramatically improved I expect them to be uncompetitive with contemporary n-th generation fission reactors, particularly when potential customers throw in a risk premium for an immature technology. The engineering problems of keeping a tokamak running at high power for 30 years will be immense, in part because the reactor structure will quickly become far too radioactive for hands-on maintenance, and in part because the entire 'first wall' will have to be replaced every few years due to neutron damage (D-3He could ameliorate that last problem, but increases the reactor cost greatly due to the much lower reactivity of this fuel combination and much higher plasma pressure required.)

The lunar Helium 3 idea is a totally crackpot idea.

Let's first not not look at the technological difficulty, just look at the availability issue. According to this source:

1.There are one million tons of He3 on the top lunar soil, no deeper than a few feet.

2.These one million tons of He3 can provide the U.S. for a one thousand years of electricity.

The number sounds big but they actually are NOT quite that big. The U.S. only consumes 1/4 of the world's total electricity, or maybe less. So to provide the whole world with electricity, the one million ton lunar He3 is only good for 250 years, not 1000 years.

Now, the world's electricity consumption is only 1/6 of total energy consumption. So to provide the world with all energy supply, not just electricity, you need to further divide the number by 6. Which results in 43 years of supply.

Only 43 years of energy supply, even if you mine every little bit of He3 on the moon 100%. That doesn't look like an encouraging solution.

Now, how do you mine the He3 from the moon? Just cook the lunar soil to 700C and it will come out. Sounds easy? But's it's easier said than done. The He3 is uniformly distributed on the whole surface of the moon, to a depth of one meter. The moon is a pretty big place. Let's calculate how much lunar soil you need to dig up and cook:

The moon's radius is 1738,000 meters. So its surface area is 4*PI*1738000^2 = 3.8x10^13 M^2. Multiply by one meter depth, that's a volume of 3.8x10^13 M^2, at about 5 kilogram mass per cubic meter, that's a total mass of 1.9x10^17 kilogram.

You are talking about cooking 190 trillion tons of lunar soil to 700C, in order to extract just one million ton of He3. The concentration of He3 is only 5 parts per billion!!! Each ton of lunar soil cooked will yield just 5 miligram of He3.

Where do you get all the energy to cook 190 trillian tons of lunar soil to 700C temperature? The extracted He3, even if all their energy is released, is far from being enough to even cook the soil from which they were extracted.

The whole history of humanity has never cooked anything remotely approaching 170 trillion tons, to any temperature remotely close to 700C. All the hot water for bath/shower humen ever cooked (which is to no more than 50C), from the Roman era to today, is about one trillion ton of hot water.

It's a complete ridiculous idea to believe that extracting He3 from lunar surface could be of any usage in terms of energy.

Quantoken

While I agree that the difficulty of extracting lunar 3He is usually glossed over by advocates, and I agree its practicality is dubious, the situation is not quite as extremely bad as you depict:

• The density of regolith is more like 2 g/cc rather than 5 as you assume.
• 3He is primarily in the small diameter component, so you can save energy by sieving to remove large chunks.
• 3He is also concentrated in the ilmenite fraction, so regions high in ilmenite are more easily processed.
• 3He concentrations depend on the equilibrium between the rate of implantation and the rate of diffusion out of regolith grains. The latter is slower at low temperature, so 3He concentrations may be higher near the poles.
• Counterflow techniques can be used to recycle the heat, so a given unit of heat can process many masses of material before being radiated away.

The limit on the total amount still applies, and becomes even stronger if one is limiting oneself to only the richer deposits. The advocates would claim that we'd be mining Uranus (or, perhaps, the atmosphere of an even more suitable, but yet undiscovered, large Kuiper belt planet) by the time the lunar 3He had run out.

Yeah, it's a stretch.

Quantoken:

You don't have to mine the whole Moon before the first check comes in.

How to cook the dust? The Sun shines on the Moon. Use a solar cooker.

We could power the entire world for only 43 years? You're right, hardly worth pursuing.

Nevermind.

How many times have we had to revise the estimate of how many years of petroleum we have left?

Quantoken: How do you come up with the figure of 1000 years? Every estimate I have seen has the total He3 potential being on the order of 10,000 years. If we use your reasoning, starting at 10,000 years, instead of 1,000, the Earth could be powered by He3 from the moon for 430 years. That is certainly worth pursuing.