Difference between revisions of "Helium"

Revision as of 15:11, 4 April 2007

Helium 3 is a rare isotope of the element Helium, consisting of a nucleus with two protons and one neutron. The approved abbreviation (for physics use) for Helium-3 is ³He, however, the abbreviation He3 is also seen.

Since most of the Earth's helium is produced by alpha-decay of Uranium isotopes, resulting in ⁴He (the most common isotope of Helium), ³He is rare on Earth. It is comparatively more abundant in non-terrestrial sources, although even in non-terrestrial sources, only a small fraction of helium atoms are Helium 3.

The Moon is a source of ³He, which is implanted into the lunar regolith by the solar wind. Helium is present in the soil in quantities of ten to a hundred (weight) parts per million, and 0.003 to 1 percent of this amount (depending on soil) is ³He.

Helium 3 as a Fusion Reaction Fuel

It has been proposed that ³He might be a possible fuel for a Nuclear Fusion reactor to produce energy using the nuclear reaction:

²D + ³He --> ⁴He + ¹H

This reaction has the advantage over the more-commonly proposed D-T fusion reaction that the reaction produces only charged particles (an alpha particle and a proton), with no production of neutrons. However, the corresponding difficulty is that the D-³He reaction has an ignition barrier that is twice as high as the barrier to igniting D-T fusion, because of the fact that the Helium nucleus has twice the charge of a Tritium nucleus.

Gerald Kulcinski's group at the Fusion Technology Institute of the University of Wisconsin-Madison has operated an experimental He3 fusion reactor for an extended period, on a non-governmental research budget (ref: Hedman, Eric; (Monday, January 16, 2006). "A fascinating hour with Gerald Kulcinski" (HTML). The Space Review. Jeff Foust, Ed. Retrieved on 2007-03-04), however the reactor has not achieved energy balance or breakeven.

So far, D-³He fusion has not yet demonstrated net energy production ("break even"). Commercial He3 reactors are long way in the future.

Value of Lunar Helium 3 in Today's Market

Since He3 has a high market value today, it might be worth collecting He3 from the Moon today simply to sell into the existing terrestrial market.

Current market price for He3 is about $46,500 per troy ounce ($1500/gram, $1.5M/kg), more than 120 times the value per unit weight of Gold and over eight times the value of Rhodium.

Question: can we reduce the cost of recovering He3 from the lunar surface to that level, e.g. $1500 per gram? What would be the capital cost of setting up a small He3 production facility on Luna?

Would it depress the market price today? That depends on the size of the market, and there is not much data on that.

The US Tritium and helium-3 stockpile sizes are classified, because they give a hint as to how many US nuclear weapons are still functional. According to Wikipedia “approximately 150 kilograms of it (He3) have resulted from decay of US Tritium production since 1955.” We could assume a similar quantity has been accumulated in the ex-USSR, and perhaps additionally from other thermonuclear powers (UK, France, China).

Today, the world's supply of Helium-3 can probably be counted in hundreds of kilograms, value of 100 kg would be $150M. So the total stockpile value today is probably about half a billion USD.

The US DOE does sell He3 commercially, but how much of the present stockpile has actually been sold on the open market? Not sure if that number is publicly available.

But for arguments sake let us start at the level of collecting 100kg of He3 from the Moon and assume its value would be $150M.

Well alas even those number do not look good.

The cost of soft landing even a small probe on to the lunar surface would easily cost that much or more. How much He3 could a small lander manufacture? How many grams per day?

Well that of course depends on the production method.

A commonly discussed method is cooking the regolith to about 1400 degF or 760 deg C. This would require a lot of energy, requiring the lander to have either a nuclear source (either Nuclear Fission or RTG, or large solar panels (see Solar Power).

Basalt has specific heat capacity of 0.24 cal/g/degC or 0.84 KJ/kg degK.

To heat 1kg of basalt by 700degC requires about 600 KJ

Best lunar regolith (in the Maria regions) is 0.01 ppm of He3

So the 600 KJ will yield 0.01 milligrams of He3

So 600 Watts power source could produce 0.01 mg He3 per second = 0.6 mg/minute = 36mg/hour = 864mg/day = 315 grams per year

Whether this business concept is viable depends on how quickly we want to amortize our investment.

Let us say our target is to produce 100 kg He3 in one year, then we need a power source of about 200 KW. That would give us a revenue stream of $150M per year assuming the He3 market does not become flooded and the price drops.

How much would it cost to set up a 200KW power source on the Moon?

A Solar Power based system would be in darkness 50% of the time, so would need to operate at 400 KW. If it were on a lunar polar mountain top it might be in near continuous illumination. Let us assume that best case of 100% lighting.

Assuming PV 10% efficiency and a fully steerable array, this would need an area of about 2,000 square meters, or about 45 metres square.

A simple non-PV solar reflector could be near 100% efficient, needing only 200 sq-m or 14 metres square, or aperture.

Setting up a 14 m aperture mirror on the Moon would be a major engineering challenge, although fortunately would not need to be particularly accurate, certainly nothing like as difficult as an astronomical telescope mirror.

How much would it weigh?

Would a Nuclear Fission power plant have better performance per kilogram of lander payload?

Maybe other contributors are interested to develop these lines of thinking.

More thermal analysis needs to be done. For example, might it be possible to recycle the heat using some form of cogeneration. Such as use the hot waste regolith, after it has been processed, to pre-heat the next incoming batch of raw dust, and thus reduce the number of solar joules needed?

That could greatly reduce the size of solar array needed and/or significantly increase the system mass throughput.

Extraction

Baked Regolith

A commonly discussed method is cooking the Regolith to about 1400 degF or 760 deg C.

Here is a reference with some details (1989, H. H. Schmitt et al):

http://fti.neep.wisc.edu/pdf/fdm817.pdf

They describe three steps: 1) heat to a few hundred deg C to drive off the volatiles 2) fractional distillation to decant off the heavy volatiles 3) separate He3 from the He4 using standard superleak process

Two challenges are devising a method to process large quantities of regolith as the He3 is at a low concentration, and providing a high power thermally efficient heat source on the Moon.

This would require a lot of energy, requiring the lander to have either a nuclear source, or large solar panels.

Basalt has specific heat capacity of 0.24 cal/g/degC or 0.84 KJ/kg degK.

To heat 1kg of basalt by 700degC requires about 600 KJ

Best lunar regolith (Maria) might be 0.01 ppm of He3

So the 600 KJ will yield 0.01 milligrams of He3

So 600 Watts power source could produce 0.01 mg He3 per second = 0.6 mg/minute = 36mg/hour = 864mg/day = 315 grams per year