Helium

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Revision as of 09:49, 31 May 2009 by Jriley (talk | contribs) (Helium 3 Fusion and a Lunar Settlement Window)
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Helium
He
In situ availability: trace
Necessity:
Atomic number: 2
Atomic mass: 4.002602
group: 18
period: 1
normal phase: Gas
series: Noble gases
density: 0.1786 g/L
melting point: 0.95K,
-272.2°C,
-458.0°F
boiling point: 4.22K,
-268.93°C,
-452.07°F
N/AN/AN/A
H ← He → N/A
FNeN/A
Atomic radius (pm): 31 pm
Bohr radius (pm):
Covalent radius (pm): 32
Van der Waals radius (pm): 140
ionic radius (pm): -
1st ion potential (eV): 24.59
Electron Configuration
1s2
Electrons Per Shell
2
Electronegativity:
Electron Affinity: Unstable anion
Oxidation states: -
Magnetism:
Crystal structure: Hexagonal or body centered cubic

Helium is a component of the solar wind, and hence is one of the volatiles found (in parts per million level) in Lunar regolith. It is a Noble gas in group 18 and is the second element in the Periodic Table of the Elements. This element has two stable isotopes: 3 and 4.

The most common isotope, Helium-4, has a nucleus of two protons and two neutrons, and two electrons. The less common isotope Helium-3 has two protons and one neutron.

3He

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 thermo-nuclear reaction (Deuterium-Helium-3):

2H + 3He --> 4He + 1H+

This reaction has the advantage over the more-commonly proposed Deuterium-Tritium fusion reaction

(2H + 3H) --> 4He + Neutron

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 2H -3He reaction has an ignition barrier that is twice as high as the barrier to igniting 2H-3H 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 2H-3He fusion reactor for an extended period, on a non-governmental research budget [1], however the reactor has not achieved energy balance or "break even". So far, 2H-3He fusion has not yet demonstrated net energy production ("break even"). The development of commercial 2H-3He reactors is dependent upon demonstrating "break even."

Helium 3 Fusion and a Lunar Settlement Window

Mining Helium 3 from the lunar regolith for generation of power on Earth is a very attractive economic foundation for a lunar settlement economy. A number of powerful historic forces are pushing the human race in this direction, but the hurdles that must be over come are daunting.

Human civilization needs a source of electrical power to maintain itself. Currently we are running on fossil fuels that are a limited resource and dump of huge amounts of greenhouse gases into Earth's atmosphere. Even given the immense effort that it will take to develop fusion as a power source, fusion is currently one of our best possibilities for addressing the global warming problem.

Current fission reactors will not meet this 21st century needs. They are limited by the possibly of nuclear proliferation, safe handling of the radioactive wastes, the amount of high grade ore available, and problems with decommission of radioactive power plants at end-of-life.

There are several possible fusion fuels (Deuterium, Tritium, Helium 3, and Boron 11) that could be used. Only one, Helium 3, comes from the Moon.

Each fuel has different prospect for use. The relative economic values can be judged by: (1) ease of ignition, (2) possibility of power generation, and (3) safety of wastes produced. Three of the top five possibles are rated below:

Fuel Lawson Criterion Relative Power Density Neutronicity
Deuterium-Tritium 1 1 0.80
Deuterium-He3 16 80 0.05
Proton-Boron 11 500 2500 0.001

The Lawson Criterion is a index of how difficult the reaction is to initiate with respect to the Deuterium-Tritium reaction. The Relative Power Density gives an idea of how much power might be harnessed commercially. The Neutronicity shows how much of the energy produced comes off in the form of fast neutrons which produce most of the radioactive wastes.

This basic comparison suggests a possible economic window of opportunity for lunar Helium 3 mining. The easiest fusion fuel, Deuterium-Tritium, comes from the seas of Earth, but the Tritium must be converted in conventional fission reactors and the fusion facility would slowly become radioactive and turn into a huge pile of radioactive waste after about 40 years of operation.

The Helium 3 reaction is more difficult to initiate, but produces more energy with each reaction and produces negotiable radioactive wastes. Its problem is that the bulk of Helium 3 will have to be mined on the Moon at great cost.

As fusion technology progresses, we will likely someday be able to fuse Boron 11. This is far more difficult to do, but yields far more energy while generating truly negotiable radioactive wastes. All this fuel's constituent parts are available at low cost on Earth.

This suggests a window of opportunity for a lunar Helium 3 mining settlement. The following historic events need to take place to open this window: (1) it is determined that dumping carbon dioxide into Earth atmosphere must be stopped no mater what the cost, (2) wind and solar are not up to the job alone, (3) Deuterium-Tritium power production is accomplished, (4) Deuterium-Helium 3 power production is demonstrated, and (5) we build a lunar mining settlement. There is nothing unreasonable in this list, although there is also nothing certain.

This window would start to close when commercial Boron 11 fusion is demonstrated. The extablished lunar settlement will then have to find other means of economic support.

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.

Questions:

  • Can the cost of recovering He3 from the lunar surface be reduced 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? This depends on the size of the market, and there is little data.

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.” One 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 be counted in hundreds of kilograms, and the value of 100 kg would be $150M. So it may be assumed that the total stockpile value today is roughly 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 is an open question. Assuming that someone were to start at the level of collecting 100kg of He3 from the Moon and assume its value would be $150M, the cost of soft landing even a small probe on to the lunar surface may easily cost that much or more. How much He3 a small lander manufacture and how many grams per day have yet to be determined and production will rely on the method of processing.

A commonly discussed method is cooking the regolith to about 1400 degrees Fahrenheit or 760 degrees Celsius[2]. 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 the 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 need a large amount of energy, requiring the lander to have either a nuclear source (either Nuclear Fission or RTG), or large solar panels. Basalt has specific heat capacity of 0.24 cal/g/degreeC or 0.84 KJ/kg degreeK. To heat 1kg of basalt by 700 degrees Celsius requires about 600 KJ. The highest concentration of He3 in the Maria regions is 0.01ppm in the regolith. This means that 600 KJ will yield 0.01 milligrams of He3. Using these numbers, a 600 Watt power source could produce 0.01 milligrams of 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 a group or entity wants to amortize their investment. If an arbitrary target is to produce 100 kg He3 in one year, then a power source of about 200 KW would be needed. That would give a revenue stream of $150M per year if the He3 market does not become flooded and the price drops.

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. Assuming a best case scenario of 100% lighting, 10% photo voltaic efficiency and a fully steerable array, this would need an area of about 2,000 square meters, or about 45 meters on a square side. A simple non-PV solar reflector could be near 100% efficient, needing only 200 square meters or about 14 meters on a square side, or aperture. Setting up a 14 meter aperture mirror on the Moon would be a major engineering challenge, although it would not need to be particularly accurate as in the case of an astronomical telescope mirror.

Open Questions:

  • How much would a 14 meter aperture mirror weigh?
  • Would a Nuclear Fission power plant have better performance per kilogram of lander payload?

More thermal analysis needs to be done, as it may be possible to recycle the heat using some form of cogeneration. One possibility is to use the hot processed regolith to pre-heat the next incoming batch of raw dust, and thus reduce the number of solar joules needed. This could greatly reduce the size of solar array needed and/or significantly increase the system mass throughput.

Applications

An He-Ne laser
  • Medical Lung Imaging
According to Wikipedia:
http://en.wikipedia.org/wiki/Helium_3
Details on this experimental application of He3: http://cerncourier.com/main/article/41/8/14
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