Originally Posted by TinFoilHat
Note - the following is the result of me sitting down and doing some back-of-the-envelope calculations. I may be off on some of my figures or math. If you have any corrections or comments, please post them.
Helium-3 mined from the moon has gotten some attention lately as a fuel
source for future fusion reactors. It would take as little as 25 tons of
He3 to satify the annual current energy needs of the united states - a
quantity which could be carried in a single shuttle mission. We don't yet
have reactors which can burn He3, or a shuttle-sized vehicle capable of
bringing such a load back from the moon, but these are things we should be able to develop in the future. So it sounds like a good idea, right?
I decided to run some order-of-magnitude calculations to look at the
economic feasability of the project. The results suprised me.
In order to understand the economic rational for mining He3, we have to
look at what makes it a desireable fusion fuel. For economic feasability
we have to look not just at He3 but compare it to alternative power sources. Deteurium-Tritium fusion is the easiest form of fusion we are likely to use as a power source. It occurs at a temperature lower than any other fusion, releases a lot of energy per reaction, and we will never run out of fuel for it. The downside is that much of the reaction energy is released in the form of fast neutrons. Converting fast neutrons into electricity is inefficient, and the reactor walls will suffer neutron embrittlement and need periodic replacement, generating a steady stream of highly radioactive waste.
He3 fusionis more difficult to achieve, taking about twice the temperature,
and releasing less energy per reaction. On the plus side the reaction is
nearly aneutronic (the He3-D reaction releases no neutrons, but some D-D
reactions will also occur, releasing some fairly low-energy neutrons) and
the energy released can be converted into electricity at higher efficiency.
So, the question is, is extracting He3 from the moon cheaper and easier than dealing with the radioactive waste products from the D-T reactors?
He3 is present in the lunar soil at a concentration of about 1 part in
200 million. Extracting 25 tons of He3 means that your lunar extraction
plant will need to process a whopping 5 billion tons of lunar material
annualy.
To extract He3 from lunar ore, we need to scoop up the material, sift and/or crush it to break up the clumps and rocks, bake it in an airtight oven for a while, seperate out the He3 from the other gases released, and then dump the spent lunar ore somewhere it won't get in the way of your continuing operation.
To estimate the cost of building and running the extraction plant, I looked
at existing stripmining equipment used on Earth. Lunar equipment will
operate on similar principles, although of course many of the engineering
details will differ. Lunar equipment operates in 1/6 earth's gravity, so
the structure can be lighter. On the other hand, lunar mining equipment
will have to be self-powered. For the purposed of this essay I'm assuming
that all of the mining equipment is solar powered. The other option is
nuclear power - but manufacturing so many nuclear power plants will
generate more radioactive waste than we save by using He3 fusion, so that option is a non-starter.
Being solar powered means the equipment can only operate during the lunar day. For simplicity I assume that we can get 4000 hours of operation out of our lunar equipment per year. I'll also ignore breakdowns and downtime for maintinence.
Lunar equipment will also need to deal with severe monthly temperature
swings. Heavy mining equipment will need liquid-cooled radiator loops to
keep from overheating during the lunar day, and will need to be kept from
freezing during the lunar night. The lunar equipment in this example will
also need to be able to operate for years, autonomously or remote
controlled at best, with little to no maintinence.
For the first step, gathering the ore, I looked at bucket-wheel excavators
used in strip-mines. A good bucket-wheel excavator can dig up about 8 times its own weight in ore an hour. For this thought experiment I'll assume our self-powered, self-contained lunar soil excavator can scoop up 10 times its own mass per hour.
Secondly, the ore needs to be transported from the excavator to the
processing plant. Modern mine trucks can carry nearly twice their own
weight in ore. Lunar mine trucks should be able to beat this. I will
assume that our transport system can carry 20 times its own mass of ore
from the excavator to the plant per hour.
Then the ore needs to be sifted and/or crushed. Fine powder is going to
more readily heat up and release He3 - putting rocks and bolders into our
ovens is a waste of time. Earthly crushing plants can handle anywhere from 2 to 8 times their own weight of rock per hour. I'll assume our lunar
processer can crush and/or sift 10 times its own mass per hour.
The ovens to heat the ore will probably use large mirrors to concentrate
sunlight on the material. The oven chamber itself needs to be airtight,
to keep the scant quantities of He3 from escaping. The low-pressure gases given off from the ore need to be pumped out into a seperation system to extract the trace He3 from the other gases released. Some of the other gases may also be useful for local use, such as oxygen for a manned lunar settlement.
I don't have much to go on to estimate the mass of the solar oven. 10 times its own mass of ore per hour seems like a reasonable quantity.
Finally the spent ore needs to be carted off and dumped somewhere. Again, assume lunar ore trucks can transport 20 times their own mass per hour.
On top of the ore-handling equipment, you'll probably need centralized
control and communication systems, mapping gear and location beacons, a storage yard for spare parts and repair vehicles to put them into effect,
and finally a system to transport the refined He3 back to Earth.
Looking at our target numbers of 25 tons of helium per year, and 4000 hours of working daylight per year, we see that our lunar He3 plant needs to process 1,250,000 tons of lunar ore per hour. Adding up the masses of the required equipment above, we can see that setting up a lunar processing plant to provide our yearly needs of He3 will mean sending over half a million tons of equipment and supplies to the moon. That's rather a lot. Using a rough figure of 40 million per ton of payload delivered to the moon, it will cost over 20 trillion dollars just to send the He3 collection system there. That's not counting the costs to build the hardware, or to support and supply it once in operation.
I submit that we can deal with the nuclear waste generated by D-T fusion
plants for much, much less than 20 trillion dollars.
There are probable more efficient ways to do the He3 extration than the
method I outlined above. But even if you can find a way to do it at ten
times the mass efficiency, that's still 2 trillion dollars to get it there.
Now, it may be that we'll someday develop a way to put things onto the moon for a fraction of today's prices. Or perhaps we will have a large lunar base, with a massive mining operation to produce oxygen and metal alloys for local operations, and the He3 will be a welcome by-product. My point is that if my assumptions and math are correct, with today's technology the cost of building the industrial infrastructure required will make the project economically unfeasable.
And by the time we can afford to build this kind of system, we may have
better options for energy. Hydrogen-Boron fusion looks to be very hard
to achieve, but is also clean and uses fuel that we have here on earth in
effectively unlimited quantities.
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22-December-2006, 10:23 PM
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