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jfribrg
2004-Sep-23, 01:18 PM
The current issue of Popular Mechanics mentioned the prospect of using He-3 for powering commercial fusion reactors (and also of mining the moon for it). I did some web searches and found mostly pie-in-the-sky articles from pro space-exploration sites about how all the world's problems will be solved with He-3. Most of these sites implied that the technology exists to for commercial He-3 fusion reactors, but the big bad oil companies don't want us to know... There were also some web sites I found that talked about how it may never be a viable source of energy because D-D fusion holds more promise than D-He3. One site even "proved" that electrostatic confinement can never produce energy at the breakeven level without violating the 2nd law of thermodynamics.

Here are a couple of questions:
1) What is the current state of He3 fusion? How close are we to breakeven ?

2) Are there terrestrial sources of He3? Tritium decays into He3 with a half-life of 12 years. How much tritium do we produce/have today? Would mining the trace amounts of He3 found on Earth be cheaper than a moon mining program?

TinFoilHat
2004-Sep-23, 01:56 PM
Break-even He3-He3 or He3-D will be harder to achieve than D-D fusion - they require higher temperatures and give less energy per fuel fused. And we haven't yet made a D-D fusion reactor that can generate energy in any commercially useful fashion. We'll want to have D-D fusion working well before we even start to think about He3 fusion, and we won't have that for a while. And the only advantage of He3-He3 fusion is that it fuses without giving off neutrons, so the walls of the fusion reactor don't become so radioactive over time.

IMHO, He3 might become a valuable resource sometime far in the future. Right now we're nowhere near even having a reactor that can generate energy from it.

Glom
2004-Sep-23, 02:23 PM
I thought the inroad was D-T fusion and I thought the breakeven had already been achieved with it (although not in a viable way).

Eta C
2004-Sep-23, 02:30 PM
Popular Mechanics is well known for putting forth fringe ideas that have little or no validity and claiming they're the next great discovery and will change our lives in the next few years (for good or for ill). Examples include:

Cold fusion
Isomer bombs (based on hafnium isomer transitions
Zero Point Energy
The aforementioned He-3 fusion

And I could go on. Frankly, PM is a rag and little better than a glossy version of the Weekly World News IMO. Remember, this is the magazine that back in the 50's predicted that we'd all be using auto-planes to fly to work by now.

TinFoilHat
2004-Sep-23, 03:00 PM
I thought the inroad was D-T fusion and I thought the breakeven had already been achieved with it (although not in a viable way).
Yep. They have achieved D-T break-even in the sense that the total energy being generated in the core is greater than the energy being used to maintain the reaction, although as of yet nobody is capturing and using the energy. D-T fusion is the easiest type of fusion to achieve - lower temperature and better energy return than D-D, D-He3, or He3-He3. But it's also the worst as far as neutron emission is concerned - a good portion of the energy released is in the form of emitted neutrons. AFAIK the current plan for commercial fusion generators is to surround the reactor core with a lithium blanket to breed tritium.

Glom
2004-Sep-23, 03:06 PM
If Li6 and D are used, the neutron becomes the catalyst and so it means that the only neutron activation problem will be through inefficiency. Besides, neutron activation is still only a problem for decomissioning anyway and not an inherent problem throughout the operating life of the reactor. So how soon can we get D-Li reactors?

jfribrg
2004-Sep-23, 03:42 PM
As I understand it, the benefit of D-H3 fusion is the complete absense of neutrons. The other fusion reactors would require shielding. The neutrons would also damage the container more quickly (in addition to making it radioactive), reducing the operational life of the device.. Since much of the energy of other fusion reactions is lost via the neutron, D-H3 leaves more usable energy, and removes the radioactive waste issue completely.

I realize that Popular Mechanics is a pie in the sky kind of publication, but that doesn't alter the fact that serious He3 research is ongoing. Given that Jack Schmitt testified in Washington in favor of He3 research (specifically, mining the Lunar regolith to get it), then it can not be considered a crackpot idea. There's nothing wrong with writing about visions of the future, as long as you don't confuse it with today's reality.

TinFoilHat
2004-Sep-23, 05:07 PM
As I understand it, the benefit of D-H3 fusion is the complete absense of neutrons.
Not entierly. D-He3 is neutron-free, but you will unavoidably get some D-D fusion in there too, and D-D fusion releases neutrons. You need pure He3-He3 fusion to have no neutrons released.

I have also heard proposals for fusing boron and regular hydrogen. Like He3-He3 fusion, H-B fusion releases no neutrons. Unlike He3, hydrogen and boron are easily available. But H-B fusion is harder to achieve than He3-He3 fusion, and much harder to achieve than D-D or D-T fusion.

Jpax2003
2004-Sep-23, 07:06 PM
So, are there any plans for a hybrid fission fusion reactor? I don't know if it would work, but just thinking about some bomb designs and wondering if they could be adapted for continuous energy release. Perhaps if magnetic pinch of Hygrogen plasma corresponded with a plutonium sparkplug...

Glom
2005-Apr-23, 07:39 PM
Can someone give me a run down of D-D reactions and H-B reactions?

I know that
He3 + He3 -> He4 + 2p
D + T -> He4 + n
D + Li6 -> 2He4
4H -> He4 + 2e+

Presumably D-D goes
D + D -> He4
but where do the neutrons come from? Are they a catalyst like D-Li6?

Van Rijn
2005-Apr-24, 11:14 AM
The Wikipedia entry looks okay, at least it fits with what I remember. Anyway:

http://en.wikipedia.org/wiki/Nuclear_fusion

About half the time D+D produces Tritium and a proton. The other half it produces He3 and Neutron. As I recall, very rarely D+D will produce He4 and a gamma.

D+Li6 is a relatively "hard" reaction, and as with D+He3 you will also have some D+D reactions. After D+T, D+D is the next logical step.

Lithium blankets are all very well in a D+T reactor, but design isn't trivial. Neutrons are going to run into other things, like expensive superconducting electromagnets.

Van Rijn
2005-Apr-24, 11:24 AM
So, are there any plans for a hybrid fission fusion reactor? I don't know if it would work, but just thinking about some bomb designs and wondering if they could be adapted for continuous energy release. Perhaps if magnetic pinch of Hygrogen plasma corresponded with a plutonium sparkplug...

The idea of a fusion fission breeder reactor has been around for a long time - use a D+T reactor as a neutron source to breed plutonium. It may have some design and safety advantages over a fast breeder, and may be useful without breakeven. But it is only useful if you want fuel for fission reactors, so you don't hear about it so much these days, at least not in the U.S.

Glom
2005-Apr-24, 12:09 PM
The problem with that is a loss of fusion over fission advantage. In addition to non involving high level waste or radioactive reactants (as long as D-T is down using Li6) and being an easily quenchable reaction, a fusion reactor would not use anything that could be turned into a nuclear weapon. Yes, fusion fuel is used in fusion bombs (actually that's tritium which would be directly used if Li6 was used), but these components are useless without a fission ignition source, which in itself is a powerful weapon.