Dr. Nebel On Next Big Future

Art Carlson · Post by **Art Carlson** » Fri May 08, 2009 5:14 pm

djolds1 wrote:Assuming a Polywell demonstrator works in say 3-10 years, would a developed reactor be able to burn He3-He3, or does Polywell's performance "max out" with pB11?

The characteristics of He3-He3 (cross-section, reactivity, Lawson criterion) are at least an order of magnitude below those for pB11. (There is a graph of reactivity on p. 7 of this presentation.) In addition, He3 is hard to come by. There might be some circumstances where the He3-He3 reaction has advantages over p-B11, but I don't know what they would be.

Roger · Post by **Roger** » Fri May 08, 2009 6:40 pm

Art Carlson wrote:There might be some circumstances where the He3-He3 reaction has advantages over p-B11, but I don't know what they would be.

Well lets assume Polywell works, it changes everything. PB-11 to get us off the Earth, but on/near the Moon use He. Only cause there should be a ton of the stuff available.

Other wise those factories on the Moon will need to be powered by PB-11.

pfrit · Post by **pfrit** » Fri May 08, 2009 7:14 pm

Art Carlson wrote:In addition, He3 is hard to come by.

It is not that hard to come by. We mine ~180 million tons of HE a year (with most of it just vented to the atmosphere) and 7-14 ppm of that is He3. The weight difference between the two would make seperating it out pretty easy. It is very expensive compared to He4, but not when you consider the amounts of He3 used right now are miniscule. We don't seperate it out because nobody wants it. If anyone wanted it, the price would go way down. I leave it to the reader to calculate how much He3 we would need to replace all coal burnt in the world to generate electricity if there was a reasonably good tokamat design that could burn He3. Answers with boundary conditions will be due next week. Have a good weekend.

chrismb · Post by **chrismb** » Fri May 08, 2009 7:27 pm

That reactivity graph is as for a Maxwellian distribution (hence no peaks). The actual reactions are around 2 orders of magnitude less that p11B for 3He3He.

I don't know where the 3He+3He reaction got a foot-hold in thinking, though I can imagine that for IEC methods, systems might benefit from the 'mirror' reactions (D+D, T+T, 3He+3He) as all ions might be accelerated equally to the same point in an electric focus, thus maximising collision-frame energy.

(There is a 6Li+6Li reaction that kicks off in the MeV range, but I guess the energy output doesn't warrant that as being any candidate for terrestrial fusion as it is already half way to iron in terms of 6Li's binding energy per nucleon.)

chrismb · Post by **chrismb** » Fri May 08, 2009 7:29 pm

pfrit wrote:I leave it to the reader to calculate how much He3 we would need to replace all coal burnt in the world to generate electricity if there was a reasonably good tokamat design that could burn He3.

If you had a thermonuclear reactor that could run 3He, then you'd use D+3He, not 3He+3He. The D+3He has the highest specific output per nucleon for a strong-force mediated reaction. In fact, you'd run a D+D rich reaction, as the 3He would come 'free' for half of the DD reactions (along with a neutron)!!

kurt9 · Post by **kurt9** » Fri May 08, 2009 7:50 pm

The question is if you can get enough 3He using isotopic separation from terrestrial sources so you do not have to go to the moon to get it.

pfrit · Post by **pfrit** » Fri May 08, 2009 8:08 pm

kurt9 wrote:The question is if you can get enough 3He using isotopic separation from terrestrial sources so you do not have to go to the moon to get it.

Absolutely. Definitely. No Question. As for cost, I am equally certain that it would be cheaper to mine here rather than on the moon. We are dumping between 1260 tons and 2520 tons of He3 a year. How much would we need if we were burning it?

KitemanSA · Post by **KitemanSA** » Fri May 08, 2009 8:41 pm

pfrit wrote: It is not that hard to come by. We mine ~180 million tons of HE a year (with most of it just vented to the atmosphere) and 7-14 ppm of that is He3.

Source?
This info seems substantially different that what I can easily find.

rnebel · Post by **rnebel** » Fri May 08, 2009 8:44 pm

We looked at He3-He3 and concluded that the fusion reactivity was just too low, as several posters have pointed out.

pfrit · Post by **pfrit** » Fri May 08, 2009 10:51 pm

KitemanSA wrote:
pfrit wrote: It is not that hard to come by. We mine ~180 million tons of HE a year (with most of it just vented to the atmosphere) and 7-14 ppm of that is He3.
Source?
This info seems substantially different that what I can easily find.

I'm looking, but remember that most of the helium produced is from natural gas and is never removed from the natural gas. When looking I did find that the normal ratio of He3 to He4 in petrolium sources is 2-12 ppm and not 7-14 ppm. Cia estimates a global annual production of 3,021,000,000,000 cubic meters of natural gas produced a year with the us producing 545,900,000,000 cubic meters a year (2007). Assuming a concentration of .5% of helium in natural gas (us) then the amount released in natural gas production is 2,729,500,000,000 cubic meters. I do not want to do the math for converting cubic meters to tons for helium, but you are left with alot of helium. And a lot of He3

TallDave · Post by **TallDave** » Fri May 08, 2009 10:59 pm

rnebel wrote:We looked at He3-He3 and concluded that the fusion reactivity was just too low, as several posters have pointed out.

It's odd this still gets press, given that anyone can do the math on that. I guess the idea of "lunar mining" is just too exciting to go away.

icarus · Post by **icarus** » Fri May 08, 2009 11:44 pm

The purity of some of the ore deposits on the moon is exciting enough in it's own right, regardless of the He3 deposits. E.g., Si and Mg oxides ... and more.

Question is cost of refining same on Earth versus space transportation costs to go and pick it up off the ground on the moon. A compact fusion reactor may well make access to such deposits cheaper than sourcing/refining same on Earth, now where can we find one of those?

KitemanSA · Post by **KitemanSA** » Sat May 09, 2009 1:00 am

TallDave wrote:
rnebel wrote:We looked at He3-He3 and concluded that the fusion reactivity was just too low, as several posters have pointed out.
It's odd this still gets press, given that anyone can do the math on that. I guess the idea of "lunar mining" is just too exciting to go away.

I think they are thinking D-He3, not He3-He3. That would be "aneutronic" if it weren't for the D-D side reactions!

TallDave · Post by **TallDave** » Sat May 09, 2009 3:39 am

icarus wrote:The purity of some of the ore deposits on the moon is exciting enough in it's own right, regardless of the He3 deposits. E.g., Si and Mg oxides ... and more.

Question is cost of refining same on Earth versus space transportation costs to go and pick it up off the ground on the moon. A compact fusion reactor may well make access to such deposits cheaper than sourcing/refining same on Earth, now where can we find one of those?

It's an airless environment at the top of a really nasty and expensive gravity well. Even with cheap fusion rockets or space elevators I expect we'll be mining the Earth's mantle before we're getting ore from the Moon, at least where terrestrial uses are concerned. He3 mining only made sense (even as a fantasy) because it had (like all fusion fuels) ridiculously high energy density and might supposedly only exist on the Moon.

Professor Science · Post by **Professor Science** » Sat May 09, 2009 4:50 pm

Personally, If we could do asteroid or lunar mining I'd prefer that, we've hacked away at the earth long enough. getting under the mantle by way of tunnels has a very high possibility of things going wrong, and strip mining is right out.