Interesting entry in Do The Math Blog about Fusion.

[AJSA]

Perhaps somebody could explain in the blog a bit of Focus Fusion Technology.
As I can see the posts are very biased…

http://physics.ucsd.edu/do-the-math/2012/01/nuclear-fusion/

[delt0r]

I didn’t read it all in detail since he misses a pretty serious detail with D+D fusion. That is he claims D+D fusion gives 4He, which it does not. A 10sec wiki search would have told him that. There are probably other equally ignorant errors throughout the rest of the text.

Remember the golden rule. Politics is not science and scientists are bad at politics. Much of the issues with ITER and there kin are from the fact that you end up with a lot of physicists trying to be good at politics.

[jamesr]

delt0r wrote: I didn’t read it all in detail since he misses a pretty serious detail with D+D fusion. That is he claims D+D fusion gives 4He, which it does not. A 10sec wiki search would have told him that.

Yes, he’s a little misleading on that – although it is what happens eventually, as in the Sun. On Earth we have to deal with the intermediate products
The initial reactions (with a 50/50 split) are of course:
D + D -> He-3 + n
D + D -> T + p

but then you get all the other combinations
D + T -> He-4 +n
D + He-3 -> He-4 +p
He-3 +He-3 -> He-4 + 2p
He-3 + T -> He-4 + p + n
He-3 + T -> He-4 + D
n + p -> D

Which all end in He-4. Given D+T much larger cross section than DD, in a DD reactor most of the tritium gets burnt up instantly rather than causing a legacy problem. Although there is still enough that precautions need to be taken. Another important aspect is that the neutrons from the DD primary reaction are only 2.45MeV not 14.1MeV so need slightly less stopping power in the shielding/blanket.

On the whole I found the article well written, and the subsequent posts I wouldn’t term biased – I’d say they had a pretty healthy scepticism.

[asymmetric_implosion]

I thought it was pretty fair. Fusion has been “our future” for over 50 years and it hasn’t delivered. That doesn’t mean it won’t someday. There are certainly political issues around ITER and NIF. One would expect that given the scale of the projects. My opinion is ITER has more political problems than NIF because of the number of countries involved. Scientists certainly participate in it (more by force than by choice in most cases), but the few fusion guys I know well are serious scientists that recognize the massive technical challenges before them. They are have no intent on wasting money or misleading anyone. They want to figure out if their solution works or not. My opinion is that ITER will not be built to full scale if at all while NIF enjoys funding for years to come.

[delt0r]

This is not the same as claiming D+D->4He by a long shot. A very long shot.

[asymmetric_implosion]

delt0r wrote: This is not the same as claiming D+D->4He by a long shot. A very long shot.

D+D-> He-4 does happen in fusion. It is a rare branch of the reaction even in plasma. They detect it on NIF as a minority event. I was surprised to see it at the last APS- Division of Plasma Physics meeting.

[delt0r]

Yes i know. But since it produces 2 gammas the branching ratio is suppressed by the fine structure constant squared. So its like 10^-6 or something IIRC. Enough to detect with MW of fusion. Otherwise not worth considering.

[vansig]

jamesr wrote:
Yes, he’s a little misleading on that – although it is what happens eventually, as in the Sun.

I thought that the Sun uses the CNO cycle?
— http://en.wikipedia.org/wiki/CNO_cycle

[ edit ]
oh, wait. ok, only 1.7% of fusion in our sun is CNO.
(i should read more 😉

[jamesr]

vansig wrote:

I thought that the Sun uses the CNO cycle?
— http://en.wikipedia.org/wiki/CNO_cycle

No, our sun is too small for the CNO cycle to be a significant contribution. As it says in the wiki, a star needs to be at around 1.3 times as massive as the sun for its core to be hot enough to support this cycle.

The size/temperature dependence is shown here: http://csep10.phys.utk.edu/astr162/lect/energy/cno-pp.html

[Maya]

AJSA wrote: Perhaps somebody could explain in the blog a bit of Focus Fusion Technology.
As I can see the posts are very biased…

http://physics.ucsd.edu/do-the-math/2012/01/nuclear-fusion/

It looks like we have two threads going on this, so I guess I’ll post my reply here as well?

This is the first published description I’ve seen so far that accurately lays out the key challenges of fusion power. Notice that not one single proposal in existence today deigns to address them. Contrary to the view of some, I think we rather should talk about these problems not as discouragement but because that is the only way we can solve the riddles. If you don’t know what the challenges are you can’t find the solution. I can summarize the key 4 issues with fusion power that must be addressed for a _commercially viable_ (at least hundreds of gigawatts) reactor to work:
1.) It must have a much more powerful means of “heating”. RF and other methods are woefully inadequate
2.) It must contain enormous pressures far beyond what any material science we have can contain (the author refers to temperature but the two are related)
3.) It must deal with the thermal management of neutron flux. The current fueling schemes will never work. Only Hydrogen and 11 Boron can be used in any foreseeable design and even that will require enormous thermal management (H-11B still produces flux in power levels of 1 to 10 giga watts in a 21 TW reactor).
4.) It must have a definitive mechanism for dampening out _all_ orbital instabilities in the plasma. Confinement time must be very, very large b/c the materials science cannot deal with it otherwise.
Part 2 has a clever, witchy exception that as far as I know no one has figured out yet.
If you have a Ph.D. in physics (any field) and will sign an NDA I’ll explain it … I hope.

[zapkitty]

Maya wrote:
I can summarize the key 4 issues with fusion power that must be addressed for a _commercially viable_ (at least hundreds of gigawatts) reactor to work:

…?

Could you explain why you think that “at least hundreds of gigawatts” are required for a commercial fusion reactor?

Even the largest current fossil and fission power plants are at most 1-2 gigawatts per unit.

Or did I misunderstand and it’s the fusion concept you mentioned that requires that minimum size to be viable?

Maya wrote:
Only Hydrogen and 11 Boron can be used in any foreseeable design and even that will require enormous thermal management (H-11B still produces flux in power levels of 1 to 10 giga watts in a 21 TW reactor).

… a 21 terawatt pB11 reactor?

Why on Earth (and where on Earth) would you want to build a single reactor that would supply over half again the entire world’s current power needs?

The pB11 reaction is aneutronic in nature and units built along the lines envisioned by Focus Fusion, Tri-Alpha and the Polywell concept would be in the scale of a few megawatts to a gigawatt or so and their neutron flux would be pretty damn small.

It seems that you’ve set impossible-to-meet criteria and I look forward to learning what you envision as the solution to them 🙂

(PACER variant at 3-2)

[Maya]

zapkitty wrote:

…?

Could you explain why you think that “at least hundreds of gigawatts” are required for a commercial fusion reactor?

Even the largest current fossil and fission power plants are at most 1-2 gigawatts per unit.

Or did I misunderstand and it’s the fusion concept you mentioned that requires that minimum size to be viable?

Yep, you did. The discussion is about fusion. Which costs more to build and run? Savannah River Site or ITER? And remember how little power ITER will produce. Fusion power plants will be economic monsters that have to be actuarially sound. I think this is part of the problem with the public perception about fusion. There’s a lack of economic reality in the proposals. Iff you could build a fusion reactor that is super cheap, that might change, but somehow I doubt that’s going to happen.

zapkitty wrote:
… a 21 terawatt pB11 reactor?

Why on Earth (and where on Earth) would you want to build a single reactor that would supply over half again the entire world’s current power needs?

It’s about cost and economics. In order for fusion to be seen as a replacement that can idle other plants it must not only produce enough power to justify its cost, it likewise must produce power so abundantly that it expands that market. Do you think that energy consumption will remain constant over the next 50 years? I don’t. I think it will skyrocket.

zapkitty wrote:
The pB11 reaction is aneutronic in nature and units built along the lines envisioned by Focus Fusion, Tri-Alpha and the Polywell concept would be in the scale of a few megawatts to a gigawatt or so and their neutron flux would be pretty damn small.

It seems that you’ve set impossible-to-meet criteria and I look forward to learning what you envision as the solution to them 🙂

I don’t think so. I think the criteria are grounded in hard reality. I don’t believe that megawatt class fusion reactors will ever be economical in any foreseeable future. On this particular point the issue isn’t technical. It’s economic. Why would anyone build a megawatt class reactor that would be such a colossal construction for a single one-off? And the four key issues I described are part of the reason why I don’t think they’ll be economical. Even if you lower the power output you will still have phenomenal pressures to deal with. Have you looked at the structural loading on ITER; a reactor that can barely power a lawnmower? They’ve got the numbers on their website if you’re curious. Even at these totally uneconomical power levels it experiences several million kg*s of force to its structure. These pressure calculations are pretty basic calculations. And before you think that’s just because of those magnets, ask yourself, why do the magnets need to exert all that force? Answer: pressure. Now, given a favorable reaction rate ask how much fuel product must be present to get a worthwhile power density in the plasma and you will see my point. As you pack the material closer and closer the pressure necessarily goes up. Power and pressure have a defined mathematical relationship in thermonuclear fusion and they scale together as a square.

Yes, I know, youtube is replete with one venture capital con after another talking about producing fantastic amounts of fusion power with virtually no mention of pressure. But that’s not reality. It’s fiction and IP games. If that doesn’t make it clear, you can also run a power calculation and estimate equivalent reaction force against the vessel. That will also make it clear. It doesn’t matter how you _do_ the fusion. The problem here is more fundamental than that. Fusion at bottom is a power conversion problem of a magnitude of which the public is not generally well aware.

In the case of fission plants producing 1 or 2 gigawatts, the energy transfer is through very large, very heavy turbine machinery that distributes what would otherwise be pressure on a fusion reactor vessel. But, it’s still only a couple of gigawatts. But more fundamentally and to my point, fission doesn’t rely nearly as much on pressure (or reaction forces generally) … because its fission … not fusion.

If you have an Ph.D in physics and will sign an NDA I’d be glad to share my thoughts on a solution with you. I’m not being hokey or pessimistic at all. I believe there _is_ a solution, but it’s not on youtube and there is a lot of incentive out there to sell intellectual property instead of creating fusion power. If we’re going to solve the fusion riddle we have to understand what the technological impediments actually are, and very few people do. Sadly, academia isn’t helping because they want the big bucks for what they well know is nothing but pure science with no near-term application merit. That’s my take on con-Fusion Wonderland.

[vansig]

Maya,
ITER is a white elephant project designed to convince the world that fusion must be enormous, expensive, probably also dangerous, and always at least fifty years away.
while none of these is true, it looks like you’ve been propagandized.

[Maya]

vansig wrote: Maya,
ITER is a white elephant project designed to convince the world that fusion must be enormous, expensive, probably also dangerous, and always at least fifty years away.
while none of these is true, it looks like you’ve been propagandized.

I don’t think so. I just did the math.

[vansig]

the article ignores aneutronic fusion, yet claims to cover all research. that doesnt add up.

[Author]

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