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PostPosted: May 12, 2008 5:57 pm 
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I found this document today and found it very interesting. Dr. Lidsky points out essentially all of the same concerns with fusion that I developed as a student learning more about fusion technology at Georgia Tech in the late 90s.

Lawrence Lidsky: The Trouble With Fusion

Some of the quotes I liked:

Quote:
How could highly motivated and intelligent people get themselves into such a difficult situation? A fundamental reason concerns the difference between scientists’ and engineers’ view of what it means to solve a problem. Although they are usually able to agree on the definition of a “good problem,” scientists and engineers often have different perspectives as to what constitutes a “good answer.”


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PostPosted: May 13, 2008 3:15 am 
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It is a typical promotional and simplified text, which I always dislike....weather it is in favor of my goals or not.

Furthermore I found some errors in it:
Quote:
Thus, energy must be used to ignite fusion and to replace the energy continuously lost by the hot fuel. Obviously, the energy produced by the reaction must exceed the required input if the reactor is to be of any use.


First of all most of the energy isn't lost by scattering, but by Bremsstrahlung. Secondly the energy lost will be supplied by the slowing down alpha particle which is produced in the D-T reaction. The main reason to continue an input of energy is to keep control of the plasma and to shape (temperature, pressure,...) it in a way that it is optimal.

Quote:
This first wall is expected to be made of stainless steel or, better, one of the refractory metals such as molybdenum or vanadium that retain their strength at very high temperatures.


The First wall has to sustain plasma sputtering! I have never read that the EM-radiation is that severe. But as far as I know Tungsten is now proposed due to it's very low sputtering yield (although having a higher Z-value then Beryllium, it is much more preferred) Furthermore tungsten has an (n,2n') reaction peaking around 14.1MeV which multiplies the primary neutrons. Of course Be has also such a reaction but with a lower threshold and an almost constant value over it's energy range. So a lot depends on the scattering and so on. But still I believe that tungsten is the way to go (besides fluid walls, but that's for a more far away fusion future).

OK I haven't read through the entire article yet, but I don't agree (some stuff are true though, but same things could be said about any energy source...nothing is perfect). I still think and believe fusion is the energy source of the future. Unfortunately the ITER program isn't the way to achieve it (according to me)...And actually I very much doubt the fact of seeing a power fusion reactor in my life, most of all by political and economical problems associated with it...

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PostPosted: May 13, 2008 8:02 pm 
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STG wrote:
The First wall has to sustain plasma sputtering! I have never read that the EM-radiation is that severe. But as far as I know Tungsten is now proposed due to it's very low sputtering yield (although having a higher Z-value then Beryllium, it is much more preferred) Furthermore tungsten has an (n,2n') reaction peaking around 14.1MeV which multiplies the primary neutrons. Of course Be has also such a reaction but with a lower threshold and an almost constant value over it's energy range. So a lot depends on the scattering and so on. But still I believe that tungsten is the way to go (besides fluid walls, but that's for a more far away fusion future).


The problem with any first wall, including a liquid-fluoride one (which has been proposed) has to do with sputtering and plasma purity. If you get any appreciable amount of high-Z material in the plasma (and high-Z in this case ain't tungsten, it's more like lithium) then the high-Z material forms a "nucleus" of energy loss--I can't quite remember if the energy loss mechanism was bremsstrahlung but I think it was.

The purity requirement gets MUCH worse as you move off of D-T fusion into the more energetic reactions like D-D, D-He3, and p-Boron.

I just don't see any way around this basic problem, which is endemic to essentially all materials and almost every plasma geometry (with the possible exception of a mirror) that I can think of.


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PostPosted: May 14, 2008 3:24 am 
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LiF walls are idiotic! F has a far higher Z and will increase losses even more. A pure Li-metal wall makes more sense...but one has to ensure that these particles (which will have bigger radius in the tokamak I thought) are send to the divertor very fast. And then you of course have to clean your exhaust gasses of the Li.

OK I agree but no one is thinking of those highly energetic fusion reactions yet, since D-T is the easiest way out at the moment (concerning plasma physics). And I see no real problem with D-T fusion since I can imagine enough sources of T. A point is of course the 14.1 MeV neutrons...but my material knowledge isn't sufficient enough to make a discussion about that.

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PostPosted: May 14, 2008 6:54 am 
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STG wrote:
OK I agree but no one is thinking of those highly energetic fusion reactions yet, since D-T is the easiest way out at the moment (concerning plasma physics). And I see no real problem with D-T fusion since I can imagine enough sources of T. A point is of course the 14.1 MeV neutrons...but my material knowledge isn't sufficient enough to make a discussion about that.


Lidsky and the Polywell guys are thinking of the higher energy reactions. Lidsky was thinking of this back in 1983. His whole point was that D-T was the easiest but that it was a dead-end for engineering practicality because of the withering fast neutron flux, which he compares to a fission reactor and shows is much worse.


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PostPosted: May 14, 2008 7:18 am 
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D-D has also a neutronic reaction (well actually 2) so that doesn't overcome that problem. For these reactions the reaction rate is that low even at higher temperatures, that the energy density will be to low and the fusion device will have a larger size. So he's actually argumenting against himself in the article then.

The hard spectrum is a problem as it will generate a lot of (n,a), (n,t), (n,p), (n,d) reactions in the materials leading to embrittlement...I know...this creates some nice challenges, but I wouldn't talk about dead ends. On the other hand the spectrum is useful for multiplication purposes as otherwise tritium self-sufficiency can be forgotten. But as long as you're working with neutrons one will generate radioactive waste (even when one is working with protons actually). It's nuclear so nuclear waste will be involved, the only thing is that none or very little of it will be long lived. And one doesn't has to deal with meltdown scenario's, MW's of decay heat,...

And define much worse: the fast flux is higher or is to energetic? or a combination of both?

And not to be annoying, but the MSR poses also a lot of engineering challenges on the material: the neutron irradiation of the heat exchanger and piping together with corrosion and so on... All leading to embrittlement, swellling,... Making the primary circuit subjected to an increased change of leakage. And the talk about the fact that everything is just dissolved in the salt and one doesn't has to worry isn't completely justified since bubbles are formed in the salt during irradiation, meaning that these bubbles can also be released in the event of leakage. All poses challenges, but there exists solutions to everything...whether they're economic or not, that's another story.
So to make a small conclusion all technologies pose challenges, the only one which has already experimentally proven to be working is an LWR (and also CANDU I think). hundreds of LOCA experiments, accidents experiments and so on, a large fleet which is evolving to more safety due to an increase in experience...not one new reactor design can compete with that.

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PostPosted: May 14, 2008 8:35 am 
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STG wrote:
D-D has also a neutronic reaction (well actually 2) so that doesn't overcome that problem. For these reactions the reaction rate is that low even at higher temperatures, that the energy density will be to low and the fusion device will have a larger size. So he's actually argumenting against himself in the article then.


No argument from me on that point.

STG wrote:
And not to be annoying, but the MSR poses also a lot of engineering challenges on the material: the neutron irradiation of the heat exchanger and piping together with corrosion and so on... All leading to embrittlement, swellling,...


There's really no comparison between the neutron flux the first wall of a D-T fusion reactor and the heat exchanger of an MSR. The first wall of the D-T reactor will be exposed to a withering blast of 14.1 MeV neutrons with essentially no attenuation, whereas the heat exchanger of an MSR will see only a tiny fraction of the neutrons in the core (a fraction of the delayed neutrons) which will have energies on the order of hundreds of keV's, and will be "downshifted" significantly by the presence of moderation in the salt itself.

The fact that fission reactors can put neutrons to direct, productive use whereas D-T fusion reactors have to endure damaging neutrons which can't directly sustain the reaction is a major advantage for fission reactors. Also the fact that neutron in a fission reactor are basically deposited in a volume rather than on a surface as in the fusion reactor.


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PostPosted: May 14, 2008 9:04 am 
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Kirk Sorensen wrote:
There's really no comparison between the neutron flux the first wall of a D-T fusion reactor and the heat exchanger of an MSR. The first wall of the D-T reactor will be exposed to a withering blast of 14.1 MeV neutrons with essentially no attenuation, whereas the heat exchanger of an MSR will see only a tiny fraction of the neutrons in the core (a fraction of the delayed neutrons) which will have energies on the order of hundreds of keV's, and will be "downshifted" significantly by the presence of moderation in the salt itself.


Almost no attenuation = almost no interact in the first wall...This is the good thing for the Liquid Lead cooled design as the lead itself can not be damaged. Just supply sufficient Lead-Lithium to attenuate the flux.

Lower energy means also more change of absorption and so on. Further nothing prevents these delayed neutrons to interact with the fissile fuel...

I only did a qualitative statement, a just quantification (which I'm to lazy to look up, and have actually no time for) is of course needed to make any real conclusions. I don't believe it's that easy just to say it's like this and it's like that...this is the same way anti-nuclear people tackle problems.

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PostPosted: May 14, 2008 2:08 pm 
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If fusion ever is cost effective, the anti's will quickly start propoganda such as "tritium leaks through all known materials and cannot be confined; massive quantities are leaking out and poisoning our waters and concentrating up the food chain (one of my favorite lies; tritium doesn't do that, like other poisons.)"

Or "any material can be hidden in the blanket and eventually collect enough neutrons to become fissile and therefore fusion is a proliferation hazard"

Back to fission...can we use fusion as a neutron source instead of an Accelerator Driven Subritical, and use the fission energy to reach breakeven? If this was done, maybe some commercial experience can be gained and we can use this to further fusion research?


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PostPosted: May 14, 2008 2:26 pm 
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robertrichter wrote:
Back to fission...can we use fusion as a neutron source instead of an Accelerator Driven Subritical, and use the fission energy to reach breakeven? If this was done, maybe some commercial experience can be gained and we can use this to further fusion research?


Yeah, I had a professor at Georgia Tech (Weston Stacey) who wanted to do just that...

Here is the link to the colloquium he gave at UTK on January 29, 2003.

I took Fusion Reactor Design from Dr. Stacey at Georgia Tech in the fall of 1998, back when I was really interested in fusion (and before I knew about thorium).

Thing is, if you build a reactor with good reactivity coefficients, why would you ever want a subcritical reactor? Especially one using a fusion neutron source!


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PostPosted: May 14, 2008 3:20 pm 
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This is called the FDS (Fusion Driven System) and is actually in a conceptual design phase in China.

And I can assure you, fusion can breed large quantities of fissile material...

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PostPosted: May 14, 2008 3:28 pm 
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STG wrote:
This is called the FDS (Fusion Driven System) and is actually in a conceptual design phase in China.

And I can assure you, fusion can breed large quantities of fissile material...


So I guess it's not as "clean" as Michio Kaku thinks? :evil:


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PostPosted: May 14, 2008 4:18 pm 
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Kirk Sorensen wrote:
There's really no comparison between the neutron flux the first wall of a D-T fusion reactor and the heat exchanger of an MSR. The first wall of the D-T reactor will be exposed to a withering blast of 14.1 MeV neutrons with essentially no attenuation, whereas the heat exchanger of an MSR will see only a tiny fraction of the neutrons in the core (a fraction of the delayed neutrons) which will have energies on the order of hundreds of keV's, and will be "downshifted" significantly by the presence of moderation in the salt itself.

Correct me if I'm wrong, but I was under the impression that the most severe neutron flux a two fluid LFTR would be the blanket wall separator pipe or whatever that thing is called that separates the core from the thorium. There would be far less neutron damage in the structural material of a LFTR (especially where protactinium partitioning is avoided for a larger blanket) than even a LWR because the only structure that actually has to endure any flux is the blanket wall. I can only assume that this big pipe will have to be designed to be replaced every so many months/years.

Any idea how often it would have to be replaced? (In a graphite free reactor I assume less often because of the higher thermal interaction cross section of nickel for hastelloy. I imagine Davids done some investigation of this)

Kirk Sorensen wrote:
Thing is, if you build a reactor with good reactivity coefficients, why would you ever want a subcritical reactor? Especially one using a fusion neutron source!

One would assume, like many other engineering problems, the reason such ridiculous ideas are because of licencing, law, and politics. Why would we ever do something as ridiculous as build Yucca mountain when spent fuel is doing just fine on site?


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PostPosted: May 14, 2008 4:21 pm 
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Let's tackle those questions on another thread and stick to fusion on this one.


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PostPosted: May 14, 2008 4:30 pm 
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Maybe a stupid question, but is the neutron flux from inertial confinement fusors high enough to breed some U233 from Thorium? Enough so we don't need HEU, plutonium or such for a reactor start?


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