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Topic: Fusion  (Read 3469 times)

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Offline Darren

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Fusion
« on: June 21, 2012, 11:53:38 AM »
Whats the purpose of scientists wanting to create fusion or make it possible? If thats not going to produce any more energy than what is consumed, whats their purpose in trying to hard to achieve it?

Offline Borek

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Re: Fusion
« Reply #1 on: June 21, 2012, 12:30:43 PM »
They hope to break even one day. In theory there is nothing that makes it impossible (quite the contrary - we know it works in stars), unfortunately technical obstacles pile up.
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Offline Enthalpy

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Re: Fusion
« Reply #2 on: June 21, 2012, 04:05:26 PM »
The next reactor, Iter, is designed to produce more energy than it consumes, during many minutes. Still not an energy production in a technological sense, because only 40% of the produced heat is converted into electricity consumed in the coils etc, but an important symbolic step.

What Iter does NOT address properly, and for which no one on Earth seems to have a good answer, is how to produce less pollutants by D-T fusion than by uranium fission.

Basically, tritium is not available on Earth so fusion reactors have to regenerate the consumed tritium through lithium bombardment by neutrons. Regeneration needs to multiply the neutrons, and Iter will have a few blankets to demonstrate the method, for which spallation of lead by the fast neutrons seems to be the only solution.

Unfortunately, this spallation produces roughly as much radioactive waste as uranium fission does, for the same output energy. My hand estimates of the produced 203Pb and 203Hg are there for instance:
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2450
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2450#p32310 (figures)
a simulation software would find more nuclides.

This is a huge disappointment to me, and I don't really see a way out.
Tokamaks can only use tritium which can't be produced elsewhere.
Lithium and beryllium can't multiply neutrons enough, as it seems, and any heavy element like lead would produce waste.
I've tried to invite to the forum the project leader for the lithium regeneration blankets at Iter, but he didn't answer, so I cannot suppose he has better solutions.

Note this worry isn't new. PhD thesis exist for years in relation with this topic, though they bear less explicit titles. Many people in the fusion job were aware of it. I let it know more broadly through Saposjoint.net and Physicsforums.com, and now it's described on Wikipedia.

Inertial confinement, by laser or Z-striction, might perhaps offer better chances to consume other fuels like D-D sooner than tokamaks - though they still don't work with D-T.

That's hard for hydrogen fusion, alas. Not only does it cost huge investments that renewables don't, it would also emerge too late to replace oil and uranium, and now it is to be dirty too.

Marc Schaefer, aka Enthalpy

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