# Fusion vs. Fission and Nuclear Transmutation

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For the heavier elements it's the energy available when a supernova occurs.

Thank you for that correction.

I'm glad someone here knows more about Astronomical Physics than I do.

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In supernova, we have rapid neutron absorption/capture.

When neutron is absorbed by some nucleus, it's releasing energy.

You can calculate energy released by searching for isotope mass, multiplying it by 931.494 MeV, add 939.565 MeV, and subtract mass-energy of A+1 isotope in table.

f.e.

Isotope Iron-56
Protons 26 Neutrons 30
Mass 55.9349
Nucleus Energy 52089.8 [MeV]

Isotope Iron-57
Protons 26 Neutrons 31
Mass 56.9354
Nucleus Energy 53021.7 [MeV]

Energy released by neutron capture of this isotope will be 52089.8+939.565-53021.7 = ~7.6458 MeV

$_{26}^{56}Fe + n^0 \rightarrow _{26}^{57}Fe+7.6458 MeV$

Fe-57 is stable, but if we would do similar calculation for unstable isotope, additional energy from decay of that isotope would be produced.

Edited by Sensei
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And while I can see two ions smacking together in a collider to acheive fusion at low 'pressure' ( stretching the meaning since both temp and press are a measure of particle energy ), I don't see how you can call that sustained fusion ( post #19 ).

So I'm still going to need an example of an appropriate mechanism, John.

Maybe I can patent it, solve the world's energy problems, and become incredibly wealthy.

Or did you 'muddle' fusion with fission ( been waiting for that a long time, no, just kidding around ).

Edited by MigL
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Fusion works with all the elements up to about iron.

They are made in stars, and the whole lot are gases under those circumstances.

It is, in principle, possible to make a fission reactor with uranium hexafluoride vapour.

There are plenty of plans for using molten salts for fission reactions of thorium.

The physical state is not fundamental to the fission/ fusion process; it's a coincidence.

How was iron created then?

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How was iron created then?

Which isotope? Different isotope can made different way (even multiple).. Iron has 4 stable isotopes.

f.e. Iron-56 can be made from Cobalt-56:

Cobalt-56 -> Iron-56 + e+ + Ve + 3.544 MeV

Cobalt-56 + e- -> Iron-56 + Ve + 4.566 MeV

If you're not interested in details, answer "from lighter elements isotopes" should be sufficient.

Edited by Sensei
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I mean how was iron created in the first place... John's post implies that iron was not created in the Big Bang, although I may just be interpreting it wrong.

Edited by /backslash/
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I mean how was iron created in the first place... John's post implies that iron was not created in the Big Bang, although I may just be interpreting it wrong.

Only hydrogen, helium and little bit of lithium were created in the big bang. All heavier elements were created in supernovae. ("We are stardust...")

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H, He and Li were created in the big bang, the elements up to about iron were created in stars the heavier ones are created in supernovae (as Swansont pointed out)

It's complicated and this explains it a lot better than I can.

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

And while I can see two ions smacking together in a collider to acheive fusion at low 'pressure' ( stretching the meaning since both temp and press are a measure of particle energy ), I don't see how you can call that sustained fusion ( post #19 ).

So I'm still going to need an example of an appropriate mechanism, John.

Maybe I can patent it, solve the world's energy problems, and become incredibly wealthy.

Or did you 'muddle' fusion with fission ( been waiting for that a long time, no, just kidding around ).

You would be about 40 or 50 years too late for the patent.

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

Typical acceleration voltages are 10 to 100 KV so the energies are in the region equivalent to temperatures of 100 million degrees (give or take a few zeroes).

The pressure is typically something like 100 microns of mercury (feel free to turn that into SI units) which is a pretty good vacuum by engineering standards (though it's a very poor one compared to those used in surface chemistry research etc).

Edited by John Cuthber
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I apologize profusely John. Technically you are right.

Although in my previous post, I had granted that a collider can fuse light ions at low 'pressure', so arranging the equivalent of several together to fire at the same spot is essentially the same.

And I still don't think that fits the definition of 'sustained'.

The only sustained fusion reactions we know of are stars and thermonuclear bombs.

And both use hi temps AND pressures.

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(1) Technically you are right.

(2) The only sustained fusion reactions we know of are stars and thermonuclear bombs.

(1) That happens a lot.

((2) You can run a fusor for minutes to hours.

Over roughly how long do you think a fusion bomb "sustains" a nuclear reaction?

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Surely there is no such thing as a 'sustained' fusion reaction?

Two nuclei collide and fuse, if the conditions are favourable.

And that's it, game over.

Mig, with respect, I think you could rephrase to what you actually mean.

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Surely there is no such thing as a 'sustained' fusion reaction?

It is what things like JET and ITER are attempting to achieve.

OK being really pedantic, you could insist that this is a sustained series of fusion reactions. But I think the meaning was pretty obvious.

The physical state is not fundamental to the fission/ fusion process; it's a coincidence.

Of course it isn't fundamental to the reaction. But the question was about the practical difficulties of each. The engineering difficulties are, largely, due to the difference between containing solids (or possibly liquids or gases) at reasonable temperatures and pressures versus containing a plasma at extremely high temperatures (and possibly pressures).

Now maybe cold fusion, or some other process, could make fusion technology easier but if so, it isn't clear what that is yet.

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Surely there is no such thing as a 'sustained' fusion reaction?

Two nuclei collide and fuse, if the conditions are favourable.

And that's it, game over.

Mig, with respect, I think you could rephrase to what you actually mean.

Sustained in the sense that you have fuel around to keep the reactions going. A fission reactor will run out of fuel, but that reaction can be self-sustained for a macroscopic amount of time.

Further, the OP made it clear that the context here is for energy production, so while a fusor will work for a macroscopic amount of time, it does not produce a net amount of energy, so it doesn't fit the parameters set in the OP.

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If all you want to do is heat a room or boil water to run a turbine then, in principle any fusion reactor produces a net amount of energy.

Energy is released by fusion and it ends up heating the surroundings.

I gather that with a good fusor and a kilowatt or so of input power, you can get something like a nanowatt of extra heat.

Not terribly practical- but net energy production nonetheless.

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That is why I granted that you are right, and yes, that is usually the case.

My interpretation of 'sustained' is actually self-sustaining.

So, while a fusion bomb's reaction might last very briefly, It is self-sustaining ( at least till the fuel is used up ).

A 'fusor', if I recall, needs a constant input of external energy to operate.

I realise it's just semantics, but the energy of the reaction doesn't directly provide the necessary temp and press needed for the reaction as athe Sun or an H-bomb do. It has to be channeled to external equipment which provides the kinetic energy to the particles ( equivalent to temp and press ) in order to facilitate the reaction.

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If all you want to do is heat a room or boil water to run a turbine then, in principle any fusion reactor produces a net amount of energy.

Energy is released by fusion and it ends up heating the surroundings.

I gather that with a good fusor and a kilowatt or so of input power, you can get something like a nanowatt of extra heat.

Not terribly practical- but net energy production nonetheless.

I took energy to mean useful energy, i.e. you could unplug it and it would continue running

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