If you were running a fusion power plant, and I know that
cannot be contained so we can't use it yet, but if you were to combine
two hydrogen atoms to perform fusion, I know you would get helium, but
could you then split the helium to turn it back into hydrogen, just so
you can perform fusion again?
Yes, in principle. Of course, you'd have to give back all the energy
you got from fusion to do this.
All the energy you got out of the fusion reaction to make the helium would
have to go back into it to break it back into hydrogen. The best you could
for would be to break even. In real life you would lose energy during each
cycle so you would turn less of the helium back into hydrogen each cycle,
eventually resulting in having nothing but helium and heat energy.
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Update: June 2012