nuclear energy?

Question

if fission and fusion provide energy,why is it not possible to start with a nucleus,make it undergo fission and fusion repeatedly and continiously ,and obtain large amount of energy ?besides ,if it is possible ,all the energy we require can be satisfied by a single nucleus undergoing fission and fusion.

Answer 1

We can extract energy from a fission or fusion reaction only when the matter after the reaction has a lower energy than the matter before. To reverse a reaction requires adding exactly the same amount of energy that the reaction released.

Answer 2

Energy is only released in a nuclear reaction when the total mass of the products is less than the total mass of the reactants. If the reverse is true then the nuclear reaction has to absorb energy for it to occur.

For instance in a fission reaction (FPs ->fission products)
n+U->2n +FPs +200 MeV releases 200 MeV
but the fusion reaction
200 MeV + FPs + 2n-> n+U absorbs 200 MeV

Typically fusion is exothermic when small atoms are involved and fission is exothermic when large atoms are involved.

Answer 3

You can't get energy from splitting any element, only heavy elements; I think heavier than Iron. Likewise, you can only get energy by fusing light elements, not heavy elements. I think Iron is where you break even and you can't get more or less energy by fusion/fission. I don't really know why that is though, ask a nuclear physicist - or become one.