Why there is more energy released in fusion than fission?

That would be in terms of mass or per nucleon. The energy released has to do with the relative energy or binding energy per nucleon.

Each fission reaction releases about 200 MeV of energy, but the initial mass is about 236 or 240 amu (after neutron capture). The fission reaction produces two new nuclei of masses about 143 and 90 amu and 2 or 3 neutrons. The two nuclei tie up a lot of energy, but are relatively stable with respect to additional fission (i.e. they will not fission but undergo other decay modes). But the main fact is that the lighter nuclei, although they have a greater binding energy per nucleon, that binding energy per nucleon is not significantly greater than the heavier fissile nuclei.

In contrast, in fusion, the reacting nuclei (often the lightest of elements, e.g. H, He, Li, . . .) have very low binding energy per nucleon, but as we go from H to He to Li, the binding energy increases significantly. It is the large difference in BE per nucleon between say, d or t and alpha, which is responsible for the energy released as a result of fusion. Other fusion reactions are not so energetic, e.g. d + d -> p + t or n + He3.


Please review this -
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c2

 

The nuclei repel each other because of the electrostatic force between them, until they get close enough that the strong nuclear force becomes more important.

 

Neutrons from fission are fast neutrons with E > 1 MeV.

In light water reactors (LWRs), fast neutrons collide with protons in the water to slow down to thermal energies (~0.025 eV or so). Reactors using heavy water, D₂O, use neutrons of slightly higher energy to induce fission.

 

If one visits www.webelements.com, and selects the superheavy elements, there is a description of each element is made from heavy ion collisions.

Seaborgium

Darmstadtium

There are a few endothermic fusion reactions:

p + t -> ³He + n

t + ⁶Li -> ⁷Li + p + n


Basically the desirable fusion reactions are those which have a high Q value, or high mass defect or mass difference between reactants and products. It's the difference in binding energies of the reactants and products, which allows for an exothermic fusion reaction. Since Fe and Ni have the highest binding energy per nucleon, Fe or Ni fusion not produce useful energy. There would be too much lost from scattering collisions, which is a problem even for the most advantageous fusion reactions.

Stellar nucleosythesis is based on fusion under conditions which cannot be recreated on earth in large reactors because of the enourmous pressures and energy densities involved. We just cannot construct sufficiently strong magnets or materials to enable controlled fusion process on the stellar scale.