Question

What do you mean by nuclear energy? What is responsible for its release?

Answer 1

The energy obtained in a nuclear reaction, like, radioactive decay, fission, fusion is called nuclear energy.
In a nuclear change such as decay, fission or fusion the total sum of masses of product nuclei is always less than the total sum of the masses of reactant nuclei. Thus, there is a loss in mass.
According to Einstein, mass and energy are interchangeable. As per mass-energy equivalence, this loss of mass 𝛥𝑚 is released as energy E such that,
𝐸 = (𝛥𝑚) $c^2$
where c = speed of light $(3.0\times {10}^{8}m{s}^{-1}$)