Why energy is released in nuclear fission and fusion?

In fission we decompose a larger nucleus to smaller ones and fusion joins them
In both cases high powered atomic bonds, held together by nuclear force are broken and releasing large amount of energy

Nuclear fusion and nuclear fission are different types of reactions that release energy due to the presence of high-powered atomic bonds between particles found within a nucleus..

The energy released by fusion is three to four times greater than the energy released by fission. Nuclear weapon: One class of nuclear weapon is a fission bomb, also known as an atomic bomb or atom bomb. One class of nuclear weapon is the hydrogen bomb, which uses a fission reaction to "trigger" a fusion reaction.