Where does the released nuclear energy come from during fission?

Answer 1

A combination of the strong nuclear force and the electromagnetic force. The strong nuclear force provides the initial energy to split one large nucleus into two small ones, but once the smaller nuclei have separated by a short distance because both are positively charged the electromagnetic force repels them and rapidly accelerates them to very high velocity (i.e. high kinetic energy aka high temperature).

The end products (fission fragment nuclei and neutrons) have a slightly smaller mass than the mass of the original nucleus. If you plug the amount of lost mass into the equation E = M c**2, the result calculated will be exactly equal to the kinetic energy gained by the end products. So the energy came from a small part of the original mass.

So wording it another way, the energy release is driven by the strong nuclear force and the electromagnetic force, but the energy itself was released by the conversion of matter to energy.

BTW, the energy released in chemical reactions also come from matter to energy conversion, but as the energy is much smaller the lost mass is also much smaller (and usually undetectable).