In a nuclear reaction, is mass lost to produce energy?

An atomic nucleus consists of protons and neutrons bound together. The mass of any nucleus other than Hydrogen is not the same as the sum of the masses of the individual protons and neutrons.

When two lighter nuclei are fused to a make heavier nucleus, the resulting nucleus is less massive than the sum of the the two original nuclei. The mass difference is converted into energy which is why fusion reaction release energy.

Any nucleus of Iron or heavier elements require more energy to fuse to make heavier elements. So, the mass of the nucleus is heavier than the sum of the masses of the component protons and neutrons. Energy needs to be converted into mass to make heavier elements. This happens in supernova explosions where nuclei are bombarded with high energy neutrons.

Bombarding certain heavy isotopes, such as Uranium-235, with neutrons causes the nucleus to split into two. It can also release neutrons. In such fission reactions the sum of the masses of the reaction products is less than the mass of the original nucleus. This mass difference is converted into energy which is why fission reactions release energy.