What is a simple evaluation for e=mc2?

Scientists do not know how to convert the entire m of an atom (protons + neutrons + electrons) into energy. When a heavy atom (uranium?) is fissioned (split) into two (or more) particles (two unequal smaller atoms, escaping neutrons, escaping alpha particles, escaping electrons, etc.) the m of all the resulting particles is slightly less than the m of the initial atom. The tiny amount of lost m (a m discrepancy) is converted to energy (E = mc2), including kinetic energy (velocity) of the new particles. Because the m discrepancy is multiplied by the speed of light squared (a huge number!) the resulting energy if far greater than that obtained by ordinary chemical reaction involving only the electrons of atoms, with no mass discrepancy. Within the sun hydrogen atoms are fused (joined) to form helium atoms, also with a mass discrepancy that produces far more energy than an ordinary chemical reaction. In nuclear power plants, billions of fissions are still required per second to produce useful power. The interior of the earth has been maintained very hot for 5 billion years by the natural decay of heavy atoms that produce heat from mass discrepancies.