I might have misunderstood Pete, but if not, then I'd have to disagree with his earlier statement. It sounded to me as if he was describing the effect of relativistic mass, which is where the mass of an object increases as its velocity increases.
In nuclear fission, mass is most certainly destroyed and converted to energy. Actually, this happens any time matter and antimatter annihilate. The reverse process is precisely what other posts have referred to, e.g. production of e+/e- pairs.