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Well, as a first point, OP's information is wrong. While it's true that the half-life of U235 is ~700 million years, it isn't true that half of the initial mass is lost. When U235 fissions, about 200 MeV of the initial mass is converted to energy. The total initial mass is about a thousand times that, though (E=mc^2). So, after 700 million years, assuming you kept it from falling apart, the initial 15 lb lump would only lose ~1/4 of an ounce as energy.