U.S. Patent #2,708,656 (Neutronic Reactor) Enrico Fermi and Leo Szilard, May 17, 1955. The present invention relates to the general subject of nuclear fission and particularly to the establishment of self-sustaining neutron chain fission reactions in systems embodying uranium having a natural isotopic content. The highly classified patent for the first atomic pile was filed with the U.S. Patent Office in December 1944. It listed Fermi and Szilard as co-inventors and described the method by which a self-sustaining nuclear chain reaction had been achieved. The patent was finally issued on May 17, 1955 13 years after it had been started and nearly 11 years after it had been filed. By then, Fermi had been dead for six months.