 There were still many unknown factors.The properties of pure uranium-235 were relatively unknown; even more so those of plutonium, a chemical element that had only recently been discovered by Glenn Seaborg and his team in February 1941, but which was theoretically fissile.The scientists at the Berkeley conference envisioned breeding plutonium in nuclear reactors from uranium-238 atoms that absorbed neutrons from fissioning uranium-235 atoms.