Do transuranic elements such as plutonium ever occur naturally?
John W. Poston, Sr. is head of the nuclear engineering department at Texas A&M University and is a Fellow of the American Nuclear Society. He offers the following explanation. Conventional wisdom tells us that plutonium (Pu) does not exist in nature. Plutonium and other so-called transuranic elements are considered by most to be man-made elements. Thus, they assume that when plutonium is found in the environment, human technology has put it there. This element has usually been considered synthetic because it is produced most efficiently in nuclear reactors. But in the strictest sense, the answer to the question is yes, plutonium does occur naturally. Plutonium appears at very low concentrations in nature, on the order of one part in 1011 in pitchblende, the ore of uranium (U).
