plutonium

from the name of the planet Pluto, because Pluto follows Neptune as plutonium follows neptunium. Although synthesized and identified for the first time in late 1940, the need for secrecy meant that no name was chosen until 1942. The report naming the element, written in March, was not declassified until 1948http://www.osti.gov/accomplishments/pdf/de91000226/de91000226.pdf. The name plutonium was revealed in August 1945, , with the publication of the 'Smyth Report'.