plutonium

Concise Oxford English Dictionary © 2008 Oxford University Press:
plutonium/pluːˈtəʊniəm/

▶noun the chemical element of atomic number 94, a dense silvery radioactive metal of the actinide series, used as a fuel in nuclear reactors and as an explosive in nuclear fission weapons. (Symbol: Pu)

– origin 1940s: from Pluto, on the pattern of neptunium.