Plutonium is a radioactive actinide and the element used in modern nuclear weapons. Plutonium-238 is an alpha emitter with a 87 year half-life, making it an excellent choice for radioisotope thermoelectric generators, the nuclear power plants that power long distance spacecraft.
Almost all plutonium is manufactured synthetically by bombarding uranium-238 with neutrons in a nuclear reactor. Thus it could probably be indirectly invested in by investing in uranium.
Plutonium News 2010-04-14 - (wp) - A summit goal: Making plutonium and uranium harder to get
2010-04-09 - (nyt) - Disposal of plutonium from U.S.-Russian disarmament is likely to take decades
