We continue to review some of the most important materials in heat treatment and metallurgy.
Plutonium (chemical symbol: Pu)
Plutonium (Fig. 1) is a hard, brittle, silvery metal that is highly radioactive and will quickly oxidize in air to a dull gray, yellowish color. It is the element with the highest atomic number to occur naturally on Earth, albeit in trace amounts. Small-sized chunks of the element will feel warm to the touch due to the energy it emits from alpha decay, and large-sized pieces will produce enough heat to cause water to boil. At room temperature, plutonium is as hard and as brittle as cast iron. It is a poor conductor of both heat and electricity. It is chemically reactive and will form compounds with carbon, nitrogen and silicon, as well as the halogen elements. Plutonium has only two isotopes that see use outside of basic research: Pu-238, with a half-life of 87.7 years; and Pu-239, with a half-life of 24,110 years.
Plutonium is the second transuranium element of the actinide series of elements to be discovered. It was first produced and isolated in 1940 by deuteron bombardment of uranium-238 at UC Berkeley by Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl. The team of scientists had first discovered neptunium-238, with a half-life of 2.1 days, which itself decayed into plutonium-238. It wasn’t until a year later that the much longer-lived Pu-239 was discovered. Shortly after its discovery, it was found that bombarding Pu-239 with slow neutrons caused it to undergo fission, which then releases more neutrons and a nuclear chain reaction that laid the groundwork for using plutonium as a source of nuclear energy.
Plutonium does not see much use other than for a few very specific purposes. Pu-238 sees use to generate energy for space probes, such as Cassini (Fig. 2) and Galileo, using radioisotope thermoelectric generators. When these probes move too far away from the sun to make use of solar power, they turn on these generators to create power to continue functioning into the deep reaches of space. Since Pu-239 undergoes a fission chain reaction, it can be utilized in some nuclear reactors and nuclear weapons. In fact, energy generation remains one of the its largest uses. It is estimated that one-third of all energy produced in nuclear power plants comes from plutonium (Fig. 3).
Here are a few important facts about plutonium.2
- Atomic number: 94
- Atomic weight: 244
- Melting point: 913 K (640°C or 1184°F)
- Boiling point: 3501 K (3228°C or 5842°F)
- Density: 19.84 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 7
- Group number: none
- Group name: Actinide
- Electron configuration: [Rn] 5f67s2