What is Plutonium?
Plutonium (Symbol: **Pu**) is a radioactive chemical element with atomic number **94**. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and it is the element with the highest atomic number to be found in nature in more than trace amounts.
"For the first time in the history of the world, a new element was created by man."
- Glenn T. Seaborg, on the discovery of Plutonium
Key Properties
Plutonium is a chemically complex and fascinating element. Unlike most metals, it is not a good conductor of heat or electricity. It also has six common solid allotropes (different crystal structures) with very similar energy levels, but with significantly varying densities and crystal structures. This makes it highly sensitive to changes in temperature, pressure, or chemistry.
| Atomic Number | 94 |
| Atomic Symbol | Pu |
| Element Category | Actinide |
| Atomic Weight | [244] (for most stable isotope) |
| Density (α-phase) | 19.816 g/cm³ |
| Melting Point | 640 °C (1184 °F) |
| Boiling Point | 3228 °C (5842 °F) |
Isotopes
Plutonium has 20 known isotopes, all of which are radioactive. The two most significant are:
- Plutonium-239 (Pu-239): The primary fissile isotope used for nuclear weapons. It is produced in nuclear reactors from Uranium-238. It has a half-life of 24,110 years.
- Plutonium-238 (Pu-238): A powerful alpha emitter with a much shorter half-life of 87.7 years. It generates significant heat as it decays, making it ideal for use in Radioisotope Thermoelectric Generators (RTGs).
History & Discovery
Plutonium was first produced and isolated on December 14, 1940, by Dr. Glenn T. Seaborg, Joseph W. Kennedy, Edwin M. McMillan, and Arthur C. Wahl at the University of California, Berkeley. They bombarded Uranium-238 with deuterons, creating Neptunium-238, which then beta-decayed into Plutonium-238.
Its discovery was kept secret due to the ongoing World War II. Its potential as a fissile material for a nuclear weapon was quickly recognized, leading to the establishment of the Manhattan Project. The Hanford Site in Washington was built to produce Pu-239 on an industrial scale, which was ultimately used in the "Fat Man" device for the Trinity test and the bombing of Nagasaki in 1945.
Applications: The Duality
Plutonium's unique properties have led to its use in two vastly different fields: deep-space exploration and nuclear applications (both energy and military).
1. Deep Space Exploration (Pu-238)
The heat generated by the decay of **Plutonium-238** is the power source for **Radioisotope Thermoelectric Generators (RTGs)**. These are essentially nuclear batteries that have no moving parts and can provide stable electrical power for decades.
RTGs are essential for missions where solar power is not feasible (e.g., deep space, or shadowed craters on the Moon or Mars).
- Voyager 1 & 2: Launched in 1977 and still operating, powered by RTGs.
- Cassini Mission: Explored Saturn and its moons.
- New Horizons: Flew past Pluto in 2015.
- Mars Rovers: The *Curiosity* and *Perseverance* rovers both use RTGs to power their instruments and stay warm during the Martian nights.
2. Nuclear Energy (Pu-239)
**Plutonium-239** is a fissile material and can sustain a nuclear chain reaction. It is produced as a byproduct in typical uranium-fueled nuclear reactors. This "reactor-grade" plutonium can be reprocessed and combined with uranium to create **Mixed Oxide (MOX) fuel**, allowing more energy to be extracted from the original uranium fuel and reducing the volume of high-level waste.
3. Military Use (Pu-239)
"Weapons-grade" plutonium, which has a very high concentration of Pu-239, is a primary material for modern nuclear weapons. Its ability to achieve supercriticality with a smaller mass compared to uranium made it the material of choice for more compact and efficient warhead designs.
Safety & Environment
Toxicity
Plutonium is toxic both chemically (as a heavy metal) and radiologically. The primary hazard is from **inhalation** of fine plutonium-containing particles.
- As an alpha emitter, plutonium is not dangerous *outside* the body, as the alpha particles are stopped by the skin.
- If inhaled, these particles can lodge in the lungs and irradiate surrounding tissue over many years, significantly increasing cancer risk. Ingestion is a lesser hazard, as very little is absorbed by the body.
Handling & Waste
Because of its high radioactivity and potential for criticality (in large enough amounts of Pu-239), plutonium must be handled in sealed, controlled environments like **gloveboxes**. The long half-life of its isotopes (e.g., Pu-239's 24,110 years) means that nuclear waste containing plutonium must be securely isolated from the environment for geological timescales, a major scientific and engineering challenge.