Plutonium

Plutonium is a heavy, silvery-gray chemical element with the symbol Pu and atomic number 94.

It belongs to a group of elements called actinides. While it looks bright like nickel when first exposed, it oxidizes very quickly in the air. This process creates a dull gray, yellow, or olive green coating.

Plutonium is also highly radioactive, meaning its atoms naturally release energy. This energy can make a piece of the metal feel warm to the touch. Because it can accumulate in human bones, handling the element is extremely dangerous.

One of the most unusual things about plutonium is its many allotropes. An allotrope is a different structural form of the same element.

At room temperature, plutonium usually exists in its alpha (α) form. This form is hard and brittle, similar to gray cast iron. However, the metal is very sensitive to changes in temperature and pressure. It has six common allotropes at normal pressure, and even a seventh called zeta (ζ) at high temperatures. These forms have very different densities, ranging from 16.00 g/cm³ to 19.86 g/cm³. This makes machining the metal very difficult because it changes state so easily. To make it easier to work with, scientists often alloy it with metals like gallium. This stabilizes the delta (δ) form, which is soft and malleable like aluminum.

Plutonium is also chemically very reactive. It reacts with several substances, including carbon, halogens, nitrogen, silicon, and hydrogen. If the metal is exposed to moist air, it forms oxides and hydrides. This chemical reaction can cause the metal to expand by up to 70% in volume. As it expands, the surface can flake off into a fine powder. This powder is pyrophoric, which means it can catch fire spontaneously.

Scientists first discovered plutonium through a complex process of synthesis. In late 1940 and early 1941, a team at the University of California, Berkeley, used a cyclotron to create it. They bombarded uranium-238 with deuterons to produce neptunium-238. This neptunium then underwent beta decay to become plutonium-238. The team named the element after the planet Pluto, following the naming patterns of uranium and neptunium. Because of the secrecy of World War II, they could not publish their findings until 1948.

There are many different isotopes, or versions, of plutonium.

Some isotopes are fissile, such as plutonium-239 and plutonium-241. Fissile means the nucleus can split when struck by a slow neutron, sustaining a nuclear chain reaction. This process releases massive amounts of energy. For example, fissioning just one kilogram of plutonium-239 produces an explosion equivalent to 17 kilotons of TNT.

Other isotopes, like plutonium-238, are not fissile but emit alpha particles. These particles release heat, which is useful for powering spacecraft through radioisotope thermoelectric generators.

The quality of plutonium is determined by its isotopic composition. Scientists categorize it into different grades based on the percentage of plutonium-240 it contains. Plutonium-240 has a high rate of spontaneous fission, which increases the neutron flux. This high neutron level can cause a weapon to detonate too early. Weapons-grade plutonium contains less than 7% plutonium-240. Fuel-grade contains between 7% and 19%, and reactor-grade contains 19% or more. Supergrade plutonium contains less than 4% and is used in specific Navy weapons.

While most plutonium is manufactured in specialized reactors, it can occur naturally in tiny amounts. In uranium deposits, uranium-238 can capture neutrons to create trace amounts of plutonium. Some scientists believe that heavy isotopes like plutonium-244 might have existed since the Earth was formed. However, current experiments are not yet sensitive enough to detect these primordial traces. Today, the management of plutonium waste from power plants and dismantled weapons is a major environmental and safety concern.