NRC Science 101: What is Plutonium?

NRC Science 101: What is Plutonium?

4 Comments Posted by Moderator on May 15, 2014

Maureen Conley

Public Affairs Officer

 

In earlier Science 101 posts, we talked about what makes up atoms, chemicals and matter. In this post, we will look at a specific chemical element — plutonium.
Plutonium is a radioactive, metallic element with the atomic number 94. It was discovered in 1940 by scientists studying the process of splitting atoms. Plutonium is created when uranium atoms absorb neutrons. Nearly all plutonium is man-made.
Plutonium emits alpha particles—a type of radiation that does not penetrate and has a short range. It is easy to contain, but plutonium can be long-lived. For the few who may be exposed to it, plutonium can deposit in the bones and lungs, and could increase an individual’s cancer risk. To protect these people, exposure limits are set very low.
In spite of its potential dangers, plutonium has unique properties that make it useful. It gives off heat and has been used in heart pacemakers and generators that power space program instruments. While it was made during World War II for use in atomic bombs, it can also fuel nuclear reactors.
Nuclear reactors that produce commercial power in the United States today create plutonium when their uranium fuel fissions. Some of the plutonium itself fissions—part of the chain reaction of splitting atoms that is the basis of nuclear power. Any plutonium that does not fission stays in the spent fuel.
Plutonium comes in 15 different forms, known as isotopes. Each has a different number of neutrons, so the isotopes are identified by different numbers to indicate their mass. Some isotopes are “fissionable”—which means the atom’s nucleus is unstable and can easily split apart if it is struck by a neutron.
The different isotopes have different “half-lives” – the time it takes for one-half of a radioactive substance to decay. Pu-239 has a half-life of 24,000 years and Pu-241’s half-life is 14.4 years. The plutonium isotope with the shortest half-life, 20 minutes, is Pu-233. Naturally occurring Pu-244 has the longest half-life—80 million years. Substances with shorter half-lives emit stronger radioactive energy, so they decay more quickly than those with longer half-lives.
Like any radioactive isotopes, plutonium isotopes transform when they decay. They might become different plutonium isotopes or different elements, such as uranium or neptunium.
More than 1,600 metric tons of plutonium have been produced worldwide, some for weapons use and most of the rest in electricity production. A commercial power reactor creates many isotopes of plutonium, including Pu-239, Pu-240, Pu-241, and Pu-242. This is known as “reactor-grade” plutonium. In contrast, “weapons-grade” plutonium is almost pure Pu-239 (more than90 percent). This form requires a specially designed and operated reactor. Plutonium production reactors operated by the U.S. government during the Cold War have all shut down.
The NRC is reviewing an application for a facility in South Carolina that could mix plutonium removed from U.S. nuclear weapons with uranium to create mixed oxide (MOX) fuel. By irradiating the MOX fuel in a commercial power reactor, the weapons-grade plutonium becomes reactor-grade and no longer useful for weapons.