Backgrounder on Plutonium
Plutonium is a radioactive, metallic element with the atomic number 94. Created when uranium atoms absorb neutrons, it was discovered in 1940 at the University of California, Berkeley. Plutonium was made during World War II for use in atomic bombs. It can also fuel nuclear reactors.
Nearly all plutonium is man-made. It does occur naturally in very small amounts in a type of ore called pitchblende. Once the main source of uranium and radium, pitchblende contains one part per trillion of natural plutonium.
Plutonium emits alpha particles—a type of radiation that does not penetrate and has a short range so it is easy to contain. But plutonium can be long-lived. It can deposit in the bones and lungs, and could increase an individual's cancer risk. Therefore, exposure limits are set very low.
In spite of its potential dangers, plutonium has several unique properties that make it useful. It gives off heat and has been used in generators that power space program instruments and heart pacemakers.
Isotope and Half-Lives
There are 15 different forms, or isotopes, of plutonium. 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. Pu-239 and Pu-241 are the most abundant of the fissionable isotopes of plutonium.
The different isotopes of plutonium 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.
When radioactive isotopes decay, they transform. They might become different plutonium isotopes or different elements, such as uranium or neptunium.
Source of Plutonium
Today's light water reactors—used to make commercial power in the United States—create plutonium when the uranium in their fuel fissions. Some of the neutrons released by uranium interact with other uranium atoms to form plutonium. 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 before the fuel is removed from the reactor remains in the spent fuel. Some countries reprocess spent fuel to extract plutonium and uranium, which can be mixed and made into new nuclear fuel.
Plutonium in the Body
Most people will never come into contact with plutonium. But for scientists and engineers that have worked with it, it is important to know about the potential health effects. The most common form of plutonium is plutonium oxide—a compound of plutonium bonded with oxygen. In this form, plutonium does not easily dissolve. Plutonium oxide's impact on human health depends on how it enters the body. If someone eats or drinks it, a large percentage will be eliminated rapidly as waste. If plutonium oxide is inhaled, some of it remains in the lungs. Between 20 and 60 percent can remain, depending upon the size of the particles and other factors. The rest is eliminated from the body within several days. Of the plutonium remaining in the lungs, about half will be removed each year. Some is excreted, some lodges in the lymph nodes, and a very small amount migrates, settling in other organs but mainly in bones. If plutonium oxide gets into an open wound, it may move directly into bones and other organs, mainly the liver.
The next most common form is plutonium nitrate—a compound of plutonium, oxygen and nitrogen. This chemical dissolves somewhat more readily than the oxide. Plutonium nitrate behaves much like plutonium oxide in the body but moves out of the lungs more rapidly.
Research shows plutonium can be chemically toxic and contribute to tumor development. But the most common form of plutonium has more radiological than chemical toxicity.
Production and Disposition
Over 1,600 metric tons of plutonium has been produced worldwide, some for weapons use and most of the rest in electricity production. A commercial power reactor creates plutonium in its many isotopic forms, 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 (over 90 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.
With the end of the Cold War, the United States and the former Soviet Union began dismantling thousands of nuclear weapons. This work created a surplus of high enriched uranium and plutonium. To keep the surplus from falling into the wrong hands, the U.S. and Russia agreed to mix their plutonium with uranium to make mixed oxide (MOX) fuel for power reactors. Irradiating the "weapons-grade" plutonium in a commercial reactor makes it "reactor-grade" and no longer useful for weapons. There would be no reprocessing or subsequent reuse of the MOX spent fuel. It would be disposed in a deep geologic repository along with other high-level nuclear waste.