John W. Poston, Sr. is head of the nuclear engineering department at Texas A&M University and is a Fellow of the American Nuclear Society. He offers the following explanation.

Conventional wisdom tells us that plutonium (Pu) does not exist in nature. Plutonium and other so-called transuranic elements are considered by most to be man-made elements. Thus, they assume that when plutonium is found in the environment, human technology has put it there.

This element has usually been considered synthetic because it is produced most efficiently in nuclear reactors. But in the strictest sense, the answer to the question is yes, plutonium does occur naturally. Plutonium appears at very low concentrations in nature, on the order of one part in 1011 in pitchblende, the ore of uranium (U).

 Image: Seaborg Center GLENN T. SEABORG and his colleague Edwin M. McMillan won the 1951 Nobel Prize in Chemistry for their discovery of plutonium and other transuranic elements.

The element plutonium was discovered by Nobel Laureate Glenn T. Seaborg and his colleagues in February 1941. It was the second transuranic element to be discovered; neptunium (Np) was identified in 1940. The 60-inch cyclotron at University of California at Berkeley produced the first isotope of plutonium, Pu-238. It was made by bombarding a U-238 target with deuterons, producing Np-238. This material had a brief radioactive half-life--the time required for half of the atoms in a sample to decay or transform--of 2.12 days. The radionuclide Np-238 decayed (by emitting beta-radiation) to Pu-238, which has a half-life of 87.7 years.

The isotope Pu-239 was produced on March 28, 1941 by bombarding a U-238 target with neutrons to produce U-239 (half-life of 23.5 minutes). This radionuclide decayed by beta emission to Np-239 (half-life of 2.12 days), which subsequently decayed by beta emission to Pu-239 (which has a very long half-life of 24,600 years).

Plutonium is produced in nature through the reasonably well-understood process discussed above. Uranium is a naturally occurring element that is ubiquitous in the Earth's crust. The isotopes of uranium decay primarily by alpha-particle emission, but there is also a process called "spontaneous fission" that occasionally competes with alpha decay.

In spontaneous fission, the nucleus splits ("fissions") and additional neutrons are released. There is a possibility that these released neutrons are absorbed (captured) by another U-238 nucleus. If this occurs, it triggers a process that produces Pu-239 in a manner similar to that discussed above. Thus, we have plutonium produced naturally in the environment (admittedly in trace quantities). This reaction has been going on since the creation of the Earth.

In 1971, Darlene Hoffman of Los Alamos National Laboratory discovered trace quantities of another isotope of plutonium in the environment. Pu-244 was found in Precambrian Age phosphate from southern California. This isotope of plutonium had a radioactive half-life of 80 million years. Scientists have postulated that, because of its long radioactive half-life, this isotope has existed since the creation of Earth about 4.5 billion years ago.

Some scientists also suspect that there was another isotope of plutonium (Pu-241) in the Earth's crust at its formation. The reasons are that the radioactive progeny (daughter radionuclides) of Pu-241 exhibit some similarities properties to those of U-235, U-238 and Thorium-232. But whereas the three other radionuclides have very long half-lives on the order of the age of the Earth, Pu-241 has a short half-life of about 13 years. Thus, it would have decayed away long before scientists could have discovered it.

Currently, most of the plutonium found in the Earth's environment results from human activities--in particular, the testing of nuclear weapons in the atmosphere. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has estimated that early above-ground nuclear weapons tests released about three tons of Pu-239/240 into the atmosphere; nearly 80 percent of this atmospheric plutonium is in the northern hemisphere.

The practice of testing nuclear weapons above ground has been essentially stopped but there is still some plutonium in the environment, including the upper atmosphere. In addition, there are also small (but measurable) quantities of plutonium near a few nuclear facilities in the U.S. and in Russia.

Thus, contrary to popular belief, plutonium does occur naturally in the environment and is not solely a manmade material. According to Glenn Seaborg, perhaps we should rethink the number of naturally occurring elements and recognize that rather than 92, there are really 94 such elements.

Gregory A. Lyzenga, a physicist at Harvey Mudd College in Claremont, Calif., adds this additional information.

Plutonium does occur naturally, but at very low concentrations. Indeed, it is all but unobservable, except by very sensitive modern analytical techniques. The reason that plutonium (and other transuranic elements) are so rare in nature is that being radioactive, they decay with a characteristic half-life.

All of the heavy elements making up the Earth were formed in nuclear reactions during supernova explosions, occurring millions or even billions of years before the Earth formed. Any element formed at that time with a half-life much less than the Earth's age--or 4.5 billion year--has nearly all decayed into lighter elements by now.

In addition to the few atoms of plutonium that may have survived since the Earth's formation, a very small, steady inventory of such unstable elements is also maintained in the environment by naturally occurring nuclear reactions (for example, those involving cosmic rays). Even so, the levels are very small