█ LARRY GILMAN
Uranium is a radioactive, metallic element with 92 protons and a variable number of neutrons in the nucleus of each atom. There are 16 isotopes of uranium, the most common being uranium-238 ( 238 U). The second-commonest isotope of uranium, 235 U, is used for building nuclear weapons, generating electricity, and propelling some submarines, aircraft carriers, and other vessels. Heat released by uranium decay also keeps Earth's interior hot, providing the energy for continental drift and volcanic eruptions.
Uranium was discovered in 1789 by German chemist Martin Heinrich Klaproth (1743–1817), and its property of radioactivity was discovered by French physicist Henri Becquerel (1852–1908) in 1896. 235 U was first isolated in kilogram quantities by the United States during World War II, and was used in war by the United States in the bomb that destroyed the city of Hiroshima, Japan in 1945. Since that time uranium has been mined in many countries and purified in large quantities for both bombs and fuel. Worldwide, several hundred nuclear reactors produce electricity from uranium, while tens of thousands of nuclear weapons (mostly held by the United States and the Russian Federation) rely on uranium either as their primary explosive (in fission bombs) or as a trigger explosive (in fusion bombs).
Uranium atoms are unstable; that is, their nuclei tend spontaneously to fission or break down into smaller nuclei, fast particles (including neutrons), and high-energy photons. The fission of an isolated uranium nucleus is a randomly timed event; however, collision with a neutron may trigger a uranium nucleus to fission immediately. Crowding large numbers of uranium atoms together can enable the neutrons emitted by a few nuclei undergoing fission to cause other nuclei to fission, whose released neutrons in turn trigger still other nuclei, and so on. If this chain reaction proceeds at a constant rate, it may be used to generate electricity; if it proceeds at an exponentially increasing rate, a nuclear explosion results.
Only 0.71% of natural uranium is 235 U, the major isotope directly useful for nuclear power and weapons. Many tons of ore must therefore be refined to produce a single kilogram of 235 U. The amount of 235 U needed to make a bomb, however, is not great: about 15 lb (7 kg). Quantities of uranium sufficient for many thousands of bombs are thus available around the world; some 21 countries export uranium, with Canada, Australia, and Niger being the three largest producers.
The most common isotope of uranium, 238 U, comprises 99.28% of the uranium in the Earth's crust. 238 U is comparatively stable, with a half-life of 4.5 billion years, and so is not directly useful for power and nuclear weapons. It is added to some antitank and antiaircraft ammunition to increase their density and thus their penetrating power. Depleted-uranium munitions, as these weapons are termed, were used extensively by the United States during the Gulf War of 1991 and in the Kosovo conflict of 1999. Because of their slight radioactivity, there is ongoing debate about whether they may cause long-term health problems in areas where they have been used.
238 U is also a major ingredient of most reactor fuel. In reactor cores, this 238 U is bombarded by neutrons, which transmute some of it into the element plutonium. Plutonium can be used directly for power and weapons; the first and third nuclear weapons ever exploded were produced by the United States using plutonium transmuted from 238 U, and a number of other countries, including India, Israel, Pakistan, and North Korea, have developed the capability to obtain plutonium for bombs by the same means.
Both 235 U and plutonium must be in fairly concentrated form for use in bomb manufacture. Alloys that have been diluted by 238 U or other substances result in bulkier explosive devices; at sufficiently great dilution, a nuclear explosion is not obtainable. (However, some experts say that a nuclear explosion might be obtainable from an alloy that is as little as 10% 235 U.) It follows that any organization that wishes to build an atomic weapon must either obtain fairly concentrated 235 U or plutonium by purchase or theft, or obtain them in dilute form and then concentrate them.
These obstacles have been surmounted by a number of governments, and may eventually be surmounted by terrorist organizations. Illegal traffic in weapons-grade 235 U and plutonium has accelerated since the breakup of the Soviet Union in 1991, because its successor states have been too poor and disorganized to keep nuclear material secure. Some 600 tons, or enough for about 40,000 bombs, of raw weapons-grade fissionables are
stored in poorly guarded stockpiles in the Russian Federation and other states; small quantities have already entered the black market. On over 16 occasions since 1993, police in Asia, Europe, or South America have intercepted illegally held bomb-grade uranium or plutonium, most of it from ex-Soviet sources. In 1994, police seized a metal briefcase when a civilian jetliner from Moscow landed in Munich, Germany; the briefcase contained 363.4 grams of weapons-grade plutonium. In April 2000, almost a kilogram of bomb-grade uranium was seized in the Republic of Georgia. In 2001, police in Bogota, Colombia seized some 600 grams of bomb-grade 235 U from the house of an animal feed salesman, the enrichment level of which corresponded to that of Russian fuel for submarines and icebreakers. And on September 11, 2001, four men were arrested in the ex-Soviet republic of Georgia in possession of almost 2 kilograms of bomb-grade 235 U—a large fraction of the amount required for a bomb. Since that day, the idea that stolen uranium might be used for terrorist acts has gained increased attention.
Through its Material Protection, Control, and Accounting Program, the United States has spent about $550 million since 1993 to help safeguard uranium and plutonium stocks in Russia, supplying complete security systems or partial protection for about a third of the material considered most vulnerable by the U.S. Department of Energy.
█ FURTHER READING:
Ladika, Susan. "Tracing the Shadowy Origins of Nuclear Contraband." Science no. 5522 (2001): 1634.
Stone, Richard. "Nuclear Trafficking: 'A Real and Dangerous Threat'." Science no. 5522 (2001): 1632–36.