Fissile Material

Substances whose nuclei can be split to produce a nuclear fission reaction. Fissile materials are the fuels that supply the power generated by nuclear weapons and nuclear reactors. The most commonly used fissile materials are forms of uranium and plutonium.

Uranium

Uranium is a radioactive metallic element that is deposited in rocky ores and found in very small amounts in plants and animals. Although it is softer than steel and easily molded into different shapes, uranium is [Page 266]extremely dense. It was first discovered by the German chemist Martin Heinrich Klaproth in 1789 and named after the recently discovered planet Uranus. Over 100 years later, in 1896, French physicist Henri Becquerel found that uranium was radioactive. In fact, it was Becquerel's work with uranium that led him to discover the phenomenon of radioactivity. The radioactive and fissile properties of uranium led to its use in the world's first atomic bomb.

Naturally occurring uranium is found in three main forms or isotopes (an isotope is an atom with an electromagnetic charge). The most abundant is U-238, which makes up more than 99% of all natural uranium; the other isotopes are U-235 and U-234. Uranium isotopes can be separated from one another in a process called enrichment. This increases the concentration of one isotope relative to the others. Enriched uranium is the most widely used fuel in nuclear power plants. Reactor-grade enriched uranium contains 3% to 7% U235, compared to less than 1% in naturally occurring uranium. Weapons-grade uranium, which is used in nuclear weapons, contains more than 90% U-235.

Plutonium

Like uranium, plutonium is a metallic element that is pliable but extremely dense; plutonium is slightly denser than uranium. It is a much more “recent” element than uranium, having been discovered in 1940 by a team of U.S. scientists at the University of California, Berkeley. The researchers bombarded uranium atoms with a heavy isotope of hydrogen called deuterium, and the reaction produced a radioactive substance they called plutonium. The newly discovered element proved critical to the Manhattan Project, which was launched by the United States at about this time to develop the world's first atomic bomb. The U.S. government built large nuclear reactors to produce plutonium, which fueled one of the two bombs developed by the Manhattan Project. All of the plutonium used in nuclear reactors and weapons is produced in such reactors, called breeders.

The main isotopes of plutonium are P-238 and P-239. Both are extremely radioactive and small amounts of either actually feel warm to the touch; slightly larger amounts can boil water. The P-238 isotope is often used as a power source for space probes because a very small amount can generate electricity for many years. Before the development of lithium-based heart pacemakers in 2003, P-238 was commonly used for that purpose. In contrast to these peaceful applications, the P-239 isotope has become the element most commonly used in nuclear weapons. Early tests showed that plutonium-based nuclear devices could produce more powerful explosions than uranium-based ones. Like uranium, plutonium must be enriched before it can be used in nuclear weapons. Weapons-grade plutonium consists of about 90% P-239 isotopes.

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