Plutonium is a radioactive metallic element. Although it is occasionally found in nature, mostly all of our plutonium is produced artificially in a lab. The official chemical symbol for plutonium is Pu, coming from its first and third letters. Its atomic number is ninety-four. Plutonium is able to maintain its solid state until very high temperatures, melting at six hundred and forty degrees Celsius, and boiling at three thousand four hundred and sixty degrees. The density of Plutonium, at twenty degrees centigrade, is 19.86 grams per cubic centimeter.
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Plutonium was discovered, in the laboratory, by Glenn Theodore Seaborg, and his associate Edward M. McMillan. The two shared the Nobel prize in 1951 for their discoveries of Plutonium, Americium (Am), Curium (Cm), Berkelium (Bk), and Californium (Cf). In addition, Seaborg later contributed to the discovery of three more radioactive elements, Einsteinium (Es), Mendelevium (Md), and Nobelium (No). Plutonium was Seaborg’s first discovery. Its name came from Pluto, the planet after Neptune for which Neptunium was named. In 1940, at the University of California at Berkeley, he bombarded a sample of Uranium with deuterons, the nuclei in atoms of deuterium, transmuting it into plutonium. Shortly after, Seaborg was able to isolate plutonium 239, an isotope used in atomic bombs.
Plutonium is a highly dangerous and poisonous element because it rapidly gives off radiation in the form of alpha particles. Alpha particles, which are identical to the nucleus of a helium atom, consist of two protons and two neutrons tightly bound together. Although the particles can only travel about five centimeters in the air, they can cause great damage when they enter the body, causing cancer and other serious health problems. Beyond the danger of their radiation, Plutonium will spontaneously explode when a certain amount, called critical mass, is kept together. Soon after the discovery of Plutonium, it was discovered that at least two oxidation states existed. It is now known to exist in oxidation states of +3, +4, +5, and +6.
Currently, there are fifteen known isotopes of Plutonium, with mass numbers ranging between 232 and 246. The most important isotope is plutonium 239, or Pu-239. When struck by a neutron, this isotope undergoes a process called fission. In fission, When struck by a neutron, the nucleus of the plutonium atom is split into two nearly equal parts, and energy is released. Although the energy released by one atom is not much, the splitting of the nucleus releases more neutrons, which strike more plutonium atoms. This process, called a chain-reaction, produces enormous amounts of energy. This energy is often used to power nuclear reactors, or to provide the energy for nuclear weapons. Although Pu-239 is such an efficient use for energy, disposing of its waste has become a major problem. When uranium is converted to Pu-239, a waste with a half-life of around 24,100 years is produced.
Another large problem for scientists creating power with plutonium is actually getting the chain-reaction to work. Often, only the first few atoms struck by the deuterons convert to Plutonium. Unfortunately for the scientists, the whole problem is a matter of probabilities and chance. There are four factors that determine whether the reaction occurs. They are
1) escape, 2) non-fission capture by uranium, 3) non-fission capture by impurities, and 4) fission capture. The first three factors cause the uranium to lose neutrons, the last is what causes the reaction. If the loss of neutrons is less than that of those produced, by fission capture, the reaction occurs. Otherwise, plutonium is not made, and the chain-reaction stops immediately.
Using the chain-reaction system, the first operating nuclear reactor of a reasonable amount of power was built in 1943. It was called the X-10 reactor. The core of the reactor was a twenty- four foot cube of graphite blocks, with 1248 fuel channels each 1.75 inches square. Each hole was fueled with four inch long uranium rod, jacketed in aluminum to protect against oxidation. The entire core was surrounded by a seven-foot thick concrete shield, with openings at one end to replace the uranium rods. At a cost of $1,000,000 for the building and $2,000,000 each for the graphite and uranium, this plant produce about 190 Mevs per fission.
In addition to its uses as fuel for a reactor or in a bomb, plutonium has some practical, everyday uses as well. For example, the original plutonium isotope, Pu-238, is used today to power pacemakers for people with deficient hearts. Also, isotopes Pu-242 and Pu-244, which occurs naturally, are used in studying chemicals and metals.
The half-life of atoms of plutonium was very important to seaborg and his assistants back in 1940. In fact, all of his other radioactive discoveries were based on the finding of Pu-238. For example, Pu-241 decays with a half-life of about thirteen years emitting negatively charged beta particles, or electrons. It then converts to Am-241, an isotope of americium, which emits alpha particles for 470 years, before turning into Am-242, which converts to Cm-242, an isotope of curium, in only sixteen hours. The Cm-242 emits alpha particles for about 162 days before ending the decay of Plutonium 241.
Chemical Equation for Producing Plutonium:
92U-238 —-> 92U-239 —-> 93Np-239 —-> 94Pu-239