Plutonium
Plutonium is the heaviest primordial element by virtue of its most stable isotope, plutonium-244, whose half-life of about 80 million years is just long enough for the element to be found in trace quantities in nature.Plutonium is mostly a byproduct of nuclear reactions in reactors where some of the neutrons released by the fission process convert uranium-238 nuclei into plutonium.
Both plutonium-239 and plutonium-241 are fissile, meaning that they can sustain a nuclear chain reaction, leading to applications in nuclear weapons and nuclear reactors. Plutonium-240 exhibits a high rate of spontaneous fission, raising the neutron flux of any sample containing it. The presence of plutonium-240 limits a sample's usability for weapons or reactor fuel, and determines its grade.
Plutonium-238 has a half-life of 88 years and emits alpha particles. It is a heat source in radioisotope thermoelectric generators, which are used to power some spacecraft. Plutonium isotopes are expensive and inconvenient to separate, so particular isotopes are usually manufactured in specialized reactors.
A team led by Glenn T. Seaborg and Edwin McMillan at the University of California, Berkeley laboratory first synthesized plutonium in 1940 by bombarding uranium-238 with deuterons. Trace amounts of plutonium were subsequently discovered in nature. Producing plutonium in useful quantities for the first time was a major part of the Manhattan Project during World War II, which developed the first atomic bombs. The first nuclear test, "Trinity" (July 1945), and the second atomic bomb used to destroy a city (Nagasaki, Japan, in August 1945), "Fat Man", both had cores of plutonium-239. Human radiation experiments studying plutonium were conducted without informed consent, and a number of criticality accidents, some lethal, occurred during and after the war. Disposal of plutonium waste from nuclear power plants and dismantled nuclear weapons built during the Cold War is a nuclear-proliferation and environmental concern. Other sources of plutonium in the environment are fallout from numerous above-ground nuclear tests (now banned).
Occurrence
Trace amounts of at least three plutonium isotopes (plutonium-238, 239, and 244) can be found in nature. Small traces of plutonium-239, a few parts per trillion, and its decay products are naturally found in some concentrated ores of uranium, such as the natural nuclear fission reactor in Oklo, Gabon. The ratio of plutonium-239 to uranium at the Cigar Lake Mine uranium deposit ranges from 2.4 × 10−12 to 44 × 10−12. Even smaller amounts of primordial plutonium-244 occur naturally due to its relatively long half-life of about 80 million years. These trace amounts of 239Pu originate in the following fashion: On rare occasions, 238U undergoes spontaneous fission, and in the process, the nucleus emits one or two free neutrons with some kinetic energy. When one of these neutrons strikes the nucleus of another 238U atom, it is absorbed by the atom, which becomes 239U. With a relatively short half-life, U-239 decays to neptunium-239 (239Np), and then 239Np decays into 239Pu.
Since the relatively long-lived isotope plutonium-240 occurs in the decay chain of plutonium-244 it should also be present, albeit 10,000 times rarer still. Finally, exceedingly small amounts of plutonium-238, attributed to the incredibly rare double beta decay of uranium-238, have been found in natural uranium samples.
Minute traces of plutonium are usually found in the human body due to the 550 atmospheric and underwater nuclear tests that have been carried out, and to a small number of major nuclear accidents. Most atmospheric and underwater nuclear testing was stopped by the Limited Test Ban Treaty in 1963, which was signed and ratified by the United States, the United Kingdom, the Soviet Union, and other nations. Continued atmospheric nuclear weapons testing since 1963 by non-treaty nations included those by China (atomic bomb test above the Gobi Desert in 1964, hydrogen bomb test in 1967, and follow-on tests), and France (tests as recently as the 1980s). Because it is deliberately manufactured for nuclear weapons and nuclear reactors, plutonium-239 is the most abundant isotope of plutonium by far.
| Symbol | Pu | |
| Atomic Number | 94 | |
| Atomic Weight | 244.0642 | |
| Oxidation States | +3, +4, +5, +6 | |
| Electronegativity, Pauling | 1.3 | |
| State at RT | Solid, Metal | |
| Melting Point, K | 914 | |
| Boiling Point, K | 3505 |
Appearance and Characteristics
Harmful effects:
Plutonium is harmful due to its radioactivity.
Plutonium and its compounds are also toxic. It collects in the bones and the liver where it can remain for a long period of time.
Characteristics:
- Plutonium is a silvery radioactive metal that tarnishes in air to give a yellow oxide coating.
- It has six allotropic forms, which vary widely in crystal structure and density.
- The metal is chemically reactive, forming compounds with carbon, nitrogen, and silicon and the halogens.
- Plutonium has five oxidation states (+3 to +7). These produce different colors in solution. For example, in 1 M perchlorate:
III: Pu3+ (blue lavender)
IV: Pu4+ (yellow brown)
V: PuO2+ (pink) (in sodium perchlorate)
VI: PuO22+(yellow)
VII: PuO52+ (olive green) (in sodium hydroxide).
- If you were to touch a small piece of plutonium metal (please don’t!) it would feel warm because of the energy released by alpha decay. A larger piece of the metal could boil water.
Uses of Plutonium
- Plutonium-239, which can undergo nuclear chain reactions, is used in nuclear bombs and nuclear reactors
- Plutonium-238 is used as a long-lived heat and power source for space probes. (Its intrinsic heat output is approximately 0.5 watts per gram.) The Pioneer and Voyager space probes used plutonium-238 nuclear batteries as a power source.
- Three radioisotope heater units (each containing 2.7 grams of plutonium-238 dioxide) were used as heat sources on the Pathfinder Mars robot lander. Each radioisotope heater unit produces about one watt of heat.
- Early pacemaker batteries also used tiny amounts of plutonium-238.
- The image on the left shows the decay of one atom of plutonium-238. This releases 5.6 million electron volts of energy. To get an idea of what this means, consider NASA’s Curiosity Mars rover, which will be powered by 4.8 kg of plutonium dioxide.
- During its first 87.7 year half-life, the plutonium will produce about 4800 gigajoules of energy. To generate the same energy using natural gas (mainly methane) the Mars rover would need to carry about 86 metric tons of methane and 345 metric tons of oxygen.
The separation of uranium from plutonium.
