More than forty years of the Cold War left the world with huge nuclear warheads arsenals. Today scientists have started to work on ways to dispose of accumulated nuclear weapons. High-enriched uranium (HEU) can be disposed of by downblending it to an original state. As for weapon-grade plutonium the situation is much more complicated. It cannot be returned to its natural state, because it was artificially created by man using a nuclear reactor. After careful consideration, ten years ago the United States and Russia agreed about the way plutonium will be disposed of. When added to nuclear reactor fuel, plutonium could be burned to a non-usable as a weapon condition. The full scale disposal of plutonium, which was born in 1940, is expected to start in 2018.
Plutonium is well known for its use in nuclear warheads. A less known fact about plutonium, which makes it truly unique, is that this transuranic element cannot be found in any significant quantities in nature, only traces. In quantities necessary for military or scientific use plutonium must be made in a so-called “breeder” nuclear reactor. The breeder reactor produces or breeds plutonium one atomic nucleus at a time.
The discovery of plutonium is associated with two names: Glenn Seaborg (1912 -1999) and Edwin McMillan (1907-1991). Seaborg was the scientist who actually discovered plutonium. But McMillan was the first to predict the discovery of plutonium and map out the way to create it. McMillan’s certainty was based on his previous success with the discovery of another transuranic element neptunium and the knowledge of the periodic table of chemical elements. In the periodic table he could see uranium, then neptunium, then empty cell for non-discovered yet transuranic element. He knew that new discovery was very close, but that was another person who made it happened.
But let’s start the story about plutonium from the very beginning. Russian chemist Dmitri Mendeleev (1834-1907) could be considered as the godfather of all chemical elements discovered in second half of 19th and 20th centuries. When he made his discovery of the periodic table of the chemical elements in 1869, the amazing tool had been given to scientists around the world. In the periodic table chemical elements were arranged in periods and groups, where specific trends between elements could be established. As a direct result chemists and physicists received the ability to predict properties of elements even before they had been discovered. This ability was successfully used to artificially create transuranic elements neptunium and plutonium, which follow in the periodic table after uranium.
The knowledge of the periodic table of the chemical elements alone was not enough to create grounds for discovery of plutonium. Some more scientific findings were needed. They started to come in 1938, when Otto Hahn (1879-1968) and Fritz Strassman (1902-1980) from Germany reported that in their experiment after uranium was bombarded with neutrons they unexpectedly found another chemical element – barium. They were not sure how that happened, and where the barium came from. Other scientists Lise Meitner (1878-1968) and Otto R. Frisch (1904-1979) explained the result of the mentioned above experiment as nuclear fission. Shortly after that a chain reaction of nuclear fission was discovered in April 1939 by Frederic Joliot-Curie (1900-1958). This last discovery was critical for future creation of plutonium. Let’s take a closer look why.
Nuclear fission could be described as a metamorphosis. When an atom of uranium-235 absorbs a free neutron, it transforms into a very unstable or, in another words, highly excited (sometimes even scientific term sounds very clearly) atom of uranium -236, which breaks apart into fragments. The fragments are not uranium anymore. They are newly created barium-141 and krypton-92. During this process of transmutation large amounts of energy are released as radiation and kinetic energy. At the same time three neutrons are freed. These free neutrons in turn continue chain reaction of new fissions by bombarding other atoms of uranium-235 and being absorbed. More and more energy is released. This is where the power of a nuclear bomb comes from, which makes it thousands of times more explosive than a comparable mass of conventional chemical explosive.
Luckily for mankind natural uranium contains less than 1% of uranium-235 and more than 99% of uranium-238, which does not support chain reaction. Otherwise every country with uranium ore deposits would be a nuclear power. The process of uranium enrichment is both time consuming and challenging. Uranium-238 and uranium-235 cannot be separated based on their chemical qualities, which are identical. The separation can be done gradually based on small mass differences. Repetition of the same process of enrichment makes processed uranium with slightly higher concentration of uranium-235. When the desired level of enrichment is achieved, the process is stopped. First military use of high-enriched uranium took place during the Second World War. The uranium-235 filled bomb “Little Boy” was dropped on the Japanese city of Hiroshima on August 6, 1945.
Use for uranium-238 was found by the scientific team led by Glenn Seaborg and Edwin McMillan in 1940, when they were bombarding uranium with particles of heavy hydrogen in the cyclotron at Berkeley, California. McMillan was the first to identify the new chemical element neptunium as a result of fission of uranium-238. He also predicted the future discovery of another chemical element – plutonium. Very soon, but without McMillan, who was moved to work on another project, Seaborg’s team was able to locate plutonium as another product of fission of uranium-238.
Here is how it works. After absorbing a neutron uranium-238 transforms into uranium-239 with a half-life of 23 minutes, which decays emitting an electron to become another radioactive element neptunium-239. This new element does not have a long life either (half-life 56 hours), and it decays to plutonium-239. The final element has the traits desired by scientists for its ability to sustain a chain reaction of nuclear fission, which makes it usable as a weapon. The first nuclear-weapons test “Trinity” was conducted on July 16, 1945. The testing device was loaded with plutonium. The second atomic bomb “Fat Man”, which was dropped on the Japanese city of Nagasaki on August 9, 1945, also used plutonium.
The world has been living with plutonium long enough. For seventy years everybody, even people who has no idea about an existence of plutonium, has lived under the threat of nuclear war. Finally in September 2000 after long negotiations the United States and Russia agreed to begin dispose of plutonium. Both countries signed the Plutonium Management and Disposition Agreement. According to this Agreement, a total of 68 metric tons of weapon-grade plutonium will be disposed of starting in 2018. What does it mean for us? For the first time after seventy years since its discovery plutonium from 17,000 nuclear warheads waits for its disposal. Today the world has a better chance to survive. We are eight years away from the beginning of plutonium disposal.
1. National Nuclear Security Administration. Plutonium Disposition
2. The Institute for Energy and Environmental Research: Uranium: Its Uses and Hazards
3. Federation of American Scientists: Plutonium Production
4. Chemical Achievers: Glenn Theodore Seaborg
5. Wikipedia: Dmitri Mendeleev
6. Wikipedia: Periodic Table
7. Wikipedia: The Manhattan Project
8. Wikipedia: Nuclear Fission
9. Wikipedia: Enriched Uranium
10. Wikipedia: Trinity (nuclear-weapon test).