Uranium & Plutonium & Fission April 6, 2011Posted by Lofty Ambitions in Science.
Tags: Nobel Prize, Nuclear Weapons, Physics, Radioactivity
Last week, we wrote about the distinction between radiation and radioactivity. We also want to make some distinctions among the radioactive substances that are involved in the ongoing disaster in Japan. (For the latest official updates, click HERE.) Uranium is the radioactive element used in all the nuclear reactors at the Fukushima Daiichi plant. One reactor uses fuel that is a combination of uranium and another radioactive element, plutonium—a combination sometimes referred to as MOX.
What makes uranium and plutonium so useful for generating energy is their ability to fission. This same physical process is also responsible for nuclear weapons. The discovery and characterization of fission is one of the great scientific detective stories of the first-half of the 20th century.
Told many times, the narrative begins with Henri Becquerel’s discovery of radioactivity via observing photographic plates clouded by uranium-containing compounds (potassium uranyl sulfate). The story picked up speed as a group of future Nobel Prize winners blasted away at the fundamental materials of universe by, in lock-step fashion, discovering and then making use of what, at that time, were thought to be the most elementary particles (JJ Thomson and the electron, Ernest Rutherford and the nucleus, James Chadwick and the neutron, and the work of Pierre and Marie Curie, all Nobel Prize winners). There are near misses in the hunt for fission by Fermi in 1934 and the Curies in 1938. (Read our previous post on Marie Curie HERE.)
This foundational work, necessary for the follow-on discovery of fission, was centered, in no small part, on characterizing the aforementioned elementary particles, which at that time consisted of whizzing, negatively charged electrons, moon-like satellites forever orbiting a planetary core of positively charged protons and neutrons.
In the end, the discovery of fission came from a German team consisting of Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Frisch (Meitner’s nephew). While there are numerous fission reactions, depending upon the elements that are being split, the fission process that their work characterized was one which involved uranium, U-238 specifically. The most common form of uranium, U-238, consists of a preternaturally energetic ensemble of 92 protons, 146 neutrons, and 92 electrons (in a one-to-one balance with the positively charged protons).
The fission process consists of flinging a neutron at the nucleus of a U-238 atom and then getting out of the way as newly created atoms of lighter elements and more neutrons are released. The output of the German team’s fission process is an atom of barium, an atom of krypton, and the release of three further neutrons.
For their work on fission, Hahn won the 1944 Nobel prize in Chemistry (ironically awarded in 1945, just months after Hiroshima and Nagasaki), Meitner was driven out of Germany for having Jewish parentage, and Frisch was able to give fission its name based on the process’s similarity to cellular fission.
Uranium is present in detectable and usable amounts in the earth’s crust. It can be mined, processed, and machined into power plant fuel rods and nuclear weapons. Plutonium, on the other hand, cannot be found in nature. It was manufactured in a particle accelerator, created in the machinery of men and the runnup to World War II. Although there were many scientists in the late 1930s and early 1940s working on the problem of transuranics (those elements lying beyond uranium in the periodic table), the nuclear chemist Glenn Seaborg, another Nobel laureate, is generally credited with “discovering” plutonium. In the scientific literature prior to plutonium’s discovery, its existence was conjectured, and the hypothetical beastie was referred to as eka-osmium, after the practice of naming as yet undiscovered elements eka + the element lying above it in the periodic table (in this case, osmium).
Plutonium’s discoverer (creator?) Glenn Seaborg is a character of special fascination to us here at Lofty Ambitions. Seaborg was born in 1912 in Ishpeming, Michigan. This is the hometown of Anna’s grandfather, Popsi, who was born in 1902. Surely their paths or their families’ paths crossed in the small Upper Peninsula town.
Last week, the news reported that traces of plutonium have been found outside the Fukushima Daiichi plant. That’s bad because tiny amounts of plutonium are very toxic to humans and because plutonium stays radioactive for thousands of years, its half-life being 24,000 years. Half-life is on our list of blog topics (and a favorite reference in a poem of Anna’s).
Despite its toxicity, early Manhattan Project scientists managed to take plutonium into their bodies in measurable quantities, and yet many of them lived relatively long and healthy lives. They even formed a club—the UPPU—standing for You Pee Plutonium. Plutonium’s symbol is Pu.
At this point, most of what is being released into the environment as a result of the nuclear accident in Japan isn’t uranium and plutonium, but, rather, the fission products that result from the melting and burning fuel rods. Next week, we will likely talk about these fission products, including the iodine and cesium that are suddenly making news. And just how long all this stays around.