Enriching Uranium Two Centuries Ago This Year, Martin Scarce Then) Or Molybdenum, and Was Also Canadian-Born Physicist Arthur J
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HISTORICAL NOTE Enriching Uranium Two centuries ago this year, Martin scarce then) or molybdenum, and was also Canadian-born physicist Arthur J. Heinrich Klaproth discovered the element used in some ceramics. In its natural com Dempster-is the best candidate for fission uranium while investigating the bluish pounds, uranium had been used for nearly work. Unfortunately, Zl5U is only 0.7% of black mineral pitchblende (uraninite) 2000 years as a coloring agent in the manu natural uranium; so, if the process of fis found in Saxony. facture of glasses and glazes-glass dating sion in uranium was to be put to any practi Mineralogists in the late 1700s tended to from 79 A.D. contains uranium oxide. Ura cal use, some way had to be found to classify pitchblende as an ore of zinc, but nium compounds were also used as a dye enrich the concentration of Zl5U. when Klaproth processed the pitchblende or stain in the leather and wood industries, In 1942, at the height of World War II, Dr. he discovered specks of a black substance, yielding colors from a pale yellow to a Vannevar Bush, head of the Carnegie Insti a new metal that he called "uranit:' The bright green. The main interest in process tution in Washington, DC, advised U.S. following year he renamed his new ele ing pitchblende ore then was to extract the President Franklin Roosevelt about the ment "uranium;' in honor of the new accompanying radium, which was consid possibilities of enormous energy to be re planet Uranus, discovered by Sir William ered much more valuable. The uranium it leased from a mass of fissioning uranium, Herschel eight years before in 1781. self was seen as a relatively undesirable and how it could be used to create a For the next 50 years Klaproth and other byproduct. weapon. Bush also suggested the possibil chemists assumed that the black specks ity of producing enriched uranium or the derived from pitchblende were pure ura also fissionable new element, plutonium nium metal. Klaproth later managed to cre (its existence was a closely kept secret, ate uranium nitrate, sulfate, formate, and The main interest in known only to American and British scien acetate compounds. Klaproth, a Berlin processing pitchblende tists). apothecary, was also an outstanding ana President Roosevelt then turned the pro lytical chemist. His other claims to fame in was to extract the ject over to the U.S. Army and ordered that clude the discovery of three additional radium. Uranium was the idea be converted to full-scale produc elements-cerium, titanium, and zirco tion of fissionable uranium. By May 1942, nium. seen as a relatively the Manhattan Project scientists proposed In 1841, however, Eugene Melchior Peli undesirable byproduct. five feasible methods: two involved pro got showed that the "pure metal" was ac duction of plutonium; the other three fo tually uranium oxide, U02• Peligot passed cused on enriching uranium. These latter chlorine over the heated substance and In 1934 Enrico Fermi and his team found three were the electromagnetic, the centri studied the products of the reaction, realiz that neutrons striking uranium led to beta fuge, and the gaseous-diffusion methods. ing that his original material was an oxide. activity, but it wasn't until four years later Much discussion and debate ensued as He then prepared the actual metal by re in Germany that Otto Hahn and Fritz to which of the five proposed methods ducing uranium tetrachloride with potas Strassman realized that the bombardment should be chosen. Each seemed to have sium in a heated platinum crucible; the of neutrons actually caused the uranium the same likelihood of success (or failure) potassium chloride dissolved, leaving a nucleus to break down into radioactive iso as the others, and each would entail enor black powder, which was uranium metal. topes of lighter elements, with the subse mous hours of work and resources to de This metal was given an atomic weight of quent release of energy. velop. But in the urgency of war and the 120, but when Dmitri Ivanovich Mende The following year, 1939, Fermi sug rush to develop the bomb, they could not leev formulated the periodic table in 1869, gested that a split uranium nucleus might risk choosing the wrong method. In exas he realized that uranium did not have the also release neutrons that could then sus peration, they chose an unorthodox properties expected for such an atomic tain the fission reaction in other uranium course, which was okayed by President weight. Instead, he suggested 240 as the nuclei. In such a sustained fission reaction, Roosevelt-all five methods would be de atomic weight. Uranium remained the one pound of uranium (one cubic inch) veloped simultaneously. heaviest of the elements until the first tran could yield the same amount of energy as After beginning work, the researchers suranic element was artifically created in three million pounds of coal. This year also discarded the centrifuge method as un 1940. marks the 50th anniversary of the discov feasible, and they selected only one of the Not until 1896 did Henri Becquerel dis ery of nuclear fission. two plutonium production methods, leav cover radioactivity in uranium, which Uranium is a dense, hard metal, concen ing the Manhattan Project to concentrate stimulated renewed interest in what had trated at about four parts per million in the on three of the original five proposals: cre been considered a relatively uninteresting Earth's crust, an abundance similar to ating plutonium in a graphite-moderated element. Bequerel wrapped a photo tungsten and tantalum and considerably pile, and using the gaseous-diffusion and graphic plate in black paper and exposed it more common than gold, silver, mercury, electromagnetic methods for enriching to the fluorescent salt, potassium uranyl or iodine. Since it is highly reactive, ura uranium. sulfate. He found that distinct parts of the nium forms many naturally occurring The gaseous-diffusion method, led by photographic plate had been exposed. compounds, some water soluble. It is very Dr. Manson Benedict, was implemented at Later tests proved that all uranium salts, as malleable and ductile, a poor conductor, Oak Ridge, Tennessee, in a plant called K- well as metallic uranium and the mineral and stongly electropositive. Uranium has 25. Benedict and his team had to develop a pitchblende, also had the same effect. 14 known unstable isotopes and no stable workable cascade of thousands of diffu Uranium, however, found only limited ones. sion stages through which gaseous ura use in practical applications. It became an Subsequent studies showed that the Zl5U nium hexafluoride would pass, each stage occasional substitute for tungsten (very isotope-discovered in 1935 by the separating out the heavier and more abun- 70 MRS BULLETIN/OCTUBER 1989 HISTORICAL NOTE dant isotope 238 U and enriching the 0.7% of only 30% 235 U. To create the pounds of en which led to new techniques for cutting the desirable 135 U. The principle of gaseous riched uranium needed for weapons and welding glass tubing. diffusion had been known for over a cen would require an enormous industrial Ultimately, the Manhattan Project fo tury; in the 1920s German chemist Gustav complex with magnets larger than any pre cused the resources of the United States to Hertz had separated isotopes of neon by viously imagined. But Lawrence was per an extent that has never been matched, not cycling the gas through many different fil suasive and had faith in his ideas; he even with the Apollo program. Sufficient tering stages. Unfortunately, the UF6 gas ultimately convinced the project engineers quantities of fissionable uranium were cre was so corrosive that it destroyed the pipes to undertake the task. ated to make the atomic bombs tested at and pumps used in the process! And, be The electromagnetic production plants, Alamogordo, New Mexico on July 15, cause of the scarcity of the 135 U isotope, known as Y-12 and also located at Oak 1945, and the "Little Boy" bomb dropped enormous quantities of the gas were Ridge, encountered great technical prob on Hiroshima, Japan on August 6, 1945. needed to produce even minute amounts lems, dealing with magnets, vacuums, The bomb dropped on Nagasaki three of pure 235 U. electric power supplies, and control de days later used plutonium instead of ura Ernest 0. Lawrence and his team at vices of totally new, never-before-used de nium for the fissionable material. Both Berkeley concentrated on the electromag signs and magnitudes. The magnets were bombs abruptly changed the course of the netic method, which used a cyclotron-style each 250 feet long and composed of thou war and forever changed the context of device to· separate the slightly lighter iso sands of tons of steel, with magnetic fields global politics and survival. tope from the abundant 238 U in a vacuum great enough to require that all movable The manufacturing, enrichment, and chamber with large magnetic fields. Some equipment in the vicinity be created of separation techniques developed by the of the Manhattan Project engineers be nonferrous metals or nonmagnetic steel Manhattan Project continue to be used to came exasperated at the theoretical work and these materials were already badly produce uranium for nuclear power plants Lawrence did with his "calutron" device needed for other war efforts. An entire se as well as weapons. These uses have taken and at the minuscule amounts of 235 U he ries of beryllium copper tools was de uranium from its rather mundane early was able to separate. Lawrence's giant signed and fabricated for use with the history to its present status as one of the magnets and round-the-clock work were project.