Uranium—It's Hot!! and Back by Popular Demand
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Volume 9, Number 2 C OLORADO G EOLOGICAL S URVEY Fall 2006 Uranium—It’s Hot!! And Back by Popular Demand Pure elemental uranium is a slightly parent element. This decay is radioactive metal, silvery white in color a natural constant in that half and dense (almost as dense as gold). Ele- of a given amount of uranium- mental uranium metal does not occur 238 (238U—the 238 superscript in nature because it readily combines refers to the atomic weight of with oxygen to form several uranium the atom) will decay to lead in oxide minerals and compounds. The about 4.5 billion years. It does most significant property of uranium is not decay directly to lead but that it is the “parent” element in a passes through several daugh- radioactive decay series that eventually ter elements including tho- leads to formation of a particular iso- rium, radium, radon (a gas), tope of lead. Radioactive decay means and bismuth on its path to a that certain elements, such as uranium, stable isotope of lead. See Fig- will over a specific period of time, give ure 1 for a much-simplified off atomic particles—electrons, protons, chart of the uranium-lead and neutrons—leading to changes in radioactive decay series. the atomic weight and Uranium is found in nature number of the in three main isotopes. 238U is the most common constitut- ing about 99.3 percent of all natural uranium; 235U consti- tutes about 0.7 percent of all Burro No. 7 Mine, Slick Rock, Colorado. (Photograph by Jim uranium; and other isotopes Cappa, 2005) such as 234U form trace amounts. 235U is important because it is stantial energy. 235U is more radioactive readily split as part of the fission process than 238U because of its shorter half-life, (Fig. 2) and in the process releases sub- about 700 million years, and as such, the ratio of 238U to 235U has changed Figure 1. Simplified uranium-lead radioactive decay series. over geological time. In the earth’s past, Abbreviations and explanation: Elements: U = uranium, there was a higher percentage of 235U Th = thorium, Ra = radium, Rn = radon, Po = polonium, than today. Scientists believe that this Pb = lead, Bi = bismuth. Half-lives: By = billion years, higher percentage of 235U (about 3 per- y = years, d = days, min = minutes. Subatomic particles; α = alpha particle, essentially y a helium nucleus consist- cent) led to a natural nuclear fission ing of two protons and two neutrons. β = beta particle a event that occurred about 1.7 billion high-energy electron or positron. years ago in present-day western Africa. Inside this issue: The Discovery of Uranium, its Uses and Future, the History of Uranium Mining in Colorado, Geology of Uranium Deposits in Colorado, Cleanup of Contaminated Mill Sites, and other useful information about Uranium CGS ROCKTALK Vol. 9, No. 2 1 From the Division Director— The boom is back! Colorado, A Natural Nuclear Reaction in which has supplied uranium Africa 1.7 Billion Years Ago to the world for more than a In the early 1970s, scientists noted that something century, is seeing the return of was strange about the uranium ore being mined at interest in this important the Oklo deposit in Gabon, West Africa. The ore energy resource. Prospectors was depleted in the fissionable isotope of uranium, are dusting off their scintil- 235U, and resembled the percentage found in spent lometers and heading for the nuclear fuel-about 0.25–0.30 percent (it should hills. Their apparent success is have been 0.7 percent). At the time, this uranium indicated by the filing of 3,000 deposit was formed (about 2,000 million years ago) new mining claims in the state the percentage of 235U was close to 3 percent, about last year. the same as what is used in a modern nuclear reac- Why the boom? Currently, tor. Scientists think that water seeped into the the 435 nuclear reactors in the world need 180 million deposit at about 1.7 billion years ago and allowed pounds of uranium per year, but the world only pro- the neutrons emitted from uranium to slow enough duces 110 million pounds. The shortfall has been made to initiate a chain reaction, which then over time up by depleting stockpiles from the last boom cycle and heated the water into steam causing the chain reac- by conversion of nuclear weapons. Both are diminish- tion to cease. This process may have happened ing in supply and consequently the price of uranium numerous times until the amount of 235U was increased from $10 per pound in 2003 to more than depleted enough to cause the cessation of the chain $55 per pound today. reaction. China and India’s increase in the use of nuclear energy The products of the nuclear reaction remained increases the demand for uranium. However, the United within the Oklo deposit and were never dissolved States also needs a lot of uranium because we are the or spread by ground water for 1.7 billion years. This world’s leading producer of nuclear energy. Indeed, we fact gives scientists hope that nuclear waste stored produce more nuclear energy than France, Spain, Ger- in geological formations may be immobile for, at many, Sweden, and the United Kingdom combined. least, millions of years. One of the best kept secrets in our country is that U.S. production of electricity from nuclear plants has increased over the decade since the last nuclear power plant came on line in 1996 (it was ordered in the early 1970s). This was achieved by increasing the operating Discovery of Uranium and Radioactivity efficiency of the plants. Today we produce nearly twice In 1789, uranium oxide was first recognized as the mineral as much nuclear power as any other country in the pitchblende (now considered as a variety of uraninite). The world, an amount equal to 30 percent of the world’s mineral was recovered from a silver mine in Joachimsthal, nuclear energy. Bohemia, now a part of the Czech Republic, by an amateur The increase in interest in uranium is but an indica- chemist, Martin Klaproth. Klaproth named the compound tor of growing interest in all of Colorado’s rich natural uranium in a tribute to his friend, William Herschel, a famous resources. China and India’s appetite for natural resources composer and astronomer, who had discovered the planet creates worldwide shortages and drives up prices of Uranus (Uranus is the Greek god of the Heavens). However, nearly every energy and mineral commodity. Colorado it was not until 1841 that the true metal, uranium, was iso- communities are already suffering from China-caused lated from its oxide form by a French chemist, Eugene Peligot. shortages of steel and cement. Prices for such diverse In 1896, another French scientist, Henri Bequerel, left some commodities as gold, copper, aluminum, molybdenum, uranium in a drawer with a photographic plate for a few days chromium, titanium, selenium, and antimony have sky- and to his surprise found the photographic plate to be rocketed in the 21st century with percentage price exposed. This lucky “experiment” was the beginning of atomic increases ranging from a low of 48 percent to more than research and the understanding of radioactivity. 1,700 percent! As shortages grow and prices increase, Marie Curie and her husband Pierre conducted various Colorado can expect to see increasing pressure to develop experiments on uranium ore, some of which came from Colo- our rich array of energy and mineral resources. rado, from 1898 through the 1920s in their lab in Paris. They recognized the process of radioactivity and discovered two Vince Matthews of uranium’s daughter products, polonium and radium. In the 1930s and early 1940s, scientists discovered that one 2 CGS ROCKTALK Vol. 9, No. 2 isotope of uranium, 235U, was fission- able—that is when an atom of 235U is bombarded by neutrons its nucleus splits into two equal parts, usually an atom of barium and krypton, at the same time releasing substantial energy in the form of heat and two or several more neu- trons (Fig. 2). The released neutrons then collide with other atoms of 235U, releas- ing more energy and neutrons, a process known as a chain reaction. The chain reaction released such large amounts of energy that it led to the development of nuclear weapons, ending World War II. After the end of the war scientists put the energy released by fission to work to heat water, create steam, turn a tur- bine to generate electricity—the major use of uranium today. Figure 2. Atomic fission of uranium. Explanation: Solid blue circles = protons, White circles = neutrons; Yellow circles = electrons. Uses of Uranium The very first uses of uranium, mostly radium; again, the manufacturers court case in which five “Radium Girls” in the form of complex uranium oxides, espoused their curative effects. Radium sued U.S. Radium Corporation was set- were as an agent in glass manufacture. was also being used to paint luminous tled in 1928 and exposed the deadly A small amount of uranium was added dials on clocks, watches, altimeters, and effects of radium. Its use declined dras- to the glass giving yellow and green col- other instruments. Young girls were tically after that. ors to the finished product. The glass is hired to paint the radium on the dial In the 1930s, scientists came to under- called Vaseline glass because it resem- faces and they were instructed to point stand the nature of radioactivity and the bles the color of Vaseline (Fig. 3). Ura- their paintbrushes with their lips.