Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. The uranium nucleus binds between 141 and 146 neutrons, establishing six isotopes, the most common of which are U-238 (146 neutrons) and U-235 (143 neutrons). All isotopes are unstable and uranium is weakly radioactive. Uranium has the second highest atomic weight of the naturally occurring elements, lighter only than plutonium-244. Its density is about 70% higher than that of lead, but not as dense as gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock, and water, and is commercially extracted from uranium-bearing minerals such as uraninite. In nature, uranium is found as uranium-238 (99.2742%), uranium-235 (0.7204%), and a very small amount of uranium-234 (0.0054%). Uranium decays slowly by emitting alpha particles. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
A well-kept secret is that the United States is the world’s largest generator of nuclear power by far: double the second-ranked country, and triple the third-ranked country. However, most of our uranium (>90%) must be imported.
So far, this hasn’t been a significant problem for us, but it could soon become a problem as more nuclear plants are built around the world, as world supplies become more constrained, and as the prices of uranium rise.
Uranium occurs in nature in the form of water soluble oxides. Naturally occurring levels of uranium in groundwater is an issue on Colorado.
The production of uranium in the United States, and the world, has not kept up with the demand, and the demand is increasing. Worldwide, 439 reactors—with a combined capacity of approximately 370 Gigawatts of electricity—require 66,500 tons of uranium. In 2005, 41,600 tons were supplied by mines—less than 2⁄3 of the required supply. The remainder was made up by the down-blending of weapons-grade material or was removed from existing stockpiles. The stockpiles are now largely depleted. This situation has increased the price of uranium dramatically. The cost of uranium in 2007 was eight times what it was in 2003.
Looking at future demand, the World Nuclear Association lists an additional 222 reactors proposed around the world, 93 already planned or ordered, and 34 being built. This will nearly double the demand for uranium. Nations planning to enter the group of nuclear power generators include Bangladesh, Belarus, Egypt, Indonesia, Iran, Israel, Kazakhstan, North Korea, Thailand, Turkey and Vietnam. The U.S. demand for 2008 will be approximately 19,000 tons, while the production in the U.S. was only 1700 tons in 2006, according to the World Nuclear Association. It’s no secret why there has been renewed interest in Colorado’s uranium resources.
Uranium is a widespread and ubiquitous element. It has a crustal abundance of 2.8 parts per million, slightly more than tin. Primary deposits of uranium tend to concentrate in granitic or alkalic volcanic rocks, hydrothermal veins, marine black shales, and Precambrian age placers.Secondary (or epigenetic) deposits of uranium are formed later than the surrounding rocks that host the mineral deposit. Uranium is soluble in oxidizing aqueous solutions, especially the U+6 valence state, and can be redistributed from primary source rocks into porous sedimentary rocks and structures by groundwater and form secondary(epigenetic) uranium mineral deposits.
Uranium Injection Mining
Organic material in the pore spaces of rocks create a reducing environment in the water. The oxidizing, uranium-bearing waters passing through the rock precipitate uranium in the rock where the reducing environment exists. Sometimes whole logs (organic matter) buried in the rocks become rich in uranium deposited through this process. This process is what allows in-situ mining of the uranium. Humans pump oxidizing water into the uranium ore, dissolve it, bring the bring the solution to the surface and precipitate out the uranium.
Epigenetic deposits of uranium in sedimentary rocks form the bulk of uranium deposits in Colorado. Epigenetic deposits in Colorado include the many mines of the Uravan, Cochetopa, Maybell, and Rifle districts, and other scattered places including the Front Range and Denver Basin. Primary uranium deposits in Colorado occur in hydrothermal veins, especially in the Front Range.
Colorado is the 4th largest uranium-producing state. Colorado has produced 16% of all the uranium ever mined in the US.
Production
Currently, the United States imports about 92% of the uranium it needs each year. About $16 million worth of uranium was produced in the state in 2008, and $7 million in 2005. Colorado has about 32 active mining permits for uranium, but no present production (2017). It is very common for a mine to be shut down until prices and demand make it economically viable to re-open the facility.
There are two major forms of uranium mining: Hard-rock mining and In-Situ Leaching (ISL).
Hard-rock mining involves determining where a vein of commercially valuable ore is located and extract it using mechanical means until the supply is exhausted.
In situ leaching (ISL), also known as solution mining, or in situ recovery (ISR) in North America, involves leaving the ore where it is in the ground, and recovering the minerals from it by dissolving them and pumping the pregnant solution to the surface where the minerals can be recovered. Consequently there is little surface disturbance and no tailings or waste rock generated. However, the orebody needs to be permeable to the liquids used, and located so that they do not contaminate groundwater away from the orebody.
The economics of uranium mining are difficult because of a lack of milling capacity in the country. Uranium production is from one mill (White Mesa, Utah) fed by four or five underground mines and several in-situ leach (ISL). A proposed new mill (Piñon Ridge Mill) in the Paradox Valley in western Colorado has been wending its way through the permitting process. However, in April 2012 Energy Fuels Resources Corporation (EFRC, a Colorado-based subsidiary of Energy Fuels Inc.of Toronto) agreed to take over all Denison Mines’ US assets and operations, including the currently operated White Mesa mill, in a C$106 million merger. This diminished the priority of building the Piñon Ridge mill, and in September 2013 the company put plans for it on hold.
Colorado’s Uravan Mining District, located in the southwestern portion of the state, is the oldest uranium district in the nation. The Uravan belt has about 1,200 historic mines that produced over 63 million pounds of uranium and 330 million pounds of vanadium from 1948 to 1978.
Reserves
The US Energy Information Administration updated its Uranium Reserve estimates in 2010, using 2008 data. Further updates were discontinued. Colorado was lumped in with Utah and Arizona. The three states are estimated to have about 200 million pounds of U3O8 at a price of $100 per pound. The price is currently hovering around $60 per pound, but reached $140 per pound in 2007.From 1989 to 2004, the price averaged about $10 per pound. The three-state group is third in the nation behind Wyoming and New Mexico.
Uses
Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissile isotope. Uranium-238 is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. While uranium-238 has a small probability for spontaneous fission or even induced fission with fast neutrons, uranium-235 and to a lesser degree uranium-233 have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction. This generates the heat in nuclear power reactors, and produces the fissile material for nuclear weapons. Depleted uranium (U-238) is used in kinetic energy penetrators and armor plating.
Depleted uranium was used as a colorant in uranium glass, also known as vaseline glass, producing orange-red to lemon yellow hues. It was also used for tinting and shading in early photography.
Uranium was originally used as a catalyst to produce ammonia. From this basic material, nitric acid is produced and ammonium nitrate is manufactured. A large part of our food supply is dependent on this catalytic process due to the widespread use of ammonium nitrate as fertilizer for commercial food crops. Iron catalysts have largely replaced the use of uranium catalysts in ammonia production due to lower cost.
Colorado Geological Survey. “RockTalk V09N2, Fall 2006 – Uranium, It’s Hot! ” RockTalk, Fall 2006.
