Radon is a tasteless, colorless, odorless elemental noble gas in the family with helium, neon, argon, krypton and xenon. Its chemical symbol is Rn and it has an atomic number of 86. It is the densest noble gas and one of the densest gases that exist. As with the other noble gases, radon is chemically inert and rarely forms compounds with other substances. It was discovered in 1900 by Friedrich Ernst Dorn as a gas given off by radium. Other gases generated by radioactive decay were subsequently discovered and eventually recognized as being radon. Some of its properties were discovered and reported in 1910.

          Radon is highly radioactive and has no stable isotopes. The 39 isotopes of radon vary in atomic weight from 193 to 228. 6 of the isotopes have the same atomic weight of other isotopes but they what are called metastable nuclear isomers which means that one or more of the neutrons and/or protons in their nuclei are in an excited state. They have much longer half lives than most of the excited states of a particular isotope.

         Radon gas is formed by the radioactive decay of uranium or thorium. When these naturally occurring elements decay, they generate whole chains of decay products with the ultimate end of the chain being non-radioactive and stable lead. Different isotopes of radon are generated by different decay chains. Radioactive radium and actinium which are also part of decay chains spawn radon isotopes. Radon isotopes spawn radioactive daughter products of other elements in turn as part of the decay chains including other isotopes of radon. Radon half lives vary from Rn-222, a decay products of uranium, which has a half life of 3.8 days to Rn-214 .27 microseconds (millionths of a second).

        Because uranium is so ubiquitous in the natural environment, radon gas is also ubiquitous. All natural soil constantly emits tiny amounts of Radon. Being dense radon tends to accumulate where there is poor air circulation like in basements and attics. There is great variation in the amount of radon in homes because of differences in underlying geology, water, naturally occurring proportion of uranium, ventilation, etc. Radon concentration in the USA is measured in picocuries per liter of air.

         Radon gas is very toxic and is the primary way in which ionizing radiation threatens human beings. Isotopes of radon emit alpha or beta particles. The most common isotope of radon is Rn-222 which is an alpha emitter. Most of the daughter products of radon decay are solids so when radon decays, the decay products can wind up sticking to dust particles. When these are inhaled, they accumulate in the lungs and can lead to lung cancer. A clear correlation has been shown between radon concentration and lung cancer. Radon is thought to cause 21,000 cases of lung cancer per year in the United States, second only to cigarette smoking. Radon gas exposure is a major problem in uranium mining and processing.

         Radon is commercially produced in small amounts for use in radiation therapy for cancer treatment. Artificially produced radon and radon in old mines is a common treatment for arthritis in many European countries. Radon produced by decay of radium paint was once used to create objects that would glow in the dark but this use has been discontinued.

From the U.S. EPA:

          Radon is a tasteless, colorless, odorless elemental noble gas in the family with helium, neon, argon, krypton and xenon. Its chemical symbol is Rn and it has an atomic number of 86. It is the densest noble gas and one of the densest gases that exist. As with the other noble gases, radon is chemically inert and rarely forms compounds with other substances. It was discovered in 1900 by Friedrich Ernst Dorn as a gas given off by radium. Other gases generated by radioactive decay were subsequently discovered and eventually recognized as being radon. Some of its properties were discovered and reported in 1910.

          Radon is highly radioactive and has no stable isotopes. The 39 isotopes of radon vary in atomic weight from 193 to 228. 6 of the isotopes have the same atomic weight of other isotopes but they what are called metastable nuclear isomers which means that one or more of the neutrons and/or protons in their nuclei are in an excited state. They have much longer half lives than most of the excited states of a particular isotope.

         Radon gas is formed by the radioactive decay of uranium or thorium. When these naturally occurring elements decay, they generate whole chains of decay products with the ultimate end of the chain being non-radioactive and stable lead. Different isotopes of radon are generated by different decay chains. Radioactive radium and actinium which are also part of decay chains spawn radon isotopes. Radon isotopes spawn radioactive daughter products of other elements in turn as part of the decay chains including other isotopes of radon. Radon half lives vary from Rn-222, a decay products of uranium, which has a half life of 3.8 days to Rn-214 .27 microseconds (millionths of a second).

        Because uranium is so ubiquitous in the natural environment, radon gas is also ubiquitous. All natural soil constantly emits tiny amounts of Radon. Being dense radon tends to accumulate where there is poor air circulation like in basements and attics. There is great variation in the amount of radon in homes because of differences in underlying geology, water, naturally occurring proportion of uranium, ventilation, etc. Radon concentration in the USA is measured in picocuries per liter of air.

         Radon gas is very toxic and is the primary way in which ionizing radiation threatens human beings. Isotopes of radon emit alpha or beta particles. The most common isotope of radon is Rn-222 which is an alpha emitter. Most of the daughter products of radon decay are solids so when radon decays, the decay products can wind up sticking to dust particles. When these are inhaled, they accumulate in the lungs and can lead to lung cancer. A clear correlation has been shown between radon concentration and lung cancer. Radon is thought to cause 21,000 cases of lung cancer per year in the United States, second only to cigarette smoking. Radon gas exposure is a major problem in uranium mining and processing.

         Radon is commercially produced in small amounts for use in radiation therapy for cancer treatment. Artificially produced radon and radon in old mines is a common treatment for arthritis in many European countries. Radon produced by decay of radium paint was once used to create objects that would glow in the dark but this use has been discontinued.

From the U.S. EPA:

          When enriched uranium is used in nuclear reactors, the exhausted fuel consists mainly of U-238 with small amounts of U-235, plutonium and minor actinides such as neptunium, americium, curium, berkelium, californium, einsteinium, and fermium. There are commercial facilities in France, the United Kingdom and Japan for reprocessing spent fuel. Reprocessing is also carried out at nuclear weapons facilities. Reprocessing is currently carried out in eleven countries.

          Originally, spent fuel was reprocessed primarily for the purpose of extracting plutonium for use in nuclear weapons. With the spread of commercial reactors, plutonium began to be used in Mixed Oxide (MOX) reactors.

          CANDU reactors are designed to use natural uranium as a fuel so spent fuel can be used in these reactors because the U-235 remaining is still present in a higher ratio than naturally occurring uranium. It only needs to be processed physically in order to be utilized with no chemical processing required.

          Reprocessed uranium and depleted uranium can be used in fast breeder reactor blankets but there is currently no commercial market for this use. Currently the price of uranium is too low to justify re-enrichment of spent fuel to supply commercial reactors but that could change if the price of uranium continues to rise.

          The current main method of reprocessing is Plutonium and Uranium Recovery by Extraction (PUREX). The spent fuel is dissolved in nitric acid and the resulting liquid is filtered. Organic solvents are used to created compounds of plutonium and uranium which are then chemically separated and extracted. A modified version of PUREX known as UREX just extracts the uranium which constitutes most of the spent fuel. TRUEX is another extraction technique which removes the americium and californium and lowers the alpha radioactivity of the spent fuel making disposal easier.

          A number of obsolete chemical processing methods have been abandoned in favor of the processes listed above. Possible new industrial processes have been developed in the laboratory and additional theoretical methods have been proposed including an electrochemical/ion exchange method , a high temperature pyrprocessing method that utilizes molten salts and molten metals, electrolysis and other new techniques.

          Reprocessing is a dirty dangerous process. The spent fuel must be pulverized behind heavy shielding with protection against dust. When boiled in nitric acid, radioactive gases are generated. After the plutonium and uranium have been extracted, toxic radioactive liquids are left behind which must eventually be solidified for disposal.

          Reprocessing can reduce the volume of waste but does not reduce radioactivity. Studies of the cost of a fuel cycle with reprocessing versus single use with waste disposal have been conducted but are not definitive because it is not possible to exactly define what the costs of disposal will  be. There has been controversy surrounding reprocessing because of possible contributions to nuclear weapon proliferation, vulnerability to terrorism, political problems that arise when attempting to site a reprocessing plant.

Closing the nuclear fuel cycle – from Argonne National Laboratory:

          When uranium metal is processed to increase the proportion of U-235, a byproduct of the process is a great deal of uranium metal containing smaller amounts of U-235 than the natural proportion of 0.72 %. This byproduct is known as “depleted” uranium(DU). The U-238 in DU emits alpha particles which contain 2 protons and 2 neutrons. These alpha particles only travel a few centimeters in open air and can be blocked by a sheet of paper or plastic, a layer of clothing.

          DU can enter the human body through in breathing, drinking, eating or skin contact and penetration by fragments of uranium munitions. The exact chemical properties of the uranium, the amount of the contamination, the way it enters the body and other factors determine its effect on health. The uranium is absorbed by tissue and distributed by diffusion, blood circulation, air passages and digestive tract and eventually excreted out of the body. All of these processes have their own impact on how the depleted uranium may damage the exposed individual.

          In addition to being mildly radioactive, DU is a toxic heavy metal although not as toxic as mercury. The danger from toxicity is about a million times greater than the danger from radioactivity. DU causes damage to the kidneys as it is eliminated from the body. It can pass the blood-brain barrier and accumulate in the different parts of the brain to cause neurological problems. DU causes problems with bone formation and can inhibit wound healing. The lungs are especially vulnerable to damage by the toxicity of DU with lesions, fibrosis, edema, swelling and hemorrhaging. There can be conjunctivitis, inflammation and ulceration of the eyes from DU. Red blood cell count and hemoglobin can be reduced in the blood.. If pregnant women are exposed, there can be birth defects in the child. The radioactivity of DU can cause lung, lymph and brain cancer.

          The use of DU in munitions has resulted in serious contamination in theaters of war such as Iraq and Afghanistan. When the munitions explode or impact, huge amounts of DU dust are spread over wide areas and then inhaled by soldiers and civilians. There is no effective way to clean up such wide spread contamination. The zone of contamination will spread as wind storms pick up the dust and distribute it more widely. Rain will wash it down streams and rivers into lakes and the sea.

          Since the first Gulf War in 1991, millions of Iraqis have been exposed to higher levels of radiation that normal background radiation in the natural environment from the hundreds of tons of DU munitions used. Since 1995, there has been a documented increase in the number of cases low level radiation exposure related diseases such as leukemia, birth defects, breast cancer and other illnesses in Iraq. The age of leukemia victims has been declining and the higher incidence levels can be correlated with the area with higher concentrations of DU. Gulf War Syndrome may be related to exposure of soldiers to DU.

          There have been charges that the Pentagon has deliberately concealed the amount of DU munitions used in Iraq and Afghanistan, downplayed the extent of the contaminated areas, and dismissed health concerns of the Iraqi people and returning veterans. Various countries and NGOs have made repeated calls for a ban on the use of DU in munitions due to these serious concerns about the health effects of DU dust in theaters of war.

          When uranium metal is processed to increase the proportion of U-235, a byproduct of the process is a great deal of uranium metal containing smaller amounts of U-235 than the natural proportion of 0.72 %. This byproduct is known as “depleted” uranium. To produce 1 pound of 10% enriched uranium, 24 pounds of uranium must be processed leaving 23 pounds of deplete uranium. Depleted uranium usually contains from 0.2 % to 0.4 percent U-235. Processes have been developed to recover more U-235 from the deplete uranium as the price of uranium has risen. Deplete uranium metal is 1.67 times as dense as lead and almost as dense as gold or tungsten. In a powdered or vaporized state, it is highly flammable.

           The U-238 in depleted uranium emits alpha particles which contain 2 protons and 2 neutrons. These alpha particles only travel a few centimeters in open air and can be blocked by a sheet of paper or plastic, a layer of clothing or even human skin. Their primary danger to human health lies in their danger when inhaled or swallowed.

            Depleted uranium is store near the uranium processing facilities. It is mainly stored in steel cylinders in a crystalline solid form of uranium hexafluoride (UF₆). Each cylinder contains about 14 tons of UF₆. As of 2008, there were about 760,000 tons of UF₆ in the US in Kentucky and Ohio. These stores of UF₆ pose an environmental threat because the UF₆ can interact with water moisture in the air to produce  solid uranyl fluoride(UO₂F₂)  and hydrogen fluoride(HF) gas both of which are highly toxic. Fortunately, the solid UO₂F₂ tends to plug leaks in the steel cylinder which would allow the HF gas to escape.

            In the 1970, research on the use of depleted uranium as a projectile was begun in response to developments in armor plating for tanks. It has also been used as armor plating because of its density.

            Armor piercing incendiary ammunition is currently in use by the U.S. military. In calibers of 20 to 30 mm, it is fired from tanks, armored personnel carriers, jet fighters, helicopters and naval vessels. Long thin penetrators made of depleted uranium are fired from tanks to defeat armored tanks and other vehicles. When they penetrate the armor of a tank, they can disintegrate, catch fire and burn everything inside the vehicle. Grenades, cluster bombs and mines were also developed by the U.S. military but they are no longer used.

         There are minor civilian uses for depleted uranium such as shielding for radiographic cameras, chemical reagents, detectors in high energy physics and other scientific and industrial application. Other civilian uses for depleted uranium that have been discontinued include coloring agents for glass and ceramics, trim weights in aircraft and keels in sailboats.

Depleted uranium penetrator of a 30 mm round:

         If the concentration of uranium in the ore from underground mines or open pit mines is too low to be processed in the mill, the heaps of ore are subjected to leaching liquids such as acids, alkaline chemicals or peroxide solutions. The liquid flows down through the heap and dissolves uranium minerals. The liquid runs down a layer of plastic under the heap and collects in pools.  

          If the concentration of uranium in the ore from underground mines or open pit mines is high enough it is crushed to separate the uranium minerals from the rock matrix and to remove impurities. Then it is subjected to chemical processing similar to the leaching described above to produce a liquid rich in uranium..

          In-situ leaching processes which rely on ground water and leaching chemicals pumped down into the orebody produce enriched liquid like the results of heap leaching and ore processing.

          In these processes of ore processing, there can be problems with radioactive dust from ore heaps, release of radon gas from the heaps, and pollution of surface and ground water from leaching liquids.

          The liquid produced from these processes also contains other elements like molybdenum, vanadium, selenium, iron, lead and arsenic. These are removed with organic solutions or ion exchange resins. Finally the liquid is filtered and the uranium compounds are precipitated and dried to produce a yellow powder called yellowcake which contains about 80% of triuranium octoxide (U₃O₈). It also contains other uranium oxides such as uranium dioxide (UO₂) and uranium trioxide (UO₃). Yellowcake is sent to enrichment facilities to produce different forms of uranium oxide and uranium metal for various applications.

          Yellowcake can be smelted to produce purified uranium dioxide. Such natural unenriched uranium  is used in pressurized heavy-water reactors and some other nuclear systems.

          The natural ratio of the two primary uranium isotopes is 99.27 percent U-238 to .75% percent U-235. U-235 is the only naturally occurring element isotope that can be made to fission by the absorption of slow moving neutrons. This capability of U-235 is referred to as being fissile and is the key to creating certain types of nuclear reactors and creating nuclear weapons.  In order to be utilized for these purposes, the uranium metal must have the percentage of U-235 increased above the natural ratio.

          In order to produce enriched uranium metal, the isotopes of U-238 and U-235 must be separated and then recombined to yield increased percentages of U-235. In gas separation, yellowcake is processed in combination with fluoride to produce uranium hexafluoride gas (UF₆). Next, isotopes of uranium are separated by gas diffusion or gas centrifuge. There are other aerodynamic processes that use special nozzles or vortex tubes to separate the different isotopes in gaseous form.  There are laser separation techniques that rely on the ability of lasers to be tuned to just excite the U-235 isotope. Electromagnetic separation vaporizes the uranium metal and then uses magnetic fields to accelerate and deflect the different isotopes. Chemical methods have been developed as well as plasma separation techniques that utilize superconducting magnets and plasma physics, .to increase the percentage of the U-235 isotope.

          When the percentage of U-235 is less than 20%, the resulting metal is called low-enriched uranium and it is used large civilian reactors. Above 20% percentage of U-235, the resulting metal is called highly-enriched uranium. This form is used in compact nuclear reactors in naval warships and submarines. Further processing that takes the U-235 percentage above 90% yields uranium suitable for nuclear weapons.

Range mine and mill in Australia:

             Uranium is mined in 20 countries with a world annual production in the range of 60,000 tons. Just 10 mines in six countries provide over half of the total world production of uranium ore. These six countries produce over 85% of the annual mined uranium in the world.

             The McArthur River mine in Saskatchewan, Canada is the largest producing uranium mine in the world with an output of 7686 tons of uranium oxide in 2011. The deposit was discovered in 1988 with full production commencing in the year 2000. It is the largest deposit of high-grade ore on Earth. It is an underground mine operated by Cameco. The McArthur River mine accounts for 14% of the world production of uranium.

             The Olympic Dam mine is located in South Australia. It produced about 3353 tons of uranium oxide in 2011. The deposit was discovered in 1975 and started production in 1988. It is an underground mine operated by BHP Billiton. Copper production is the primary purpose of the mine with the uranium oxide a minor by-product of operation. The Olympic Dam mine accounts for 6% of the world production of uranium.

             The Arlit mines in north Niger produced around 2726 tons of uranium in 2011. Uranium was first mined in Niger in 1969 by a French company. The Arlit mines are open pit types operated by the Somair and Areva companies.  These mines produce about 5% of the annual world uranium production..

             The Torkuduk mine is located in Kazakhstan. It is operated by Katco Joint Venture with Areva and it produced 2608 tons of uranium in 2011. It is a in-situ leaching type operation. The mine started production in 2005 at one site and 2007 at a second site. The Torkuduk mine produces about 5% of annual world uranium production.

             The Ranger mine is located in the Northern Territory of Australia and it produced 2240 tons in 2011. The deposit was discovered in 1969 and commenced operation in 1980. The first orebody was exhausted in 1995 and a second orebody began producing uranium in 1997. It is currently operated by ERA. It is an open pit type mine and it accounts for 4% of annual world uranium production.

             Kraznokamensk is the name of both a mine and a town in eastern Russia near the border with China and Mongolia. The town was constructed to house miners for a deposit discovered in 1963 and uranium production commenced in 1968. It is operated by AtomRedMetZoloto and it produced 2191 tons of uranium oxide in 2011 by underground mining. The waste produced by the mine has created the largest amount of uranium mine tailings in the world and seriously contaminated areas of the town. Kraznokamensk accounts for about 90% of Russian uranium production and about 4% of annual world uranium production.

             The Budenovskoye 2 mine is located in southern Kazakhstan. It is operated by Karatau Joint Venture of the Russian company Kazatomprom and the Canadian company Uranium One and it produced 2175 tons of uranium in 2011. It is an in-situ leaching type operation. The mine started production in 2007 and produces  about 4% of annual world uranium production.

             The Rossing mine is located in Namibia in south Africa on the Atlantic coast. It is a low grade orebody that produced 1822 tons in 2011. A 70% share is owned by the Rio Tinto company. It is one of the largest open pit uranium mines in the world and it produced about 4% of annual world uranium production.

             The Budenovskoye 2 mine is located in southern Kazakhstan. It is operated by Karatau Joint Venture of the Russian company Kazatomprom and the Canadian company Uranium One and it produced 2175 tons of uranium in 2011. It is an in-situ leaching type operation. The mine started production in 2007 and produces about 4% of annual world uranium production.

             The Inkai and South Inkai mines in Kazakhstan together produced 3150 tons of uranium in 2011 by in-situ leaching. They each produce about 3% of annual world uranium production.

             Uranium mines in 14 other countries produce the remaining 15% of the annual world uranium production.

             Aerial view of the McArthur River uranium mine by Scott Prokop:

           Uranium is mined all over the world. There a number of different techniques that have been developed to extract uranium from various types of sources.

          Open pit mining is a technique where the material (called the overburden) above a uranium deposit is removed by drilling and blasting. The uranium bearing ore is blasted to pulverize it and then excavated with earth moving equipment. Water is sprayed over the pit to reduce the dust released into the atmosphere. Open pit mines devastate a landscape and pollute streams, rivers and lakes.

          For deposits that are too far underground to use the open pit techniques, tunnels are dug to the uranium ore. Shafts called crosscuts are dug horizontally into the vein of ore at different levels. Tunnels called drifts are branched out from the crosscut. Slopping tunnels are then dug between the crosscut levels. Blasting and digging are used to remove the ore. One type of mining fills in the holes created by ore removal and the other just leaves large holes. Flatter org bodies extract the ore in “rooms” and leave pillars of ore to hold up the ceiling of the mine. Radon gas poses a significant danger in this type of mining.

          In heap leaching, oxides of uranium are piled in heaps on layers of plastic on leveled ground with a slight gradient. Sulfuric acid is sprayed on the heaps of ore and the uranium is chemically leached from the ore. The solution of acid and uranium runs down the plastic layer and accumulates in pools. The solution in the pools is pumped out and taken to facilities for further processing. This type of mining poses a serious danger of pollution of surface water in the area of the mine.

          In-situ leaching is a technique where the ore is left in the ground and leaching chemicals are pumped into the ground water in the ore body. The resulting solution is then pumped to the surface and taken away for processing. Depending on the chemistry of the ore, acidic or alkaline chemicals are used. In order to do in-situ leaching, water carrying leaching chemicals has to be able to percolate through the ore body. There is also the concern that the leaching chemicals may pollute ground water away from the mine.

          Uranium is present in low concentrations in sea water. Research on recovering uranium from the world’s oceans has been carried out since the 1960s. Titanium oxide was tested for extraction and then abandoned because of low efficiency. Japanese researchers have found that a special irradiate polymer has the ability to collect heavy metals when immersed in sea water. This techniques is ten times as efficient as the titanium oxide approach.

           Each technique has benefits and drawbacks. In general, pollution of ground water, surface water, land and the atmosphere are major concerns as is the threat of radon gas and uranium dust to the health of the workers.

Rössing open pit uranium mine in Namibia.

          Uranium is a common element. It forms compounds with many other elements and is present in a wide variety of minerals. Four common geological processes distribute uranium minerals in many different forms across the earth. Only a few of the many minerals are considered suitable for extraction to obtain useful uranium and only a few of their deposits are currently exploited. .About one third of the worlds uranium resources are in the form of unconformity-related deposits. Another quarter are found in sedimentary sandstone deposits.

            The biggest producer of uranium in the world is the Central Asian country of Kazakhstan which used to be part of the Soviet Union. It contains about one fifth of the explored uranium reserves on Earth and produces about 36% of the world’s mined uranium. The deposits were formed by a variety of processes including sedimentation, veins, phosphorite, lignite, and oxidation mineralization. There are is a major deposit in the north, a deposit in the west near the Caspian Sea and four major deposits in the south east of the country.

           Canada accounts for 17% percent of the uranium mined in the world. All of Canada’s uranium production comes from unconformity-related deposits at McClean Lake and McArthur River in the Atahbasca Basin of Saskatchewan in the far north.  A third deposit being developed in that area near Cigar Lake is the largest undeveloped high-grade deposit in the world.

            The majority of Australia’s 11% share of world uranium production is taken from the breccia complex deposit at Olympic Dam in Southern Australia. Australia also extracts uranium from big deposits of the unconformity-related type in the Alligator Rivers area of the Northern territories and the Rudall River area of Western Australia.

            Niger in is a large landlocked mostly desert country in Western Africa which provides 8% of the world’s mined uranium. The principle deposits are the sandstone type and are located in the north east of the country near the Sahara desert.

            Namibia produces around 6% of the world’s uranium. It is near the southern tip of Africa on the Atlantic Coast. Magmatic processes created the Namibia uranium deposits.

            Russian mines produce 5.5% of the uranium extracted from the world’s mines. Volcanic processes were a major generator of the Russian deposits. The principle deposits of uranium being mined in Russia is in the Krasnokamensky District of Zabaykalsky Krai in the far East on the border with Mongolia and China.

            Uzbekistan is another of the Central Asian countries that used to be part of the Soviet Union. It is locate east of the Caspian Sea south of Kazakhstan. Uranium deposits are of the sandstone and black shale variety and Uzbek mines produce 4.5% of the world’s uranium production.

            The United States extracts most of its 3% of the world’s mined uranium from sandstone deposits in what is referred to as the Western Cordillera region which extends all the way from Alaska to Mexico and covers the western part of the U.S. from the Rocky Mountains to the Pacific Coast. The Powder River Basin in Wyoming, the Colorado Plateau and the Gulf Coast Plain in Texas also contain uranium bearing sandstone deposits.

            The eight countries listed above account for over 91% of the annual uranium mine production in the world. Ukraine, China, Malawi, South Africa, India, Brazil, Czech Republic, Romania, Germany, Pakistan and France together account for 8% of the mined uranium in the world.

          The primary uranium minerals in commercial ores are uraninite (UO₂), pitchblende (U₃O₈), coffinite (U(SiO₄), brannerite (UTi₂O₆), davidite ((REE)(Y,U)(Ti,Fe³)₂₀O₃₈) and thucholite (Uranium-bearing pyrobitumen). There are a number of other common uranium minerals which form hydrated crystals incorporating water molecules.

          The mineralogy of the host minerals, the reduction-oxidation potential of the uranium mineral and the porosity which determines water infiltration are important factors in the formation of uranium deposits. Since uranium is highly soluble, it can be easily moved around by the flow of water underground. This contributed to the variety of places and manners in which uranium may accumulate. The way in which uranium interacts with other elements and compounds in melted rock also influences its distribution.

          Combinations of surface weathering, sedimentation, diagenetic, magmatic and hydropthermal geological processes mentioned in a previous post produce fifteen general types of uranium deposits.

         The richest uranium ore deposits are found near unconformities. An unconformity is a break in between two layers of rock that have been laid down at different times. In the case of uranium deposits, the two layers are a quartz rich sedimentary layer and a metamorphic layer has been altered by heat and pressure. These deposits were formed between two billion five hundred million years ago and five hundred million years ago.

         The second best uranium ore deposits form in sedimentary deposits on continental shelves and freshwater areas such as river deltas, lakes, etc. In an oxygen rich environment, the uranium dissolves and then moves with the water. When it encounters an oxygen poor or reducing environment, it precipitates out of solution.

         Tabular deposits occur parallel to groundwater flow in sandstone. The ores are rich but the deposits are small.

         Roll front uranium deposits form when ground water dissolves the uranium in sandstone and, after flowing underground, collides with some sort of organic matter rich in carbon. The uranium precipitates out at the “front” when the water encounters the organic material.

         Basal channel deposits form from moving ground water like the tabular and roll front deposits, but the deposition occurs along channels of moving surface water such are rivers. When the water evaporates along desert margins or in shallow saline ponds, the uranium is deposited.

         Quartz-pebble conglomerate deposits are created by the separation and movement of particles of uranium in flows of surface water and their deposit in river beds, river deltas and lakes. These deposits generally contain large quantities of low grade ore

          Breccia complex deposits contain uranium along with iron oxide, copper, gold, silver and rare earth elements. Hydrothermal processes enriched the uranium content of  the quartz-hematite breccias.

          Vein deposits are uranium minerals filling in cracks, veins, fractures and breccias in steeply dipping fault systems. Magmatic processes in molten rock create the veins and later hydrothermal activity can concentrate the uranium. Some veins contain a variety of other metals in combination with the uranium.

          Intrusive uranium deposits form when magma is forced into older rocks deep within the Earth’s crust.

          Marine sedimentary deposits of phosphorite (which contain large amounts of phosphorus) sometimes contain uranium.

          Collapsed breccia pipe deposits are created when vertical cylindrical cavities formed by groundwater dissolving limestone are filled with fragments of rock when they collapse. Uranium fills cavities and coats other rocks.

          Volcanic deposits of uranium may be formed by magmatic processes in the molten rock or later mineralization by groundwater and chemical processes. Such deposits are usually small with low grade ore.

          Surface deposits can form in peat bogs, karst caverns and in soil from the weathering of shallow sedimentary deposits of uranium.

          Metasomite deposits are the result of uranium minerals being distributed in rocks that have been subjected to sodium metasomatism which is chemical alteration by hot subsurface solutions of sodium.

          Metamorphic deposits were laid down by sedimentary or magmatic processes and then remained unaltered by any other processes.

          Lignite is a soft brown young coal derived from wood. Some deposits contain significant amounts of uranium minerals.

          Black shale deposits form in oxygen-free submarine sedimentation processes. The uranium is not mineralized by organic materials due to the lack of oxygen. These deposits are considered very low grade ores.

          There are many other types of uranium minerals but these fifteen types constitute the pool from which uranium ores are chosen for extraction.

Sedimentary layers:

          Uranium is a very common element present in greater quantities than silver. The term “clarke” refers to the average concentration of a particular element in the Earth’s crust.  The clarke of uranium is about 4 parts per million while the clarke of silver is about 1 part per million, the clarke of aluminum is 82,000 parts per million and iron is 63,000 parts per million.

          The term “ore” refers to a mineral deposit which contains a sufficient concentration of a valuable metal to make extraction of that metal profitable. Mineral are formed by geological processes. The four processes that are the most important for formation of uranium minerals are:

1. The accumulation of uranium ores by tiny flakes pickup up as rain water runs off the land into a body of water when the flakes settle into of sedimentary deposits. When the body of water disappears, the soft sediment eventually become solid rock.
2. After sedimentary deposit, there are changes in the distribution of particular elements referred to as “diagenesis.” These are low temperature low pressure changes as the sediment is compressed, liquids are squeezed out, chemicals precipitate out of solution. Oxygen in the sediment may combine with uranium forming oxides.
3. “Magmatic segregation” is a process in which minerals become locally concentrated during the circulation, cooling and crystallization of molten rock. As the molten rock cools, different minerals solidify at different temperatures. Then they may move up or down based on their density.
4. During “hydrothermal circulation”, water penetrates down rock and then is moved back up out of the rock by a source of heat such as underlying magma. As the water rises it carries minerals out of the rock it is moving through and ultimately deposits those minerals.

          In Gabon, in Africa, a combination of these processes concentrated uranium to the point where a natural “reactor” was formed that generated heat for hundreds of thousands of years. J. Marvin Herndon has a theory that there is a natural nuclear fission “georeator” driven by an accumulation of uranium at the Earth’s core. He believes that this reactor is responsible for the magnetic field of the Earth that permits life to exist.

            Most known uranium deposits formed near areas that experienced volcanism and intrusions of magma. Although uranium is present in many minerals, the quantities are too small to make extraction profitable. Concentration is the most important qualification for a commercial ore. The two most concentrated uranium minerals are pitchblende and uraninite which can contain up to 85% uranium. Carnotite, torbernite, tyuyamunite, autunite, uranophane, and brannerite uranium minerals that may contain up to 60% uranium. In addition to concentration, a good ore must have uranium that is not bound up in a complicated chemical compound that would make it difficult to extract. The distribution of the uranium through the mineral deposit is also important. If it is too sparse in the deposit, then that deposit would not make a good ore.

            Uranium may be present in throughout the crust of the earth and may be found in many minerals but deposits of commercial grade uranium ores are not all that common and are highly sought after.

Picture of pitchblende by Geomartin: