Uranium Production

How Uranium is Produced

and Used

General Information

Uranium (U) – a weakly-radioactive metallic chemical element in the actinide series of the periodic table. Uranium atom consists of 92 protons, 92 electrons (including 6 valence ones) and between 141 and 146 neutrons, thus having six isotopes, the most common of which are U-238 and U-235 (99.27% and 0.72% of natural uranium).

Characteristics

• Atomic weight. Uranium is the heaviest naturally occurring element, also having high density - it is 18.7 times as dense as water.

• Fissionability. Uranium atom can be split into two lighter elements, releasing neutrons and nuclear binding energy. U-235 is the only naturally occurring fissile (easily splitting) isotope of uranium. Another fissile isotope, U-233, can be produced from natural thorium. U-238 is only fissionable with fast neutrons , but can be transmuted to fissile plutonium-239 in a nuclear reactor. A nuclear chain reaction can be induced with slow neutrons and maintained only when U-235 or U-233 are present among U-238 in sufficient concentrations (i.e. when U-238 is enriched).

• Weak radioactivity. All uranium isotopes are unstable and decay slowly by emitting an alpha-particle. Long half-life (U-238 – 4.5 billion years) makes uranium useful in dating the age of Earth and other objects.

Contemporary uses of uranium exploit its unique properties.

Induced nuclear fission of U-235

Uranium is covered with black oxide coating in the air.

Discovery and Studies

I century AD -

Uranium in its natural oxide form had been used as a

yellow, red and orange colorant in glassmakin

g and pottery.

1789 - the German chemist Martin

Klaproth obtained uranium

oxide from pitchblende, supposed it was an unknown element

and named it.

1841 - Eugène-Melchior Péligot isolated metallic uranium.

1896 - Antoine

Becquerel discovered radioactivi-ty by using uranium.

1898 - Pierre and

Marie Curie found

radioactive radium and polonium in

uranium ore.

Industrial mining of uranium

was started to extract radium.

1934-1939 - Research of radioactive elements

resulted in the discovery

of nuclear fission.

Importance of uranium as

a weapon material and

energy source had been widely acknowled-

ged.

1942 – Under the Manhattan

Project, Enrico Fermi

initiated the first

self-sustained nuclear chain

reaction.

1945, August 6th

– Uranium-

based “Little Boy”

bomb was detonated

over Hiroshima

1954, June 27th – the

first commercial

-scale nuclear power station began

operation in Obninsk,

USSR.

Martin Henrich Klaproth

One of the Manhattan Project’s calutrons in Oak Ridge, Tennessee

Military Application

Fissile explosive material for nuclear weapons. Early nuclear bombs used U-235 or plutonium-239 derived of U-238. More recent and complicated devices are based on uranium-cased plutonium causing hydrogen isotopes to undergo fusion.

Armour-piercing ammunition – due to high density.

Counterweights and balances in military equipment.

Anti-radioactive shielding – due to low radioactivity and high density (more effective than lead).

Nuclear explosion

Bullet shell made of depleted uranium

Civilian Application

Fuel for nuclear power plants. Controlled fission chain reaction creates heat by splitting U-235

atoms, making water steam spin a turbine to drive a generator,

producing electricity. One kilogram of uranium-235 can theoretically produce about 80 terajoules of

energy or as much as 3000 tonnes of coal. Nuclear plants typically use

fuel enriched to 3% of U-235. By now over 440 nuclear reactors

operate all over the world.

Making of high-energy X-rays

Radiometric dating

Formerly used in glass-making, pottery, photography

Uranium Minerals

The primary uranium ore mineral is uraninite (UO2) or pitchblende (UO3, U2O5), commonly collectively referred to as U3O8(the most stable form).

A large variety of secondary uranium minerals are known, many of which are brilliantly coloured and fluorescent. The most common are gummite, autunite (with calcium), saleeite (magnesium) and torbernite (with copper); and hydrated uranium silicates such as coffinite and uranophane (with calcium).

Uraninite or Pitchblende Autunite crystals under the microscope

Torbernite, an important secondary uranium mineral

Cuprosklodowskite - CuUO2SiO2OH2•6H2O.

Uranium MineralsPrimary uranium minerals

Name Chemical Formulauraninite UO2

pitchblende U3O8, rare U3O7

coffinite U(SiO4)1–x(OH)4x

brannerite UTi2O6

davidite (REE)(Y,U)(Ti,Fe3+)20O38

Secondary uranium mineralsName Chemical Formulaautunite Ca(UO2)2(PO4)2 x 8-12 H2O

carnotite K2(UO2)2(VO4)2 x 1–3 H2O

gummite gum like amorphous mixture of various uranium minerals

seleeite Mg(UO2)2(PO4)2 x 10 H2O

torbernite Cu(UO2)2(PO4)2 x 12 H2O

tyuyamunite Ca(UO2)2(VO4)2 x 5-8 H2O

uranocircite Ba(UO2)2(PO4)2 x 8-10 H2O

uranophane Ca(UO2)2(HSiO4)2 x 5 H2O

zeunerite Cu(UO2)2(AsO4)2 x 8-10 H2O

Uranium Ore DepositsUranium on Earth is 40 times more common than silver and 500 times more common than gold, and its tiny concentrations can be found almost everywhere, but the challenge is to find areas with adequate concentrations for economically viable extraction.

IAEA classification (1996) lists 15 main deposit types, of which the most economically significant are unconformity-related and sandstone deposits. The latter are common in the Southern Kazakhstan. Uranium reserves distribution

Uranium Mining• The contemporary economically viable

methods of uranium extraction include: Open pit mining Underground mining In-situ leaching Uranium recovery from seawater has been also

tried and found uneconomical.

Uranium mining distribution in 2007

The worldwide production of uranium in 2009 amounted to 50 572 tons, of which 27% was mined in Kazakhstan. Uranium mines operate in some 20 countries, 58% of world production comes from 10 mines in six countries, these six accounting for 85% of the world’s mined uranium. The diagram to the right shows distribution of uranium mining volumes by countries in 2007. Though the early uranium boom of 1950s made Moab, Utah “the world’s uranium capital” and large deposits were discovered in Siberia around that time, nowadays the “Big Three” of uranium market are Canada, Australia and Kazakhstan.

Open-Pit and Underground Mining

These mining methods are no different from mining other metals or minerals, and gold, silver or copper are often mined in association with uranium.

• If orebodies lie close to the surface, they can be accessed by open cut mining. Overlying rock is removed by blasting and excavation, and ore is extracted from a large pit. Disadvantages of the method include much waste material, high airborne dust levels (suppressed by water) and significant alteration of landscape, e.g. damage to fertile soils.

• Underground mining is employed when deposits occur at larger depths. A shaft is sunk in the vicinity of ore veins, and crosscuts are driven horizontally from the shaft to the veins at different levels. Ultimately the shaft and tunnels form a structure of the mine with several stopes, from which the ore is extracted. Though less waste material is removed from underground mines, underground mining of uranium is more dangerous for workers due to radiation exposure and release of Radon-222 gas causing cancer.

Uranium mine near Moab, Utah

In Situ Leach Mining

Some orebodies lie in groundwater in porous unconsolidated material (such as gravel or sand) and may be accessed simply by oxygenating the groundwater and pumping it out with dissolved uranium – this is in-situ leach (ISL) mining. ISL mining means that removal of the uranium minerals is accomplished without any major ground disturbance. Weakly acidified groundwater (or alkaline groundwater where the ground contains a lot of limestone with a lot of oxygen in it) is circulated through an enclosed underground aquifer. The leaching solution dissolves the uranium before being pumped to the surface treatment plant where the uranium is recovered as a precipitate. Most US and Kazakh uranium is produced by this method.

The general scheme of an ISL wellfield

In Situ Leach Mining: Leaching

The uranium ions are selectively transferred into the solution in the result of chemical processes involving decomposition of uranium-bearing substances by leachants and ion exchange. The resulting “pregnant” solution proceeds through several stages, circulating in a closed technological cycle.

In Situ Leach Mining: Processing Unit

As the process is complete, the uranium bearing solution is supplied into sorption columns. The ion-exchange resin intakes the uranium ions from the pregnant solution. Saturated resin is supplied to desorption columns. At this stage the uranium is stripped off the resin by a nitrate solution, producing eluate, a solution of high uranium content. The denitrated resin returns back to the sorption columns. For uranium precipitation from the eluate Kazatomprom uses caustic soda. Obtained slurry is filtered and dried into (“torrefaction”) yellow cake. In the refinery the yellow cake is dissolved for organic extraction, a process allowing for additional purification by organic substances. Following precipitation, drying and thermal decomposition operations the refinery produces U3O8.

In Situ Leach Mining: Recirculation

The solution resulted after sorption collected in settling ponds to reduce remaining solid impurities. The refined solutions after adding sulfuric acid are returned to the well house. From the well house solutions go via a distribution unit to the wellfield under the pressure of 5-6 atmospheres. The leaching solution penetrates through filters of the injection wells to the underground productive horizon where the uranium transfers into the solution. With the use of wellfield pipeline system including the submersible pumps or an airlift or their combination the uranium-bearing solutions are supplied to the surface through the productive wells. The uranium bearing solutions are accumulated from a number of productive wells in accumulating tanks. From the collecting tanks the solutions proceed to sorption columns.

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