Uranium enrichment and extraction from its ores

• By• Mohamed Atef Mohamed

CONTENTS• WHAT IS URANIUM• CHARACTERISTICS OF URANIUM • Uranium isotopes• Nuclear fuel cycle• MINING OF URANIUM • MILLING OF URANIUM Process • Uranium Enrichment• enrichment grades• Enrichment methods(Diffusion techniques/Centrifuge techniques/Laser

techniques)• FUEL FABRICATION• REACTOR• REPROCESSING

WHAT IS URANIUM? Is a radioactive chemical elements, Atomic number is 92, Among the most prominent qualities: heavy, silvery white, poisonous, metallicAnd a piece of metal pure uranium looks close to the silver metal or steel but very heavy relative to their size. A density of about 20 grams / cubic centimeter, which means that one cubic meter of uranium weighs about 20 tons!

CHARACTERISTICS OF URANIUM

• Uranium is weakly radioactive metal.• It is malleable, ductile, slightly paramagnetic.• Strongly electropositive and is a poor electrical

conductor.• Uranium metal has very high density .• Uranium-235 was the first isotope that was found to

be fissile.• Uranium-235 can be used to make an atomic bomb.

The first nuclear bomb used in war relied on uranium fission.

Uranium isotopes• the most important isotopes of uranium 235U , 234U, 233U,

238U Mostly Uranium-238 by 98% And uranium-235 by 0.7% • (Atomic isotope 235) is available for fission, which gives this

isotope fission huge amounts of energy, It does not automatically splits, but when exposed to a stream of neutrons turns into plutonium 239, Which has automatic fission property, and exists in uranium ore by a small percent 0.7% and is used in nuclear reactors and atomic bombs made from it works initiator of the hydrogen bomb.

• (Atomic isotope 238) and exists in ore by 99% and it non-fissionable, Which is enriched for use in nuclear reactors and is used in the studies and the diagnosis is also used to improve agriculture and chemotherapy is used to track the arrival of the drug to sites by in-vivo. And is used in reactors generating nuclear fuel

-(Atomic isotope 233) also fissionable by Neutrons ,It can be used in atomic reactors which working with helium generated high temperature -(Atomic isotope 234) and present as an impurity in the ore.

Nuclear fuel cycle

• The nuclear fuel cycle is the series of industrial processes which involve the production of uranium and use in nuclear energy power reactors. It involves

• Mining of Uranium • Milling of Uranium • Enrichment of Uranium • Fabrication • Reprocessing of used fuel

MINING OF URANIUM

• The process or business of extracting the precious or valuable metals from the earth either in their native state or in their ores is called mining. Mining of Uranium can be;

• Underground mining • Open pit mining • In situ leaching

Depending on the depth in the ground of the seam of rock containinguranium, the deposit is either mined using surface (open-cast or open-pit) or sub-surface (underground) mining. The uranium ore is extractedthrough mechanical means such as blasting, drilling, pneumatic drilling,picks and shovels, and then transported to the surface.

In situ leaching

• This method also produces „yellowcake“. It is different from theconventional method in that it uses a chemical process to separate theuranium in the earth’s crust from the surrounding rock. The uraniumsolution is then pumped to the surface.

The chemical solution is injected into a drilled hole into the rock at theperiphery of the uranium deposit. This liquid loosens the uranium fromthe rock and binds it; in other words, the uranium is „flushed“ out of the rock. This solution, now supplemented with uranium, is then brought up to the surface through another borehole.

MILLING OF URANIUM Process • of conversion of extracted

uranium ore to a usable form called yellow cake is called milling The ore is first crushed to a fine powder by passing raw uranium ore through crushers and grinders. This is further processed with concentrated acid, alkaline, or peroxide solutions to leach out the uranium. Yellowcake is what remains after drying and filtering.

CONVERSION OF YELLOW CAKE TO UF6

• Yellow cake is converted into UF6 through series of chemical reactions because in enrichment Uf6 will b used.

Uranium EnrichmentNuclear power plants use uranium for fuel. One type of uranium atom – uranium-235 (U235) – is easily split to produce energy. U235 makes up less than 1 percent of natural uranium. To make fuel for reactors, this natural uranium is “enriched” to increase the U235 to between 3 and 5 percent.

enrichment grades• Slightly enriched uranium (SEU)• Slightly enriched uranium (SEU) has a 235U concentration of

0.9% to 2%. This new grade can be used to replace natural uranium (NU) fuel in some heavy water reactors like the CANDU. Fuel designed with SEU could provide additional benefits such as safety improvements or operational flexibility, normally the benefits were considered in safety area while retaining the same operational envelope.

• Safety improvements could lower positive reactivity feedback such as reactivity void coefficient. Operational improvements would consist in increasing the fuel burnup allowing fuel costs reduction because less uranium and fewer bundles are needed to fuel the reactor. This in turn reduces the quantity of used fuel and its subsequent management costs.

Low-enriched uranium (LEU)

• Low-enriched uranium (LEU) has a lower than 20% concentration of 235U. For use in commercial light water reactors (LWR), the most prevalent power reactors in the world, uranium is enriched to 3 to 5% 235U. Fresh LEU used in research reactors is usually enriched 12% to 19.75% U-235, the latter concentration being used to replace HEU fuels when converting to LEU.

Highly enriched uranium (HEU)

• Highly enriched uranium (HEU) has a greater than 20% concentration of 235U. The fissile uranium in nuclear weapon primaries usually contains 85% or more of 235U known as weapon(s)-grade, though theoretically for an implosion design, a minimum of 20% could be sufficient (called weapon(s)-usable) although it would require hundreds of kilograms of material and "would not be practical to design. For criticality experiments, enrichment of uranium to over 97% has been accomplished.

Reprocessed uranium (RepU)

• Reprocessed uranium (RepU) is a product of nuclear fuel cycles involving nuclear reprocessing of spent fuel. RepU recovered from light water reactor (LWR) spent fuel typically contains slightly more U-235 than natural uranium, and therefore could be used to fuel reactors that customarily use natural uranium as fuel, such as CANDU reactors.

• It also contains the undesirable isotope uranium-236 which undergoes neutron capture, wasting neutrons (and requiring higher U-235 enrichment) and creating neptunium-237 which would be one of the more mobile and troublesome radionuclides in deep geological repository disposal of nuclear waste.

Enrichment methods

• Isotope separation is difficult because two isotopes of the same elements have very nearly identical chemical properties, and can only be separated gradually using small mass differences. (235U is only 1.26% lighter than 238U.)

• This problem is compounded by the fact that uranium is rarely separated in its atomic form, but instead as a compound (235UF6 is only 0.852% lighter than 238UF6.) A cascade of identical stages produces successively higher concentrations of 235U. Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.

• There are currently two generic commercial methods employed internationally for enrichment: gaseous diffusion (referred to as first generation) and gas centrifuge (second generation) which consumes only 2% to 2.5% as much energy as gaseous diffusion, with centrifuges being at least a "factor of 20" more efficient.

• Later generation methods will become established because they will be more efficient in terms of the energy input for the same degree of enrichment and the next method of enrichment to be commercialized will be referred to as third generation. Some work is being done that would use nuclear resonance; however there is no reliable evidence that any nuclear resonance processes have been scaled up to production

Diffusion techniques• Gaseous diffusion• The UF6 contains both U235 and the more plentiful U238, which is

heavier. A gaseous diffusion plant processes UF6 in a vessel with small holes in its walls. A U235 molecule will travel faster and strike the walls more often than a molecule of U238, so more U235 flows through the walls.

• The gaseous-diffusion process depends on the separation effect arising from molecular effusion (i.e., the flow of gas through small holes). On average, lighter gas molecules travel faster than heavier gas molecules and consequently tend to collide more often with the porous barrier material. Thus, lighter molecules are more likely to enter the barrier pores than are heavier molecules. For UF 6 , the difference in velocities between molecules containing 235 U and 238 U is small (0.4 percent), and, consequently, the amount of separation achieved by a single stage of gaseous diffusion is small. Therefore, many cascade stages are required to achieve even LEU assays.

• UF6 is a solid at room temperature but becomes a gas when heated above 135 degrees Fahrenheit. The solid UF6 is heated to form a gas, and the gaseous diffusion enrichment process begins. The process separates the lighter U-235 isotopes from the heavier U-238. The gas is forced through a series of porous membranes with microscopic openings. Because the U-235 is lighter, it moves through the barriers more easily. As the gas moves, the two isotopes are separated, increasing the U-235 concentration and decreasing the concentration of U-238.

Schematic of the gaseous diffusion process

• Diffusion equipment tends to be rather large and consumes significant amounts of energy. The main components of a single gaseous-diffusion stage are (1) a large cylindrical vessel, called a diffuser or converter, that contains the barrier; (2) a compressor used to compress the gas to the pressures needed for flow through the barrier; (3) an electric motor to drive the compressor; (4) a heat exchanger to remove the heat of compression; and (5) piping and valves for stage and interstage connections and process control.

• The entire system must be essentially leak free, and the compressors require special seals to prevent both out-leakage of UF 6 and in-leakage of air. The chemical corrosiveness of UF 6 requires use of metals such as nickel or aluminum for surfaces exposed to the gas (e.g., piping and compressors). In addition to the stage equipment, auxiliary facilities for a gaseous-diffusion plant could include a large electrical power distribution system, cooling towers to dissipate the waste process heat, a fluorination facility, a steam plant, a barrier production plant, and a plant to produce dry air and nitrogen.

• The production of a sustainable, efficient separating membrane (barrier) is the key to the successful operation of a diffusion plant. To obtain an efficient porous barrier, the holes must be very small (on the order of one-millionth of an inch in diameter) and of uniform size. The porosity of the barrier must be high to obtain high flow rates through the barrier. The barrier must also be able to withstand years of operation while exposed to corrosive UF 6 gas. Typical materials for the barrier are nickel and aluminum oxide.

• Gaseous diffusion is unlikely to be the preferred technology of a proliferator due to difficulties associated with making and maintaining a suitable barrier, large energy consumption, the requirement for procuring large quantities of specialized stage equipment, large in-process inventory requirements, and long equilibrium times.

Thermal diffusion• Thermal diffusion utilizes the transfer of heat across a

thin liquid or gas to accomplish isotope separation. By cooling a vertical film on one side and heating it on the other side, the resultant convection currents will produce an upward flow along the hot surface and a downward flow along the cold surface. Under these conditions, the lighter 235 U gas molecules will diffuse toward the hot surface, and the heavier 238 U molecules will diffuse toward the cold surface. These two diffusive motions combined with the convection currents will cause the lighter 235 U molecules to concentrate at the top of the film and the heavier 238 U molecules to concentrate at the bottom of the film.

• The thermal-diffusion process is characterized by its simplicity, low capital cost, and high heat consumption. Thermal diffusion in liquid UF 6 was used during World War II to prepare feed material for the EMIS process.

• A production plant containing 2,100 columns (each approximately 15 meters long) was operated in Oak Ridge for less than 1 year and provided a product assay of less than 1% 235 U. Each of these columns consisted of three tubes. Cooling water was circulated between the outer and middle tubes, and the inner tube carried steam. The annular space between the inner and middle tubes was filled with liquid UF 6.

Gas Centrifuge

• The gas centrifuge process has been used in Europe for about 35 years. It uses many rotating cylinders (centrifuges) that are connected in long lines. UF6 gas is placed in the cylinder, which spins at high speed, creating a strong centrifugal force. Heavier U238 gas molecules move to the cylinder wall, while lighter U235 collects near the center. The centrifuge enrichment requires much less electricity than either of the older technologies.

• the gas centrifuge uranium-enrichment process has been highly developed and used to produce both HEU and LEU. It is likely to be the preferred technology of the future due to its relatively low-energy consumption, short equilibrium time, and modular design features.

• In the gas centrifuge uranium-enrichment process, gaseous UF6 is fed into a cylindrical rotor that spins at high speed inside an evacuated casing. Because the rotor spins so rapidly, centrifugal force results in the gas occupying only a thin layer next to the rotor wall, with the gas moving at approximately the speed of the wall. Centrifugal force also causes the heavier 238 UF6 molecules to tend to move closer to the wall than the lighter 235 UF6 molecules, thus partially separating the uranium isotopes. This separation is increased by a relatively slow axial countercurrent flow of gas within the centrifuge that concentrates enriched gas at one end and depleted gas at the other. This flow can be driven mechanically by scoops and baffles or thermally by heating one of the end caps Zippe centrifuge .

• The main subsystems of the centrifuge are (1) rotor and end caps; (2) top and bottom bearing/suspension system; (3) electric motor and power supply (frequency changer); (4) center post, scoops and baffles; (5) vacuum system; and (6) casing. Because of the corrosive nature of UF6 , all components that come in direct contact with UF6 must be must be fabricated from, or lined with, corrosion-resistant materials. The separative capacity of a single centrifuge increases with the length of the rotor and the rotor wall speed. Consequently, centrifuges containing long, high-speed rotors are the goal of centrifuge development programs (subject to mechanical constraints).

• A single centrifuge might produce about 30 grams of HEU per year, about the equivalent of five Separative Work Unit (SWU). As as a general rule of thumb, a cascade of 850 to 1,000 centrifuges, each 1.5 meters long, operating continuously at 400 m/sec, would be able to produce about 20-25 kilograms of HEU in a year, enough for one weapon. One such bomb would require about 6,000 SWU.

Laser Enrichment

• U235 can also be separated using specially-tuned lasers. Lasers can increase the energy in the electrons of a specific isotope, changing its properties and allowing it to be separated. Laser enrichment is more technically complicated but consumes less power and is more efficient. Two laser enrichment methods have been developed, but neither has been used commercially.

• Present systems for enrichment processes using lasers fall into two categories: those in which the process medium is atomic uranium vapor and those in which the process medium is the vapor of a uranium compound. Common nomenclature for such processes include "first category- atomic vapor laser isotope separation (AVLIS or SILVA)" and "second category- molecular laser isotope separation (MLIS or MOLIS).

The atomic vapor laser isotopeseparation(AVLIS)• process is based on the fact that 235 U atoms and 238 U atoms

absorb light of different frequencies (or colors). Although the absorption frequencies of these two isotopes differ only by a very small amount (about one part in a million), the dye lasers used in AVLIS can be tuned so that only the 235 U atoms absorb the laser light.

• As the 235 U atom absorbs the laser light, its electrons are excited to a higher energy state. With the absorption of sufficient energy, a 235 U atom will eject an electron and become a positively charged ion. The 235 U ions may then be deflected by an electrostatic field to a product collector. The 238 U atoms remain neutral and pass through the product collector section and are deposited on a tails collector.

• The AVLIS process consists of a laser system and a separation system. The separator system contains a vaporizer and a collector. In the vaporizer, metallic uranium is melted and vaporized to form an atomic vapor stream. The vapor stream flows through the collector, where it is illuminated by the precisely tuned laser light.

• The AVLIS laser system is a pumped laser system comprised of one laser used to optically pump a separate dye laser, which produces the light used in the separation process. Dye master oscillator lasers provide precise laser beam frequency, timing, and quality control. The laser light emerging from the dye master oscillator laser is increased in power by passage through a dye laser amplifier. A total of three colors are used to ionize the 235 U atoms.

Molecular laser isotope separation (MLIS)

• There are two basic steps involved in the MLIS process. In the first step, UF 6 is irradiated by an infrared laser system operating near the 16 mm wavelength, which selectively excites the 235 UF 6 , leaving the 238 UF 6 relatively unexcited.

• In the second step, photons from a second laser system (infrared or ultraviolet) preferentially dissociate the excited 235 UF 6 to form 235 UF 5 and free fluorine atoms. The 235 UF 5 formed from the dissociation precipitates from the gas as a powder that can be filtered from the gas stream.

• LIS is a stagewise process, and each stage requires conversion of the enriched UF 5 product back to UF 6 for further enrichment. CO 2 lasers are suitable for exciting the 235 UF 6 during the first step.

• In terms of the gas flow for the MLIS process, gaseous UF 6 mixed with a carrier gas and a scavenger gas is expanded through a supersonic nozzle that cools the gas to low temperatures. Hydrogen or a noble gas are suitable as carriers.

• A scavenger gas (such as methane) is used to capture the fluorine atoms that are released as a result of the dissociation of 235 UF 6 molecules.

FUEL FABRICATION

• Fuel fabrication convert enriched UF6 into fuel for nuclear reactors. (UO2) The UO2 powder is pressed into fuel pellets.The pellets are then encased in metal tubes to form fuel rods, which are then arranged into a fuel assembly and ready for introduction into a reactor.

REACTOR

• inside a nuclear reactor the nuclei of uranium-235 atoms split (fission) and, in the process, release energy. This energy is used to heat water and turn it into steam. The steam is used to drive a turbine connected to a generator that produces electricity. The fissioning of uranium is used as a source of heat in a nuclear power plant in the same way that the burning of coal, gas or oil is used as a source of heat in a fossil fuel power plant

USED FUEL

• After 12-24 months the 'spent fuel' is removed from the reactor because it is no longer practical to continue to use as a fuel .The amount of energy produced from a fuel bundle varies with the type of reactor and the policy of the reactor operator. Typically, some 46 million kilowatt-hours of electricity are produced from one tonne of natural uranium. The production of this amount of electrical power from fossil fuels would require the burning of over 20,000 tonnes of black coal or 8.5 million cubic metres of gas.

REPROCESSING

• Used fuel is about 95% uranium-238 but it also contains up to 1% uranium-235 that has not fissioned, about 1% plutonium and 3% fission products In a reprocessing facility the used fuel is separated into its three components: uranium, plutonium and waste, by chopping up the fuel rods and dissolving them in acid to separate the various materials. It enables recycling of the uranium and plutonium into fresh fuel, and produces a significantly reduced amount of waste

References

• http://www.globalsecurity.org/index.html• Production & Enrichment of Uranium• http://en.wikipedia.org/wiki/Enriched_uranium• . وتخصيبه. اليورانيوم ليال حمو مصطفى د أ• URANIUM ENRTCHMENT PLANT

CHARACTERISTICSATRAINING MANUAL FOR THE IAEA (J. M. Whitaker)

• Uranium enrichment methods(Avtor: Stanko Manojlović)

• Extractingg uranium from its ores by D.C. Seidel*