uranium and enrichment

Uranium and enrichment

History

For many years, uranium was primarily as a coolant in ceramic glazes, producing colors that   ranged from orange - red to lemon yellow. It was also used for tinting in early photography . Its radio active  properties were not recognized until 1896, and its potential for use an energy source was not realized until middle of 20th century. It is also used in weapon application , and small nuclear reactors to produce isotopes for medical and industrial purposes around world.

Origin of uranium

Small amounts of uranium are found almost every where in soil, rock and water. However  concentrated deposits are normally cover over with vegetates uranium has been mined in Canada ,the south west U.S.,Australia , part of former Soviet Union, Namibia , South Africa  Niger and elsewhere.

Properties of uranium

Uranium is a radio active element that occurs naturally in varying but small amounts in soil, rocks, water, plants, animal and all human being. It is heaviest naturally occurred element, with atomic number 92. In its pure form, uranium is a silver colored heavy metal  that is a nearly twice as dense as lead. In nature, uranium atom exists as several isotopes which are identified by total number of protons and neutrons in the nucleus : uranium U-238, uranium U-235 and uranium  U-234 (isotopes of element have same number of protons with nucleus but different number of neutrons ) . In a tipped sample of natural uranium most weight 99.27 per cent consists of atom of U-238, about 0.72 per cent U-235 and a very small amount (0.0055 per cent by weight ) is uranium U-234

Radio active isotopes of  uranium

The three naturally occurring isotopes of  uranium are each radio active, which means the nuclei spontaneously disintegrate or ‘decay’ radio activity emitted from uranium isotopes consists of Alpha  similar  to visible light except with higher energy and more penetrating power). The rate at which is the nuclei is an isotope sample decay is called activity , which is the  number disintegrated that occur for second. The activity of isotope sample decreases with the time as atoms disintegrate . Each isotope has its own -half life which is the time it takes for half of atoms in a sample of isotope decay and activity of sample to be proportionally reduced.

Uranium -235 and fission

In addition to being naturally radio active, the uranium U-235 isotopes of uranium is capable of fission, the spitting of nucleus into two parts,  triggered by absorption of neutron. When this spitting occurs , consider energy is released, which makes uranium U-235 valuable as a fuel in reactor used to generate electricity and for use in national defense. 

Enrichment process

Using uranium as fuel in the types of nuclear is common in united states requires that uranium enriched so that the percentage of the uranium U-235 isotope is increased, typically to 3 per cent to 5 per cent. Uranium enrichment is a isotopic separation process that increases the proportion of uranium -235 isotope in relation to uranium U-238 in natural uranium.

The enrichment process used in the West involves combining with fluorine to make uranium hexafluoride (UF6) followed by gaseous diffusion . The UF6 out put from gaseous diffusion is into two streams- one in increased, or enriched in its percentage of U-235 and other is reduced or depleted in its percentage of U-235. The  “deflected  UF-6”  after gaseous diffusion the enriched uranium hexafluoride is for further process, while depleted UF-6 is generally stored.

The uranium enrichment process was first developed in the year 1940 as part of Manhattan project.

Gaseous diffusion

In a gaseous diffusion , uranium hexafluoride  is divided into two separate stream and U-235 selectively diffusion s from one  stream to other.

Before processing by gaseous diffusion uranium is first converted from uranium oxide( U308) to hexafluoride (UF6). The uranium hexafluoride (UF6) is heated and converted  from solid to  a gas . This gas is then forced  though a series of compressors and converts that contain porous barriers. Because  uranium U-235 has slightly lighter isotope mass than U-238 , UF6  molecules made with uranium U-235 diffuse  though the barriers at slightly higher rate than the molecule containing uranium  U-238. At the end of process, there are two UF6 stream, with one stream having higher concentration  of U-235 than other, the steam having greater U-235 concentration is referred to as  enriched UF6  while stream that is reduced  in concentration of U-235 is referred as depleted UF6. The depleted UF6 can be converted to other chemical forms, as depleted uranium oxide or  depleted uranium metal.

Centrifuge technique  

The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder rotation creates a storing centrifuge force so that heavier gas molecule containing U-238 uranium move toward the out side of the cylinder and lighter gas molecule rich in U-235 collect closer to centre. It requires much less energy to achieve the same separation than older gaseous diffusion process, which it has largely replaced and so is the current method of choice and is termed second generation. It has separation factor per stage 1.3 relative to gaseous diffusion of 1.005 which translates to about one -fifth of energy requirements. Gas centrifuge technique produce about 54 per cent of world enrichment uranium.

Low enriched uranium

Low enriched uranium (LEU) has a lower than 20 per cent concentration of U-235 for use in light water reactors  in the world. Uranium is enriched 3 per cent to 5 per cent in U-235. LEU  used in research reactors is usually  enriched  12 per cent to 19.75 per cent in U-235

Highly enriched   uranium

Highly enriched uranium (HEU ) has greater than 20 per cent concentration of U-235  or U-233 . The fission  uranium in nuclear weapon usually contain 85 per cent or more U-235 known as weapon grade
though  a crude in efficient weapon 20 per cent weapon usable. For critical experiments enrich uranium to over 97 per cent has been accomplished.