Isotope of Uranium which, in its purest form, is used as fuel for nuclear reactors and weapons.
U-235 will eventually degrade into stable Lead 206.
Uranium is a heavy metal, heavier than gold, and not only does it have the largest atoms of any natural element, the atoms that comprise Uranium have far more neutrons than protons. This does not enhance their capacity to split, but it does have an important bearing on their capacity to facilitate an explosion.

There are two isotopes of Uranium. Natural Uranium consists mostly of isotope U-238, which has 92 protons and 146 neutrons (92+146=238). Mixed with this isotope, one will find a 0.6% accumulation of U-235, which has only 143 neutrons. This isotope, unlike U-238, has atoms that can be split, thus it is termed "fissionable" and useful in making atomic bombs. Being that U-238 is neutron-heavy, it reflects neutrons, rather than absorbing them like its brother isotope, U-235. (U-238 serves no function in an atomic reaction, but its properties provide an excellent shield for the U-235 in a constructed bomb as a neutron reflector. This helps prevent an accidental chain reaction between the larger U-235 mass and its `bullet' counterpart within the bomb. Also note that while U-238 cannot facilitate a chain-reaction, it can be neutron-saturated to produce Plutonium (Pu-239). Plutonium is fissionable and can be used in place of Uranium-235 {albeit, with a different model of detonator} in an atomic bomb.

Both isotopes of Uranium are naturally radioactive. Their bulky atoms disintegrate over a period of time. Given enough time, (over 100,000 years or more) Uranium will eventually lose so many particles that it will turn into the metal lead. However, this process can be accelerated. This process is known as the chain reaction. Instead of disintegrating slowly, the atoms are forcibly split by neutrons forcing their way into the nucleus. A U-235 atom is so unstable that a blow from a single neutron is enough to split it and henceforth bring on a chain reaction. This can happen even when a critical mass is present. When this chain reaction occurs, the Uranium atom splits into two smaller atoms of different elements, such as Barium and Krypton.

When a U-235 atom splits, it gives off energy in the form of heat and Gamma radiation, which is the most powerful form of radioactivity and the most lethal. When this reaction occurs, the split atom will also give off two or three of its `spare' neutrons, which are not needed to make either Barium or Krypton. These spare neutrons fly out with sufficient force to split other atoms they come in contact with. See chart below In theory, it is necessary to split only one U-235 atom, and the neutrons from this will split other atoms, which will split on and so forth. This progression does not take place arithmetically, but geometrically. All of this will happen within a millionth of a second.

The minimum amount to start a chain reaction as described above is known as SuperCritical Mass. The actual mass needed to facilitate this chain reaction depends upon the purity of the material, but for pure U-235, it is 110 pounds (50 kilograms), but Uranium is never quite pure, so in reality more will be needed.

Uranium is not the only material used for making atomic bombs. Another material is the element Plutonium, in its isotope Pu-239. Plutonium is not found naturally (except in minute traces) and is always made from Uranium. The only way to produce Plutonium from Uranium is to process U-238 through a nuclear reactor. After a period of time, the intense radioactivity causes the metal to pick up extra particles, so that more and more of its atoms turn into Plutonium.

Uranium-235 is very difficult to extract. In fact, for every 25,000 tons of Uranium ore that is mined from the earth, only 50 tons of Uranium metal can be refined from that, and 99.3% of that metal is U-238 which is too stable to be used as an active agent in an atomic detonation. To make matters even more complicated, no ordinary chemical extraction can separate the two isotopes since both U-235 and U-238 possess precisely identical chemical characteristics. The only methods that can effectively separate U-235 from U-238 are mechanical methods.

U-235 is slightly, but only slightly, lighter than its counterpart, U-238. A system of gaseous diffusion is used to begin the separating process between the two isotopes. In this system, Uranium is combined with fluorine to form Uranium Hexafluoride gas. This mixture is then propelled by low- pressure pumps through a series of extremely fine porous barriers. Because the U-235 atoms are lighter and thus propelled faster than the U-238 atoms, they could penetrate the barriers more rapidly. As a result, the U-235's concentration became successively greater as it passed through each barrier. After passing through several thousand barriers, the Uranium Hexafluoride contains a relatively high concentration of U-235 -- 2% pure Uranium in the case of reactor fuel, and if pushed further could (theoretically) yield up to 95% pure Uranium for use in an atomic bomb.

Once the process of gaseous diffusion is finished, the Uranium must be refined once again. Magnetic separation of the extract from the previous enriching process is then implemented to further refine the Uranium. This involves electrically charging Uranium Tetrachloride gas and directing it past a weak electromagnet. Since the lighter U-235 particles in the gas stream are less affected by the magnetic pull, they can be gradually separated from the flow.

Following the first two procedures, a third enrichment process is then applied to the extract from the second process. In this procedure, a gas centrifuge is brought into action to further separate the lighter U-235 from its heavier counter-isotope. Centrifugal force separates the two isotopes of Uranium by their mass. Once all of these procedures have been completed, all that need be done is to place the properly molded components of Uranium-235 inside a warhead that will facilitate an atomic detonation.

Supercritical mass for Uranium-235 is defined as 110 lbs (50 kg) of pure Uranium.

Depending on the refining process(es) used when purifying the U-235 for use, along with the design of the warhead mechanism and the altitude at which it detonates, the explosive force of the A-bomb can range anywhere from 1 kiloton (which equals 1,000 tons of TNT) to 20 megatons (which equals 20 million tons of TNT -- which, by the way, is the smallest strategic nuclear warhead we possess today. {Point in fact -- One Trident Nuclear Submarine carries as much destructive power as 25 World War II's}).

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