uranium-235
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What is uranium-235?
Uranium-235 (U-235) is a radioactive isotope of uranium, containing 92 protons and 143 neutrons in its nucleus. It is the only naturally occurring fissile material, meaning its nucleus undergoes nuclear fission when colliding with a slow neutron.
U-235 makes up about 0.72 percent of naturally occurring uranium, with a half-life of 704 million years, decaying into thorium-231 and eventually lead-207. For it to be useful, uranium-235 must be separated from the more abundant uranium-238 through methods like gaseous diffusion or gas centrifugation. Low-enrichment uranium (2–3% uranium-235) is used as fuel for light-water nuclear reactors, while fully enriched uranium (97–99% uranium-235) is used in nuclear weapons.
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How is uranium-235 different from other types of uranium?
Uranium-235 (U-235) is a radioactive isotope of uranium, notable because it is the only naturally occurring fissile material. This means its nucleus can undergo nuclear fission when it collides with a slow neutron, releasing a significant amount of thermal energy, gamma rays, and more neutrons.
Other uranium isotopes can undergo fission, but U-235 does so more readily and emits more neutrons per fission. Naturally occurring uranium consists of a mixture of isotopes: uranium-238 (99.27%), uranium-235 (0.72%), and uranium-234 (0.006%). For many applications, uranium-235 must be separated from the more abundant uranium-238.
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Why is uranium-235 important in nuclear energy?
Uranium-235 (U-235) is the only naturally occurring fissile material, meaning its nucleus can undergo nuclear fission when colliding with a slow neutron, releasing a significant amount of thermal energy, gamma rays, and additional neutrons. This process is crucial for nuclear energy because these released neutrons can strike other uranium-235 nuclei, causing them to fission and release more neutrons, leading to a chain reaction.
In nuclear reactors, this chain reaction is controlled to produce heat, which then generates electrical power. When completely fissioned, 1 kg of uranium-235 releases energy equivalent to 17 kilotons of TNT. The practical significance of actinoids arises from the fissionability, or potential for splitting, of nuclei of certain of their isotopes.
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How does uranium-235 undergo nuclear fission?
Uranium-235 (U-235) is a radioactive isotope that undergoes nuclear fission when its nucleus collides with a slow neutron. This collision causes the U-235 nucleus to split into two fragments, each containing roughly half the number of protons and neutrons of the original nucleus.
During this splitting, a significant amount of thermal energy, gamma rays, and two or more neutrons are released. These released neutrons can then strike other surrounding uranium nuclei, causing them to fission and release more neutrons, leading to a chain reaction. If this chain reaction is uncontrolled, it can result in an explosion, such as in an atomic bomb. However, in a nuclear reactor, the chain reaction is carefully maintained at a constant level for power generation.
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What is meant by uranium enrichment?
Uranium enrichment is the process of concentrating the uranium-235 isotope from natural uranium for use in nuclear reactors and weapons. Natural uranium contains only about 0.72% uranium-235, with the remainder being mostly uranium-238.
To increase the concentration of uranium-235, methods like gaseous diffusion and gas centrifuging are employed. Gaseous diffusion involves uranium hexafluoride gas passing through a porous barrier, where the lighter uranium-235 isotope diffuses faster than the heavier uranium-238. Gas centrifuging uses high-speed centrifuges to separate the isotopes based on their mass. These methods increase the proportion of uranium-235, enabling efficient nuclear fission.
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What are the uses and risks of uranium-235 in society?
Uranium-235 (U-235) is a radioactive isotope of uranium, notable as the only naturally occurring fissile material.
Uses
- Nuclear Fuel Low-enrichment uranium-235 fuels light-water nuclear reactors.
- Nuclear Weapons Fully enriched uranium-235 is used in atomic bombs.
- Chain Reactions U-235 readily undergoes fission, emitting multiple neutrons, which sustains controlled chain reactions in nuclear reactors.
Risks
- Radioactivity Nuclear reactors contain large amounts of radioactive isotopes. Escape of radioactivity can cause increased probability of cancer, cellular damage, developmental abnormalities in children, and possibly death.
- Accidents Accidents can lead to emergency situations despite preventive and mitigating measures.
- Health Concerns Exposure to high levels of ionizing radiation can have deleterious effects.
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uranium-235 (U-235), radioactive isotope of the element uranium with a nucleus containing 92 protons and 143 neutrons. Uranium-235 is the only naturally occurring fissile material; that is, the uranium-235 nucleus undergoes nuclear fission when it collides with a slow neutron (a neutron with a kinetic energy less than 1 electron volt).
Uranium-235 constitutes about 0.72 percent of all naturally occurring uranium. (Most naturally occurring uranium is uranium-238.) It has a half-life of 704 million years, decaying to thorium-231, with the radioactive decay chain eventually ending in the stable isotope lead-207.
Uranium-235 must be separated from the more plentiful isotope uranium-238 for its various uses. Any of several methods—gaseous diffusion, gas centrifugation, liquid thermal diffusion—can be employed to separate and concentrate the fissile uranium-235 isotope into several grades, from low-enrichment (2 to 3 percent uranium-235) to fully enriched (97 to 99 percent uranium-235). Low-enrichment uranium is typically used as fuel for light-water nuclear reactors; fully enriched uranium is typically used for nuclear weapons.
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Gases can diffuse through the small pores present in many materials. The diffusion proceeds in a random manner as gas molecules bounce off the walls of the porous medium. The average time a molecule of gas takes to traverse such a barrier depends on its velocity and certain other factors. According to the kinetic theory of gases, at a given temperature a lighter molecule will have a larger average velocity than a heavier one. This result provides the basis for a separation method. Natural uranium in the form of uranium hexafluoride (UF6) gas is diffused from one compartment of a chamber to another through a porous barrier. Since the molecules of 235UF6 travel at a higher velocity than those of 238UF6, they pass into the second compartment more rapidly than the latter. Because the percentage of uranium-235 increases only slightly after traversal of the barrier, the process must be repeated hundreds of thousands of times to obtain the desired concentration of the isotope. This method was one of those used by the American Manhattan Project in World War II to make the uranium-235 for the first atomic bombs.
Because gaseous diffusion uses large amounts of energy, it has since been supplanted by isotope separation with centrifuges. The UF6 gas is fed into a high-speed centrifuge. The centrifuge is balanced very well at the top and bottom and spins at an extremely high rate. Because of the relative centripetal forces that each atom experiences, the lighter species235UF6 tends to concentrate near the centre of the spinning centrifuge, while the heavier 238UF6 accumulates along the wall. These mixtures are then siphoned off.
Many isotopes of uranium can undergo fission, but uranium-235 undergoes fission more readily and emits more neutrons per fission than other such isotopes. When a neutron strikes the nucleus of an atom of the isotope uranium-235, it causes that nucleus to split into two fragments, each of which is a nucleus with about half the protons and neutrons of the original nucleus. In the process of splitting, a great amount of thermal energy, as well as gamma rays and two or more neutrons, is released. Under certain conditions, the escaping neutrons strike and thus fission more of the surrounding uranium nuclei, which then emit more neutrons that split still more nuclei. This series of rapidly multiplying fissions culminates in a chain reaction in which nearly all the fissionable material is consumed, in the process generating the explosion of what is known as an atomic bomb. When completely fissioned, 1 kg (2.2 pounds) of uranium-235 releases the energy equivalently produced by 17,000 tons, or 17 kilotons, of TNT. In a nuclear reactor the chain reaction is maintained at a controlled, nearly constant level. Nuclear reactors are so designed that they cannot explode like atomic bombs.