Uranium is a radioactive actinide metal that is widely known as the fuel for nuclear power plants. It is an atom with 92 protons and 92 electrons that melts at approximately 1132°C. It was first discovered by a German chemist, Martin Heinrich Klaproth in 1789 in a mineral pitchblende. Although it is a metal, uranium is oxidized rapidly in the air. Therefore, it does not occur in the form of pure metal, instead it exists most generally as uranium oxide (U3O2) in ores.
The metal that was named after the planet Uranus is more common than silver and mercury. It is because uranium was made in supernovas circa 6.6 billion years ago. At present, its radioactive decay, which is the process of an atomic nucleus losing energy through radioactive emission, becomes the primary heat source inside the Earth. This heat affects convection as well as continental drift.
As a nuclear power it can generate a massive amount of energy. Uranium fuel can be used in a reactor for up to averagely five years. The American Nuclear Energy Agency (NEA) estimated that uranium resources and reactors could last for more than 200 years at current consumption rate of 70,000 tons per year. One uranium fuel pellet with the size of a small marble can produce the same amount of energy produced by 481 m3 of natural gas, 807 kg of coal, or 185 L of oil. Moreover, it does not emit harmful pollutant like fossil fuel does, which makes it a safer, cleaner, and more efficient alternative source of power.
Like any other atoms, uranium also exists in several different forms called isotopes. Uranium isotopes are different in their number of neutron, uncharged atomic particle, in their nucleus. The most common isotopes of natural uranium are the fertile uranium-238 (U-238), which makes up of 99.3% and the fissile uranium-235 (U-235), which makes up of 0.7%.
Uranium-235 is fissile because when hit with a neutron, it will split into two lighter atoms, release energy, and shoot three neutrons.The released three neutrons will hit other U-235 atoms. This chain reaction from a fission releases a sufficient energy to power a nuclear reactor. However, there is only 0.7% of it, thus the major isotope U-238 has to be enriched so that it contains more concentration of U-235.
Unfortunately, uranium enrichment can cost a fortune, so a more cost-efficient way to fuel a nuclear reactor is to use the fertile U-238. Although U-238 cannot sustain a chain reaction, it is fertile, meaning it can absorb neutrons and be converted into plutonium-239, which is fissile and able to sustain a chain reaction. 
Uranium is not only useful for nuclear power plants, but it is also useful for medical uses and radioactive dating. In medicine, many radioactive isotopes used for diagnosis or treatment, such as X-ray and radiotherapy for cancer patients, are made from uranium. Moreover, uranium-lead dating is the most advanced method of radioactive dating. It is used to discover the age of rocks or fossils from 1 million years to more than 4.5 billion years ago due to the half-life of uranium that reaches 4.5 billion years.
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