What is Nuclear Fission?

In the process of nuclear fission, a heavy nucleus (mass number > 200) is split to form smaller nuclei of intermediate mass and one or more neutrons. This process releases a large amount of energy because the heavy nucleus is less stable than its products.

The first nuclear fission reaction studied was that of uranium-235 bombarded with slow neutrons, whose velocity is comparable to that of air molecules at room temperature. Under these conditions, uranium-235 undergoes nuclear fission. This reaction is very complex, since more than 30 different elements are found in the nuclear fission products.

Although it is possible to cause fission in many heavy nuclei, only the fission of uranium-235 (naturally occurring) and that of the artificial isotope plutonium-239 are of any practical significance. The outstanding feature of the fission of uranium-235 is not the enormous amount of energy released, but the fact that the fission produces more neutrons than were captured at the beginning of the process. Because of this property it is possible to obtain a nuclear chain reaction, i.e. a sequence of self-sustaining nuclear fission reactions.

The neutrons generated in the initial stages of fission can induce fission in other uranium-235 nuclei, which in turn produce more neutrons, and so on. In less than a second, the reaction becomes uncontrollable, releasing a large amount of heat into the surroundings.

There are two types of fission reactions. For a chain reaction to take place, the sample must have enough uranium-235 to capture the neutrons; otherwise, many neutrons will escape from the sample and the chain reaction does not progress (as depicted in the figure). In this situation, the mass of the sample is said to be subcritical. When the amount of fissile material is equal to or greater than the critical mass, i.e. the minimum mass of fissile material necessary to generate a nuclear chain reaction. In this case, the uranium-235 nuclei will capture most of the neutrons and a chain reaction will form.

Nuclear Reactors and Nuclear Fission

Nuclear reactors are an example of the peaceful application of nuclear energy. They work by causing fission and using the heat released to boil water. A turbine converts this steam into electric current. Although the process is simple, the difficulty lies in keeping the reaction under control. To do this, carbon or boron rods (control system) are used, which work by trapping neutrons, thus damping the fission when necessary. The nuclear fuel used in nuclear reactors is natural uranium. One gram of uranium produces energy equivalent to 2,500 kg of coal.

The negative aspect of a nuclear reactor is the radioactive waste it produces, since there is currently no recycling method, so they must be stored in lead-shielded drums buried deep in the ground, waiting for their remaining radioactivity to die out. In addition, nuclear disasters can occur due to accidents or terrorist attacks on nuclear reactors. This highlights the potential danger of having a nuclear reactor near a city. In South America, nuclear technology is also present, as Argentina and Brazil have more than three nuclear reactors used for different purposes.

The Atomic Bomb and Nuclear Fission

The first application of fission was the atomic bomb, here the critical mass of uranium is separated and the fission reaction is induced by means of a conventional explosive. At the start of the chemical explosion the critical mass of Uranium-235 is brought together and the chain reaction takes place.

They are one of the most nefarious inventions in human history. Nuclear weapons use the principle of nuclear fission to release an enormous amount of energy and heat. The power of atomic bombs is measured in kilotons (kt), which is equivalent to 1,000 tons of T.N.T., or it can be expressed in megatons. or it can be expressed in megatons (Mt), which is equivalent to 1,000,000 tons of T.N.T.

Nuclear weapons are used by the various world powers as a deterrent against possible enemy attacks. The problem lies in the possibility of certain governments or terrorist groups using these weapons against humanity or a certain ethnic group for destructive purposes.

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