Notes On Nuclear Fission - ICSE Class 10 Physics
Nuclear fission was discovered in 1938 by German scientist Otto Hahn and his associate Fritz Strassmann. Further research was carried out by Lise Meitner.   Principle When the nucleus of a heavy element is bombarded with neutrons, it splits into two or more smaller nuclei, and releases energy. The process is called nuclear fission. When a nucleus is bombarded by a slow neutron, it is captured by the nucleus and a compound nucleus is formed. The captured neutron imparts some extra energy, known as excitation energy, to the nucleus. As a result, the nucleus gets into the excited state. The strong nuclear attractive forces try to keep the nucleus in spherical shape, but the excitation energy tries to deform it. As a result, strong oscillations are set up inside the compound nucleus. Initially, these oscillations distort the shape of the compound nucleus from a sphere to an ellipsoid. If the excitation energy is sufficiently large, then the ellipsoid nucleus attains an elongated dumbbell shape. At this stage, the distance between the nucleons increases, and the effect of the nuclear attractive forces decreases. The electrostatic repulsive force between the two portions of the dumbbell drives them away from each other. When a nucleus of uranium 235 is bombarded by slowly moving thermal neutrons, it split into two smaller fragments identified as (barium 141) and (krypton 92). The amount of energy released in each fission reaction is 200 MeV. n1 + 92U235 → 56Ba141 + 36Kr92 + 3n1 When conditions are favourable, the neutrons released in the first fission reaction can trigger further fission reactions. If this continues, a series of self-sustained nuclear fission reactions take place. A nuclear chain reaction is a process in which a series of self-sustained nuclear fissions take place as the neutrons produced in each fission cause additional fissions. If the number of fissions is allowed to multiply in an uncontrolled manner, it may lead to an explosion. If there is no control over the number of fission reactions taking place, then such a chain reaction is called an uncontrolled chain reaction, in which the number of fissions increases a thousand-fold within a fraction of second.  The basic principle of the atom bomb is an uncontrolled chain reaction. To avoid leakage of neutrons and to have a sustained chain reaction, the sample of  fissionable material must have a minimum size and mass. This minimum size of the sample is called “critical size” and the corresponding mass is called “critical mass”. Nuclear reactor The enormous energy generated in nuclear fission reactions can be utilised only when the reaction is controlled. A chain reaction can be controlled by absorbing the excess neutrons released in the nuclear fission reactions. Nuclear reactors are based on controlled chain reactions. A nuclear reactor gives fresh nuclear fuel, and is called a breeder reactor. However, it also produces a lot of radioactive waste, which is extremely hazardous to both human beings and the environment if not handled carefully.

#### Summary

Nuclear fission was discovered in 1938 by German scientist Otto Hahn and his associate Fritz Strassmann. Further research was carried out by Lise Meitner.   Principle When the nucleus of a heavy element is bombarded with neutrons, it splits into two or more smaller nuclei, and releases energy. The process is called nuclear fission. When a nucleus is bombarded by a slow neutron, it is captured by the nucleus and a compound nucleus is formed. The captured neutron imparts some extra energy, known as excitation energy, to the nucleus. As a result, the nucleus gets into the excited state. The strong nuclear attractive forces try to keep the nucleus in spherical shape, but the excitation energy tries to deform it. As a result, strong oscillations are set up inside the compound nucleus. Initially, these oscillations distort the shape of the compound nucleus from a sphere to an ellipsoid. If the excitation energy is sufficiently large, then the ellipsoid nucleus attains an elongated dumbbell shape. At this stage, the distance between the nucleons increases, and the effect of the nuclear attractive forces decreases. The electrostatic repulsive force between the two portions of the dumbbell drives them away from each other. When a nucleus of uranium 235 is bombarded by slowly moving thermal neutrons, it split into two smaller fragments identified as (barium 141) and (krypton 92). The amount of energy released in each fission reaction is 200 MeV. n1 + 92U235 → 56Ba141 + 36Kr92 + 3n1 When conditions are favourable, the neutrons released in the first fission reaction can trigger further fission reactions. If this continues, a series of self-sustained nuclear fission reactions take place. A nuclear chain reaction is a process in which a series of self-sustained nuclear fissions take place as the neutrons produced in each fission cause additional fissions. If the number of fissions is allowed to multiply in an uncontrolled manner, it may lead to an explosion. If there is no control over the number of fission reactions taking place, then such a chain reaction is called an uncontrolled chain reaction, in which the number of fissions increases a thousand-fold within a fraction of second.  The basic principle of the atom bomb is an uncontrolled chain reaction. To avoid leakage of neutrons and to have a sustained chain reaction, the sample of  fissionable material must have a minimum size and mass. This minimum size of the sample is called “critical size” and the corresponding mass is called “critical mass”. Nuclear reactor The enormous energy generated in nuclear fission reactions can be utilised only when the reaction is controlled. A chain reaction can be controlled by absorbing the excess neutrons released in the nuclear fission reactions. Nuclear reactors are based on controlled chain reactions. A nuclear reactor gives fresh nuclear fuel, and is called a breeder reactor. However, it also produces a lot of radioactive waste, which is extremely hazardous to both human beings and the environment if not handled carefully.

#### Activities

Activity1: Hko.gov.hk has developed an interactive simulation regarding nuclear fission. And how today's civilization depends this for power generation. It also explaines about the generation energy from the collision of uranium with fast moving neutron. Go to Activity Activity2: Nd.edu has developed an interactive simulation regarding nuclear fission. Explaining each of the stage while propagating into a chain reaction with the suitable images. Go to Activity Activity3: Chem.ox.ac.uk has developed an interactive simulation regarding nuclear fission reaction mechanism and propagation of the reaction with suitable images. Go to Activity Activity4:  Doitpoms.ac.uk has developed an interactive simulation reagarding nuclear fission reaction inside nuclear reactor. Go to Activity Activity5:  Visionlearning.com has developed an interactive simulation regarding controlled nuclear reaction and un-controlled nuclear reaction. Go to Activity

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