The mass of uranium or other fissile element that is required in order to sustain a chain reaction is called the critical mass. The process of nuclear fission is best known within the context of fission bombs and as the process that operates within nuclear power plants.
Designing a workable fission bomb presents many technical challenges. A mass of fissile material that exceeds the critical mass is unstable, so you must begin with a smaller, non-critical mass and somehow create one within a few microseconds. In the original design, this was accomplished by taking two non-critical pieces and forcing them together very rapidly. Each of the uranium fragments are less than a critical mass, but when they collide, they form a mass capable of sustaining the nuclear chain reaction.
The assembly stays together for a few microseconds before the energy released from the fission blows it to pieces. The trick is designing nuclear devices like this is to keep them together long enough so that enough energy is released. At the point where the chain reaction can become self-sustaining, this is referred to as critical mass.
In an atomic bomb, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. The amount of a fissionable material's critical mass depends on several factors; the shape of the material, its composition and density, and the level of purity.
A sphere has the minimum possible surface area for a given mass, and hence minimizes the leakage of neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction. Nuclear chain reaction : A possible nuclear fission chain reaction. In the first step, a uranium atom absorbs a neutron, and splits into two new atoms fission fragments , releasing three new neutrons and a large amount of binding energy.
In the second step, one of those neutrons is absorbed by an atom of uranium, and does not continue the reaction. Another neutron leaves the system without being absorbed. However, one neutron does collide with an atom of uranium, which then splits and releases two neutrons and more binding energy. In the third step, both of those neutrons collide with uranium atoms, each of which splits and releases a few neutrons, which can then continue the reaction. This chain reaction can be controlled using neutron poisons and neutron moderators to change the portion of neutrons that can cause more fissions.
Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if unsafe conditions are detected. The amount and nature of neutron moderation affects reactor controllability and safety. Since moderators both slow and absorb neutrons, there is an optimum amount of moderator to include in a given geometry of reactor core. In a nuclear reactor, the neutron population at any instant is a function of the rate of neutron production and the rate of neutron loss.
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. In U reactors, this time might be a long as many minutes. A common type of startup neutron source is a mixture of an alpha particle emitter such as Am americium with a lightweight isotope such as 9 Be beryllium Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants convert the energy released from nuclear fission.
The heat is removed from the reactor core by a cooling system that generates steam. The steam drives a turbine which runs a generator to produce electricity. Privacy Policy. Skip to main content. Nuclear Chemistry. Search for:. Nuclear Fission Nuclear Fission Nuclear fission occurs when an atom splits into two or more smaller atoms, most often the as the result of neutron bombardment. Learning Objectives Describe the process of nuclear fission.
Nuclear fission occurs with heavier elements, where the electromagnetic force pushing the nucleus apart dominates the strong nuclear force holding it together. In order to initiate most fission reactions, an atom is bombarded by a neutron to produce an unstable isotope, which undergoes fission. When neutrons are released during the fission process, they can initiate a chain reaction of continuous fission which sustains itself. Key Terms fissile : Capable of undergoing nuclear fission.
Neutron Bombardment In order to initiate fission, a high-energy neutron is directed towards a nucleus, such as U. The Atomic Bomb Atomic bombs are nuclear weapons that use the energetic output of nuclear fission to produce massive explosions. Learning Objectives Describe the chemical reaction which fuels an atomic bomb.
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