Basic Nuclear Fission

Introduction:

Nuclear fission is the process of splitting atoms, or fissioning them. This page will explain to you the basics of nuclear fission. Before we talk about that, however, I would like to discuss marbles. Everyone's played with marbles at one time or another, right? Well, imagine about 200 marbles lying on a flat surface, all jumbled together, and roughly forming a circle. What would happen if someone took another marble and threw it at them? They would fly all around in different directions and groups, right? That is exactly what happens in nuclear fission. The filled circle is like an atom's nucleus. The marble being thrown is like a "neutron bullet". The only differences are that the marbles are protons and neutrons and the protons and neutrons aren't in a filled circle, but in the actual atom are in the shape of a sphere. Of course, an atom is also a bit more complicated than a pack of marbles.

Choosing the Bullet:

When we spoke about the marble analogy earlier, we said that the marble being thrown is a like a "neutron bullet". But what does this mean, and why not use another type of particle to "throw" at a nucleus to fission it? First, what particles with distinct mass are available to launch at a nucleus? Think back to our lesson on radioactivity. Recall that two particles emitted by radioactive elements are the  particle and the neutron. (There are other particles emitted too, but they are generally much smaller than the neutron and the  particle.) Recall that the  particle is essentially a 4He nucleus. Now, let's review the structure of an atom. Remember that an atomic nucleus is made up of positive protons and neutral neutrons? Because of this, the nucleus carries an overall positive charge. So, if we were to launch another particle with a positive charge at a nucleus, it wouldn't get there. Why wouldn't it get there? The answer lies in magnetism. Have you ever used magnets? If you have, you'd know that two like poles of a magnet repel each other. A positive particle and the positive nucleus would repel each other in the same way. The  particle is positive. Why? Well, it's composed of two protons and two neutrons. Its positive protons give it a positive charge. Because it's positive, it would get repelled away from another positive nucleus. So, the only thing left is the neutron. The neutron is electrically neutral and thus would not get repelled from a positive nucleus.

Fissile Isotopes:

Fissile isotopes are isotopes of an element that can be split through fission. Only certain isotopes of certain elements are fissile. For example, one isotope of uranium, 235U, is fissile, while another isotope, 238U, is not. Other examples of fissile elements are 239Pu and 232Th. An important factor affecting whether or not an atom will fission is the speed at which the bombarding neutron is moving. If the neutron is highly energetic (and thus moving very quickly), it can cause fission in some elements that a slower neutron would not. For example, thorium 232 requires a very fast neutron to induce fission. However, uranium 235 needs slower neutrons. If a neutron is too fast, it will pass right through a 235U atom without affecting it at all.

Splitting the Uranium Atom:

Uranium is the principle element used in nuclear reactors and in certain types of atomic bombs. The specific isotope used is 235U. When a stray neutron strikes a 235U nucleus, it is at first absorbed into it. This creates 236U. 236U is unstable and this causes the atom to fission. The fissioning of 236U can produce over twenty different products. However, the products' masses always add up to 236. The following two equations are examples of the different products that can be produced when 235U fissions: 

• 235U + 1 neutron  2 neutrons + 92Kr + 142Ba + ENERGY
• 235U + 1 neutron  2 neutrons + 92Sr + 140Xe + ENERGY

Nuclear Fission If a massive nucleus like uranium-235 breaks apart (fissions), then there will be a net yield of energy because the sum of the masses of the fragments will be less than the mass of the uranium nucleus. If the mass of the fragments is equal to or greater than that of iron at the peak of the binding energy curve, then the nuclear particles will be more tightly bound than they were in the uranium nucleus, and that decrease in mass comes off in the form of energy according to the Einstein equation. For elements lighter than iron, fusion will yield energy. The fission of U-235 in reactors is triggered by the absorption of a low energy neutron, often termed a "slow neutron" or a "thermal neutron". Other fissionable isotopes which can be induced to fission by slow neutrons are plutonium-239, uranium-233, and thorium-232. Uranium-235 Fission

Index Fission concepts

HyperPhysics***** Nuclear For the generation of electrical power by fission, see Nuclear power. "Splitting the atom" redirects here. For the EP, see Splitting the Atom. Nuclear physics Radioactive decay Nuclear fission Nuclear fusion Classical decays[show] Advanced decays[show] Emission processes[show] Capturing[show] High energy processes[show] Nucleosynthesis[show] Scientists[show] v · d · e An induced fission reaction. A slow-moving neutron is absorbed by a uranium-235 nucleus turning it briefly into a uranium-236 nucleus; this in turn splits into fast-moving lighter elements (fission products) and releases three free neutrons. In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which thenucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons andphotons (in the form of gamma rays), and releasing a tremendous amount of energy. The two nuclei produced are most often of comparable size, typically with a mass ratio around 3:2 for commonfissile isotopes.[1][2] Most fissions are binary fissions, but occasionally (2 to 4 times per 1000 events), three positively-charged fragments are produced in a ternary fission. The smallest of these ranges in size from a proton to an argon nucleus. Fission is usually an energetic nuclear reaction induced by a neutron, although it is occasionally seen as a form of spontaneous radioactive decay, especially in very high-mass-number isotopes. The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum-tunnelling processes such as proton emission, alpha decay and cluster decay, which give the same products every time. Fission of heavy elements is an exothermic reaction which can release large amounts of energyboth as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). In order for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes possible a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon. The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very dense source of energy. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over thedestructive potential of nuclear weapons may counterbalance the desirable qualities of fission as an energy source, and give rise to ongoing political debate over nuclear power. Contents  [hide] 1 Physical overview 1.1 Mechanics 1.2 Energetics 1.2.1 Input 1.2.2 Output 1.3 Product nuclei and binding energy 1.4 Origin of the active energy and the curve of binding energy 1.5 Chain reactions 1.6 Fission reactors 1.7 Fission bombs 2 History 2.1 Discovery of fission 2.2 The fission chain reaction 2.3 Manhattan Project and beyond 2.4 Natural fission chain-reactors on Earth 3 See also 4 References 5 External links R Nave

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Uranium-235 Fission In one of the most remarkable phenomena in nature, a slow neutron can be captured by a uranium-235 nucleus, rendering it unstable toward nuclear fission. A fast neutron will not be captured, so neutrons must be slowed down by moderation to increase their capture probability in fission reactors. A single fision event can yield over 200 million times the energy of the neutron which triggered it! More detailed illustration Comparison with fusion Some history. Detailed energy release calculation

Index Fission concepts

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Uranium Fuel Natural uranium is composed of 0.72% U-235 (the fissionable isotope), 99.27% U-238, and a trace quantity 0.0055% U-234 . The 0.72% U-235 is not sufficient to produce a self-sustaining critical chain reaction in U.S. style light-water reactors, although it is used in Canadian CANDU reactors. For light-water reactors, the fuel must be enriched to 2.5-3.5% U-235. Uranium is found as uranium oxidewhich when purified has a rich yellow color and is called "yellowcake". After reduction, the uranium must go through an isotope enrichment process. Even with the necessity of enrichment, it still takes only about 3 kg of natural uranium to supply the energy needs of one American for a year.

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Fissionable Isotopes While uranium-235 is the naturally occuring fissionable isotope, there are other isotopes which can be induced to fission by neutron bombardment. Plutonium-239 is also fissionable by bombardment with slow neutrons, and both it and uranium-235 have been used to make nuclear fission bombs. Plutonium-239 can be produced by "breeding" it from uranium-238. Uranium-238, which makes up 99.3% of natural uranium, is not fissionable by slow neutrons. U-238 has a small probability for spontaneous fission and also a small probability of fission when bombarded with fast neutrons, but it is not useful as a nuclear fuel source. Some of the nuclear reactors at Hanford, Washington and the Savannah-River Plant (SC) are designed for the production of bomb-grade plutonium-239. Thorium-232 is fissionable, so could conceivably be used as a nuclear fuel. The only other isotope which is known to undergo fission upon slow-neutron bombardment is uranium-233. Nuclear fission For the generation of electrical power by fission, see Nuclear power. "Splitting the atom" redirects here. For the EP, see Splitting the Atom. Nuclear physics Radioactive decay Nuclear fission Nuclear fusion Classical decays[show] Advanced decays[show] Emission processes[show] Capturing[show] High energy processes[show] Nucleosynthesis[show] Scientists[show] v · d · e An induced fission reaction. A slow-moving neutron is absorbed by a uranium-235 nucleus turning it briefly into a uranium-236 nucleus; this in turn splits into fast-moving lighter elements (fission products) and releases three free neutrons. In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which thenucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons andphotons (in the form of gamma rays), and releasing a tremendous amount of energy. The two nuclei produced are most often of comparable size, typically with a mass ratio around 3:2 for commonfissile isotopes.[1][2] Most fissions are binary fissions, but occasionally (2 to 4 times per 1000 events), three positively-charged fragments are produced in a ternary fission. The smallest of these ranges in size from a proton to an argon nucleus. Fission is usually an energetic nuclear reaction induced by a neutron, although it is occasionally seen as a form of spontaneous radioactive decay, especially in very high-mass-number isotopes. The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum-tunnelling processes such as proton emission, alpha decay and cluster decay, which give the same products every time. Fission of heavy elements is an exothermic reaction which can release large amounts of energyboth as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). In order for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes possible a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon. The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very dense source of energy. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over thedestructive potential of nuclear weapons may counterbalance the desirable qualities of fission as an energy source, and give rise to ongoing political debate over nuclear power. Contents  [hide] 1 Physicaloverview 1.1 Mechanics 1.2 Energetics 1.2.1 Input 1.2.2 Output 1.3 Product nuclei and binding energy 1.4 Origin of the active energy and the curve of binding energy 1.5 Chain reactions 1.6 Fission reactors 1.7 Fission bombs 2 History 2.1 Discovery of fission 2.2 The fission chain reaction 2.3 Manhattan Project and beyond 2.4 Natural fission chain-reactors on Earth 3 See also 4 References 5 External links