Nuclear Fission
Visão Geral
Somewhere inside a working reactor a single neutron is drifting between atoms at a few kilometres a second. It is not aimed and nobody launched it. In a moment it will wander close enough to one particular nucleus to be swallowed, and that nucleus will tear itself in half and fling out two or three more neutrons of its own. Each of those can do the same thing to a neighbour. Left to itself the count goes 1, 2, 4, 8, 16, and twenty steps later it is over a million. Held in check, it goes 1, 1, 1, 1 for years, and a country keeps its lights on.
The difference between those two sequences is a set of metal rods that can be lowered a few centimetres. This lesson takes you through what fission actually is, why a uranium-235 nucleus has to catch a neutron before it can split, and how the arithmetic of one generation feeding the next turns a single split into a power station or into a disaster. You will learn to draw the chain-reaction diagram the specification asks for, count neutrons in and out of a reactor core, compare the energy of one split nucleus with the energy of burning, and finish by weighing the one problem nobody has fully solved: what to do with the waste.
Objetivos
- Nuclear fission is the splitting of a large and unstable nucleus and the release of energy.
- There are two fissionable substances in common use in nuclear reactors: uranium-235 and plutonium-239. Students should be aware that the majority of nuclear reactors use uranium-235.
- For fission to occur a uranium-235 or plutonium-239 nucleus must first absorb a neutron to make the nucleus unstable. The nucleus undergoing fission splits into two smaller nuclei, releasing two or three neutrons and energy. The amount of energy released during nuclear fission is much greater than that released in a chemical reaction involving a similar mass of material.
- A chain reaction occurs when neutrons from the fission go on to cause further fission. In a nuclear reactor control rods absorb fission neutrons to ensure that on average only one neutron per fission goes on to produce further fission and energy transfer. Students should be able to sketch or complete a labelled diagram to illustrate how a chain reaction may occur.
- Nuclear reactions produce waste which may be dangerous due to its radioactive nature and may remain so for a long time, depending upon its half life and products. The disposal of such waste needs to be managed with care and is a factor that may influence the use of nuclear power for the generation of electricity.
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- What must happen to a uranium-235 nucleus before it can undergo fission? A. It must absorb a neutron B. It must emit an alpha particle C. It must absorb a proton D. It must be heated until it melts Answer: A
- Which pair are the two fissionable substances in common use in nuclear reactors? A. Uranium-235 and uranium-238 B. Uranium-238 and barium-141 C. Uranium-235 and plutonium-239 D. Plutonium-239 and krypton-92 Answer: C
- How many neutrons are released when a uranium-235 nucleus undergoes fission? A. None B. Exactly one C. Two or three D. More than fifty Answer: C
- What is the purpose of the control rods in a nuclear reactor? A. To slow the fission neutrons down so they are absorbed more easily B. To absorb fission neutrons so that on average one neutron per fission causes further fission C. To carry energy away from the core to the boiler D. To hold the fuel rods apart inside the core Answer: B
- A chain reaction begins with one fission. On average two neutrons from each fission go on to cause further fission. How many fissions occur in the fifth generation? A. 8 B. 16 C. 25 D. 32 Answer: B
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