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Nuclear fusion and fission. Fusion is … combination of nuclides into a new and heavier nucleus Happens in stars, such as our sun H + H He + energy Fission is … breaking apart (through decay) of an unstable heavy nucleus into smaller nuclei (and particles). Discovery. - PowerPoint PPT Presentation
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Nuclear fusion and fission
• Fusion is …– combination of nuclides into a new and heavier
nucleus– Happens in stars, such as our sun– H + H He + energy
• Fission is …– breaking apart (through decay) of an unstable
heavy nucleus into smaller nuclei (and particles)
Discovery
• Italian disccovery: Enrico Fermi was bombarding Uranium nuclei with neutrons, believed he had synthesized a “trans-Uranic” element, but could not verify it
Next
• Lise Meitner worked out that the “transuranics” were merely radioisotopes of known elements
• This proved that nuclei could be shattered
The natural conclusion
• Because of the mass excess of heavy isotopes…• Einstein’s conclusion that mass and energy are
equivalent…• Fermi and Meitner’s demonstration of the
fission of Uranium…• …came the realization that the energy holding a
nucleus together could be released if the nucleus itself was shattered
The Manhattan Project
Neutron release drives nuclear fission
• In a nuclear reaction, the heavy nuclei is struck by a “slow” neutron
• The neutron is absorbed, which renders the nuclei unstable
• The nuclei disintegrates into two new nuclei• Three neutrons are released (they are “fast”
neutrons”)• The binding energy of the nucleus is released
Chain reactions
• Because more neutrons are released than began the process, each neutron is capable of impacting another nucleus and inducing fission in that nucleus
• The reaction then is self-sustaining, a “chain reaction”
• This released a spectacular amount of energy (binding energy + conversion of a portion of mass into energy)
fission
Driving the reaction
• Critical mass – the minimum amount of fissile material needed to provide the number of neutrons needed to sustain the reaction
• If each nuclei releases >1 neutron the reaction will accelerate out of control
• If each nuclei release 1 neutron the reaction is controlled
• If each nuclei releases <1 neutron, the reaction will slow down and stop
Nuclear energy
Nuclear power• Just like an old-fashioned
steam engine – except: • Controlled release of
neutrons achieves a slow steady release of energy (heat)
• Energy heats water• Flow of neutrons
determined by “control rods”• Does produce unstable
(radioactive) waste nuclei