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Nuclear reactions

Nuclear reactions. Using the strong nuclear force to produce useful energy. Micro-world Macro-world Lecture 17. Strong Nuclear Force. It is very strong It overcomes the electrical repulsion between positively charged protons that are only 10 -15 m apart. It acts over a very short range

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Nuclear reactions

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  1. Nuclear reactions Using the strong nuclear force to produce useful energy Micro-world Macro-world Lecture 17

  2. Strong Nuclear Force • It is very strong • It overcomes the electrical repulsion between positively charged protons that are only 10-15m apart. • It acts over a very short range • It is not felt by nucleons when they are more than 10-15m apart. • It is selective • It is felt by neutrons & protons, but not by electrons

  3. Nuclear “bullets” Protons are repelled by electrical the repulsion force of the positively nucleus. Only protons with KE of a few MeV or more can get within the range of the strong nuclear force & produce “nuclear reactions” + + + + + + + F + + v + + + + + + + + Producing nuclear reactions with protons (or any other charged nuclei) is a challenge + + +

  4. Neutron induced nuclear reactions Neutrons don’t feel the electrical force so even very slow, low-energy neutrons can strike the nucleus & produce “nuclear reactions” + + + v + + + + + + + + + + + + + Low energy neutrons are effective nuclear “bullets” + + +

  5. Nuclear fission 235142 92 n+ 92U  56Ba + 36Kr + 2n

  6. Energy balance in a fission reaction 141Ba+ 92Kr+2n 200 MeV  KE  heat 235U+n

  7. Chain reaction Use the neutrons produced by one fission to initiate another fission Enrico Fermi

  8. Requirements for A-bomb • Fissionable material: 235U or 239Pu • Critical mass • Mechanism

  9. Critical Mass Mcrit Enriched 235U 50kg 239Pu 10kg

  10. Fissionable Material Fortunately, only certain nuclear isotopes undergo the fission process: 235U only 0.7% of naturally occurring U (99.3% is 238U, which doesn’t fission) 239Pu doesn’t occur naturally, but is produced in nuclear reactors …. There are other fissionable isotopes, e.g. 233U & 232Th, but they are very rare

  11. Little boy (235U) (doughnut-like)

  12. Fat man (239Pu)

  13. Devastation Hiroshima Aug 6 1945 8:15AM 80,000 people killed immediately; ~100,000 people were exposed to lethal radiation & died painful slow deaths

  14. Hiroshima aftermath

  15. Devastation Nagasaki Aug 9 1945 10:45AM 39,000 people killed immediately; ~70,000 people were exposed to lethal radiation & died painful slow deaths

  16. Nagasaki aftermath

  17. Nuclear fusion Here the nuclei have to start out with large energy in order to overcome the electrical repulsion Two light nuclei fuse together to form a heavier one 2H +3H  4He + n

  18. Energy balance in a fusion reaction 4He+n 12.3 MeV  KE  heat 2H+3H

  19. Need to overcome electric repulsion Protons need ~2MeV energy to get within 10-15 m of each other (where strong nuclear force can be felt) + + This requires super-high temperatures (several Million degrees). Such high temperatures exist in the core of the Sun or in an Atomic-Bomb explosion

  20. H-bomb: powered by nuclear fusion Nuclear fusion bomb Nuclear fission bomb “detonator” produces the high temperature required to initiate fusion processes

  21. Brighter than 1000 suns 1000 times the power of an A-bomb!!

  22. Dangers of teaching nuclear physics Oh, and I suppose it was me who said ‘what harm could it be to give the chickens a book on nuclear physics?’

  23. Fusion in the Sun The core temperature is ~14 million degrees Here a tiny fraction of the protons have enough thermal energy to undergo fusion

  24. Solar fusion processes + 1.4 MeV + 5.5 MeV + 12.9 MeV

  25. pp-cycle 6 protons  4He + 2 protons + 2 “positrons” + 2neutrinos

  26. Energy balance in the pp-cycle 4He 25 MeV  KE  heat 4 protons + 2 neutrinos

  27. How do we know what goes oninside the Sun?

  28. Superkamiokande

  29. Superkamiokande

  30. Direction of neutrinosdetected in Superkamiokande

  31. Sun as seen by a neutrino detector

  32. Neutrinos come directly from solar core

  33. Neutrinos are everywhere test T est

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