Fission

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ADVANTAGES Nuclear power generation does emit relatively low amounts of carbon dioxide (CO2). The emissions of green house gases and therefore the contribution of nuclear power plants to global warming is therefore relatively little. This technology is readily available, it does not have to be developed first. It is possible to generate a high amount of electrical energy in one single plant

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Fission

  1. 1. Nuclear Fission energy for war and peace Nuclear fission is a process, by which a heavy nuclide splits into two or more pieces Nuclear fission reactions release a lot of energy. Nuclear energy has been used for peace and for war.
  2. 2. Discovery of Induced Nuclear Fission * O. Hahn, L. Meitner, and F. Strassmann in Berlin * F. Joliot and I. Curie in Paris * Enrico Fermi in Rome All three groups thought the reactions to be 238 U (n,  ) 239 U 92 (,  ) 239 E 93 (,  ) 239 E 94
  3. 3. Discovery of Induced Nuclear Fission Hahn (chemist), Meitner (physicist), and F. Strassmann (analytical chemist) used H 2 S to precipitate the radioactive products. The half-life measurements indicated to them that not one but many elements were produced . Meitner used barium ions, Ba 2+ , as a carrier and precipitated the radioactive products from the neutron bombardment.
  4. 4. Discovery of Induced Nuclear Fission Atomic weight of Ba 2 + is 137 Neutron induced uranium nuclear fission reactions
  5. 5. Induced Nuclear Fission A simplified view of neutron induced fission: n + 235 U  xxx E yy + uuu E ww + 3 n
  6. 6. Discovery of Induced Nuclear Fission The Official History of the Manhattan Project : Dr. Meitner brought the discovery of neutron induced fission to Copenhagen as she, a non-Aryan, exiled from Germany in 1938. She told Frisch, who told N. Bohr and Bohr told Fermi Fermi fond out only 235 U underwent fission, for example: 235 U + n  142 Cs 55 + 90 Rb 37 + 4n neutrons are releases
  7. 7. Nuclear Fission Energy Fission Energy
  8. 8. Problem: If a 235 U atom splits up into two nuclides with mass number 117 and 118, estimate the energy released in the process. Nuclear Fission Energy
  9. 9. If a 235 U atom splits up into two nuclides with mass number 117 and 118, estimate the energy released in the process. Nuclear Fission Energy From handbooks Stable nuclides with mass numbers 117 and 118 are 117 Sn 50 , and 118 Sn 50 and masses are given below the symbols 235 U  117 Sn 50 + 118 Sn 50 235.043924 = 116.902956 + 117.901609 + Q fe Q fe = 0.2394 amu (931.5 MeV) / (1 amu) = 223 MeV. Discussion: The fission reaction equation is over simplified. Usually, neutrons are released too.
  10. 10. Nuclear Fission Energy Assume the neutron induced fission reaction to be, 235 U + n  142 Cs 55 + 90 Rb 35 + 4 n. explain the results and estimate the energy released. Solution : The neutron-rich fission products are beta emitters: 142 Cs  142 Ba +  (~1 min) 90 Rb  90 Sr  +  (half-life, 15.4 min) 142 Ba  142 La +  (11 min) 90 Sr  90 Y +  (27.7 y) 142 La  142 Ce +  (58 min) 90 Y  90 Zr (stable) +  (64 h) 142 Ce  142 Pr +  (5  10 15 y) 142 Pr  142 Nd (stable) +  (19 h)
  11. 11. Solution – cont. For the energy, consider the reaction and mass balance: 235 U 92  142 Nd 60 + 90 Zr 40 + 3 n + Q 235.04924 = 141.907719 + 89.904703 + 3x1.008665 + Q Q = (235.043924 - 141.907719 - 89.904703 - 3x1.008665) = 0.205503 amu ( 931.4812 MeV / 1 amu ) = 191.4 MeV per fission( 1.6022e-13 J / 1 MeV ) = 3.15e-11 J Assume the neutron induced fission reaction to be, 235 U + n ® 142 Cs 55 + 90 Rb 35 + 4 n. explain the results and estimate the energy released. Nuclear Fission Energy – cont.
  12. 12. Nuclear Fission Energy Estimate the energy released by the fission of 1.0 kg of 235 U. Solution From the results of the previous two examples, energy released by 1.0 kg uranium-235 is estimated below: (3.15e-11 J) 1000 g = 8.06e13 J (per kg). Discussion This is a large amount of energy, and it is equivalent to the energy produced by burning tones of coal or oil. 1 mol 235 g 6.023e23 1 mol
  13. 13. Nuclear Fission Energy Kinetic energy of fission fragments Prompt (< 10 –6 s) gamma (  ) ray energy Kinetic energy of fission neutrons Gamma (  ) ray energy from fission products Beta (  ) decay energy of fission products Energy as antineutrinos ( v e ) 167 MeV 8 8 7 7 7 Energy (MeV) distribution in fission reactions
  14. 14. The Cyclotron and Fission Research Particle accelerators machines to speed up particles Linear accelerators Cyclotrons
  15. 15. The Cyclotron and Fission Research 7 Li (p, n) 7 Be 3 T (p, n) 3 He 1 H (t, n) 3 He 2 D (d, n) 3 He 2 D (t, n) 4 He 3 T (d, n) 4 He Fusion reactions studied using the cyclotron
  16. 16. The Cyclotron and Fission Research Threshold* Energy range (keV) Reaction energy(keV) narrow-energy neutron 51 V (p, n) 51 Cr 2909 5.6-52 45 Sc (p, n) 45 Ti 1564 2.36-786 57 Fe (p, n) 57 Co 1648 2-1425 __________________________________ * The threshold energy is the minimum energy of proton required for the reaction. Neutrons of desirable energy is required for fission research.
  17. 17. The Cyclotron and Fission Research For neutron sources from the cyclotron, energy can be varied. Energy dependence of neutron induced fission studied. The cross section data enabled nuclear reactor design. fast neutrons - 10 MeV to 10 KeV) slow neutrons - 0.03 to 0.001 eV for neutron induced fission
  18. 18. The Synthesis of Plutonium Fast neutrons provided the reactions: 238 U + n  239 U +  239 U  239 Np +  (t 1/2 23.5 min) (t 1/2 2.35 d) 239 Np  239 Pu +  Short notations 238 U (n,  ) 239 U ( ,  ) 239 Np ( ,  ) 239 Pu or 238 U (n, 2  ) 239 Pu
  19. 20. Uniting Political and Nuclear Power Neutron induced fission reactions release energy and neutrons , thus it is possible to start a chain reaction for nuclear power. Dictator Hitler (political power in 1933) made many scientists in Austria, Hungary, Italy and Germany uncomfortable and they came to the U.S.A. Hitler invaded Poland, Hungary, Slovak and other European countries. Nuclear fission was discovered in Germany, and nuclear power threatens the world. Leo Szilard, Eugene Wigner, and Edward Teller drafted a letter and Einstein signed the letter for president Roosevelt of U.S. to use political power for nuclear power.
  20. 21. Uniting Political and Nuclear Power F.D. Roosevelt (Einstein’s address omitted) . . . . . . . (address omitted) Sir: Some recent work by E.Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the partof the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations: . . . . (m iddle part omitted) . . . . I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated. Yours very truly,
  21. 22. Thermal Neutrons Conclusion : Slow neutrons (0.03 to 0.001 eV) are more effective for inducing fission of 235 U Fast neutrons (10 MeV to 10 KeV) favours neutron capture reaction of 238 U Light atoms are effective moderators Experiment : Neutron bombarded samples surrounded by water, wood, and paraffin are more radioactive - Fermi’s group discovered
  22. 23. Thermal Neutrons - Moderators Light atoms are effective moderators
  23. 24. Thermal Neutrons Cross Sections Cross section (  ) a measure of reaction probability Thermal neutron cross sections (  c ) Thermal neutron cross section for fission (  f ) 1 H 2 H 12 C 14 N 16 O 113 Cd  c /b 0.33 0.00052 0.0034 1.82 0.0002 19,820 Moderators: H 2 O vs. D 2 O vs. C Fermi’s avoided N 2 in his first nuclear reactor and used Cd for emergency
  24. 25. Thermal Neutrons Cross Sections Uranium for Fission Fuel in Nuclear Reactor 113 Cd 233 U 235 U 238 U  c /b 19,820 46 98 2.7  f /b 530 580 2.7×10 -6 t 1/2 /y 1.6×10 5 7×10 8 4.5×10 9
  25. 26. Plutonium Isotopes 236 Pu 237 Pu 238 Pu 239 Pu 240 Pu 241 Pu 242 Pu  f 150 2100 17 742 0.08 1010 0.2 t 1/2 2.9y 45 d 88 y 24131y 6570 y 14y 3.8×105y Neutrons Capture Cross Sections of Cadmium Isotopes 106 Cd 108 Cd 110 Cd 111 Cd 112 Cd 113 Cd 114 Cd  c / b 1 1 0.1 24 2.2 19,820 0.3 Abundance/% 1.25 0.89 12.45 12.80 24.13 12.22 28.37 Thermal Neutrons Cross Sections
  26. 27. Fission Products nuclides produced in nuclear fission Data on fission products are required for reactor design, operation, and accident responses. The study of fission products requires the separation, identification, and quantitative determination of various elements and isotopes. Fission products emit  particles until they are stable. Mass number range: 40 - 170 Elements range: all the elements in the 4 th , 5 th , and 6 th periods. including the lanthanides.
  27. 28. Fission Products Fission yield is the relative amounts of nuclides formed in fission reactions. The fission yield curve shown here shows most fission reactions split fission atoms into two unequal fragments.
  28. 29. Nuclear Fission Products Fission-product and their decay data are needed for social and environmental concerns, and for the management of used fuel. Fission nuclides usually have very short half lives. Typical medium-life fission products: 85 K 10.7 y, 90 Sr 29 y, 137 Cs 30 y, Typical long-life fission products: 126 Sn 1.0e5 y, 126 Tc 2.1e5 y, 91 Tc 1.9e6 y, 135 Cs 3.0e6 y, 107 Pd 6.5e6 y, and 129 Tc 1.6e7 y. Xenon poisoning: 115 Xe,  c = 2,640,000 b, and t 1/2 = 9.2 h
  29. 30. First Fission Nuclear Reactor Fermi’s group assembled natural uranium into an atomic pile to test the feasibility of a sustained chain fission reaction. Key elements: fuel, neutron moderator, control rod, neutron detector, and radioactivity detector Dec. 2, 1942, Fermi achieved sustained chain reaction, and the first fission reactor provided data for future design of nuclear reactors. Today, more than 400 power nuclear reactors provided energy world wide, more than 100 of them in the US.

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