Nuclear Fission & Fusion

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Ashlie Hodge
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1 Nuclear Fission & Fusion 1

2 Nuclear Fission 2

3 There is a delicate balance between nuclear attraction and electrical repulsion between protons in the nucleus. Nuclear Fission If the uranium nucleus is stretched into an elongated shape, the electrical forces may push it into an even more elongated shape. 3

4 Nuclear Fission Nuclear forces yield to electrical ones and the nucleus separates. 4

5 Nuclear Fission 5

6 Nuclear Fission 6

7 Nuclear Fission A chain reaction is a self-sustaining reaction in which the products of one reaction event stimulate further reaction events. 7

8 Nuclear Fission Typical fission reaction > 200,000,000 ev Explosion of 1 TNT molecule ~ 30 ev 1 ev = 1.6 x J 8

9 Nuclear Fission The combined mass of the fission fragments and neutrons produced in fission is less than the mass of the original uranium nucleus plus the original neutron that triggered the fission! 9

10 Nuclear Fission A fission reaction can be self-sustaining! Each new neutron can cause the fissioning of two or three other atomic nuclei, releasing more energy and from four to nine more neutrons. 10

11 Nuclear Fission Why do chain reactions not occur in naturally occurring uranium deposits? Naturally occurring uranium is too impure to undergo a chain reaction spontaneously. 11

12 Nuclear Fission 12

13 Nuclear Fission 13

14 Nuclear Fission 14

15 Nuclear Fission Critical Mass is the amount of mass for which each fission event produces, on average, one additional fission event. 15

16 Nuclear Fission 16

17 Nuclear Fission The difficulty in bomb making is separating enough U-235 from all the U-238 in uranium ore. 17

18 Nuclear Fission The vast quantities of energy stored in an atomic nucleus can be harnessed to provide energy for daily life. This is Fission Power. 18

19 Fission Power All energy producing power plants convert water to steam that drives turbines to generate electricity. Various fuels are used: coal, oil, gas, and now, fissionable isotopes like uranium. 19

20 The fissioning of U-235 requires that the stimulating neutrons have the proper speeds. Fission Power 20

21 Fission Power This is similar to the selective absorption of various frequencies of light. Just as various elements absorb and reflect light differently, various isotopes of the same element can have quite different nuclear properties and absorb neutrons differently. 21

22 Fission Power Only relatively slow neutrons will initiate fission in these two isotopes. 22

23 Fission Power Enrico Fermi led the construction of the first nuclear reactor or atomic pile, in a squash court underneath the grandstands of the University of Chicago's Stagg Field. 23

24 Fission Power 24

25 Fission Power 1 kg of uranium fuel (a chunk smaller than a baseball) yields more energy than 30 freight car loads of coal! About 20% of electric energy in the US is generated by nuclear fission reactors. 25

26 Benefits of Fission Power 1. Plentiful electricity 2. Conservation of the many billions of tons of coal, oil and natural gas that every year are literally converted to heat and smoke 3. Elimination of megatons of sulfur oxide and other poison, as well as greenhouse gas Carbon Dioxide that are released into the air each year by the burning of these fossil fuels. 26

27 Downside of Fission Reactors Generation of radioactive waste products. 27

28 Plutonium & Breeding in 1930, what seemed to be a planet beyond Neptune was discovered and named Pluto In 1846, the planet Neptune was discovered beyond Uranus 28

29 Plutonium & Breeding When a U-238 nucleus absorbs a neutron, no fission occurs. 29

30 Plutonium & Breeding With a half-life of 24 minutes, it emits a beta particle and becomes an isotope of the first synthetic element beyond uranium the transuranic element called neptunium. 30

31 Plutonium & Breeding Np-239 is a beta emitter and soon emits a beta particle and becomes an isotope of plutonium, specifically Pu

32 Plutonium & Breeding The nucleus of Pu-239, like U-235, will undergo fission when it captures a neutron. Pu-239 is even more fissionable than U

33 Plutonium & Breeding Plutonium is chemically toxic in the same sense as are lead and arsenic. It attacks the nervous system and can cause paralysis; death can follow if the dose is sufficiently large. Plutonium in any form, is radioactively toxic to humans and other animals. It is more toxic than uranium, although less toxic than radium. 33

34 Plutonium & Breeding A remarkable feature of fission power is the breeding of plutonium from nonfissionable U

35 Plutonium & Breeding 35

36 Plutonium & Breeding Some breeding occurs in all fission reactors, but a breeder reactor is specifically designed to breed more fissionable fuel than is put into it. 36

37 Mass-Energy Equivalence E = mc 2 37

38 Mass-Energy Equivalence The more energy that is stored in a particle, the greater is the mass of that particle. The mass of a nucleus is not simply the sum of the masses of the individual nucleons that compose it! 38

39 Mass-Energy Equivalence Think of the amount of force required to pull a nucleon out of the nucleus through a sufficient distance to overcome the attractive strong nuclear force. Enormous work would be required! 39

40 Mass-Energy Equivalence Mass Spectrometer 40

41 Mass-Energy Equivalence C-12 mass per nucleon amu Mass of proton + electron amu Mass of lone neutron amu 41

42 Mass-Energy Equivalence (Number of Protons) 42

43 Mass-Energy Equivalence Average mass per nucleon = Total mass of atom Number of nucleons in nucleus 43

44 Mass-Energy Equivalence 44

45 Mass-Energy Equivalence It turns out that the mass of a nucleus is NOT equal to the sum of the masses of its parts! So where is the missing mass? 45

46 Mass-Energy Equivalence The missing mass come out as ENERGY! And lots of it! 46

47 Mass-Energy Equivalence 47

48 Mass-Energy Equivalence All nuclear power today is by way of nuclear fission. 48

49 Nuclear Fusion Fusion is the opposite process of fission. Fusion is when nuclei of light atoms are fused to form nuclei of heavier atoms again with the release of much energy! 49

50 Accelerate a proton at high energy toward a target that contains other protons. Nuclear Fusion But that involves just a few protons and releases very little energy. 50

51 Nuclear Fusion Large Hadron Collider 51

52 Nuclear Fusion Fusion occurs in the sun s interior due to its environment of enormous heat and gravitational contraction. Fusion in the sun is called thermonuclear fusion because it is brought about by high temperatures. 52

53 Nuclear Fusion Prior to the development of the fission bomb, the temperatures required to overcome mutual proton repulsion and initiate nuclear fusion on Earth were unattainable. 53

54 Nuclear Fusion Nothing less energetic than a fission bomb can ignite a thermonuclear bomb! 54

55 Nuclear Fusion 55

56 Nuclear Fusion Mass per Nucleon versus Atomic Number 56

57 Nuclear Fusion Mass per Nucleon versus Atomic Number 57

58 Nuclear Fusion Deuterium Deuterium He-3 58

59 Nuclear Fusion Deuterium Tritium He-4 59

60 Nuclear Fusion Both fission and fusion reactions can only occur if there are two or more products of the reaction to carry away ENERGY! 60

61 Nuclear Fusion Nuclear energy is released when atoms split apart. Now we learn that nuclear energy is released when atoms combine. Is this a contradiction? How can energy be released by opposite processes? 61

62 Controlled Fusion 62

63 Controlled Fusion 63

64 Controlled Fusion Mass per Nucleon versus Atomic Number If we want energy out of the reaction, we should always move downhill. Fuse light elements or fission heavy elements. In both cases, decreased mass per nucleon is the name of the game. 64

65 Controlled Fusion Fusion processes in stars cease at elements close to iron, where the energy graph starts to slope upward. Stars are cooled at this stage, fusion ceases. 65

66 Controlled Fusion OR Which releases more energy? 66

67 Controlled Fusion MeV OR MeV 67

68 Controlled Fusion 68

69 Controlled Fusion 69

70 Controlled Fusion 70

71 Controlled Fusion 71

72 Controlled Fusion One final question: Sometimes pairs of iron nuclei are fused in stellar cores. Does this fusion further heat the star or cool it? 72

40 Nuclear Fission and Fusion. Nuclear fission and nuclear fusion reactions release huge amounts of energy.

40 Nuclear Fission and Fusion. Nuclear fission and nuclear fusion reactions release huge amounts of energy. Nuclear fission and nuclear fusion reactions release huge amounts of energy. In 1939, just at the beginning of World War II, a nuclear reaction was discovered that released much more energy per atom than