nuclear fission & fusion - classical cottage school...

1

NuclearFission & Fusion

2

Nuclear Fission

3

Nuclear Fission

There is a delicate balance between nuclear attraction and electrical repulsion between protons in the nucleus.

If the uranium nucleus is stretched into an elongated shape, the electrical forces may push it into an even more elongated shape.

4

Nuclear Fission

Nuclear forces yield to electrical ones and the nucleus separates.

5

Nuclear Fission

6

Nuclear Fission

7

Nuclear Fission

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

8

Nuclear Fission

Typical fission reaction > 200,000,000 eV

Explosion of 1 TNT molecule ~ 30 eV

1 eV = 1.6 x 10-19 J

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!

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.

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.

12

Nuclear Fission

13

Nuclear Fission

14

Nuclear Fission

15

Nuclear Fission

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

fission event.

16

Nuclear Fission

17

Nuclear Fission

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

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.

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.

20

Fission Power

The fissioning of U-235 requires that the stimulating

neutrons have the proper speeds.

21

Fission Power

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.

This is similar to the selective absorption of various frequencies of light.

22

Fission Power

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

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.

24

Fission Power

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.

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.

27

Downside of Fission Reactors

◼ Generation of radioactive waste products.

28

Plutonium & Breeding

In 1846, the planet Neptune was discovered beyond Uranus

in 1930, what seemed to be a planet beyond

Neptune was discovered and named Pluto

29


When a U-238 nucleus absorbs a neutron, no fission occurs.

30


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.

31


Np-239 is a beta emitter and soon emits a beta particle and becomes an isotope of plutonium, specifically Pu-239.

32


The nucleus of Pu-239, like U-235, will undergo fissionwhen it captures a neutron. Pu-239 is even more fissionable than U-235.

33


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.

34


A remarkable feature of fission power is the breeding of plutonium from nonfissionable

U-238.

35


36


Some breeding occurs in all fission reactors, but a breeder reactor is specifically designed to breed

more fissionable fuel than is put into it.

37

Mass-Energy Equivalence

E = mc2

38


◼ 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!

39


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!

40


Mass Spectrometer

41


C-12 mass per nucleon ➔ 1.00000 amu

Mass of proton + electron ➔ 1.007825 amu

Mass of lone neutron ➔ 1.00867 amu

42


(Number of Protons)

43


Average mass per nucleon = Total mass of atom

--------------------------------------Number of nucleons in nucleus

44


45


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?

46


The missing mass come out as ENERGY! And lots of it!

47


48


All nuclear power today is by way of nuclear fission.

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!

50

Nuclear Fusion

Accelerate a proton at high energy toward a target that contains other protons….

But that involves just a few protons and releases very little energy.

51

Nuclear Fusion

Large Hadron Collider

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 thermonuclearfusion because it is brought about by high temperatures.

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.

54

Nuclear Fusion

Nothing less energetic than a fission bomb can ignite a thermonuclear bomb!

55

Nuclear Fusion

56

Nuclear Fusion

Mass per Nucleon versus Atomic Number

57

Nuclear Fusion

Mass per Nucleon versus Atomic Number

58

Nuclear Fusion

Deuterium Deuterium He-3

59

Nuclear Fusion

Deuterium Tritium He-4

60

Both fission and fusion reactions

can only occur if there are two or

more products of the reaction to

carry away ENERGY!

Nuclear Fusion

61

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?

Nuclear Fusion

62

Controlled Fusion

63

Controlled Fusion

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.

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.

66

Controlled Fusion

OR Which releases more

energy?

67

Controlled Fusion

OR

+ 200 MeV

+ 17.6 MeV

68

Controlled Fusion

69

Controlled Fusion

70

Controlled Fusion

71

Controlled Fusion

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?

nuclear fission & fusion - classical cottage school...

Documents