1. Fission, Fusion and Nuclear Energy
Chapter 31

2. What is Nuclear Fission?
Nuclear Fission is the splitting of a large nucleus into two smaller nuclei of similar size with the release of Energy and Neutrons.

3. Fission is produced in a large nucleus by bombarding it with neutrons.
The neutrons act as a trigger for the reaction.
During fission very large amounts of energy are given out, about 200 MeV per nucleus.
More neutrons are produced in the fission reaction. These can produce further fission.

4. Nuclear Fission

5. Fission in Uranium-235 (i)
If U-235 is bombarded with fast or slow neutrons it undergoes fission.
It is much more likely to undergo fission with slow neutrons.
The two nuclei formed during fission have similar masses, but vary from one fission to the next.
The products formed are called the Fission Fragments.
Most of the energy released goes to the kinetic energies of the fission fragments. These are very often themselves radioactive.

6. Fission in Uranium-235 (ii)
The neutrons released are fast neutrons and may trigger further fission.
If at least one neutron from each atom that undergoes fission produces further fission we have a Chain Reaction.
In a small sample of fissionable material many neutrons escape and a chain reaction cannot occur.
If the size of the sample is increased, a size is reached - the Critical Size - whereby a chain reaction can occur.
For U-235 this is about the size of a tennis ball and has a mass of about 10 kg.

7. A Fission Chain Reaction

8. Nuclear Fission Reactor

9. The Fuel Rods contain the fissionable material, which is natural uranium or uranium slightly enriched with U-235. These are the source of the energy.
The Control Rods absorb neutrons. Placing them into the core of the reactor slows the chain reaction. Placing them in fully stops the reaction. As they are removed the rate at which the chain reaction occurs increases.
The Moderator slows down fast neutrons so that they produce further fission in U-235 rather than be absorbed by the U-238. (The fast neutrons collide with the nuclei of the atoms in the moderator; this slows them down).

10. Nuclear Fission Bomb
In a Fission Bomb at least two pieces of fissile material of sub-critical mass are very suddenly brought together and an uncontrolled chain reaction occurs with an enormous release in energy.
The material used is either Plutonium 239 or Uranium 235.
The devastation produced in Hiroshima

11. The devastation produced in Hiroshima

12. ENVIRONMENTAL IMPACT OF FISSION REACTORS
The mining of Uranium Ore releases radon gas, which can cause lung cancer in miners. The area around the mine may also contain radioactive materials.
Containment of radioactive materials within the reactor. There have been accidents in reactors with the release of radioactivity into the atmosphere or leakage from the cooling system. Major accidents are rare, but the consequences are very serious.
The removal and treatment of spent fuel rods, i.e. nuclear reprocessing. These are withdrawn from the core and transferred to a cooling pond to cool down. They are then transported to a reprocessing plant to separate Uranium and Plutonium from the fission products. There are problems associated with the transport of these materials.
Radioactive waste. The remaining waste products pose a terrorist threat and must be stored securely for a very long time.

13. Give three Advantages and three Disadvantages of a Nuclear Fission Reactor as a source of energy.
Advantages: No C​O​2​ emissions
No greenhouse gases
Less dependence on fossil fuels
Disadvantages: Possibility of very serious accidents
Large amounts of radioactive waste
products
Terrorist threat if radioactive materials
get into the wrong hands

14. What is Nuclear Fusion?
Nuclear Fusion is the joining of two small nuclei to form a larger nucleus with the release of energy.
Nuclei are positively charged. Fusion can only occur if the two nuclei have sufficient kinetic energy to overcome the coulomb repulsion between them. This is done by heating them to extremely high temperatures.

15. An isotope of Hydrogen of mass number 1 combines with an isotope of Hydrogen of mass number 2 to form an isotope of Helium with the release of a gamma ray.
Write an equation for this nuclear reaction.

16. Give three advantages of fusion over fission as a source of power?
At present why is a fission reactor a more viable source of energy than a fusion reactor?
Give an example of an uncontrolled fusion reaction on earth.
The fuel is plentiful, readily available and cheap.
Vast amounts of energy can be released; more energy released per gram than from any other source.
Very little radioactive waste.
Fission is much easier to start and fission is much easier to control.
The hydrogen bomb.

17. E = m c2 THE EQUIVALENCE OF MASS AND ENERGY
In 1905, Einstein, in his Special Theory of Relativity, first proposed that Mass is a Form of Energy.
Thus mass and energy are not independent quantities.
Mass can be Converted into Energy and Energy can be Converted into Mass.
Change in Energy = Change in Mass  (Speed of Light)2
( in Joules ) ( in kilograms ) ( in m s-1 )
E = m c2

18. Mass-Energy Conservation in nuclear reactions.
In a nuclear reaction the mass of the products is often significantly different from the masses of the reactants.
If the mass of products is greater than the mass of reactants then energy must be supplied.
If the mass of products is less than the mass of reactants then energy is given out.
The change in mass m, is related to the energy E given out or taken in by Einstein's equation:

19. What is meant by Background Radiation?
The low-level radiation that everybody is exposed to every day is called Background Radiation. This is radiation that is always in the environment.
Most Background Radiation comes from:
Radioactive rocks in the Earth’s crust
Radiation coming from space (known as Cosmic Rays)
Human-made radioactive materials.

20. Experiment to measure Background Radiation
Use the equipment shown.
Measure the background count rate.
Note how the count rate varies with time, showing the random nature of background radiation.