Fission and fusion

Question 1

How do nuclear reactions produce energy?

Answer 1

• Nuclear reactions (fission, fusion, and radioactive decay) release energy.
• Energy is released due to mass being converted into energy, following E = mc².
• In fission, energy is released when a large nucleus splits into smaller ones.
• In fusion, energy is released when two small nuclei combine to form a larger nucleus.

Question 2

How is a U-235 nucleus split in nuclear fission?

Answer 2

• A slow-moving neutron collides with a uranium-235 nucleus, making it unstable.
• The nucleus splits into two smaller radioactive daughter nuclei.
• A small number of free neutrons are also released.
• This process releases a large amount of energy as kinetic energy of the fission products.
• The energy is used to heat water into steam to drive turbines in a nuclear power station.

Question 3

What are the products of U-235 fission?

Answer 3

• Two radioactive daughter nuclei (e.g., Krypton-92 and Barium-141).
• A small number of free neutrons, which may cause further fission reactions.
• Energy release in the form of kinetic energy of the fission fragments.
• Gamma radiation may also be emitted.

Question 4

How does a chain reaction occur in fission?

Answer 4

• Free neutrons from one fission event collide with other U-235 nuclei.
• This causes more fission reactions, releasing more neutrons and energy.
• If uncontrolled, the reaction becomes explosive (e.g., nuclear bomb).
• In a nuclear reactor, the reaction is controlled using control rods to absorb excess neutrons.
• The chain reaction ensures a continuous and controlled energy output in nuclear power plants.

Question 5

What is the role of control rods and the moderator in a nuclear reactor?

Answer 5

• Control rods (boron/cadmium):
  
  • Absorb excess neutrons to regulate the chain reaction.
  • Prevent overheating or a meltdown.
  • Can be lowered or raised to control the reaction rate.
• Moderator (graphite/water):
  
  • Slows down fast neutrons so they can be absorbed by U-235.
  • Ensures neutrons move at the right speed for efficient fission.
• Both work together to keep the reactor stable and safe.

Question 6

Why is shielding used in a nuclear reactor?

Answer 6

• Nuclear reactors produce dangerous radiation, including gamma rays and neutrons.
• Thick lead and concrete shielding absorb radiation to protect workers and the environment.
• Reduces exposure to harmful ionising radiation, preventing cell damage and mutations.
• Prevents radiation leaks, ensuring nuclear safety.

Question 7

How does nuclear fusion differ from nuclear fission?

Answer 7

• Fission: A large nucleus splits into smaller nuclei, releasing energy.
• Fusion: Two small nuclei combine to form a larger nucleus, releasing energy.
• Fusion requires extreme conditions (high temperature and pressure), while fission occurs more easily.
• Fission produces radioactive waste, while fusion produces harmless helium.
• Fusion occurs in stars, while fission is used in nuclear power plants.

Question 8

What happens during nuclear fusion?

Answer 8

• Two light nuclei (e.g., deuterium and tritium) fuse to form a heavier nucleus (helium).
• Some mass is lost and converted into energy, following E = mc².
• This process releases much more energy per reaction than fission.
• Fusion occurs in stars, including the Sun, where hydrogen nuclei combine to form helium.
• Scientists aim to replicate fusion for clean energy, but it requires extreme temperatures.

Question 9

What is the energy source for stars?

Answer 9

• Nuclear fusion powers stars by converting hydrogen into helium.
• The Sun’s energy comes from the fusion of hydrogen nuclei under high temperature and pressure.
• This process has been ongoing for billions of years and will continue until the hydrogen is exhausted.

Question 10

Why does nuclear fusion require high temperatures and pressures?

Answer 10

• Atomic nuclei are positively charged, so they experience electrostatic repulsion.
• High temperatures (millions of degrees) provide enough energy for nuclei to overcome repulsion.
• High pressure (e.g., in the Sun’s core) forces nuclei closer together, increasing the chance of fusion.
• Without extreme conditions, nuclei would repel each other, preventing fusion.
• This is why fusion power on Earth is difficult, as scientists need to replicate the Sun’s conditions.