Fission & Fusion (CIE IGCSE Physics)

Nuclear Fission

• There is a lot of energy stored within the nucleus of an atom
  • This energy can be released in a nuclear reaction such as fission

• Nuclear fission is defined as:

The splitting of a large, unstable nucleus into two smaller nuclei

• Isotopes of uranium and plutonium both undergo fission and are used as fuels in nuclear power stations
• During fission, when a neutron collides with an unstable nucleus, the nucleus splits into two smaller nuclei (called daughter nuclei) as well as two or three neutrons
  • Gamma rays are also emitted

Large nuclei can decay by fission to produce smaller nuclei and neutrons with a lot of kinetic energy

• The products of fission move away very quickly
  • Energy transferred is from nuclear potential energy to kinetic energy
• The mass of the products (daughter nuclei and neutrons) is less than the mass of the original nucleus
  • This is because the remaining mass has been converted into energy which is released during the fission process

• The processes involved in nuclear fission can be shown in different ways as diagrams
• These diagrams show how the reaction happens in a way that is easy to understand

A neutron is fired into the target nucleus, causing it to split

• The diagram above is useful because it shows clearly the different parts of the fission reaction
• An example of a nuclide equation for fission is:

energy

• Where:
  • is an unstable isotope of Uranium
  • is a neutron
  • us an unstable isotope of Krypton
  • is an unstable isotope of Barium
• The above equation represents a fission reaction in which a Uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Krypton nucleus and a Barium nucleus, releasing three neutrons in the process
  • The sum of top (nucleon) numbers on the left-hand side equals the sum of top number on the right-hand side:

    235 + 1 = 92 + 141 + (3 × 1)

  • The same is true for the lower (proton) numbers:

    92 + 0 = 36 + 56 + (3 × 0)

Nuclear Fusion

• Small nuclei can react to release energy in a process called nuclear fusion
• Nuclear fusion is defined as:

When two light nuclei join to form a heavier nucleus

• This process requires extremely high temperatures to maintain
  • This is why nuclear fusion has proven very hard to reproduce on Earth

• Stars use nuclear fusion to produce energy
• In most stars, hydrogen atoms are fused together to form helium and produce lots of energy

Two hydrogen nuclei are fusing to form a helium nuclei

• The energy produced during nuclear fusion comes from a very small amount of the particle’s mass being converted into energy
• Albert Einstein described the mass-energy equivalence with his famous equation:

E = m × c²

• Where:
  • E = energy released from fusion in Joules (J)
  • m = mass converted into energy in kilograms (kg)
  • c = the speed of light in metres per second (m/s)
• Therefore, the mass of the product (fused nucleus) is less than the mass of the two original nuclei
  • This is because the remaining mass has been converted into energy which is released when the nuclei fuse

• The amount of energy released during nuclear fusion is huge:
  • The energy from 1 kg of hydrogen that undergoes fusion is equivalent to the energy from burning about 10 million kilograms of coal

• An example of a nuclide equation for fusion is:

+ energy

• Where:
  •   is deuterium (isotope of hydrogen with 1 proton and 1 neutron)
  • is hydrogen (with one proton)
  • is an isotope with helium (with two protons and one neutron)

Step 1: Calculate the nucleon number on the left side of the equation

235 + 1 = 236

Step 2: Calculate the nucleon number on the right side of the equation 

96 + 137 + (N × 1) = 233 + N

Step 3: Equate the nucleon number for both sides of the equation 

236 = 233 + N

Step 4: Rearrange for the number of neutrons, N

N = 236 – 233 = 3