Fission & Fusion (CIE IGCSE Physics)

• Nuclear fission & fusion are nuclear reactions that change the nucleus of an atom to produce high amounts of energy from the energy stored in the nucleus of an atom

Nuclear fission

• Nuclear fission is defined as:

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

• During fission:
  • A neutron collides with an unstable nucleus
    • The neutron and the nucleus are the reactants
  • The nucleus splits into two smaller nuclei (called daughter nuclei) and two or three neutrons
    • The daughter nuclei and the neutrons are the products of the reaction
  • Gamma rays are also emitted

Nuclear fission process

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

Nuclear fission nuclide equations

• An example of a nuclide equation for the fission of uranium-235 is:

• Where:
  • is an unstable isotope of uranium
  • is a neutron
  • is an unstable isotope of krypton
  • is an unstable isotope of  barium

Nuclear fission of uranium-235

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

Nuclear fission mass and energy values

• Energy is conserved in a nuclear fission reaction

• In the example:

• The sum of the nucleon (top) numbers of the reactants (left-hand side) is equal to the sum of the nucleon numbers of the products (right-hand side):

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

• The products of fission move away very quickly
  • During a fission reaction, energy is transferred from nuclear energy store of the parent nucleus to the kinetic energy store of the reactants 
• The mass of the products 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
• Large isotopes with a large nucleon number, such as uranium and plutonium, both undergo fission and are used as fuels in nuclear power stations

Nuclear fusion

• Nuclear fusion is defined as:

When two light nuclei join to form a heavier nucleus

• Stars use nuclear fusion to produce energy
  • In most stars, hydrogen nuclei (light nuclei) are fused together to form a helium nucleus (heavier nucleus) and massive amounts of energy is produced

Nuclear fusion of hydrogen

Two hydrogen nuclei fuse to form a helium nucleus

• Nuclear fusion requires extremely high temperature and pressure 
  • So fusion is very hard to reproduce on Earth

Nuclear fusion nuclide equations

• An example of a nuclide equation for fusion is:

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

Nuclear fusion mass and energy values

• The energy produced during nuclear fusion comes from a very small amount of a particle’s mass converted into energy
• Therefore, the mass of the product (fused nucleus) is less than the mass of the two original nuclei (reactants)
  • The remaining mass has been converted into the energy 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