Fission & Fusion (Cambridge International IGCSE Physics)

Spec point — 5.1.4 · Describe the processes of nuclear fission and nuclear fusion

• as the splitting or joining of nuclei,
• to include the nuclide equation
• and qualitative description of mass and energy changes without values

Fission & fusion

• 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:

U _ 92 ^ 235 + n _ 0 ^ 1 -> Kr _ 36 ^ 92 + Ba _ 56 ^ 141 + 3 n _ 0 ^ 1 + energy

• Where:

  • U _ 92 ^ 235 is an unstable isotope of uranium
  • n _ 0 ^ 1 is a neutron
  • ${}_{36}^{92}\mathrm{Kr}$ is an unstable isotope of krypton
  • ${}_{56}^{141}\mathrm{Ba}$ 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:

U _ 92 ^ 235 + n _ 0 ^ 1 -> Kr _ 36 ^ 92 + Ba _ 56 ^ 141 + 3 n _ 0 ^ 1 + energy

• 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):

$235+1=92+141+(3\times1)$

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

$92+0=36+56+(3\times0)$

• 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 products

• The mass of the products is fractionally 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 a huge amount 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:

H _ 1 ^ 2 + H _ 1 ^ 1 -> He _ 2 ^ 3 + energy

• Where:

  • H _ 1 ^ 2 is deuterium (isotope of hydrogen with 1 proton and 1 neutron)
  • ${}_{1}^{1}\mathrm{H}$ is hydrogen (with one proton)
  • ${}_{2}^{3}\mathrm{He}$ 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 fractionally 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

Worked Example A nuclide equation for nuclear fission is stated as:

U _ 92 ^ 235 + n _ 0 ^ 1 -> Rb _ 37 ^ 96 + Cs _ 55 ^ 137 + N n _ 0 ^ 1

Calculate the number of neutrons, N emitted in this reaction.

[2]

Answer:

Step 1: Calculate the sum of the nucleon numbers of the reactants

• The reactants are on the left-hand side of the equation
• The nucleon numbers are the top numbers in the nuclide notation

$235+1=236$

Step 2: Calculate the sum of the nucleon numbers of the products

• The products are on the right-hand side of the equation

$96 + 137 + ( N \times 1 ) = 233 + N$

Step 3: Equate the total nucleons of the reactants and products

$236=233+N$ [1 mark]

Step 4: Rearrange for the number of neutrons, N

$N=236-233$

$N=3$ [1 mark]