Fission & Fusion (Cambridge (CIE) IGCSE Physics): Revision Note

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

Nuclear fission nuclide equations

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

${}_{92}{}^{235}\mathrm{U}+{}_0{}^1\mathrm{n}\to {}_{36}{}^{92}\mathrm{Kr}+{}_{56}{}^{141}\mathrm{Ba}+3{}_0{}^1\mathrm{n}+\mathrm{energy}$

• Where:

  • ${}_{92}{}^{235}\mathrm{U}$ is an unstable isotope of uranium

  • ${}_0{}^1\mathrm{n}$ 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

Nuclear fission mass and energy values

• Energy is conserved in a nuclear fission reaction

• In the example:

${}_{92}{}^{235}\mathrm{U}+{}_0{}^1\mathrm{n}\to {}_{36}{}^{92}\mathrm{Kr}+{}_{56}{}^{141}\mathrm{Ba}+3{}_0{}^1\mathrm{n}+\mathrm{energy}$

• 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):

$235+1=92+141+(3\times 1)$ 

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

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

• 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

• 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:

${}_1{}^2\mathrm{H}+{}_1{}^1\mathrm{H}\to {}_2{}^3\mathrm{He}+\mathrm{energy}$

• Where:

  • ${}_1{}^2\mathrm{H}$ 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