Fission & Fusion (Cambridge O Level Physics)

• Nuclei can join together, or split up, to form new nuclei
• These processes are known are
  • Nuclear fission
  • Nuclear fusion

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

How does nuclear fission work?

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

• 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

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

How does nuclear fusion work?

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:

• 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)

• The processes involved in nuclear fission can be shown in different ways, such as diagrams and nuclear equations

Fission of Uranium-235

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

• 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:
  • ${}_{92}{}^{235}\mathrm{U}$ is an unstable isotope of Uranium
  • ${}_0{}^1\mathrm{n}$ is a neutron
  • ${}_{36}{}^{92}\mathrm{Kr}$ us an unstable isotope of Krypton
  • ${}_{56}{}^{141}\mathrm{Ba}$ is an unstable isotope of Barium

• This equation represents a fission reaction in which
  • A Uranium-235 nucleus is hit by a neutron
  • It splits into two smaller nuclei – a Krypton nucleus and a Barium nucleus
  • Three neutrons are released in the process which can go on to trigger further fission reactions
• The sum of the 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 + (2 × 0)

• Only one extra neutron is required to induce a Uranium-235 nucleus to split by fission
• During the fission, it produces two or three neutrons which move away at high speed
• Each of these new neutrons can start another fission reaction, which again creates further excess neutrons
• This process is called a chain reaction

Chain Reaction Analogy

The neutrons released by each fission reaction can go on to create further fissions, like a chain that is linked several times – from each chain comes two more

Controlled Chain Reactions

• In a nuclear reactor, a chain reaction is required to keep the reactor running
• When the reactor is producing energy at the correct rate, the number of free neutrons in the reactor needs to be kept constant
  • This means some must be removed from the reactor

• To do this, nuclear reactors contain control rods
• These absorb neutrons without becoming dangerously unstable themselves

Uncontrolled Chain Reactions

• Because each new fission reaction releases energy, uncontrolled chain reactions can be dangerous
• The number of neutrons available increases quickly, so the number of reactions does too
• A nuclear weapon uses an uncontrolled chain reaction to release a huge amount of energy in a short period of time as an explosion