Nuclear Fission (OCR GCSE Physics A (Gateway))

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 or fusion

• 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

Chain Reactions

• It is rare for nuclei to undergo fission without additional energy being put into the nucleus

• When nuclear fission occurs in this way it is called spontaneous fission

• Usually, for fission to occur the unstable nucleus must first absorb a neutron

• Take, for example, uranium-235, which is commonly used as a fuel in nuclear reactors

• It has a very long half-life of 700 million years

• This means that it would have a low activity and energy would be released very slowly

  • This is unsuitable for producing energy in a nuclear power station

• During induced fission, a neutron is absorbed by the uranium-235 nucleus to make uranium-236

• This is very unstable and splits by nuclear fission almost immediately

  • During the fission, it produces two or three neutrons which move away at high speed

The fission of a nucleus, such as uranium, to produce smaller daughter nuclei with the release of energy

• Each of these new neutrons can start another fission reaction, which again creates further excess neutrons

  • This process is called a chain reaction

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

• 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