Fission (WJEC GCSE Physics): Revision Note

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

• It is rare for a nucleus to undergo fission without additional energy being put into it

  • When nuclear fission occurs in this way it is known as spontaneous fission

• Usually, for fission to occur, the reaction must be induced in fissile nuclei, such as uranium or plutonium

  • Fissile nuclei are nuclei which can undergo fission

• During induced fission, a slow-moving neutron is absorbed by a fissile nucleus

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

• The nucleus splits into two smaller nuclei (called daughter nuclei) as well as two or three neutrons

  • A large amount of energy in the form of gamma rays is also emitted

Nuclear Fission of Uranium-235

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

• Only one extra neutron is required to induce a uranium-235 nucleus to split it 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

Visualisation of 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

Fission Products

• The radioactive isotope uranium-235 is the most commonly used fuel in nuclear reactors

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

• Several possible daughter nuclei can be produced from the fission of uranium-235

• These fission products also tend to have a range of half lives, from short to very long, which means:

  • They will have dangerously high levels of radioactivity for many years

  • Therefore, they must be stored and disposed of very carefully