Nuclear Fission (OCR A Level Physics) : Revision Note

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

Spontaneous Fission

• 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

Induced 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 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

Chain Reactions

• 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

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