Spontaneous & Induced Fission (SL IB Physics)

• 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 to convert nuclear energy into electrical energy
• 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
• This is because energy is transferred from the nuclear potential energy store of a nucleus to the kinetic energy store of the fission fragments
  • This energy is then carried away as heat

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
  • This will be slow moving, often called a 'thermal' neutral
• 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 only if it was to have additional neutrons added

• 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