Nuclear Fission (Edexcel IGCSE Physics)

• Nuclear fission is defined as:

The splitting of a large, unstable nucleus into two smaller nuclei

• Some isotopes of uranium and plutonium are known as fissile materials
• This means they can undergo fission under the right conditions
• This makes them ideal to use as fuels in nuclear power stations

Spontaneous and induced 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

• Usually, for fission to occur, the unstable nucleus must first absorb a neutron
• This makes a nucleus more unstable so that it decays almost immediately
  • When nuclear fission occurs in this way it is called induced fission

Fission of uranium-235

• Uranium-235 is commonly used as a fuel in nuclear reactors
• It has a very long half-life of 700 million years
  • This means that it has a low activity and releases energy very slowly
  • This is unsuitable for producing energy in a nuclear power station

• Therefore, the fission of uranium-235 must be induced
• During induced fission, the uranium-235 nucleus absorbs a neutron and becomes uranium-236
• Uranium-236 is very unstable and splits by nuclear fission almost immediately to produce
  • two smaller daughter nuclei
  • two or three neutrons

When a uranium-235 nucleus is struck by a neutron, it breaks into two smaller daughter nuclei and 2 or 3 neutrons

• During fission, when a neutron collides with an unstable nucleus, the nucleus splits into
  • two smaller nuclei (daughter nuclei)
  • two or three neutrons
  • gamma rays are also emitted

• One of the many decay reactions uranium-235 can undergo is shown below:

When fission is induced in a uranium-235 nucleus it may split into two smaller daughter nuclei, such as barium-144 and krypton-89

• The products of the fission reaction move away very quickly
• This is because energy is transferred from the nuclear potential energy stored in the original nucleus into the kinetic energy of the products
• In a nuclear power station, this energy can be harnessed and converted into electrical energy