IB Physics Sub-topic E4 Notes

Fission

Now that you have learned the basics of atoms and radiation, you need to understand how energy is produced in nuclear reactors. There are two types of nuclear reactions:

1. Fusion - the joining of two nuclei to form a larger nucleus, releasing lots of energy. 

   An example is the fusion of H-2 and H-3 to form He-4 and one neutron. This is the process that occurs in the sun.

2. Fission - the splitting of a large nucleus into two smaller nuclei, releasing less energy, although still a large amount. 

   An example is the fission of a U-235 nucleus by a neutron into Ba-141, Kr-92 and 3 neutrons. This is the process that occurs in nuclear reactors.

Whilst fusion has a higher energy output, it currently requires a higher energy input to initiate and is thus not industrially viable for now. Thus, fission is the current reaction used in nuclear reactors.

Chain reaction

In a reactor, fission can be initiated one of two ways:

1. Spontaneously - radioactive elements like Uranium are already unstable and will undergo radioactive decay by themselves without any external energy input. However, this occurs at low baseline levels and yields little energy.
2. Neutron-induced - a neutron is fired at Uranium nucleus, colliding and making it more unstable. This instantly induces fission . 

Following the initiation of fission reactions, remember that a Uranium nucleus will release three neutrons. At sufficient speeds, these can set off the fission of three other nuclei, with each setting off another three etc, called a chain reaction.

Due to the exponential growth of fission in a chain reaction, harnessing fission power is incredibly dangerous and needs to be well-monitored and controlled.