Fission and the Thermal Nuclear Reactor

Induced Fission

In 1938, Hahn and Strassmann discovery discovered that when uranium nucleus is bombarded with neutrons, it splits into two approximately two equally fragment. This process is known as induced fission. 

Uranium contains 99% U-238 which does not undergo nuclear fission. So, fuel rods have to be enriched so they contain 3% U-235 which does undergo fission. 

 Neutrons released from a fission event are capable of starting further fission events; this is called a chain reaction. 

The energy released from a fission reaction occurs because the fragments repel each other with sufficient force to overcome the strong nuclear force trying to hold them together, therefore the nuclei and neutrons gain kinetic energy. 

The two new nuclei are closer bound together which makes them more stable. The energy released is therefore the change in binding energy, usually of magnitude 200 MeV.

Example of fission event:

U-235 + n = Ba-144 + Kr-90 +2n     E = Δmc^2

Thermal Nuclear Reactor

Control Rods: The function of these is to absorb neutrons from fission events to prevent a chain reaction. The rods are automatically adjusted to keep the number of neutrons in the core constant so only one neutron from one fission event causes another fission event. 

Moderator: Neutrons need to be significantly slowed down to cause further fission reactions. This is why the fuel rods are surrounded by a moderator so the neutrons are slowed down by colliding with the moderator atoms. This is why these reactors are thermal,…