Nuclear reactors controlled by two methods
1. With the help of control rod-
By adjusting height of control rods in reactor core according to requirement we can control the chain reaction.
When control rods are pushed deep in the core then it absorbs all neutrons from the fission process. Hence chain reaction will stop automatically.
However, when control rods are being withdrawn then more & more neutrons causes fission process & hence intensity of chain reaction(heat produced) will increased.
Therefore pulling out of control rods from the core, power of nuclear reaction will increase. Whereas by pushing control rod towards the core it will reduce.
2. By adjusting value of multiplication factor (k)-
At the time of starting chain reaction, value of multiplication factor (k) should be kept greater than one.
At the time of steady condition chain reaction, value of multiplication factor (k) should be kept equal to one.
At the time of shutting down chain reaction, value of multiplication factor (k) should be kept less than one. In this way nuclear reactor is controlled.