Question

What is controlled and uncontrolled chain reaction

Answer 1

A Chain Reaction is simply the repeated 'break down' of a molecule due to a nuclear reaction. ... The next slide is just one example of anuncontrolled nuclear reaction.Controlled Chain Reactions. Each time a nucleus splits and releases a neutron, a large amount of energy is released

Answer 2

if more than one neutrons emitted in a particular fission cause further fissions then no.of fission increaces with time and reaction is very fast huge amount of energy is released as a violent explosion and reaction is called uncontrolled.
if by some means the reaction is controlled in such a way that only one neutron emitted in a fission causes another fission then reaction rate is constant and reaction is called controlled reaction..