How to control a chain reaction??

A neutron bombards a fissionable material ,releases energy and in the process releases at least two more neutrons.These neutrons under suitable conditions would cause further disintegration of the fissionable material.This is a chain reaction.If we have no control over this process,it becomes an uncontrolled chain reaction and readily causes a huge explosion.But if we use some moderators which slow down the speed of the neutrons as they pass through them,the speed of the neutrons can be kept perfectly under control.Further cadmium is a material which absorbs neutrons without causing disintegration.Thus,by using more or less of the cadmium in the process the number of neutrons can be fully controlled.So in the ultimate analysis,when the number and speed of the neutrons is in our hands,the chain reaction once started can be completely kept under control.This is the controlled chain reaction.By properly controlling  a chain reaction ,K,the reproduction factor can be made just equal to one and thus a constant level of energy release can be obtained.This energy may be suitably altered to give electrical power.Thus,we can get uninterrupted constant output of energy with the help of a suitably controlled chain reaction.As an example a nuclear submarine doesn't have to surface out for months and practically it becomes undetected in enemy waters.