Within the favorable conditions of the reactor the uranium fuel reacts spontaneously and builds up a chain reaction which has to be kept under control.
This control is achieved by absorbing or mopping up surplus atomic particles called neutrons, which are produced during fission and which are the agents of the chain reaction.
Rods of boron, a neutron-absorbing material are used for this purpose. They are lowered into the reactor core from above through a special control rod channels and they can be raised or lowered ton increase or decrease the heat produced by the reactor.
These rods which are in effect the brakes of the running reactor have to be very carefully calibrated to show their absorbing power at any level of insertion. In the same way a flow of gas through each fuel channel has to be measured to ensure that the rate matches exactly the job it has to do in carrying away the heat from each particular channel. Too much cooling would be wasteful and inefficient and too little would cause the fuel elements to overheat.
Much time has to be spent on such tests and measurements between the stage of critically - the point during the initial fuel loading when sufficient fuel is placed in the reactor to sustain a chain reaction - and the taking of the reactor up to power to ensure that these conditions are correctly matched.
At Hunterston the time taken from the stage of criticality to switching of each reactor into the grid took only five months, a period shorter by far than previous best.