How Nuclear Reactors Controlled

Q. How are nuclear reactors controlled? Explain two different methods in brief.

1. To maintain a sustained controlled nuclear reaction, for every 2 or 3 neutrons released, only one must be allowed to strike another uranium nucleus.
2. If this ratio s less than one then the reaction will die out, if it is greater than one there will be atomic explosion.
3. A neutron absorbing element must be present to control the amount of free neutrons in the reaction space.
4. Most reactors are controlled by means of control rods that are made of a strongly neutron-absorbent material such as boron or cadmium.
5. The movement of these control rods out of the reactor will allow the neutron being captured by them to produce fission reaction at a brisk rate, thus increasing the power generated.
6. The neutron flux density in the reactor is measured by sensors. As per the output from sensors, signal is given to the control system which operates the control rods.
7. The position of control rods is adjusted such that desired power output is obtained.
8. In addition to the need to capture neutrons, the neutrons often have too much kinetic energy.
9. These fast neutrons are slowed through the use of a moderator such as heavy water and ordinary water.
10. Some reactors use graphite as a moderator, but this design has several problems.
11. Once the fast neutrons have been slowed, they are more likely to produce further nuclear fissions or be absorbed by the control rod.