Control of Nuclear Reactor

Control of Nuclear Reactor

Control of Nuclear Reactor by Using Control Rods

• If no control is provided the nuclear fission produces a chain reaction, if such a reaction occurs at uncontrolled rate it will become a divergent chain reaction resulting into automatic bomb.
• In order to control the chain reaction control rods are used, which are made of neutron absorbing material. 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. 
• 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. The position of control rods is adjusted such that desired power output is obtained. 

The absorbing controls (control rods) should have the following properties :

1. They must have high cross-sectional area for absorption.
2. They must be adequately strong.
3. They must have low mass number in order to allow rapid movement with slight Inertia effects,
4. They must be able to shut down the reactor almost instantly under all conditions.
5. They must stop any instability of the core power.
6. They must provide adequate control of power during operation.
7. They must provide good resistance to corrosion.
8. They must be stable under heat and radiation.
9. They must have reasonable heat transfer properties.
10. They must be economical.

• Boron and Cadmium possess the above properties for being used as control rods. The rods are about 8 cm in diameter steel tubes and carry boron steel Inserts. The length is little more than the height of the core.