Nuclear reactor parts and their functions
We have been discussing the Nuclear reactor main parts and their functions. We have so far seen the reactor core and moderator. Now continuing to our topic, let’s discuss the remaining components of a nuclear reactor. Like Control rod, Coolant, Reflector, Thermal shielding, Reactor vessel, and Biological shield.
Control rod
The name describes its function itself that these rods are used for controlling the rate of fission of U-235.
The material for these rods is mainly boron-1c, cadmium that absorbs some of the slowed neutrons.
Chain reaction should be maintained steady during the reactor’s operation; this chain reaction is controlled either by removing the rods or inserting neutron-absorbing materials. Hence, the material used for the control rod must have a high neutron absorbing capacity.
These control rods are inserted into the reactor core from the top of the reactor vessel. The main function of these rods is to regulate fissioning in the reactor by absorbing excess neutrons.
These rods are movable. They can easily be moved in and out of the holes of the reactor core assembly. The position of control rods decides the chain reaction. If we want to increase the fission rate of the chain reaction, the control rods are slightly moved out to absorb fewer neutrons, and the reaction becomes fast.
Coolant
Whenever we use a coolant in any system, it is to exchange the heat or cool down the system.
Coolant is a medium through which the heat generated in the reactor is transferred to the heat exchanger for further utilization in power generation.
When the water is used as a coolant in the reactor, it takes up heat and gets converted into steam, which is directly used to drive the steam turbine.
It performs the additional function of keeping the reactor interior at the desired temperature. Sometimes the same medium is used as a moderato and coolant through different materials.
For example, ordinary water is used as a coolant and moderator in boiling water reactors. Pressurized water is used as a coolant and moderator in pressurized water reactors.
Qualities of good coolant
- A good coolant should not absorb neutrons
- It should be non- Oxidizing
- Coolant should be non-toxic and non-corrosive
- Have high chemical and radiation stability and good heat transfer capacity.
Materials of coolant
- Air
- Helium
- Hydrogen
- CO2 amongst the gases
- Light and heavy water amongst the fluids
- Molten sodium and lithium amongst the metals
Reflector
A reflector is placed around the core to avoid the leakage of neutrons from the core.
As a result, the reflector is completely surrounded by the reactor core within the thermal shielding arrangements a result, it. It bounces back most of the neutrons and tries to escape from the fuel core.
The reflector helps conserve nuclear fuel. However, the reflector gets heated due to the collision of neutrons; hence the coolant is necessary.
A reflector must have good neutron scattering properties and preferably a small tendency to absorb the neutrons. Therefore, it is often moderator material or the material that can be used for both moderators and a reflector.
Thermal shielding
Thermal shielding is required in the nuclear reactor to protect from deadly α, β- particle radiations, and ϒ- rays and neutrons given off by the process of fission within the reactor.
This thermal shielding is generally constructed from iron. It gets heated during the process and prevents the reactor wall from getting heated. Instead, coolant flows over the shielding takes all the heat away.
Reactor vessel
The reactor core reflector and thermal shielding are all enclosed in the main body of the reactor; the enclosed body is called a reactor vessel.
The reactor vessel is a strong-walled container and provides entrance and exit to the coolant and passage for its flow through and around the reactor core.
There are holes at the top to allow the control rod to pass through them. The reactor core (fuel and moderator assembly) is usually placed at the bottom of the vessel.
Biological shield
The whole reactor is enclosed in a biological shield to prevent the escape or leak away of the fast neutrons, slow neutrons β- particle radiations, and ϒ- radiations as these radiations are harmful to the living beings.
The lead iron or dense concrete shields are used for this purpose.
