Nuclear Reactor has following important parts.
1. Fissile materials or fuel:
This is the main or most important part of the reactor which contains a fissile material such as uranium-235 as a reaction fuel in the form of rods. These rods are kept in aluminum containers and placed at some distances in graphite or heavy water as shown in fig. So that neutrons produced by the fission in one rod should reach another rod after passing through the graphite or heavy water. This centre part of the reactor is called reactor core. In this core the fission of uranium by neutrons is carried out in the form of self-sustained controlled Chain reaction. This process containues till the fissile materials remains in sufficient amount. The fission process occurring in the core is a main source of energy.
The function of moderator is to reduce or slow down the velocity of the neutrons produced in the fission. The energy of neutrons in the fission is about 1MeV. We know that more is the velocity of neutrons produced; more will be rate of loss of neutrons and the capacity of capturing neutrons, they are allowed to pass through the moderator.
A moderator is a a substance of low atomic weight whose nuclei do not capture neutrons but only scatter them, thus reducing the velocity of neutrons by large amount during collisions. Suitable substances for use as moderator are heavy water and carbon (in the form of graphite). By the collision of neutrons with the atoms of moderator, their energy reduces up to 0.025 eV which is equivalent to the thermal energy. Normal water or beryllium oxide can also be used as a moderator in addition to the heavy water or graphite.
3. Control rods-
Reactor once started, continues in operation without any obstruction. But it is essential to control the fission process of uranium inside the core otherwise the chain reaction may become explosive. And consequently the reactor will be damaged. The chain reaction can be controlled by controlling the number of neutrons in it.
The control system works on the principle of absorbing excess neutrons produced during the fission of fissile material and is achieved by introducing neutron absorbing material called control rods. These rods are made of materials such as cadmium or boron. Which are good absorbers of slow neutrons. These rods are inserted through the graphite block and the can be raised or lowered in side the core to control the excess neutrons produced in it.
When chain reaction starts in the reactor, a huge amount of heat energy is released. If this heat energy remains in the reactor, then it can increased the temperature of the core up to million degrees Celsius. QSo it becomes essential to remove heat produced by fission as fact as it is liberated otherwise it may damage the reactor. The material used to extract heat from the core is called coolant. The coolant is generally pumped and circulated through the core of the reactor in the form of liquid or gas so as to maintain a uniform temperature. The coolant should have following properties:
- High boiling point
- High specific heat
- Easy to pump
- Cheap and
- Chemically stable
The material proposed for coolants can be any one of the air, CO₂, Normal water, heavy water, liquid sodium etc. Heat is then transferred from coolant to the steam or a hot gas which can be used to generate electric power by means of turbine.
In a nuclear reactor various types of highly energetic radioactive rays are emitted. These rays are very harmful for the living beings those are nearby it. In order to avoid these rays thick walls of cement and concrete known as shields are constructed around the reactor so that rays must not come out of the reactor.
For the operation of reactor some neutrons are always present in the core. To start reactor, all the controlling rods except one are taken out. Now fissile material U-235 starts disintegrating by the neutrons present in the reactor and as a resultdfast neutron are produced. These fast neutrons strike the moderator (graphite or water).
They also slowed down to thermal levels and proceed to produce further fission. For continuous operation of the reactor the size of the nuclear fuel should be greater than its critical size. To maintain the power level constant the control rods are inserted ar a suitable rate so that explosion in the reactor could be avoided. Heat energy so produced is taken out by circulating coolant and is used for producing electric power.