8 Main Parts of a Nuclear Reactor | Power Plants | Electricity

Reactor is that part of nuclear power plant where nuclear fuel is subjected to nuclear fission and the energy released in the process is utilised to heat the coolant which may in turn generate steam or be used in a gas turbine. The main function of the reactor is to control the emis­sion and absorption of neutrons.

A nuclear reactor consists of the following basic compo­nents: 1. Reactor Core 2. Moderator 3. Control Rods 4. Coolant 5. Reflector 6. Thermal Shielding 7. Reactor Vessel 8. Biological Shield.

Part # 1. Reactor Core:

It contains a number of fuel rods made of fissile material. They may be diluted with non-fissionable material for better control of the reaction or to reduce the damage from fission product poisoning. As the uranium gets oxidised rapidly, the fuel rods should be clad with aluminium, stainless steel or zirconium. The size of core, just sufficient to maintain a chain reaction is the “critical” size. It can be brought down by using enriched uranium as fuel.

It is desirable to use core as cubical or cylindrical in shape rather than spherical, as it facilitates the re-fuelling operation and simplifies the process of circulation of coolant through the core. With this configuration, the core has a series of parallel fuel elements in the form of thin plates or small rods, with coolant flowing axially and additional moderator or reflector material surrounding the assembly.

For using the reactor to convert the fertile material into fissionable material, the material to be converted should be put around the core so that the neutrons, which otherwise would escape the core, would be utilized for conversion. This arrangement also simplifies the process of separation of the converted material during fuelling reprocessing.

Part # 2. Moderator:

Neutrons produced by the fission proc­ess are ejected from the nucleus at a very high velocity of about 1.5 × 10⁷ m/s and therefore, have a very large kinetic energy and are termed as fast neutrons. The elements which can undergo a fission reaction with fast neutrons are U-233, U-235 and Pu-239.

Natural uranium contains only 0.7% U-235. Fast neu­trons are slowed down by elastic scattering process and chain reaction can still occur. But during this process, there is a possibility of their getting absorbed by U-238 and the chain reaction may not be maintained. If the proportion of U-235 in the metal is increased to more than 10%, the above ab­sorption effect can be overcome and a chain reaction is pos­sible. This occurs in fast reactors but the enriching process is expensive.

For more effective use in nuclear reactor, it is desirable to slow down the fast neutrons to speeds corresponding to the speed of molecules in a gas at NTP (i.e., to a speed of about 2.2 × 10³ m/s). Such neutrons are known as slow or thermal neutrons. The absorption properties of U-238 are very much reduced with thermal neutrons. Thus, if natural uranium is bombarded by thermal or slow neutrons, the chain reaction can be maintained. This is accomplished with the help of ‘moderator’ which is mixed with the fissile material in a suitable manner.

Thus the purpose of moderator material in the reactor core is to moderate, or reduce the neutron speeds to a value that increases the probability of fission occurrence.

The good moderator material should have the following properties:

1. It should have a light weight nucleus, so that it does not absorb the neutron as it collides.

2. It must not react with neutrons because neutrons captured in nuclear reactions are lost to the fission process and this makes the reactor inefficient.

3. It should be chemically inert and it should neither corrode nor erode.

4. It should not undergo harmful physical or chemical changes when bombarded by neutrons.

5. The average neutron-nucleus collision should lead to large neutron energy loss.

6. It should not be costly.

The fast neutrons collide with the nuclei of moderator material, loose their energy and get slowed down. As per simple laws of mechanics, if a neutron collides with a nu­cleus of equal mass it will loose all its energy and it will come to standstill, whereas if it collides with a heavier nu­cleus it will loose very little energy and there will be not much change in the magnitude of velocity but its direction will change.

Thus it is evident that only elements at the top of periodic table or compounds with small molecular weights are suitable as moderator materials. Such elements are- Hy­drogen, Deuterium, Helium, Lithium, Beryllium, Boron, Car­bon, Nitrogen, and Oxygen.

Other properties of good moderator are:

(i) High scattering cross section.

(ii) Low neutron absorption cross section.

Out of the elements having small atomic mass, gases (oxygen, nitrogen, hydrogen and deuterium) are unsuitable owing to their low density and the consequent small number of collisions. Helium and beryllium are costly. Boron and lithium have high absorption cross section. At present, the common moderator materials are graphite, ordinary water and heavy water.

Graphite is simple to fabricate and handle and does not pose any containment problem. However, if continued bombing is maintained, this may create some stress problem. Ordi­nary water is cheap but it has high neutron absorption and can be used only with enriched uranium. This can be used as a coolant at moderate temperature and pressure.

Heavy wa­ter is costlier per unit weight, as compared to graphite or ordinary water; as a result containment is a serious problem for heavy water than for ordinary water. For the same power output, the size of the reactor using heavy water is more compact as compared to one using ordinary water. It can be used with ordinary uranium. It is used in many reactors inspite of its heavy cost.

The moderator and the fuel can either be intimately mixed or the fuel may be scattered throughout the moderator in discrete lumps. These two arrangements are called homogenous and heterogeneous arrangements respectively.

Part # 3. Control Rods:

Control rods are meant for controlling the rate of fission of U-235. These are made of boron-10, cadmium or hafnium that absorbs some of the slowed neutrons.

In a reactor, nuclear chain reaction has to be initiated when started from cold, and the chain reaction is to be maintained at a steady value during the operation of reactor. Also the reactor must be able to shut down automatically under emergency conditions. All this requires a control of reactor so as to prevent the melting of fuel roads, disintegra­tion of coolant and destruction of reactor as the amount of energy released is enormous.

Chain reaction is controlled either by removing the fuel rods or by inserting neutron absorbing materials. The mate­rials used for control rods must have very high absorption capacity for neutrons.

The control rods are inserted into the reactor core from the top of the reactor vessel. These rods regulate the fissioning in the reactor by absorbing the excess neutrons. These rods can be moved in and out of the holes in the reactor core assembly. If the fissioning rate of the chain reaction is to be increased, the control rods are moved out slightly so that they absorb less number of neutrons and vice versa.

Part # 4. Coolant:

It is a medium through which the heat gen­erated in the reactor is transferred to the heat exchanger for further utilisation in power generation. Sometimes when water is used as a coolant it takes up heat and gets converted into steam in the reactor which is directly used for driving steam turbines.

Coolant flows through and around the reactor core. It performs the additional function of keeping the interior of reactor at the desired temperature. Sometimes, the same medium is used as the coolant as well as the moderator though sepa­rate materials are used more commonly.

A good coolant should not absorb neutrons, should be non-oxidising, non-toxic and non-corrosive and have high chemical and radiation stability and good heat transfer capa­bility.

Air, helium, hydrogen and CO₂ amongst the gases, light and heavy water amongst the liquids, and the molten sodium and lithium amongst the metals are the materials used as coolants.

Ordinary water is used both as coolant and moderator in boiling water reactors and pressurised water is used both as coolant and moderator in pressurised water reactors. Water has good thermal capacity per unit volume and is, therefore, a good heat transport medium. In reactors using water as coolant, the power consumption of circulating pump is low. Heavy water (D₂O) is even more efficient than light water.

Liquid metals (e.g., sodium and potassium) are used as coolant in fast reactors which have large heat release from a small core. They have high heat transfer capability and low vapour pressure. Reactors employing liquid metal coolants can operate at high temperature. They are employed in liquid metal fuelled reactors.

Carbon dioxide is colourless and odourless and has low neutron absorption cross section. When dry, it does not react with mild steel of the pressure vessel and the supports of the core. However, it does react with graphite and therefore, special steps are to be taken in the design of the reactor so as to inhibit the reaction between CO₂ and graphite. It is used in Magnox and advanced gas cooled reactors.

Part # 5. Reflector:

A neutron reflector is placed around the core and used to avoid the leakage of neutrons from the core. This completely surrounds the reactor core within the thermal shielding arrangement and bounces back most of the neutrons that escape from the fuel core. This conserves the nuclear fuel, as the low speed neutrons thus returned are useful in continuing the chain reaction.

The reflector gets heated due to collision of neutrons with its atom; therefore, its cooling is essential. The reflector should have good neutron scattering properties and preferably a small tendency to absorb neutrons. It is often a moderating material and sometime the same material is used both for moderator and reflector.

Part # 6. Thermal Shielding:

The shielding is usually constructed from iron and help in giving protection from the deadly α- and β-particle radiations and γ rays as well as neutrons given off by the process of fission with in the reactor. In this manner it gets heated and prevents the reactor wall from getting heated. Coolant flows over the shielding to take away the heat.

Part # 7. Reactor Vessel:

The reactor core, reflector and thermal shielding are all enclosed in the main body of the reactor and are called the reactor vessel or tank. It is a strong walled container and provides the entrance and exit for the coolant and also the passages for its flow through and around the reactor core. There are holes at the top to allow the control rods to pass through them. The reactor core (fuel and moderator assembly) is usually placed at the bottom of the vessel. The reactor vessel has to withstand high pressures (up to 21 MPa).

Part # 8. Biological Shield:

The whole of the reactor is enclosed in a biological shield to prevent the escape or leak away of the fast neutrons, slow neutrons, β-particles and γ rays as these radiations are very harmful for living organisms. Lead iron or dense concrete shields are used for this purpose.

Reactor Control:

All power plant reactors are provided with the means to regulate the fission process so that energy is generated ac­cording to the load requirements and in an emergency the reactor can be quickly shut down. Fission control is affected by regulating the neutron population or flux as per power requirement by providing for absorption of excess neutrons through such substances which have high neutron absorption coefficient.

These are called the poisons. Cadmium and car­bon are two such substances which are inserted with the help of adjustable control rod. The position of the control rods is automatically regulated by electrochemical and electronic sensing objects which measure the neutron flux density in the reactor and actuate the control rods to regulate power generation.

All the neutrons released in the fission reaction are not used up in propagating the chain reaction but some of these are lost to the surroundings. For maintaining chain reaction, it is therefore, essential that the number of neutrons after the fission should be slightly more than the number before it to allow for the escape or leak of neutrons from the reactor core. The ratio is known as mul­tiplication factor.

The multiplication factor k for any reactor in defined as:

Unity value of k indicates that the chain reaction will continue at a steady rate (critical). If k is less than unity, the chain reaction will stop and the system is called subcritical. While for k exceeding unity, the chain reaction will build up and the system is called super critical.

The desirable requirement of power reactors is that the system should be critical (i.e., k = 1). The critical size of a thermal reactor is one that produces neutrons just enough to balance those lost and absorbed and at the same time main­tains the chain reaction.

For reactor control, the value of k is to be controlled. At the time of starting of reactor, value of k is kept above unity so as to build up the chain reaction. This increases the power level. Once the required power level has been attained, k is reduced to unity and is kept at this value as long as the output rate is to be maintained. For decreasing the output (power level) k is reduced to slightly less than unity till the required power level is attained and at this point k is brought back to unity and maintained. Similarly for shutting down the reactor k is reduced below unity; the chain reaction will stop.