What Is The Purpose Of The Control Rods In A Nuclear Power Plant?

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

A nuclear reactor consists of the following bones components: 1. Reactor Core 2. Moderator 3. Control Rods four. Coolant v. 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 production poisoning. As the uranium gets oxidised rapidly, the fuel rods should exist clad with aluminium, stainless steel or zirconium. The size of core, merely sufficient to maintain a chain reaction is the "disquisitional" size. It can be brought down by using enriched uranium every bit fuel.

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

For using the reactor to catechumen the fertile cloth into fissionable material, the material to exist converted should be put around the cadre then that the neutrons, which otherwise would escape the core, would exist utilized for conversion. This arrangement as well simplifies the process of separation of the converted material during fuelling reprocessing.

Function # ii. Moderator:

Neutrons produced past the fission process are ejected from the nucleus at a very high velocity of well-nigh 1.five × 10⁷ m/s and therefore, take 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 procedure and chain reaction can yet occur. Merely during this procedure, 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 x%, the higher up absorption effect tin exist overcome and a chain reaction is possible. This occurs in fast reactors but the enriching process is expensive.

For more constructive employ 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.east., 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 exist 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 cadre 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:

one. It should have a light weight nucleus, and so that it does non absorb the neutron equally it collides.

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

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

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

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

6. It should non be costly.

The fast neutrons collide with the nuclei of moderator material, loose their free energy and become slowed down. Every bit per uncomplicated laws of mechanics, if a neutron collides with a nucleus of equal mass it will loose all its free energy and it volition come to standstill, whereas if it collides with a heavier nucleus it will loose very little free energy and in that location volition be not much change in the magnitude of velocity only its direction volition change.

Thus it is evident that but elements at the meridian 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 skillful moderator are:

(i) High handful cantankerous section.

(ii) Low neutron absorption cross section.

Out of the elements having pocket-size diminutive mass, gases (oxygen, nitrogen, hydrogen and deuterium) are unsuitable attributable to their low density and the consistent small number of collisions. Helium and beryllium are costly. Boron and lithium accept high assimilation cross section. At present, the common moderator materials are graphite, ordinary h2o and heavy water.

Graphite is uncomplicated to fabricate and handle and does not pose any containment trouble. Still, if continued bombing is maintained, this may create some stress trouble. Ordinary water is cheap just it has high neutron absorption and can be used simply with enriched uranium. This tin can be used as a coolant at moderate temperature and pressure level.

Heavy water is costlier per unit weight, every bit 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 i using ordinary water. Information technology can be used with ordinary uranium. Information technology 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 detached lumps. These 2 arrangements are called homogenous and heterogeneous arrangements respectively.

Role # iii. Control Rods:

Command 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 exist initiated when started from cold, and the chain reaction is to be maintained at a steady value during the operation of reactor. Likewise the reactor must be able to close downwardly 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 past removing the fuel rods or past 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 cadre from the meridian of the reactor vessel. These rods regulate the fissioning in the reactor by arresting 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 # four. Coolant:

It is a medium through which the estrus generated in the reactor is transferred to the heat exchanger for further utilisation in power generation. Sometimes when water is used as a coolant information technology 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. Information technology performs the additional function of keeping the interior of reactor at the desired temperature. Sometimes, the same medium is used every bit the coolant equally well every bit the moderator though sepacharge per unit materials are used more than commonly.

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

Air, helium, hydrogen and CO₂ amongst the gases, light and heavy h2o 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 every bit coolant and moderator in pressurised h2o reactors. Water has good thermal capacity per unit volume and is, therefore, a good heat send medium. In reactors using water as coolant, the power consumption of circulating pump is low. Heavy water (D₂O) is even more than efficient than lite water.

Liquid metals (east.g., sodium and potassium) are used equally coolant in fast reactors which accept large heat release from a pocket-sized core. They have loftier estrus transfer capability and low vapour pressure level. Reactors employing liquid metal coolants tin 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, information technology does react with graphite and therefore, special steps are to be taken in the pattern of the reactor so as to inhibit the reaction betwixt 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 avert the leakage of neutrons from the core. This completely surrounds the reactor core inside 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 ofttimes a moderating material and former the aforementioned material is used both for moderator and reflector.

Part # 6. Thermal Shielding:

The shielding is usually synthetic from iron and assist in giving protection from the deadly α- and β-particle radiations and γ rays likewise every bit neutrons given off by the procedure 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 stiff walled container and provides the entrance and exit for the coolant and also the passages for its flow through and effectually the reactor cadre. In that location are holes at the height to let the control rods to pass through them. The reactor core (fuel and moderator associates) is usually placed at the bottom of the vessel. The reactor vessel has to withstand loftier pressures (up to 21 MPa).

Part # 8. Biological Shield:

The whole of the reactor is enclosed in a biological shield to preclude 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. Pb iron or dumbo concrete shields are used for this purpose.

Reactor Control :

All power found reactors are provided with the ways to regulate the fission process so that energy is generated according to the load requirements and in an emergency the reactor can be apace close downwardly. Fission control is affected by regulating the neutron population or flux as per ability requirement by providing for assimilation of excess neutrons through such substances which accept high neutron absorption coefficient.

These are called the poisons. Cadmium and motorcarbon are two such substances which are inserted with the assistance of adaptable control rod. The position of the control rods is automatically regulated past 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 upward in propagating the concatenation 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 permit 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 volition keep at a steady rate (disquisitional). If k is less than unity, the chain reaction will end and the arrangement is called subcritical. While for chiliad exceeding unity, the concatenation reaction will build up and the system is called super disquisitional.

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

For reactor control, the value of k is to exist controlled. At the time of starting of reactor, value of k is kept above unity so as to build upward the concatenation reaction. This increases the ability level. Once the required ability level has been attained, k is reduced to unity and is kept at this value every bit long as the output rate is to exist 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 betoken grand is brought back to unity and maintained. Similarly for shutting down the reactor grand is reduced below unity; the concatenation reaction will stop.