The neutrons produced during the fission process will be partly absorbed by the fuel rods, moderator, coolant or structural material etc. They should have sufficient cross-sectional area for the absorption. They should be sufficient strong and should be able to shut down the reactor almost instantly under all conditions. They should be stable under heat and radiation. They should have adequate heat transfer properties. Thus power of reaction is controlled by shifting control rods which may be done manually or automatically.Ĭontrol rods should possess the following properties: Their insertion absorbs more neutrons and damps down the reaction and their withdrawal absorbs less neutrons. These rods can be moved in and out of the holes in the reactor core assembly. This is done by means of control rods.Ĭontrol rods in the cylindrical or sheet form are made of boron or cadmium. This implies that we should have some means to control the power of reactor. The energy produced in the reactor due to fission of nuclear fuel during chain reaction is so much that if it is not controlled properly the entire core and surrounding structure may melt and radioactive fission products may come out of the reactor thus making it uninhabitable. The consumption of this fuel and the power level of the reactor depend upon its neutron flux in the reactor core. The furnace is fed continuously and the heat energy in the furnace is controlled by regulating the fuel feed, and the combustion air whereas a nuclear reactor contains as much fuel as is sufficient to operate a large power plant for some months. The Control and operation of a nuclear reactor is quite different form a fossil and fueled (coal or oil fired) furnace. Table 10.3 indicates the moderating ratio for some of the material used as moderator. A high value of moderating ratio indicates that the given substance is more suitable for slowing down the neutrons in a reactor. To characterize a moderator it is best to use so called moderating ratio which is the ratio of moderating power to the macroscopic neuron capture coefficient. It should be stable under heat and radiation. It should provide good resistance to corrosion. Moderators should also possess good strength and machinability. It should have high melting point in case of solid moderators and low melting point in case of liquid moderators. It should be available in large quantities in pure form. It should have high thermal conductivity. A moderator should process the following properties: But enriched uranium is costly due to processing needed. Reactors using enriched uranium do not require moderator. Graphite, heavy water and beryllium are generally used as moderator. The slow neutrons (Thermal Neutrons) so produced are easily captured by the nuclear fuel and the chain reaction proceeds smoothly. Each such collision causes loss of energy, and the speed of the fast moving neutrons is reduced. It is done by colliding them with the nuclei of other material which is lighter, does not capture the neutrons but scatters them. To improve the utilization of these neutrons their speed is reduced. These fast moving neutrons are far less effective in causing the fission of U235 and try to escape from the reactor. In the chain reaction the neutrons produced are fast moving neutrons. U233 arid Pu239 are artificially produced from Th232 and U238 respectively and are called secondary fuel. Out of these U235 is most unstable and is capable of sustaining chain reaction and has been given the name as primary fuel. Natural uranium found in earth crust contains three isotopes namely U234, U235 and U238 and their average percentage is as follows : Figure shows the various parts of reactor, which are as follows:įuel of a nuclear reactor should be fissionable material which can be defined as an element or isotope whose nuclei can be caused to undergo nuclearįission by nuclear bombardment and to produce a fission chain reaction. A nuclear reactor is an apparatus in which heat is produced due to nuclear fission chain reaction.

Components of nuclear fission reactor