2.4.3 Nuclear plants
As coal reserve is not unlimited, there is natural threat to thermal power plants based on coal. It is estimated that within next 30 to 40 years, coal reserve will exhaust if it is consumed at the present rate. Nuclear power plants are thought to be the solution for bulk power generation. At present the installed capacity of unclear power plant is about 4300 MW and expected to expand further in our country. The present day atomic power plants work on the principle of nuclear fission of 235U. In the natural uranium, 235U constitutes only 0.72% and remaining parts is constituted by 99.27% of 238U and only about 0.05% of 234U. The concentration of 235U may be increased to 90% by gas diffusion process to obtain enriched 235U. When 235U is bombarded by neutrons a lot of heat energy along with additional neutrons are produced. These new neutrons further bombard 235U producing more heat and more neutrons. Thus a chain reaction sets up. However this reaction is allowed to take place in a controlled manner inside a closed chamber called nuclear reactor. To ensure sustainable chain reaction, moderator and control rods are used. Moderators such as heavy water (deuterium) or very pure carbon 12C are used to reduce the speed of neutrons. To control the number neutrons, control rods made of cadmium or boron steel are inserted inside the reactor. The control rods can absorb neutrons. If we want to decrease the number neutrons, the control rods are lowered down further and vice versa. The heat generated inside the reactor is taken out of the chamber with the help of a coolant such as liquid sodium or some gaseous fluids. The coolant gives up the heat to water in heat exchanger to convert it to steam as shown in figure 2.4. The steam then drives the turbo set and the exhaust steam from the turbine is cooled and fed back to the heat exchanger with the help of water feed pump. Calculation shows that to produce 1000 MW of electrical power in coal based thermal plant, about 6 × 106 Kg of coal is to be burnt daily while for the same amount of power, only about 2.5 Kg of 235U is to be used per day in a nuclear power stations.
The initial investment required to install a nuclear power station is quite high but running cost is low. Although, nuclear plants produce electricity without causing air pollution, it remains a dormant source of radiation hazards due to leakage in the reactor. Also the used fuel rods are to be carefully handled and disposed off as they still remain radioactive. The reserve of 235U is also limited and can not last longer if its consumption continues at the present rate. Naturally search for alternative fissionable material continues. For example, plutonium (239Pu) and (233U) are fissionable. Although they are not directly available. Absorbing neutrons, 238U gets converted to fissionable plutonium 239Pu in the atomic reactor described above. The used fuel rods can be further processed to extract 239Pu from it indirectly increasing the availability of fissionable fuel. Effort is also on to convert thorium into fissionable 233U. Incidentally, India has very large reserve of thorium in the world. Total approximate generation capacity and Contribution by thermal, hydel and nuclear generation in our country are given below.
Version 2 EE IIT, Kharagpur