4: Environmental Chemistry 29: Nuclear Power Plant

Transcription

1 Subject Paper No and Title Module No and Title Module Tag 4: Environmental 29, Nuclear Power Plant CHE_P4_M29 4: Environmental

2 TABLE OF CONTENTS 1. Learning outcomes 2. Introduction 2.1. Nuclear power plant 3. Nuclear power generation 3.1. Nuclear fission 3.2. Nuclear fusion 3.3. Nuclear reactor 3.4. Component and working of nuclear reactor 4. Problems of nuclear power plant 4.1. Disposal of nuclear waste 4.2. Effect of radioactivity on human health 4.3. Cost of nuclear power plant and terrorism 4.4. Nuclear accidents 4.5. Thyroid cancer 4.6. Uranium scarcity 5. Disasters caused by Nuclear power plant 5.1 Chernobyl disaster 5.2 Three Miles Island accident 6. Future power plants 7. Summary 4: Environmental

3 1. Learning Outcomes After studying this module, you shall be able to Know about the nuclear power plant and methods of generation of nuclear energy. Learn the working of a typical nuclear reactor. Identify the components and functions of each component of a nuclear reactor. Evaluate the problems associated with the continuous use of power plants and its effects on human health 2. Introduction 2.1 Nuclear Power Plant A nuclear power plant is a thermal power station in which nuclear reactor is used as a heat source. The heat is produced indirectly by the process of fission in a nuclear reactor and this heat is used to generate water vapor (steam) which drives a turbine which is connected to a generator and electricity is produced. Its power of producing electricity is cleaner and higher than any other energy resource (Theoretically uranium can give 3 million times more electrical energy than coal). A nuclear reactor is a central part of the power plant. In nuclear power plant different types of reactors, nuclear fuels, cooling circuits and moderators are used. There is no other technology which potentially matches the nuclear energy for long-run supply of electricity. Despite this no other source of electricity is more controversial than nuclear power plant. By the process of nuclear fission radioactive wastes are produced which affect human health. Also the radioactive by-products of fission reaction are not easily disposable. The other risks are associated with construction costs, operating performance and fuel price. Whether it is good or bad, we have no choice but to develop it further. In India there are 20 nuclear reactors in operation in 6 nuclear power plants till date. Worldwide there are 435 nuclear power reactors in operation, operating in 31 countries. 3. Nuclear Power Generation Nuclear energy is fundamentally produced in two different ways: nuclear fission and nuclear fusion. In the process of nuclear fission large radioisotopes, such as uranium-235 and plutonium- 239 are split to release energy, while in nuclear fusion small nuclei are combined to release energy. 3.1 Nuclear fission Nuclear power generation plants use radioactive uranium-235 as the fission fuel. It is different from other radioactive decay as it can be controlled via a chain reaction so that only that much 4: Environmental

4 energy is produced which can be harnessed for electrical power generation. For example the nuclear fission of heavier nuclei of Uranium-235 gives Krypton-92 and Bismuth-141 here, energy and three neurons are generated whereas, only one neutron is required to carry out the reaction. 3.2 Nuclear fusion In order to obtain nuclear energy two lighter nuclei can be fused together to form heavier nuclei. The energy produced by sun and stars is an example of fusion reaction. For example the nuclear fusion reaction between deuterium and tritium atom results in the formation of larger atom of helium and certain amount of energy is released. 3.3 Nuclear Reactor Nuclear reactor is a device which is used to carry out a nuclear fission reaction. There are different types of reactors based on the method of producing steam i.e. light water reactors and high temperature gas cooled reactors. 3.4 Components and working of nuclear reactor Fig. 1 shows the working of a typical nuclear power plant in which each component has its own function. Followings are the components of a nuclear power plant. 1. Nuclear fuel rods: The fuel rod is made up of the isotope which undergoes fission by capturing neutrons. Uranium-235 is the most common isotope used for the process. As the natural abundance of uranium-235 is only 0.71% therefore in order to increase the concentration enrichment process is done. 2. Nuclear reactor core: It is the portion where the nuclear reactions take place as it contained the component of fuel. It contains low-enriched uranium, control systems and structural materials. 3. Moderator: As the number of neutrons, speed of the neutrons too must be controlled. As uranium-235 nuclei are split more effectively by slow moving neurons therefore, the projectile neurons must be slowed down to optimum velocities and the substances which reduce the velocity are known as moderators. The commonly used moderators in the nuclear reactors are graphite or heavy water. 4. Coolant: The heat generated in the fission reaction is absorbed by coolant. Also if the nuclear reaction begins to go at faster rate emergency cooling is provided by the coolant. Because of the high thermal conductivities either water or liquid sodium is used as coolants. In some reactors helium gas is also used. 4: Environmental

5 Fig. 1 Nuclear power plant 5. Control rods: To ensure that the reaction is carried out in a controlled manner there must be restriction in the number of neurons. Otherwise, uranium-235 would be fissioned at an abnormally high rate and releases excessive amount of energy as a result of which reactor temperature rises. Generally for this the control rods are made up of cadmium because the element is capable in absorbing neurons. 6. Steam generator: The steam generator is used to generate steam from water with the help of heat exchangers in a nuclear reactor core. It is used to convert the mechanical energy obtained from steam turbine into electrical energy. 7. Steam turbine: The steam turbine is used to convert the heat energy of the steam into mechanical energy. The steam turbine is separately structured from the main reactor building. 8. Condenser: In nuclear power plant the function of the condenser is to condense the exhaust steam from turbine in order to increase the efficiency and also to convert the exhaust steam into water so that this water can be reused in the steam generator. 9. Containment structure: It is a screen which prevents the radiation from escaping out of the reactor, the salt of lead is used to absorb the radiation. 4: Environmental

6 4. Problems with Nuclear Power Plant There are many problems which are caused by the use of nuclear power plants for the generation of electricity. The nuclear reactions taking place in the nuclear reactors are responsible for the continued production of radioactive waste which is dangerous for human health. Also an increased risk from terrorism is associated with the use of nuclear power plants. Some major problems caused by the continuous use of nuclear power plants are: 4.1 Disposal of nuclear waste The radioactive isotopes which are produced as by products in fission reaction taking place in the nuclear reactor. According to the level of radioactivity contained in these nuclear wastes and different mechanism for their disposal, they are basically of two types namely: low level wastes and high level wastes Low level waste Low level wastes include plastic bottles, rubber, glassware and protective clothing contaminated with radioactivity. They are less harmful than high level wastes but are more bulky. The disposal of low level wastes is simple. The material is simply packed and buried High level waste The concentration of radioactivity in waste is higher than that of low level wastes. Mostly they are remnants of the fuel which is used to run the reactor for a specific time. The high level wastes can t be managed easily therefore, they are more dangerous. They are very hot therefore they are buried with special care, they are stored above ground for 10 years before burial so that intense heat will not cause fracture in the burial site. 4.2 Effect of radioactivity on human health Human health is affected by radioactivity in two ways: Ionizing radiation and radioisotopes Ionizing radiation The ionizing radiations are those radiations which cause ionization of neutral atoms. These are α, β and γ-rays. These radiations can be emitted at any stage in the generation of electricity in nuclear power plant and affect the human heath by entering the body and transferring their energy to the tissues. The energy is then absorbed by biomolecules. It causes two types of effect: somatic effect and genetic effect. Somatic effect is due to cellular damage. But genetic effects are caused 4: Environmental

7 by ionizing radiation on reproductive cells. They damage the nucleic acid molecules in the DNA of the germ cells or causes mutation Radioisotopes Radioisotopes may be inhaled by human from air or may enter the body through food chain. Radioisotopes are also absorbed by plants which are taken by humans either by eating them or by eating meat of animals that had consumed such plants. It may also be ingested through contaminated water. 4.3 Cost of nuclear power plant and terrorism The expensive cost of establishment of the nuclear power plant is also a major problem. The construction and electricity generating costs are more than other resources. Also there is a risk of terrorism because of transport of tons of radioactive waste around countries by road, rail and sea. Also if terrorists were able to seize a nuclear power plant then the harmful radioactive radiation will cause lives lost. 4.4 Nuclear Accidents If the reaction temperature increases the sealed radioactive materials in the reactor core begins to melt and radioisotopes escape out to the environment. Accidents caused by use of such plants are classified into two types: Reactivity excursion accidents and loss of coolant accidents Reactivity Excursion accidents If the control rods are not used properly, the number of neutron is not controlled and number of projectile neutrons increases then the Uranium-235 nucleus begins to split abnormally at fast rates as a result of which temperature increases and melting of core takes place. The nuclear accident occurred at Chernobyl is the cause of reactivity excursion. This can be controlled by injecting a solution of boron compounds into the reactor. As it absorb the excess neutrons Loss of coolant accidents This accident is occurred because of loss of coolant from the reactor core. The Three Mile Island accident is an example of loss of coolant. In this incident, the temperature rised and the coolant was not restored, therefore within a few seconds, the melting of core took place. Proper inspection of reactor equipment is the only possible prevention. 4: Environmental

8 4.5 Thyroid Cancer This is one of the consequences of disaster caused by reactivity excursion accidents. This type of cancer is caused by iodine-131, which is ingested mainly through milk and milk products. Children get more affected by this cancer as they consume more milk then adults. 4.6 Uranium scarcity Nuclear power plants are based on the use of uranium-235 which is a limited non-renewable resource. Therefore the continuous usage of uranium is also a matter of concern. 5. Disasters caused by nuclear power plant There are many accidents occurred due to establishment of large number of nuclear power plants. It is estimated that at least 4 serious nuclear power accidents are expected in the period of because of the growth of nuclear power in that period. During the period of , 99 nuclear accidents have been recorded. These accidents includes incidents resulted in the loss of human life or property damage. Property damage includes destruction of property, emergency response, lost product, fines, and court claims. Because the nuclear power plants are very large therefore the accidents result in a heavy money loss. Fig. 2 depicts Fukushima Daiichi nuclear disaster in Japan on 11 March people got injured due to radiation explosion. Fig. 2 Fukushima Daiichi nuclear disaster 4: Environmental

9 The two Nuclear Disasters well known in world history for their destruction are Chernobyl disaster and Three Miles Island disaster. 5.1 Chernobyl disaster The 1986 Chernobyl disaster is the world s worst nuclear accident in Ukraine. This major accident occurred due to the rapture of reactor vessel which caused steam explosions and destroyed reactor number 4. Both control rods or liquid water were absent; as a result the number of neutrons increased and temperature rises caused the increase in steam pressure. Heat was being generated from the decaying fission products. It caused increase in release of radioactive waste from the reactor. First day 200 people were injured and around 31 died. The radioactive waste was rich in iodine-131, cesium-134 and cesium-137 therefore, the accident causes long-term health problem. Fig.3 Chernobyl Disaster, Russia Immediately after the accident about one lakh people living around the plant were evacuated. In the absence of human beings, the population of wild boar, otters. Wildfowl and voles boomed. The animals, however, could not escape the consequences of radioactive landscape. They developed a high degree of mutations. For example, voles from contaminated zone have been found to have a mutation rate about 40 times that of the background rate. The rate is high enough to pass the mutations from one generation to other. The trees surrounding Tolsty Les would be cut down and burnt to produce electricity. A power plant fuelled by timber from around the crippled reactor is being constructed at Belarus. The power plant shall he equipped with modern filtering technologies and electrostatic precipitators to prevent radioactive fallout. This operation would decontaminate Belarus forests within 40 years, compared to centuries required for natural decay of radioactivity, it is feared that if nothing is done with the forest trees, they will eventually bum in forest fires with no clean-up at all. The area around Chernobyl has had already suffered several forest fires since : Environmental

10 5.2 Three Miles Island Accident The 1979 Three Miles Island Accident in USA was caused by the failure in the secondary system at the nuclear reactor. It is the most significant accident in U.S. history. The reactor was operating at 98% of its rated power when the mishap occurred Cause of the accident At 4.00 AM on March 28, 1979, a pump in the secondary steam loop of unit 2 reactor of Three Mile Island stopped functioning. Under normal circumstances, the steam would have been used to turn the turbine and then condensed. Subsequently, the condensate would have been reinjected into the reactor as coolant water. Because of the pump failure, however, reinjection of coolant stopped. In such an event, auxilliary pumps are normally called into action. But a fortnight earlier, on March 14, 1979, the primary valve of the auxilliary pump had struck open. Therefore, when the coolant was circulated through the auxiliary systems it drained off. At this stage the emergency core cooling system (ECCS) was switched on. The coolant did start to flood the reactor, but a faulty gauge indicated that the reactor was flooded to an optimum level. A negligent worker shut the ECCS without verifying the coolant level. Without the coolant the reactor temperatures rose rapidly. A partial core melt-down initiated. The damage was arrested when the coolant level was restored. As the core melt-down proceeded, steam containing about 10 MCi of radioisotopes, mainly those of xenon and krypton, vented into the atmosphere. A part of the plant s Ventilating system was also contaminated. Enormous quantities of hydrogen gas were released at the reactor site. The reactor core is made up of a zirconium alloy. As the temperature of the reactor rose, the zirconium content of the alloy started reacting with steam, releasing hydrogen. The steel walls of the reactor were indented from the pressure pulse of hydrogen gas. Other accessories showed flame damage due to combustion of hydrogen. Immediately after the accident, pregnant Women and children were evacuated to safer areas. Surprisingly, no major casualty was reported. The reason being that radioactive iodine, which normally escapes a failed reactor and causes toxic effects, was unexpectedly not released in the Three Mile Island episode. Subsequent analysis of the mishap revealed that radioactive iodine underwent dissolution in water and was passed into an auxilliary building through water pipes. This way its escape to the environment was prevented. 5.3 Nuclear power accidents in India Table 1 depicts the accidents caused by nuclear power plant in India. 4: Environmental

11 Table 1 Nuclear power accidents in India 6. Future Perspective Establishment of more nuclear power plants is the subject of current research. A number of new designs are ready to be practically used. The new designs are specifically attempt to make a safer nuclear reactor where there is less risk of destruction. As of march 2007 there are five nuclear power plants are under construction in China and seven in India. In the United States expansion of two nuclear power plant are scheduled to be completed till These are the Plant Vogtle and V.C Summer nuclear power plants. 4: Environmental

12 7. Summary Nuclear power plant is a thermal power station used for the generation of electricity. Nuclear energy is not directly obtained. The generation of electricity takes place by two different ways namely nuclear fission and nuclear fusion reaction. Theoretically, one ton of uranium can give as much electrical energy as 3 million tons of fossil fuel. Hence, no technology shows more potential than nuclear energy for long-run supply of electricity. There are many problems caused by the use of nuclear power plant. As the radioactive waste produced by the plant are harmful for humans. They contain iodine-131 which causes thyroid cancer. Also the nuclear wastes are not easily disposable which causes air, water and soil pollution Nuclear power plants are based on the use of uranium-235 which is a limited nonrenewable resource.. The nuclear power plants are provided with various safety systems which ensure no harm. The probability of a nuclear power plant mishap may be negligible, but can never be zero. Whether nuclear energy is good or bad, we have no choice but to develop it further, for the conventional energy resources are fast depleting. There are number of nuclear power plants which are under construction throughout the world. 4: Environmental