The Nuclear Fission Power Plant
Currently, about half of all nuclear power plants are located in the US. There are many different kinds of nuclear power plants, and we will discuss a few important designs. A nuclear power plant harnesses the energy inside atoms themselves and converts this to electricity. This electricity is used by all of us. A nuclear power plant uses controlled nuclear fission. Let's explore how a nuclear power plant operates and the manner in which nuclear reactions are controlled.
Earlier we talked about nuclear fission with 235U. In reality, this will not be the only isotope of uranium present in a nuclear reactor. In naturally occurring uranium deposits, less than one percent of the uranium is 235U. The majority of the uranium is 238U. 238U is not a fissile isotope of uranium. When 238U is struck by a loose neutron, it absorbs the neutron into its nucleus and does not fission. Thus, by absorbing loose neutrons, 238U can prevent a nuclear chain reaction from occurring. This would be a bad thing because if a chain reaction doesn't occur, the nuclear reactions can't sustain themselves, the reactor shuts down, and millions of people are without electrical power. In order for a chain reaction to occur, the pure uranium ore must be refined to raise the concentration of 235U. This is called enrichment and is primarily accomplished through a technique called gaseous diffusion. In this process, the uranium ore is combined with fluorine to create a chemical compound called uranium hexafluoride. The uranium hexafluoride is heated and vaporizes. The heated gas is then pushed through a series of filters. Because some of the uranium hexafluoride contains 238U and some contains 235U, there is a slight difference in the weights of the individual molecules. The molecules of uranium hexafluoride containing 235U are slightly lighter and thus pass more easily through the filters. This creates a quantity of uranium hexafluoride with a higher proportion of 235U. This is collected, the uranium is stripped from it, and the result is an enriched supply of fuel. Usually, nuclear power plants use uranium fuel that is about 4% 235U.
Parts of a Nuclear Reactor
A typical nuclear reactor has a few main parts. Inside the "core" where the nuclear reactions take place are the fuel rods and assemblies, the control rods, the moderator, and the coolant. Outside the core are the turbines, the heat exchanger, and part of the cooling system.
The fuel assemblies are collections of fuel rods. These rods are each about 3.5 meters (11.48 feet) long. They are each about a centimeter in diameter. These are grouped into large bundles of a couple hundred rods called fuel assemblies, which are then placed in the reactor core. Inside each fuel rod are hundreds of pellets of uranium fuel stacked end to end.
Also in the core are control rods. These rods have pellets inside that are made of very efficient neutron capturers. An example of such a material is cadmium. These control rods are connected to machines that can raise or lower them in the core. When they are fully lowered into the core, fission can not occur because they absorb free neutrons. However, when they are pulled out of the reactor, fission can start again anytime a stray neutron strikes a 235U atom, thus releasing more neutrons, and starting a chain reaction.
Another component of the reactor is the moderator. The moderator serves to slow down the high-speed neutrons "flying" all around the reactor core. If a neutron is moving too fast, and thus is at a high-energy state, it passes right through the 235U nucleus. It must be slowed down to be captured by the nucleus and to induce fission. The most common moderator is water, but sometimes it can be another material.
The job of the coolant is to absorb the heat from the reaction. The most common coolant used in nuclear power plants today is water. In actuality, in many reactor designs the coolant and the moderator are one and the same. The coolant water is heated by the nuclear reactions going on inside the core. However, this heated water does not boil because it is kept at an extremely intense pressure, thus raising its boiling point above the normal 100° Celsius.
The Inside of a Reactor Containment Structure
One can see the heavy concrete walls from which the structure is made. Also, a fuel rod transportation canister is in the background (blue arrow). In front of that is the pit where the reactor core would normally reside (red arrow).
The heated water rises up and passes through another part of the reactor, the heat exchanger. The moderator/coolant water is radioactive, so it can not leave the inner reactor containment. Its heat must be transferred to non-radioactive water, which can then be sent out of the reactor shielding. This is done through the heat exchanger, which works by moving the radioactive water through a series of pipes that are wrapped around other pipes. The metallic pipes conduct the heat from the moderator to the normal water. Then, the normal water (now in steam form and intensely hot) moves to the turbine, where electricity is produced.
After the hot water has passed through the turbine, some of its energy is changed into electricity. However, the water is still very hot. It must be cooled somehow. Many nuclear power plants used steam towers to cool this water with air. These are generally the buildings that people associate with nuclear power plants. At reactors that do not have towers, the clean water is purified and dumped into the nearest body of water, and cool water is pumped in to replace it.
From Fission to Electricity:
A nuclear power plant produces electricity in almost exactly the same way that a conventional (fossil fuel) power plant does. A conventional power plant burns fuel to create heat. The fuel is generally coal, but oil is also sometimes used. The heat is used to raise the temperature of water, thus causing it to boil. The high temperature and intense pressure steam that results from the boiling of the water turns a turbine, which then generates electricity. A nuclear power plant works the same way, except that the heat used to boil the water is produced by a nuclear fission reaction using 235U as fuel, not the combustion of fossil fuels. A nuclear power plant uses much less fuel than a comparable fossil fuel plant. A rough estimate is that it takes 17,000 kilograms of coal to produce the same amount of electricity as 1 kilogram of nuclear uranium fuel.
Most nuclear power plants today use enriched uranium in which the concentration of U-235 is increased from 0.7 percent U-235 to (nowadays) about 4 to 5 percent U-235. This is done in an expensive separation plant of which there are several kinds. The U-238 "tails" are left over for eventual use in "breeder reactors". The Canadian CANDU reactors don't require enriched fuel, but since they use expensive heavy water instead of ordinary water, their energy cost is about the same.
In 1993 there were 109 licensed power reactors in the U.S. and about 400 in the world. They generate about 20 percent of the U.S. electricity. (There are also a large number of naval power reactors.) The expansion of nuclear power depends substantially on politics, and this politics has come out differently in different countries. Very likely, after some time, the countries whose policies turn out badly will copy the countries whose policies turn out well.
For how long will nuclear power be available? Present reactors that use only the U-235 in natural uranium are very likely good for some hundreds of years. Bernard Cohen, a Professor Emeritus at the University of Pittsburgh, has shown that with breeder reactors, we can have plenty of energy for billions of years.
Cohen's argument is based on using uranium from sea water. Other people have pointed out that there is more energy in the uranium impurity in coal than could come from burning the coal. There is also plenty of uranium in granite. None of these sources is likely to be used in the next thousand years, because there is plenty of much more cheaply extracted uranium in conventional uranium ores.
A power reactor contains a core with a large number of fuel rods. Each rod is full of pellets of uranium oxide. An atom of U-235 fissions when it absorbs a neutron. The fission produces two fission fragments and other particles that fly off at high velocity. When they stop the kinetic energy is converted to heat - 10 million times as much heat as is produced by burning an atom of the carbon in coal. See the supplement for some interesting nuclear details.
Besides the fission fragments several neutrons are produced. Most of these neutrons are absorbed by something other than U-235, but in the steady-state operation of the reactor exactly one is absorbed by another U-235 atom causing another fission. The steam withdrawn and run through the turbines controls the power level of the reactor. Control rods that absorb neutrons can also be moved in and out to control the nuclear reaction. The power level that can be used is limited to avoid letting the fuel rods get too hot.
The heat from the fuel rods is absorbed by water, which is used to generate steam to drive the turbines that generate the electricity. A large plant generates about a million kilowatts of electricity - some more, some less.
After about two years, enough of the U-235 has been converted to fission products and the fission products have built up enough so that the fuel rods must be removed and replaced by new ones. What to do with the spent fuel rods is what causes most of the fuss concerning nuclear power.
The Nuclear Fusion Power Plant
The power plant in consideration is the product of ITER (International Thermonuclear Experimental Reactor). ITER is an international research/engineering proposal, which is intended to be an experimental project between today's studies of plasma physics and future electricity-producing fusion power plants.
The project will seek to turn seawater into fuel by mimicking the way the sun produces energy. It would be cleaner than current nuclear reactors, would not rely on enriched uranium fuel or produce plutonium. But critics argue it could be at least 50 years before a commercially viable reactor is built, if at all.
Currently there are seven national and supranational parties participating in the ITER program: the European Union (EU), India, Japan, People's Republic of China, Russia, South Korea, and the USA. On November 21, 2006, the seven participants formally agreed to fund the project. The program is anticipated to last for 30 years—10 years for construction, and 20 years of operation—and cost approximately €10 billion (US$14.6 billion), which would make it one of the most expensive modern techno scientific mega projects. It will be based in Cadarache, France. It is technically ready to start construction and the first plasma operation is expected in 2016.
ITER will be designed to produce approximately 500 MW (500,000,000 watts) of fusion power sustained for up to 400 seconds by the fusion of about 0.5 g of deuterium/tritium mixture in its approximately 840-m3-reactor chamber. Although ITER is expected to produce net power in the form of heat, the generated heat will not be used to generate any electricity.
According to the ITER consortium, fusion power offers the potential of "environmentally benign, widely applicable and essentially inexhaustible" electricity; properties that they believe will be needed as world energy demands increase while simultaneously greenhouse gas emissions must be reduced, justifying the expensive research project.
A nuclear fusion power station is the 'Holy Grail' for scientists trying to find a viable alternative to the world's depleting stocks of oil and gas. The search took on new significance as crude this week reached a record price of $60.95 a barrel in some trading.
Next week, a summit of the Group of Eight leading industrial nations in Scotland is to discuss climate change, widely blamed on burning fossil fuels for energy.
DECADES OF RESEARCH
Unlike fission reactors, which are used in existing nuclear power stations and release energy by splitting atoms apart, ITER would generate energy by combining them.
Power has been harnessed from fusion in laboratories but scientists have so far been unable to build a commercially viable reactor, despite decades of research.
The 500-megawatt ITER reactor will use deuterium, extracted from seawater, as its major fuel and a giant electromagnetic ring to fuse atomic nuclei at extremely high temperatures.
One of the biggest challenges facing scientists is to build a reactor that can sustain temperatures of about 100 million Celsius (180 million F) for long enough to generate power.
The ITER project began in 1985 but scientific challenges and wrangling between its partners over the site and financing have caused repeated delays. At their meeting in Moscow, officials from ITER partners China, the 25-nation EU, Japan, Russia, South Korea and the United States chose France over Japan for the location of the power plant. Japan will provide headquarters and research facilities.
The EU is to take on 40 percent of the project's cost, France will pay 10 percent and the remaining five partners 10 percent each. Building the reactor is expected to take about ten years at a cost of 4.6 billion euros ($6.14 billion).
But some scientists say it could take three times that long and the sides have yet to reach a final agreement on a number of issues, including financing, before the builders can move in.
Environmental campaign group Green peace estimates that if the project yields any results at all, it will not be until the second half of this century. At a time when it is universally recognized that we must reduce greenhouse gas emissions by 2050, Green peace considers it ridiculous to use resources and billions of euros on this project.
France has been a big producer of nuclear energy since the oil shocks of the 1970s and has 58 nuclear reactors, the most in the world after the United States.
One of the primary drivers for building a Nuclear Fusion power plant is an aspect of fusion energy itself. It contrasts to many other energy sources that the cost of production is inelastic. The cost of wind energy, for example, goes up as the optimal locations are developed first, while further generators must be sited in less ideal conditions. With fusion energy, the production cost will not increase much, even if large numbers of plants are built. It has been suggested that even 100 times the current energy consumption of the world is possible.
Some problems which are expected to be an issue in the next century such as fresh water shortages can actually be regarded merely as problems of energy supply. For example, in desalination plants, seawater can be converted into pure freshwater through a process of either distillation or reverse osmosis. However, these processes are energy intensive. Even if the first fusion plants are not competitive with alternative sources, fusion could still become competitive if large-scale desalination requires more power than the alternatives are able to provide.
Despite being technically non-renewable, fusion power has many of the benefits of long-term renewable energy sources (such as being a sustainable energy supply compared to presently-utilized sources and emitting no greenhouse gases) as well as some of the benefits of such much more finite energy sources as hydrocarbons and nuclear fission (without reprocessing). Like these currently dominant energy sources, fusion could provide very high power-generation density and uninterrupted power delivery (due to the fact that they are not dependent on the weather, unlike wind and solar power).
Several fusion reactors have been built, but as yet none has 'produced' more thermal energy than electrical energy consumed. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050 with the ITER project.
Future of Nuclear Fusion Power Plants
The recently completed "European Fusion Power Plant Conceptual Study" investigates the technical feasibility, the expected safety and environmental properties, and the cost of a future fusion power plant. The latest results in plasma physics, technology, and materials research provided the basis for the development of four different power plant models illuminating a wide spectrum of physical and technical possibilities. Analysis of their ecological and economic properties has confirmed favorable results of previous investigations: Present knows how indicates that accidents with severe impact on the environment in a fusion power plant are impossible and permanent disposal of waste is not necessary with recycling. The price of electricity will be equivalent to that of other environmentally benign energy technologies.
The aim of fusion research is to reproduce the generation of energy by the sun in a power plant on earth by deriving energy from fusion of atomic nuclei. The fuel is an ionized low-density gas,”plasma", composed of the two hydrogen isotopes, deuterium and tritium. This fuel is confined in a magnetic field and heated to ignite the fusion fire. Above a temperature of 100 million degrees the plasma starts to "burn": The hydrogen nuclei fuse to form helium, thereby releasing neutrons and large quantities of energy. The possibility of a fusion fire providing energy is to be shown by the international ITER (Latin for "the way") test device with a generated fusion power of 500 megawatts. ITER was planned on the basis of the materials and technologies available today, which are not yet fully optimized for fusion. This is the objective of a parallel physics and technology programme. All of this work is preparatory to a demonstration power plant; commercial plants could then supply the grid from the middle of the century.
Problems With Nuclear Power Plants
Although nuclear power plants have many advantages, they have some major problems. If anything were ever to go wrong inside the reactor, the results could be disastrous. One of the most dangerous difficulties is the possibility of a nuclear meltdown. This occurs when the core overheats in an uncontrolled manner -- the core simply melts. Such an event would release amazing amounts of radioactivity. There are many emergency cooling systems and back-ups to prevent the reactor from getting to meltdown temperatures.
One problem that was not stopped was the incident at Chernobyl. The Chernobyl plant reached 150 times its normal power level. The pressure inside the water holding tubes there became so great that finally the plant just blew itself apart. Poor construction and operation of the power plant caused the disaster. The disaster killed 31 people and 20 square miles of land are now uninhabitable. Some people say that the Chernobyl accident is responsible for many cases of cancer all across Europe. The scientists and environmentalists fighting against nuclear power, use accidents like these as their argument.
Effect on health of population near nuclear plants
Most of human exposure to radiation comes from natural background radiation. Most of the remaining exposure comes from medical procedures. Several large studies in the US, Canada, and Europe have found no evidence of any increase in cancer mortality among people living near nuclear facilities. For example, in 1991, the National Cancer Institute (NCI) of the National Institutes of Health announced that a large-scale study, which evaluated mortality from 16 types of cancer, found no increased incidence of cancer mortality for people living near 62 nuclear installations in the United States. The study showed no increase in the incidence of childhood leukemia mortality in the study of surrounding counties after start-up of the nuclear facilities. The NCI study, the broadest of its kind ever conducted, surveyed 900,000 cancer deaths in counties near nuclear facilities.
Some areas of Britain near industrial facilities, particularly near Sellafield, have displayed elevated childhood leukemia levels, in which children living locally are 10 times more likely to contract the cancer. One study of those near Sellafield has ruled out any contribution from nuclear sources, and the reasons for these increases, or clusters, are unclear. Apart from anything else, the levels of radiation at these sites are orders of magnitude too low to account for the excess incidences reported. One explanation is viruses or other infectious agents being introduced into a local community by the mass movement of migrant workers. Likewise, small studies have found an increased incidence of childhood leukemia near some nuclear power plants has been found in Germany and France. Nonetheless, the results of larger multi-site studies in these countries invalidate the hypothesis of an increased risk of leukemia related to nuclear discharge. The methodology and very small samples in the studies finding an increased incidence has been criticized. Also, one study focusing on leukemia clusters in industrial towns in England indicated a link to high-capacity electricity lines suggesting that the production or distribution of the electricity, rather than the nuclear reaction, may be a factor.
It was reported in December 2007 that a study showed that German children who lived near nuclear power plants had a higher rate of cancer than those who did not. However, the study also stated that there was no extra radiation near the nuclear power plants, and scientists were puzzled as to what was causing the higher rate of cancer.
Environmental effects of nuclear power
The primary environmental impacts of nuclear power are damage through Uranium mining, radioactive effluent emissions, and waste heat. Like renewable sources, the majority of life cycle studies have found that indirect carbon emissions from nuclear power are many times less than comparable fossil fuel plants. Nuclear generation does not directly produce sulfur dioxide, nitrogen oxides, mercury or other pollutants associated with the combustion of fossil fuels.