Nuclear reactors generate electricity by releasing the energy stored in the nuclei of uranium and plutonium atoms. To understand how a reactor works, one must look at the basic structure of an atom and the interaction of its components. An atom consists of a dense central core, called the nucleus, surrounded by electrons that may be thought of as orbiting the nucleus. The nucleus is made up of positively charged protons and neutral particles called neutrons; the strong nuclear forces hold the nucleus together, offsetting the repulsive force between the positively charged protons.
The chemical properties that we associate with an element are determined by the number of protons and electrons. The simplest atom, the hydrogen atom, has one proton in its nucleus and one electron circling the nucleus nearly. All atoms have isotopes which are different forms of that element. Isotopes have different numbers of neutrons in the nucleus, but have the same number of protons. Since it is the positive charge on the nucleus and the negative charges of the electrons that determine the chemical behavior of an element, isotopes of a given element exhibit the same chemical behavior. For hydrogen, there are three isotopes; the simplest form has no neutrons in the nucleus - only a proton. The other two isotopes of hydrogen, deuterium and tritium, contain one and two neutrons, respectively.3
Most elements have naturally occurring isotopes. Of particular interest are the isotopes of uranium, uranium 238, and uranium 235. Here, the numbers refer to the total number of neutrons plus protons found in the nucleus. Since the element is determined by the number of protons, the nucleus of an atom of uranium 235 will have the same number of protons as the nucleus of an atom of uranium 238, namely 92. The number of neutrons in the nucleus will be different, however.
As we move up the periodic chart, the neutrons and protons are bound more tightly to one another until we reach the element iron. Once we pass iron, the neutrons and protons are bound more loosely to one another. Hence, in the heavy elements such as uranium, the neutrons and protons are bound less tightly to one another. As a consequence, uranium undergoes a natural form of radioactive transformation called fission. The nucleus literally splits apart into two lighter nuclei in a process called spontaneous fission. Spontaneous fission occurs so very infrequently that it is of little technological interest. However, when struck by a neutron, the nuclei of both uranium 235 and uranium 238 may be induced to undergo fission. When they do, energy is released. If a sufficient number of nuclei fission nearly simultaneously, then a useful amount of energy is released. A nuclear reactor is a device designed to encourage the fissioning of the uranium fuel. It is important to note that the fragments or fission products that result from fission are highly radioactive and must be kept from the environment when produced in large quantities such as in a reactor.
Because of the structure of the uranium 235 nucleus, it is able to fission when struck with a neutron that is moving at any speed, even one moving relatively slowly. Uranium 238, however, requires a neutron that is moving very fast in order for fission to occur. To generate a useful amount of energy, there must be a large number of reactions occurring nearly simultaneously. In nature, the proportion of uranium that is uranium 235 is small, about 0.7%. For most power reactors, this amount is insufficient and the percentage of uranium 235 must be increased or enriched. The enrichment process relies on the slight differences in mass between uranium 235 and uranium 238 and is a very energy intensive one.
Two methods of enrichment are presently in use. The oldest method, the gaseous diffusion method, is based on the fact that a uranium 235 atom bearing molecules of uranium hexafluoride gas is lighter and moves at a higher speed than a uranium
3 The difference in mass between hydrogen and its isotopes does cause the rates of reaction for chemical processes to differ. This difference is used to increase the concentration of deuterium to produce heavy water, which is rich in the isotope deuterium.
238 atom. A gaseous diffusion enrichment plant must have thousands of separation stages since each stage produces only an extremely small amount of enrichment. The gaseous diffusion enrichment method is a very energy intensive enrichment technique and its use is decreasing in favor of the more energy efficient centrifuge enrichment technique.
In the centrifuge method, cylinders containing uranium hexafluoride gas are spun at very high speed. The difference in weight between the uranium 235 bearing molecules and uranium 238 molecules again causes a natural separation to occur. Like the gaseous diffusion method, each stage produces only an extremely small increase in the concentration of uranium 235, requiring many thousands of stages until practical levels of enrichment are obtained. For a typical commercial reactor, the concentration of uranium must be increased to between 3% and 5%. Although still energy intensive, a centrifuge based enrichment facility will use about 1/50 the energy of a gaseous diffusion based enrichment facility.
Once enriched in uranium 235, the uranium hexafluoride gas undergoes a conversion process where the uranium is extracted and converted into uranium dioxide. The uranium dioxide is then formed into pellets about 1 cm in diameter and about 2 cm in length. These pellets are then loaded into cylinders made from an alloy of zirconium which is used since zirconium does not easily absorb neutrons. The cylinders form a cladding around the fuel, preventing the release of fission products into the reactor. Neutron conservation in a reactor is extremely important since it is the neutrons that induce the fissioning of the uranium 235 nuclei. Every effort is made to ensure that neutrons are conserved to encourage their absorption in the uranium fuel pellets.
The cylinders containing the enriched uranium dioxide fuel pellets are hermetically sealed fuel rods. A hundred or more of these rods are then assembled in a square array to form a fuel assembly. Depending on the type of reactor and the number of fuel rods in each assembly, there are between 175 and 400 assemblies needed to form the core of a nuclear reactor. About 100 metric tons of uranium is used to make up a typical reactor core.
The energy released in each fission reaction is far greater than the energy released when a carbon atom is chemically combined with oxygen atoms in a coal fired power plant, each fission releasing about 100 million times as much energy. This energy is in the form of the motion of the split uranium nucleus fragments. The fragments or fission products move off very quickly from the fission site and strike other fuel nuclei, raising the temperature of the fuel. In addition, the fission process produces neutrons that then induce more fissions. If just one neutron from each fission event goes on to induce fission, then the reaction is self-sustaining and the reactor is said to be critical.
The fission products are also highly radioactive and emit various forms of radiation. Some of this radiation is absorbed in the reactor as well, further increasing the temperature of the reactor. The heat generated is then used to heat water in much the same way that the heat from the combustion of coal is used to heat water in a coal-fired power plant. The heated water is then used to generate steam, which in turn generates electricity. The exact process depends on the type of reactor.
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