Basic Process - How Reactors Work
Fuel - Thermal Fission - Fission Fragments
The majority of nuclear reactors used in power plants world-wide are called thermal reactors because they use primarily thermal fission. Thermal neutrons (in red in figure below) "move in thermal equilibrium with the environment they are in." At a temperature of 550F, this corresponds to a velocity of about 3700 meters per second.
When these neutrons strike a uranium-235 nucleus (in blue in figure below) in the Fuel, sometimes the nucleus will split, or fission, into 2 unequally sized nuclei called fission fragments (in green in figure below). The fission products are large positively-charged particles of elements like Strontium or Iodine. These fission products slow down quickly in the fuel . They are the major contributor to heat production in the fuel (~80%). This heat gets transmitted to the coolant outside the fuel.
Also produced are between 2 and 3 neutrons (in red in figure below) at a very high energy. These neutrons must slow down to the thermal energy to increase the likelihood of causing further fissions.
Courtesy Tony Bird
Moderator
A Moderator is used to slow the neutrons down to the thermal energy. The neutrons usually do not travel very far in the reactor. They get produced from the fission in the fuel, bounce around a lot in the moderator and lose most of their energy, then cause fission (sometimes) or get absorbed in the fuel (and produce a more highly energized uranium isotope, which then decays into another material by gamma and/or beta decay).
Coolant
The purpose of the nuclear fission process is to produce heat. The fission fragments that are produced by the fission then slow down in the fuel losing most of their energy. The energy is converted to heat in the fuel. A Coolant is used to remove the heat and maintain the temperature of the fuel within acceptable limits. Sometimes the coolant is used to cool the moderator which, in turn, can be used to cool the fuel. Sometimes the coolant cools the fuel by passing around the rods containing the fuel. Sometimes the moderator and the coolant are the same material (e.g. water).
More on Fuel and Uranium Isotopes
The fuel in the reactor is in the form of pellets of either uranium metal or uranium dioxide. There are 2 or more uranium (U) isotopes (iso means same; p refers to the protons, i.e. isotopes are nuclides with the same number of protons, but different numbers of neutrons) - U-235 and U-238. U-238 is the most common form of uranium found in nature. It has 92 protons and 146 neutrons = total atomic mass number of 238. 99.27 % of uranium found in nature is U-238. The rest is U-235 (0.72%) and U-234 (0.0055%). In commercial reactors the fuel often has a higher concentration of U-235 than found in nature - typically 2 to 5%, compared to the 0.72% in nature. See the heavy water moderated reactor section for more on reactors with no enrichment. We say the fuel is enriched and the process used to make fuel with the higher U-235 levels is enrichment.
U-235 will undergo thermal and fast fission, i.e. it will likely fission when hit by neutrons of any energy - but not always; sometimes the neutron will bounce off (called scattering) and sometimes it will be absorbed by the U-235 (called absorption).
U-238 will only undergo fission when it is struck by faster neutrons of certain energies. Sometimes it will absorb neutrons of certain energies. When it does this, the U-238 will undergo a series of decays to form Plutonium-239 (Pu-239). When this happens, the Pu-239 may also fission with thermal neutrons. In the typical commercial reactor, Pu-239 is created and fissions throughout the period of a cycle. Just before the reactor is shutdown for refueling almost 30% of the power generated by the reactor may be due to the Pu-239.
In Europe, some countries, e.g. France, reprocess spent nuclear fuel. The plutonium is chemically separated from the spent fuel and used with U-235 and U-238 in the preparation of new fuel to be installed in the reactor. This type of fuel is referred to as Mixed Oxide Fuel, or MOX for short. Originally, breeder reactors had been planned to produce the Pu-239 for use in MOX fuels in the thermal reactors. Because of plutonium proliferation concerns, the reprocessing option was disallowed in the United States. At that time, the United States tried to halt all reprocessing of spent fuel world wide, since separated plutonium could be used to make atomic bombs. There are 2 schools of thought regarding plutonium proliferation - some as World Nuclear Association, EdF, and British Nuclear Fuels advocate the use of plutonium as an energy source. Others, as the Canadian Coalition for Nuclear Responsibility feel that the production of plutonium is a serious disadvantage of nuclear power as an energy source.
Links for Relevant Searches
The Nuclear Reactor - How They are Constructed
A nuclear reactor consists of
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 Courtesy GE and
UC Berkeley NE Dept. |
Fuel - Each fuel rod of Zircaloy contains stacks of uranium dioxide pellets. Zircaloy (2 or 4) is an alloy of zirconium with low concentrations of tin, iron, chromium, and nickel . Sometimes the fuel may contain boron or gadolinium to aid in the control of the nuclear reaction. In some reactors as the older design Magnox gas cooled reactors, uranium metal may be used as the fuel.
The fuel rods may be individually loaded into the reactor (into tubes located in the reactor) or may be organized into fuel assemblies, as in the case of the BWR fuel assembly shown above, of a square or hexagonal design, 6-10 inches nominal width. In this case, there are about 120 to 900 fuel assemblies in a reactor. Each assembly consists of 50 to 200 fuel rods. Most reactor cores are about 12 to 15 feet high or long, depending on the reactor type.
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This photo shows a fuel assembly
(approximately 8-in x 8-in x 14-ft) being moved in the spent fuel storage
pool of a nuclear power plant. This fuel assembly has 179 fuel rods. The sketch illustrates how the
fuel pellets make up one of the rods and the fuel rods make up a fuel assembly. Click for photos and graphics of fuel assemblies. |
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Moderator - Used to slow down the neutrons to thermal energies. Sometimes the same material is used as a coolant. Typical materials that can be used as moderator include water, heavy water, and graphite. Water is often used because it is plentiful and inexpensive. In general, the better a material is at "moderating" the neutrons, the lower the fuel needs to be enriched. Heavy water is better than water, however, it is also expensive to produce. Graphite also is better than water, however, some of the effects of exposure of graphite materials to radiation are undesirable.
Coolant - Used to remove the heat from the fuel rods directly if the moderator and coolant are the same material. In cases where a separate moderator is used, coolant tubes are routed through the moderator, removing heat from the moderator directly and fuel rods directly or indirectly. Water is often used as a coolant. However, sometimes heavy water, liquid metals (sodium, potassium), or even gases (carbon dioxide) may be used. These different coolants are discussed in the Reactor Coolants section.
Control Rods - Used to regulate the distribution of power in the reactor while the reactor in operating. The most important function is to insert to shutdown or stop the nuclear fission process when required. An automatic control system, or the operator, can initiate the shutdown. In some reactors, all rods (29 to 100+, depending on reactor size) may insert in as short a time as 2 seconds. The control rods are made of materials that quickly stop the nuclear reaction by absorbing the neutrons (i.e. the materials divert the neutrons from being absorbed or causing fissions in the fuel. Materials used include boron carbide, silver, indium, cadmium, and hafnium.
Supporting Structure - Used to keep the fuel rigid either horizontally or vertically, depending on the specific design. Also is used to direct a uniform, or optimum, flow distribution through the reactor.
Vessel - Used to hold the fuel, moderator, coolant, and supporting structure. Normally, these are able to sustain high pressures. Thickness of the vessel depends on the pressure to vessel is subjected to.
Coolants
The Coolant used to cool the reactor in most commercial reactors today is water. In this case, the water is dual purpose since it also serves a moderating function (i.e. it slows the neutrons down to thermal energy to increase the likelihood of fission). Desirable properties for a coolant include:
Coolants that have been used, in test or commercial applications, include:
Power plants that have used nonwater coolants have included:
Some excellent references on coolants:
