There are two main types of nuclear reactor, characterized by the speed of the neutrons which induce fission:
- Thermal reactors. These are the predominant kind, using slower neutrons to induce fission, the basic fissile nuclide being U-235.
- Fast breeder reactors. In these less-common reactors, the fast neutrons are used directly to create (breed) fissile nuclides from fissionable nuclides; most commonly Pu-239 is bred from U-238. Pu-239 is also used in nuclear weapons.
There are many varieties of nuclear reactor, but all have the following common elements:
Fuel: The material that undergoes fission. This needn’t have the fissionable nuclides in the form of the element. The fuel is often in the form of a ceramic.
Cladding: This encases the nuclear fuel, isolating it mechanically and chemically from its immediate environment.
Moderator: Necessary in thermal reactors to slow down the neutrons produced by the fission process. Commonly, the moderator is in the form of a rod, but can be in liquid form or even be mixed with the fuel itself.
Control: This can be used to absorb excess neutrons, or even shut down the reactor in an emergency. Most often, the control material is in the form of a rod.
Core: The heart of the reactor, containing the fuel. The fuel is encased in cladding, and core must also accommodate the coolant and allow for more moderating rods or control rods to be added.
Coolant: The coolant removes heat from the reactor core into a heat exchanger. Note that the coolant itself is not cool, just that it removes heat from the core.
Reactor vessel: This contains the reactor core and the coolant. It often also acts as a reflector, reducing the loss of neutrons to the outside environment.
Generator/turbine: The heat generated by the reactor core generates steam, used to drive a turbine, which can generate electricity.
The following simulation demonstrates these main components in use.
Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.
The types of reactor are loosely grouped into generations describing the time period in which they were first used. Advances in technology have led to new designs.
The current generation of reactors can be defined by the materials used for each of these components. They include Pressurised Water Reactors (PWR), the most common reactor type, Boiling Water Reactors (BWR), CANDU or Pressurised Heavy Water Reactor (PHWR). These all include water as a coolant in some form. There are also Gas Cooled Reactors (GCR) and Advanced Gas Cooled Reactors (AGR), which use CO2 as coolant. Finally, there are also Liquid Metal Fast Breeder Reactors (LMFBR), which are cooled by a liquid metal (sodium or lead). There are also many other forms of reactors used for research purposes.
The next generation, commonly referred to as Generation IV, in some cases are just incremental improvements on these designs, but in other cases are radically different designs aimed at increasing efficiencies and reducing risk. The latter may demand materials which can sustain exposure to much more extreme environments.
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