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Heat is produced in a nuclear reactor when neutrons strike Uranium atoms causing them to fission in a continuous chain reaction. Control elements, which are made of materials that absorb neutrons, are placed among the fuel assemblies. When the control elements, or control rods as they are often called, are pulled out of the core, more neutrons are available and the chain reaction speeds up, producing more heat. When they are inserted into the core, more neutrons are absorbed, and the chain reaction slows or stops, reducing the heat.

Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste product and is dissipated as efficiently as possible. In a power reactor the heat produced is of primary importance for use in driving conventional heat engines; the beams of radiation are controlled by shielding.

Research and test reactors -- also called "non-power" reactors -- are nuclear reactors primarily used to conduct research, development and education. These reactors contribute to almost every field of science including physics, chemistry, biology, medicine, geology, archeology, and environmental sciences.

A breeder reactor is defined as a reactor that both consumes and produces fissionable fuel. Generally breeder reactors produce more fuel than they consume. Breeding is the process by which new fissionable material is created by capturing neutrons from fissions in fertile materials.

Fast breeder reactors are reactors where the fission reaction is sustained by fast neutrons. Fast breeder reactors do not require a moderator, allowing for a variety of working fluids. Two types of fast breeder reactors are Gas-Cooled Fast Breeder Reactors (GCBRs), often cooled by pressurized helium, and Liquid Metal Fast Breeder Reactors (LMFBRs), which are cooled by molten sodium.

The reactor core, at its center, has concentrations of ~20% Pu-239 and 80%. Surrounding fuel rods are 100%. The reactor has a high concentration of fissile material at its core, allowing a chain reaction to be sustained even with fast neutrons, despite the lower probability of fast neutrons causing fissions than slow neutrons.

A consequence of operating with fast-moving neutrons (hence the common name Fast Breeder Reactors, or FBR) is that there is a higher chance of transmuting U-238(uranium- 238) to Pu-239(Plutonium-239).



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