A thermal reactor uses slow or thermal neutrons. Most power reactors are of this type. These type of reactors use a neutron moderator to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutrons to low velocity thermal neutrons. The nuclear cross section of uranium-235 for slow thermal neutrons is about 1000 barns. For fast neutrons it is in the order of 1 barn.^[1]

Thermal neutrons will be more likely to cause uranium-235 to fission when it is struck by the neutrons, in this case thermal neutrons, and fewer neutrons will be captured by uranium-238. If at least one neutron from the U-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass.

Thermal reactors consists of the following:

• Neutron moderator to slow down the neutrons.
• Nuclear fuel, which is a fissile material, usually uranium.
• Containment buildings which is designed to, in any emergency, contain the escape of radiation.
• Reactor vessel that is a pressure vessel containing the coolant and reactor core.
• Radiation shielding to protect people and the environment from the harmful effects of ionizing radiation.
• Instrumentation to monitor and control the reactor's systems.

[edit] See also

• Enriched uranium
• Fast neutron
• Fast neutron reactor
• Neutron moderator
• Nuclear fission
• Uranium-235
• Uranium-238

[edit] References

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