Athermal-neutron reactor is anuclear reactor that uses slow orthermal neutrons. ("Thermal" does not mean hot in an absolute sense, but means inthermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and structure, which is much lower energy than thefast neutrons initially produced by fission.)
Mostnuclear power plant reactors are thermal reactors and use aneutron moderator to slowneutrons until they approach the averagekinetic energy of the surrounding particles, that is, to reduce the speed of the neutrons to low-velocity, thermal neutrons. Neutrons are uncharged, this allows them to penetrate deep in the target and close to the nuclei, thus scattering neutrons by nuclear forces, some nuclides are scattered large.[1]
Thenuclear cross section ofuranium-235 for slow thermal neutrons is about 1000barns, while for fast neutrons it is in the order of 1 barn.[2] Therefore, thermal neutrons are more likely to cause uranium-235 tonuclear fission than to be captured byuranium-238. If at least one neutron from the U-235 fission strikes another nucleus and causes it to fission, then thechain reaction will continue. If the reaction will sustain itself, it is said to becritical, and the mass of U-235 required to produce the critical condition is said to be acritical mass.
Thermal reactors consist of the following:
Thelight-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to , as both its coolant and ; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reactors are the most common type of , and light-water reactors are the most common type of thermal-neutron reactor.
There are three varieties of light-water reactors: the (PWR), the (BWR), and (most designs of) the (SCWR).These were early prototypes and demonstration reactors.
Commercial reactors with standardized designs and improved safety.
All of these are thermal reactors using moderators like water or graphite.
Enhanced safety, longer lifespans, and passive safety systems.