A thermal-neutron reactor is a nuclear reactor that uses slow or thermal neutrons. ("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and structure, which is much lower energy than the fast neutrons initially produced by fission.)
Most nuclear power plant reactors are thermal reactors and 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. 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.
The nuclear cross section of uranium-235 for slow thermal neutrons is about 1000 barns, while for fast neutrons it is in the order of 1 barn. Therefore, thermal neutrons are more likely to cause uranium-235 to nuclear fission than to 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 consist of the following:
The light-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).
A heavy water reactor (HWR) is a type of which uses (D2O, oxide) as a . It may also use this as the coolant, in the case of s. Due to heavy water's low neutron absorption cross section, HWRs can operate with fuel.
A gas-cooled reactor (GCR) is a that uses as a and a gas ( or in extant designs) as . Although there are many other types of reactor cooled by gas, the terms GCR and to a lesser extent gas cooled reactor are particularly used to refer to this type of reactor.
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.