Neutron moderator in the context of Neutron energy

A nuclear reactor is a device used to sustain a controlled fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. Fissile nuclei (primarily uranium-235 or plutonium-239) absorb single neutrons and split, releasing energy and multiple neutrons, which can induce further fission. Reactors stabilize this, regulating neutron absorbers and moderators in the core. Fuel efficiency is exceptionally high; low-enriched uranium is 120,000 times more energy-dense than coal.

Heat from nuclear fission is passed to a working fluid coolant. In commercial reactors, this drives turbines and electrical generator shafts. Some reactors are used for district heating, and isotope production for medical and industrial use.

Chicago Pile-1 (CP-1) was the first artificial nuclear reactor. On 2 December 1942, the first human-made self-sustaining nuclear chain reaction was initiated in CP-1 during an experiment led by Enrico Fermi. The secret development of the reactor was the first major technical achievement for the Manhattan Project, the Allied effort to create nuclear weapons during World War II. Developed by the Metallurgical Laboratory at the University of Chicago, CP-1 was built under the west viewing stands of the original Stagg Field. Although the project's civilian and military leaders had misgivings about the possibility of a disastrous runaway reaction, they trusted Fermi's safety calculations and decided they could carry out the experiment in a densely populated area. Fermi described the reactor as "a crude pile of black bricks and wooden timbers".

After a series of attempts, the successful reactor was assembled in November 1942 by a team of about 30 that, in addition to Fermi, included scientists Leo Szilard (who had previously formulated an idea for non-fission chain reaction), Leona Woods, Herbert L. Anderson, Walter Zinn, Martin D. Whitaker, and George Weil. The reactor used natural uranium. This required a very large amount of material in order to reach criticality, along with graphite used as a neutron moderator. The reactor contained 45,000 ultra-pure graphite blocks weighing 360 short tons (330 tonnes) and was fueled by 5.4 short tons (4.9 tonnes) of uranium metal and 45 short tons (41 tonnes) of uranium oxide. Unlike most subsequent nuclear reactors, it had no radiation shielding or cooling system as it operated at very low power – about one-half watt; nonetheless, the reactor's success meant that a chain reaction could be controlled and the nuclear reaction studied and put to use.

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.

A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV, on average), as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is comparatively rich in fissile material.

The fast spectrum is key to breeder reactors, which convert highly abundant uranium-238 into fissile plutonium-239, without requiring enrichment. It also leads to high burnup: many transuranic isotopes, such as of americium and curium, accumulate in thermal reactor spent fuel; in fast reactors they undergo fast fission, reducing total nuclear waste. As a strong fast-spectrum neutron source, they can also be used to transmute existing nuclear waste into manageable or non-radioactive isotopes.

A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan, India and Canada).

In a PWR, water is used both as a neutron moderator and as coolant fluid for the reactor core. In the core, water is heated by the energy released by the fission of atoms contained in the fuel. Using very high pressure (around 155 bar: 2250 psi) ensures that the water stays in a liquid state. The heated water then flows to a steam generator, where it transfers its thermal energy to the water of a secondary cycle kept at a lower pressure which allows it to vaporize. The resulting steam then drives steam turbines linked to an electric generator. A boiling water reactor (BWR) by contrast does not maintain such a high pressure in the primary cycle and the water thus vaporizes inside of the reactor pressure vessel before being sent to the turbine. Most PWR designs make use of two to six steam generators, each associated with a coolant loop.

Moderation is the process or trait of eliminating, lessening, or avoiding extremes. It is used to ensure normality throughout the medium on which it is being conducted. Common uses of moderation include:

• A way of life emphasizing perfect amounts of everything, not indulging in too much of one thing.
• A moderator may remove unsuitable contributions from the website, forum or chat room they represent in accordance with their moderation system.
• Ensuring consistency and accuracy in the marking of student assessments.
• A "moderator" is one name for the formal position of one who presides over a discussion, helping to ensure that the discussion is balanced among its many voices, for example in the Methodist church's use of the term for the heads of its conferences.
• A neutron moderator is used to slow down neutrons in a nuclear reactor.