A nuclear reactor is vessel in which controlled nuclear reactions take place within one building or container with the ultimate goal of generating electricity (steam may first be produced, and then the steam is used to generate electrical power). Nuclear reactions are controlled nuclear fission chain reactions with a neutron flux. Reasons for reactors existing include heat generation for electrical generation, marine propulsion, or heating industrial plants or other facilities; breeding nuclear fuel; the preparation of radioactive isotopes for use in nuclear medicine, industrial testing, or creating controlled sources of radiation; production of nuclear materials such as plutonium or tritium; or making materials temporarily radioactive for procedures such as neutron activation analysis. While there can be some overlap of functions, larger reactors tend to be optimized for a single purpose; part of the design failures causing the Chernobyl Disaster were that the reactor tried to be equally effective for electric power and plutonium generation.
Fundamentals of nuclear fission reactors
For power generation, nuclear reactors are the centerpiece of nuclear power plants. Up to this time, nuclear reactors for large scale power generation use energy released by nuclear fission, which is highly exothermic, meaning each fission releases a relative large amount of heat per atom split. These nuclear fission reactions take place by a controlled nuclear chain reaction in the reactor core inside the reactor. The material undergoing the fission in the core is considered the nuclear fuel. The nuclear fuel consists of fissile isotopes, atoms of isotopes of high atomic number and mass which can readily undergo fission to produce a nuclear chain reaction. The three most common fissile isotopes are uranium-235 (235U or U-235), plutonium-239 (239Pu or Pu-239), and uranium-233 (233U or U-233). Material that can be bred into such fissile isotopes may also be considered nuclear fuel. For example, uranium-238 (238U or U-238) can be bred to produce plutonium-239 and thorium-232 (232Th or Th-232) can be bred to produce uranium-233. Nuclear reactors in which this sort of breeding takes place are called nuclear breeder reactors.
Nuclear fission typically occurs when a neutron hits a fissile nucleus, splitting the nucleus into two smaller nuclei called nuclear fission products and a couple of neutrons. These newly released neutrons can then go on to cause further fission of other fissile nuclei, releasing more neutrons. A repetitive cycle of fissions and neutrons results in a chain reaction under the right conditions, which is the objective of a nuclear fission reactor. In such an operating reactor, there are many neutrons flying around in the core, and the concentration of these neutrons is often referred to as a neutron flux. The numerous nuclear fissions in an operating reactor core release heat, which is used as thermal power, the usual ultimate goal of the reactor. A reactor has a number of control rods consisting of a material which captures neutrons. These control rods can be withdrawn from or inserted into the core to control the nuclear chain reaction. Inserting all of the control rods into the core will capture the neutrons and stop the chain reaction, effectively shutting down the reactor. A quick insertion of the control rods into the core for an emergency shutdown of the reactor is called a scram. Withdrawing the control rods in a precise manner is used to start up the reactor. There are also other means used for controlling the power level of a nuclear reactor.
A fission reactor core contains fuel elements, which are like "package structures" which contain the nuclear fission fuel encased in cladding. The cladding is a solid material and pressure boundary which keeps the nuclear fuel and any fission products created inside each fuel element. The cladding is typically a metal alloy called zircalloy consisting largely of the metallic chemical element zirconium, which has a low neutron absortion cross-section, a low affinity for absorbing neutrons. Zircalloy is also reasonably strong, corrosion-resistant, and able to withstand high enough temperature for reactor operation. A common structure for a fuel element has been to have a zircalloy tube used as cladding to contain small cylindrical pellets of nuclear fuel throughout the tube length contained in the core. The fuel elements are commonly assembled into bundles called fuel modules; there are a number of such fuel modules inside a reactor core. Flowing reactor coolant fluid and a neutron moderator surround the fuel elements in the core. The same material may serve as both reactor coolant and moderator. One or more control rods, which can slide in and out, are commonly inserted into the fuel modules between fuel elements. To help the fuel in the core burn out more evenly, small neutron-absorbing poison "pellets", which typically contain boron-10 (10B or B-10), are often placed in some strategic locations inside the core. Although the initial fuel may not be particularly radioactive, once the reactor core has gone critical (been operating), the resulting fission products make the fuel elements very highly radioactive.
The cladding of each fuel element must maintain its integrity, so as not to leak out any radioactive material into the coolant or the rest of the reactor plant. Cladding also should not swell up, narrowing the reactor coolant channel between the fuel elements, so as not to slow down coolant flow, which would allow heat and temperature to build up around that channel. This means the zircalloy cladding cannot reach excessively high temperature. For this reason and to prevent undesired boiling in pressurized water reactors, reactor power level is operationally limited.
The probability of fission occurring depends on the type of fissile nucleus and the colliding neutron's kinetic energy, which is effectively proportional to the velocity of the neutron squared. Neutrons which are produced directly from fission are typically fast neutrons. To maintain a good, effective fission chain reaction, it is desirable for a neutron hitting a fissile nucleus to cause fission and to minimize the incidence of the fissile nucleus capturing the neutron. Slowly moving neutrons are more effective at causing fission. Collisions of faster neutrons with small nuclei slow down neutron. A material containing such nuclei which slow down neutrons is a neutron moderator. Slow neutrons that have a kinetic energy approximately equal to the thermal energy of surrounding molecules are referred to as thermal neutrons. The process of slowing down faster neutrons to such thermal energy with a moderator can be called thermalization of the neutrons.
The smallest nuclei are in hydrogen-1 (1H or H-1, also called light hydrogen or protium) atoms and slow down neutrons for fission the best. H-1 nuclei have a small tendency to capture neutrons also, but are good enough to maintain a fission chain reaction anyway if modestly enriched uranium is used as a nuclear fuel. The easiest way to use H-1 as a moderator is to use purified, normal water (called light water) as both a moderator and a coolant to carry away heat simultaneously. The hydrogen in such light water is practically 100% H-1. A nuclear reactor which uses such light water is called a light water reactor, of course.
The second smallest nuclei are in hydrogen-2 (2H, H-2, or D, also called heavy hydrogen or deuterium) atoms and moderate neutrons for fission quite well also. The tendency for H-2 nuclei to capture neutrons is practically negligible, which is good enough to maintain a fission chain reaction even with unenriched uranium. Similarly, the easiest way to use H-2 as a moderator is to use purified, heavy water (deuterium oxide, often symbolized as D2O) as both a moderator and coolant. Heavy water contains H-2 atoms instead of H-1. A nuclear reactor using heavy water is analogously called a heavy water reactor.
Carbon of natural isotopic abundance is a solid mostly of carbon-12 (12C or C-12) and able to withstand very high temperature when in the absence of oxygen. Carbon can be used in the form of graphite as a moderator, but not as a coolant since a solid does not flow. Some fluid such as water would then be used as the reactor coolant to carry away the heat produced. RBMK reactors of formerly Soviet Union countries used carbon as the moderator.
Reactors of any appreciable size are liquid- or gas-cooled. The most common liquid coolant is highly purified water, or "light water" to differentiate it from heavy water. Heavy water cooling, which plays a part in moderation, has specific applications in reactors that produce plutonium or tritium. For some power producing reactors, there has been continuing experimentation with liquid sodium, which has advantages for heat transfer.
Reactors designated as being for peaceful purposes are under the inspection of the International Atomic Energy Agency, which makes physical inspections, and also installs unmanned but tamperproof seals on certain reactor components, as well as on-site cameras and other instrumentation.
Soviet plutonium-producing reactors released 85krypton, detectable by air sampling. 
- ↑ Jeffrey Richelson (2006), Spying on the Bomb: American Nuclear Intelligence from Nazi Germany to Iran and North Korea, W.W. Norton, ISBN 8769393053838, p. 114