Nuclear reactor

A nuclear reactor is a unit or vessel, including associated equipment and material, in which controlled nuclear reactions take place for a variety of purposes. These reactions generally involve controlled nuclear fission chain reactions with a neutron flux. These purposes may 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.

Cooling

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.

Surveillance

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.