Nuclear reactor
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 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.
After the discovery of fission in 1938, many countries launched military nuclear research programs. Early subcritical experiments probed neutronics. In 1942, the first artificial critical nuclear reactor, Chicago Pile-1, was built by the Metallurgical Laboratory. From 1944, for weapons production, the first large-scale reactors were operated at the Hanford Site. The pressurized water reactor design, used in about 70% of commercial reactors, was developed for US Navy submarine propulsion, beginning with S1W in 1953. In 1954, small-scale nuclear electricity production began with the Soviet Obninsk plant. The first commercial nuclear power station was Calder Hall at Sellafield, England, which started operation in 1956.
Spent fuel can be reprocessed, potentially reducing nuclear waste and recovering reactor-usable fuel. This also poses a nuclear proliferation risk via production of plutonium for nuclear weapons.
Reactor accidents have been caused by combinations of design and operator failure. The 1979 Three Mile Island accident, at INES Level 5, and the 1986 Chernobyl disaster and 2011 Fukushima disaster, both at Level 7, all had major effects on the nuclear industry and anti-nuclear movement.
, there are 417 commercial reactors, 226 research reactors, and over 200 marine propulsion reactors in operation globally. Commercial reactors provide 9% of the global electricity supply, compared to 30% from renewables, together comprising low-carbon electricity. Almost 90% of this comes from pressurized and boiling water reactors. Other designs include gas-cooled, fast-spectrum, breeder, heavy-water, molten-salt, and small modular, each of which improves safety, efficiency, cost, fuel type, enrichment, or burnup, and the now-obsolete Light water graphite reactor.
Terminology
During early 1940s nuclear research, the phrase "atomic pile" was used for any assembly involving uranium and attempts at neutron multiplication, including the majority which were subcritical. After Chicago Pile-1 demonstrated a self-sustaining chain reaction, the "reactor" terminology became more common. The phrases "nuclear pile" and "atomic reactor" were also common.Critical mass experiments, while being far simpler, are sometimes referred to as research reactors, such as the Godiva device.
"Nuclear reactor" is predominantly used to refer to the nuclear fission reactor. It can also refer to a nuclear fusion reactor, of which only net negative power systems have been constructed. Radioisotope thermoelectric generators and radioisotope heater units, while deriving power from nuclear decay reactions, are not referred to as nuclear reactors as they do not induce reactions.
Operation
Just as conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms.Fission
When a large fissile atomic nucleus such as uranium-235, uranium-233, or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei,, releasing kinetic energy, gamma radiation, and free neutrons. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.To control such a nuclear chain reaction, control rods containing neutron poisons and neutron moderators are able to change the portion of neutrons that will go on to cause more fission. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring or instrumentation detects unsafe conditions.
Heat generation
The reactor core generates heat in a number of ways:- The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms.
- The reactor absorbs some of the gamma rays produced during fission and converts their energy into heat.
- Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shut down.
The fission of one kilogram of uranium-235 releases about 19 billion kilocalories, so the energy released by 1 kg of uranium-235 corresponds to that released by burning 2.7 million kg of coal.
Cooling
A nuclear reactor coolant – usually water but sometimes a gas or a liquid metal or molten salt – is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. However, in some reactors the water for the steam turbines is boiled directly by the reactor core; for example the boiling water reactor.Reactivity control
The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. Some of these methods arise naturally from the physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose.The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods are made of so-called neutron poisons and therefore absorb neutrons. When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces – often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power.
The physics of radioactive decay also affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder released immediately upon fission. The fission products which produce delayed neutrons have half-lives for their decay by neutron emission that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly when a reactor reaches the critical point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, and other points in the process interpolated in cents.
In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.
In other reactors, the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors, power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to scram the reactor in an emergency shut down. These systems insert large amounts of poison into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.
Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit. The common fission product Xenon-135 produced in the fission process acts as a neutron poison that absorbs neutrons and therefore tends to shut the reactor down. Xenon-135 accumulation can be controlled by keeping power levels high enough to destroy it by neutron absorption as fast as it is produced. Fission also produces iodine-135, which in turn decays to new xenon-135. When the reactor is shut down, iodine-135 continues to decay to xenon-135, making restarting the reactor more difficult for a day or two, as the xenon-135 decays into cesium-135, which is not nearly as poisonous as xenon-135, with a half-life of 9.2 hours. This temporary state is the "iodine pit." If the reactor has sufficient extra reactivity capacity, it can be restarted. As the extra xenon-135 is transmuted to xenon-136, which is much less a neutron poison, within a few hours the reactor experiences a "xenon burnoff transient". Control rods must be further inserted to replace the neutron absorption of the lost xenon-135. Failure to properly follow such a procedure was a key step in the Chernobyl disaster.
Reactors used in nuclear marine propulsion often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without refueling. For this reason many designs use highly enriched uranium but incorporate burnable neutron poison in the fuel rods. This allows the reactor to be constructed with an excess of fissionable material, which is nevertheless made relatively safe early in the reactor's fuel burn cycle by the presence of the neutron-absorbing material which is later replaced by normally produced long-lived neutron poisons which gradually accumulate over the fuel load's operating life.