A
nuclear reactor core is that portion of a
nuclear reactorA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate.The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the power in some ships...
containing the
nuclear fuelNuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor;...
components where the
nuclear reactionsIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
take place.
The nuclear reactor core (also referred to as the "reactor core", or the "core") is the region within a nuclear reactor where the nuclear fuel assemblies are located and the nuclear reaction consequently takes place.
Inside the core of a typical
pressurized water reactorPressurized water reactors comprise a majority of all western nuclear power plants and are one of two types of light water reactor , the other type being boiling water reactors . In a PWR the primary coolant is pumped under high pressure to the reactor core, then the heated water transfers...
or
boiling water reactorThe Boiling Water Reactor is a type of nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor . The BWR was developed by the Idaho National Laboratory and General Electric in the...
.
A
nuclear reactor core is that portion of a
nuclear reactorA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate.The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the power in some ships...
containing the
nuclear fuelNuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor;...
components where the
nuclear reactionsIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
take place.
Description
The nuclear reactor core (also referred to as the "reactor core", or the "core") is the region within a nuclear reactor where the nuclear fuel assemblies are located and the nuclear reaction consequently takes place.
Water-moderated reactors
Inside the core of a typical
pressurized water reactorPressurized water reactors comprise a majority of all western nuclear power plants and are one of two types of light water reactor , the other type being boiling water reactors . In a PWR the primary coolant is pumped under high pressure to the reactor core, then the heated water transfers...
or
boiling water reactorThe Boiling Water Reactor is a type of nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor . The BWR was developed by the Idaho National Laboratory and General Electric in the...
. are pencil-thin nuclear fuel rods, each about long, which are grouped by the hundreds in bundles called "fuel assemblies". Inside each fuel rod, pellets of uranium, or more commonly uranium oxide, are stacked end to end. Also inside the core are control rods, filled with pellets of substances like
hafniumHafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
or
cadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. The soft, bluish-white transition metal is chemically similar to the two other metals in group 12, zinc and mercury. Similar to zinc it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
that readily capture
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s. When the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the
chain reactionA chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events...
. On the converse, when the control rods are lifted out of the way, more neutrons strike the
fissileIn nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate...
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
-235 (U-235) or
plutoniumPlutonium is a rare transuranic radioactive element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen and...
-239 (Pu-239) nuclei in nearby fuel rods, and the
chain reactionA nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
intensifies.
The heat of the fission reaction is removed by the water, which also acts to moderate the neutron reactions. An alternative form of nuclear fuel would be
fissileIn nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate...
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
-233 (U-233) made by the neutron-bombardment of the common
thoriumThorium is a chemical element with the symbol Th and atomic number 90. It is a naturally occurring, slightly radioactive metal. Thorium is estimated to be about three to four times more abundant than uranium in the earth's crust...
-232. Also, fissile uranium-234 (U-234) is found as a trace addition to U-235 wherever U-235 is found. They are both good nuclear "fuels".
Graphite-moderated reactors
There are also Graphite moderated reactors in use.
One type uses solid graphite for the
neutron moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
and ordinary water for the coolant. See the Soviet-made
RBMKRBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union for use in nuclear power plants to produce nuclear power from nuclear fuel....
nuclear-power reactor. This was the type of reactor involved in the
Chernobyl disasterThe Chernobyl disaster was a nuclear reactor accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine . It is considered to be the worst nuclear power plant disaster in history and the only level 7 event on the International Nuclear Event Scale...
.
In the
advanced gas-cooled reactorAn advanced gas-cooled reactor is a type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant...
, a British design, the core is made of a graphite
neutron moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
where the fuel assemblies are located.
Carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
gas acts as a coolant and it circulates through the graphite removing heat.
There have also been several experimental reactors that use graphite for moderation, such as the
pebble bed reactorThe pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of Very high temperature reactor [formally known as the high temperature gas reactor ], one of the six classes of nuclear reactors in the Generation IV initiative...
concepts and the
molten-salt reactor experimentThe Molten-Salt Reactor Experiment was an experimental molten-salt reactor at the Oak Ridge National Laboratory ; researching this technology through the 1960s. The MSRE was a 7.4 MWth test reactor simulating the neutronic "kernel" of an inherently safe epithermal thorium breeder...
(graphite core shown at right).