Nuclear Graphite

Nuclear Graphite

Overview
Nuclear graphite is any grade of graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

, usually electro-graphite, specifically manufactured for use as a moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

 or reflector
Neutron reflector
A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, and tungsten carbide, or other materials...

 within nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

s. Graphite is an important material for the construction of both historical and modern nuclear reactors as it is one of the purest materials manufactured at industrial scale and it retains its properties (including strength
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...

) at high temperatures.

In 1942 Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

 decided to use graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 as a moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

 in the first attempt to create a self sustaining nuclear chain reaction
Nuclear chain reaction
A 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...

.
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Encyclopedia
Nuclear graphite is any grade of graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

, usually electro-graphite, specifically manufactured for use as a moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

 or reflector
Neutron reflector
A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, and tungsten carbide, or other materials...

 within nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

s. Graphite is an important material for the construction of both historical and modern nuclear reactors as it is one of the purest materials manufactured at industrial scale and it retains its properties (including strength
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...

) at high temperatures.

History


In 1942 Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

 decided to use graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 as a moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

 in the first attempt to create a self sustaining nuclear chain reaction
Nuclear chain reaction
A 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...

. The initial attempt was not successful as the graphite was not sufficiently pure. A second attempt was made with AGX graphite, manufactured by the National Carbon Company
National Carbon Company
The National Carbon Company was founded in 1886 by the former Brush Electric Company executive W. H. Lawrence, in association with Myron T. Herrick, James Parmelee, and Webb Hayes, son of U.S. President Rutherford B. Hayes, in Cleveland, Ohio. In 1890, National Carbon merged with Thomson-Houston,...

, which successfully went critical
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...

. The Pile constructed was called the Chicago Pile-1
Chicago Pile-1
Chicago Pile-1 was the world's first man-made nuclear reactor. CP-1 was built on a rackets court, under the abandoned west stands of the original Alonzo Stagg Field stadium, at the University of Chicago. The first self-sustaining nuclear chain reaction was initiated in CP-1 on December 2, 1942...

.

Similar work had been undertaken in the USSR leading to findings similar to those in the USA. In particular, the negative temperature coefficient of reactivity was found, which became a major argument in favour of Graphite moderated reactors
Graphite moderated reactors
There are several types of graphite moderated nuclear reactors that have been used in commercial electricity generation:*Gas-cooled reactors**Magnox**Advanced gas-cooled reactor *Water-cooled reactors**RBMK*High temperature gas-cooled reactors...

  (see Classification of Nuclear Reactors).

Unlike the promising results in the US and in the USSR, German investigators came to the conclusion that graphite could not be used with natural uranium
Natural uranium
Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...

 to produce a nuclear chain reaction. The purest graphite available to them at that time was a product from the Siemens
Siemens
Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG...

 Plania company, which exhibited a neutron absorption cross section of about 7.5 mb
Barn (unit)
A barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...

. Compared to that the graphites used in CP-1 exhibited average thermal absorption cross sections of 6.68 mb (AGX, National Carbon Company), 5.51 mb (Speer Carbon Company), and 4.97 mb (AGOT, National Carbon Company). (Haag [2005])

Already in December 1942, Eugene P. Wigner first called attention to the effects of fast particles on solids. It was suggested that neutrons from fission would displace atoms from their lattice sites by momentum transfer with effects on most of the properties of graphite, which has become known as the Wigner effect
Wigner effect
The Wigner effect , also known as the discomposition effect, is the displacement of atoms in a solid caused by neutron radiation....

. The possibility was even suggested that graphite bars might fuse together as chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

s at the surface of the bars are opened and closed again, and during operation of a reactor enough energy would be deposited in the graphite to open all the chemical bonds several times. Even the possibility that the graphite parts might very quickly break into small pieces could not be ruled out. However, the first power-producing reactors (X-10 Graphite Reactor
X-10 Graphite Reactor
The X-10 Graphite Reactor at Oak Ridge National Laboratory in Oak Ridge, Tennessee, formerly known as the Clinton Pile and X-10 Pile, was the world's second artificial nuclear reactor and was the first reactor designed and built for continuous operation.When President Roosevelt in December 1942...

 and Hanford B Reactor) had to be built without such knowledge. Cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...

s, which were the only fast neutron sources
Neutron source
A Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...

 available, would need several months of irradiation to produce damaging effects equivalent to one day in a Hanford reactor.

This was the starting point for large-scale research programmes to investigate the property changes due to fast particle radiation
Particle radiation
Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam....

 and to predict their influence on the safety and the lifetime of graphite reactors to be built. Influences of fast neutron radiation on strength
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...

, electrical and thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

, thermal expansitivity, dimensional stability, on the storage of internal energy (Wigner energy) and on many other properties have been observed many times and in many countries after the first results emerged from the X-10 reactor in 1944.

Although catastrophic behaviour such as fusion or crumbling of graphite pieces, has never occurred, large changes in many properties do result from fast neutron irradiation which need to be taken into account when graphite components of nuclear reactors are designed. Although not all effects are well understood yet, more than 100 graphite reactors have successfully operated in the last 60 years. A few severe accidents in graphite reactors can in no case be attributed to a lack of knowledge or insufficient properties of the graphite in use.

Purity


Care must be taken that reactor-grade graphite is free of neutron absorbing materials such as boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

, widely used as the seed electrode in commercial graphite deposition systems-- this caused the failure of the Germans' World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 graphite-based nuclear reactors.

Behaviour under irradiation


This describes the behavior of nuclear graphite, specifically when exposed to fast neutron irradiation.

Specific phenomena addressed:
  • Dimensional change (shrinkage and neutron induced swelling)
  • Change in elastic modulus
    Elastic modulus
    An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force is applied to it...

     (measured by Impulse excitation technique
    Impulse excitation technique
    -Practical:The impulse excitation technique is a nondestructive test method that uses natural frequency, dimensions and mass of a test-piece to determine Young's modulus, Shear modulus, Poisson's ratio and damping coefficient....

    )
  • Change in coefficient of thermal expansion
  • Change in thermal conductivity
    Thermal conductivity
    In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

  • Change in electrical resistivity
  • Irradiation induced creep
    Creep (deformation)
    In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....


Manufacture


Nuclear graphite for the UK Magnox
Magnox
Magnox is a now obsolete type of nuclear power reactor which was designed and is still in use in the United Kingdom, and was exported to other countries, both as a power plant, and, when operated accordingly, as a producer of plutonium for nuclear weapons...

 reactors were manufactured from petroleum coke
Petroleum coke
Petroleum coke is a carbonaceous solid derived from oil refinery coker units or other cracking processes. Other coke has traditionally been derived from coal....

 mixed with coal-based binder pitch
Pitch (resin)
Pitch is the name for any of a number of viscoelastic, solid polymers. Pitch can be made from petroleum products or plants. Petroleum-derived pitch is also called bitumen. Pitch produced from plants is also known as resin. Products made from plant resin are also known as rosin.Pitch was...

 heated and extruded into bricks, and then baked at 1,000 °C for several days. To reduce porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

 and increase density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 the bricks were impregnated with coal tar
Coal tar
Coal tar is a brown or black liquid of extremely high viscosity, which smells of naphthalene and aromatic hydrocarbons. Coal tar is among the by-products when coal iscarbonized to make coke or gasified to make coal gas...

 at high temperature and pressure before a final bake at 2,800 °C. Individual bricks were then machined
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...

 into the final required shapes.

Accidents in graphite-moderated reactors


There have been two major accidents in graphite moderated reactors, the Windscale fire
Windscale fire
The Windscale fire of 10 October 1957 was the worst nuclear accident in Great Britain's history, ranked in severity at level 5 on the 7-point International Nuclear Event Scale. The two piles had been hurriedly built as part of the British atomic bomb project. Windscale Pile No. 1 was operational in...

 and the Chernobyl disaster
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...

.

In the Windscale fire, however, the uranium fuel rather than the graphite in the reactor caught fire. The only graphite moderator damage was found to be localized around burning fuel elements.