Nuclear graphite is any grade of

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

, usually synthetic graphite, manufactured for use as a moderator or reflector within a

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

. Graphite is an important material for the construction of both historical and modern

s, due to its extreme purity and ability to withstand extremely high temperature. Graphite has also recently been used in nuclear

fusion reactors Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices de ...

such as the

Wendelstein 7-X The Wendelstein 7-X (abbreviated W7-X) reactor is an experimental stellarator built in Greifswald, Germany, by the Max Planck Institute for Plasma Physics (IPP), and completed in October 2015.stellarator's wall and a graphite island divertor have greatly improved plasma performance within the device, yielding better control over impurity and heat exhaust, and long high-density discharges. MSRE Core

History

Nuclear fission, the creation of a nuclear chain reaction in

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

, was discovered in 1939 following experiments by

Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the fields of radioactivity and radiochemistry. He is referred to as the father of nuclear chemistry and father of nuclear fission. Hahn and Lise Meitner ...

and

Fritz Strassman Friedrich Wilhelm Strassmann (; 22 February 1902 – 22 April 1980) was a German chemist who, with Otto Hahn in December 1938, identified the element barium as a product of the bombardment of uranium with neutrons. Their observation was the k ...

, and the interpretation of their results by physicists such as Lise Meitner and Otto Frisch. Shortly thereafter, word of the discovery spread throughout the international physics community. In order for the fission process to chain react, the neutrons created by uranium fission must be slowed down by interacting with a

neutron moderator In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely mo ...

(an element with a low atomic weight, that will "bounce", when hit by a neutron) before they will be captured by other uranium atoms. By late 1939, it became well known that the two most promising moderators were heavy water and

. In February 1940, using funds that were allocated partly as a result of the Einstein-Szilard letter to President Roosevelt,

Leo Szilard Leo Szilard (; hu, Szilárd Leó, pronounced ; born Leó Spitz; February 11, 1898 – May 30, 1964) was a Hungarian-German-American physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea of a nuclear ...

purchased several tons of graphite from the Speer Carbon Company and from the

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. ...

(the National Carbon Division of the Union Carbide and Carbon Corporation in Cleveland, Ohio) for use in

Enrico Fermi Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...

's first fission experiments, the so-called exponential pile. Fermi writes that "The results of this experiment was icsomewhat discouraging" presumably due to the absorption of neutrons by some unknown impurity. So, in December 1940 Fermi and Szilard met with Herbert G. MacPherson and V. C. Hamister at National Carbon to discuss the possible existence of impurities in graphite. During this conversation it became clear that minute quantities of

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

impurities were the source of the problem. As a result of this meeting, over the next two years, MacPherson and Hamister developed thermal and gas extraction purification techniques at National Carbon for the production of boron-free graphite. The resulting product was designated AGOT Graphite ("Acheson Graphite Ordinary Temperature") by National Carbon, and it was "the first true nuclear grade graphite". During this period, Fermi and Szilard purchased graphite from several manufacturers with various degrees of

neutron absorption Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

: AGX graphite from

with 6.68 mb (millibarns) cross section, US graphite from United States Graphite Company with 6.38 mb cross section, Speer graphite from the Speer Carbon Company with 5.51 mb cross section, and when it became available, AGOT graphite from National Carbon, with 4.97 mb cross section. (See also Haag 005) By November 1942 National Carbon had shipped 250 tons of AGOT graphite to the University of Chicago where it became the primary source of graphite to be used in the construction of Fermi's Chicago Pile-1, the first nuclear reactor to generate a sustained chain reaction (December 2, 1942). AGOT graphite was used to build the X-10 graphite reactor in Oak Ridge TN (early 1943) and the first reactors at the

Hanford Site The Hanford Site is a decommissioned nuclear production complex operated by the United States federal government on the Columbia River in Benton County in the U.S. state of Washington. The site has been known by many names, including SiteW a ...

in Washington (mid 1943), for the production of plutonium during and after World War II. The AGOT process and its later refinements became standard techniques in the manufacture of nuclear graphite. The neutron cross section of graphite was also investigated during the second world war in Germany by

Walter Bothe Walther Wilhelm Georg Bothe (; 8 January 1891 – 8 February 1957) was a German nuclear physicist, who shared the Nobel Prize in Physics in 1954 with Max Born. In 1913, he joined the newly created Laboratory for Radioactivity at the Reich Physi ...

, P. Jensen, and

Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent series ...

. The purest graphite available to them was a product from the

Siemens Siemens AG ( ) is a German multinational conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal divisions of the corporation are ''Industry'', '' ...

Plania company, which exhibited a

of about 6.4 mb to 7.5 mb (Haag 2005). Heisenberg therefore decided that graphite would be unsuitable as a moderator in a reactor design using natural uranium, due to this apparently high rate of neutron absorption. Consequently, the German effort to create a chain reaction involved attempts to use heavy water, an expensive and scarce alternative, made all the more difficult to acquire as a consequence of the

Norwegian heavy water sabotage The Norwegian heavy water sabotage ( nb, Tungtvannsaksjonen; nn, Tungtvassaksjonen) was a series of Allied-led efforts to halt German heavy water production via hydroelectric plants in Nazi Germany-occupied Norway during World War II, involvi ...

by Norwegian and Allied forces. Writing as late as 1947, Heisenberg still did not understand that the only problem with graphite was the boron impurity.

Wigner effect

In December 1942

Eugene Wigner Eugene Paul "E. P." Wigner ( hu, Wigner Jenő Pál, ; November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist who also contributed to mathematical physics. He received the Nobel Prize in Physics in 1963 "for his co ...

suggested that neutron bombardment might introduce dislocations and other damage in the molecular structure of materials such as the graphite moderator in a nuclear reactor (the

Wigner effect The Wigner effect (named for its discoverer, Eugene Wigner), also known as the discomposition effect or Wigner's disease, is the displacement of atoms in a solid caused by neutron radiation. Any solid can display the Wigner effect. The effect is ...

). The resulting buildup of energy in the material became a matter of concern The possibility was suggested that graphite bars might fuse together as

chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...

s at the surface of the bars when opened and closed again. 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 and

Hanford B Reactor The B Reactor at the Hanford Site, near Richland, Washington, was the first large-scale nuclear reactor ever built. The project was a key part of the Manhattan Project, the United States nuclear weapons development program during World War II. I ...

) had to be built without such knowledge.

Cyclotron A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Jan ...

s, which were the only

fast neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

sources Source may refer to: Research * Historical document * Historical source * Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence * Source (journalism), a person, publication, publishing institute o ...

available, would take several months to produce neutron irradiation 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 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. Due to the wave–par ...

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 may refer to: Physical strength *Physical strength, as in people or animals * Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human c ...

electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...

and

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

thermal expansivity Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

, dimensional stability, on the storage of internal energy (

Wigner energy Eugene Paul "E. P." Wigner ( hu, Wigner Jenő Pál, ; November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist who also contributed to mathematical physics. He received the Nobel Prize in Physics in 1963 "for his con ...

), 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 for decades since the 1940s. A few severe accidents in graphite reactors can in no case be attributed to insufficient information (at the time of design) regarding the properties of the graphite in use. In the 2010s, the collection of new material property data has improved knowledge significantly.

Purity

Reactor-grade graphite must be free of neutron absorbing materials, especially

, which has a large neutron capture cross section. Boron sources in graphite include the raw materials, the packing materials used in baking the product, and even the choice of soap (for example, borax) used to launder the clothing worn by workers in the machine shop. Boron concentration in thermally purified graphite (such as AGOT graphite) can be less than 0.4 ppm and in chemically purified nuclear graphite it is less than 0.06 ppm.

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 Neutron-induced swelling is the increase of volume and decrease of density of materials subjected to intense neutron radiation. Neutrons impacting the material's lattice rearrange its atoms, causing buildup of dislocations, voids, and Wigner ener ...

, as well as possible hardening) *Change in

elastic modulus An elastic modulus (also known as modulus of elasticity) is the unit of measurement of an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is ...

(measured by

impulse excitation technique The impulse excitation technique (IET) is a non-destructive material characterization technique to determine the elastic properties and internal friction of a material of interest. It measures the resonant frequencies in order to calculate the You ...

) *Change in

coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...

*Change in

electrical resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

*Irradiation induced creep As the state of nuclear graphite in active reactors can only be determined at routine inspections, about every 18 months, mathematical modelling of the nuclear graphite as it approached end-of-life is important. However as only surface features can be inspected, and the exact time of changes is not known, reliability modelling is especially difficult.

Manufacture

Nuclear graphite for the UK

Magnox Magnox is a type of nuclear power/production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors. The n ...

reactors was manufactured from

petroleum coke Petroleum coke, abbreviated coke or petcoke, is a final carbon-rich solid material that derives from oil refining, and is one type of the group of fuels referred to as cokes. Petcoke is the coke that, in particular, derives from a final cracki ...

mixed with coal-based binder pitch heated and

extruded Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...

into billets, and then baked at 1,000 °C for several days. To reduce

porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...

and increase

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

, the billets were impregnated with

coal tar Coal tar is a thick dark liquid which is a by-product of the production of coke and coal gas from coal. It is a type of creosote. It has both medical and industrial uses. Medicinally it is a topical medication applied to skin to treat psoriasi ...

at high temperature and pressure before a final bake at 2,800 °C. Individual billets were then

machined Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...

into the final required shapes.

Accidents in graphite-moderated reactors

There have been two major

accidents An accident is an unintended, normally unwanted event that was not directly caused by humans. The term ''accident'' implies that nobody should be blamed, but the event may have been caused by unrecognized or unaddressed risks. Most researche ...

in graphite-moderated reactors, the

Windscale fire The Windscale fire of 10 October 1957 was the worst nuclear accident in the United Kingdom's history, and one of the worst in the world, ranked in severity at level 5 out of a possible 7 on the International Nuclear Event Scale. The fire was in ...

and the

Chernobyl disaster The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR in the Soviet Union. It is one of only two nuc ...

. In the Windscale fire, an untested annealing process for the graphite was used, causing overheating in unmonitored areas of the core and leading directly to the ignition of the fire. The material that ignited was not the graphite moderator itself, but rather the canisters of metallic uranium fuel within the reactor. When the fire was extinguished, it was found that the only areas of the graphite moderator to have incurred thermal damage were those that had been close to the burning fuel canisters. In the Chernobyl disaster, the moderator was not responsible for the primary event. Instead, a massive power excursion during a mishandled test caused the catastrophic failure of the reactor vessel and a near-total loss of coolant supply. The result was that the fuel rods rapidly melted and flowed together while in an extremely high power state, causing a small portion of the core to reach a state of runaway

prompt criticality In nuclear engineering, prompt criticality describes a nuclear fission event in which criticality (the threshold for an exponentially growing nuclear fission chain reaction) is achieved with prompt neutrons alone (neutrons that are released imm ...

and leading to a massive energy release, resulting in the explosion of the reactor core and the destruction of the reactor building. The massive energy release during the primary event superheated the graphite moderator, and the disruption of the reactor vessel and building allowed the superheated graphite to come into contact with atmospheric oxygen. As a result, the graphite moderator caught fire, sending a plume of highly radioactive fallout into the atmosphere and over a very widespread area.

References

*Haag, G. 2005, Properties of ATR-2E Graphite and Property Changes due to Fast Neutron Irradiation, FZ-Juelich, Juel-4813. {{Reflist

External links

Manufacturing and Production of Graphite
IAEA Nuclear Graphite Knowledge Base
Graphite Behaviour under Irradiation
IAEA Nuclear Graphite Knowledge Base Allotropes of carbon Neutron moderators Nuclear technology