The CANDU, for Canada Deuterium Uranium, is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its use of uranium fuel. CANDU reactors were first developed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited, the Hydro-Electric Power Commission of Ontario, Canadian General Electric, other companies. There have been two major types of CANDU reactors, the original design of around 500 MWe, intended to be used in multi-reactor installations in large plants, the rationalized CANDU 6 in the 600 MWe class, designed to be used in single stand-alone units or in small multi-unit plants. CANDU 6 units were built in Quebec and New Brunswick, as well as Pakistan, South Korea and China. A single example of a non-CANDU 6 design was sold to India; the multi-unit design was used only in Ontario and grew in size and power as more units were installed in the province, reaching ~880 MWe in the units installed at the Darlington Nuclear Generating Station.
An effort to rationalize the larger units in a fashion similar to CANDU 6 led to the CANDU 9. By the early 2000s, sales prospects for the original CANDU designs were dwindling due to the introduction of newer designs from other companies. AECL responded by moving to the Advanced CANDU reactor design. ACR failed to find any buyers. In October 2011, the Canadian Federal Government licensed the CANDU design to Candu Energy, which acquired the former reactor development and marketing division of AECL at that time. Candu Energy offers support services for existing sites and is completing stalled installations in Romania and Argentina through a partnership with China National Nuclear Corporation. SNC Lavalin, the successor to AECL, is pursuing new Candu 6 reactor sales in Argentina, as well as China and Britain. Sales effort for the ACR reactor has ended; the basic operation of the CANDU design is similar to other nuclear reactors. Fission reactions in the reactor core heat pressurized water in a primary cooling loop.
A heat exchanger known as a steam generator, transfers the heat to a secondary cooling loop, which powers a steam turbine with an electric generator attached to it. The exhaust steam from the turbines is cooled and returned as feedwater to the steam generator; the final cooling uses cooling water from a nearby source, such as a lake, river, or ocean. Newer CANDU plants, such as the Darlington Nuclear Generating Station near Toronto, use a diffuser to spread the warm outlet water over a larger volume and limit the effects on the environment. Although all CANDU plants to date have used open-cycle cooling, modern CANDU designs are capable of using cooling towers instead. Where the CANDU design differs is in the details of the fissile core and the primary cooling loop. Natural uranium consists of a mix of uranium-238 with small amounts of uranium-235 and trace amounts of other isotopes. Fission in these elements releases high-energy neutrons, which can cause other 235U atoms in the fuel to undergo fission as well.
This process is much more effective when the neutron energies are much lower than what the reactions release naturally. Most reactors use some form of neutron moderator to lower the energy of the neutrons, or "thermalize" them, which makes the reaction more efficient; the energy lost by the neutrons is extracted for power. Most commercial reactor designs use normal water as the moderator. Water absorbs some of the neutrons, enough that it is not possible to keep the reaction going in natural uranium. CANDU replaces this "light" water with heavy water. Heavy water's extra neutron decreases its ability to absorb excess neutrons, resulting in a better neutron economy; this allows CANDU to run on unenriched natural uranium, or uranium mixed with a wide variety of other materials such as plutonium and thorium. This was a major goal of the CANDU design; this presents an advantage in nuclear proliferation terms, as there is no need for enrichment facilities, which might be used for weapons. In conventional light-water reactor designs, the entire fissile core is placed in a large pressure vessel.
The amount of heat that can be removed by a unit of a coolant is a function of the temperature. Building a pressure vessel of the required size is a significant challenge, at the time of the CANDU's design, Canada's heavy industry lacked the requisite experience and capability to cast and machine reactor pressure vessels of the required size; this issue was so major that the small pressure vessel intended for use in the NPD prior to its mid-construction redesign could not be fabricated domestically and had to be manufactured in Scotland instead, domestic development of the technology required to produce pressure vessels of the size required for commercial-scale heavy water moderated power reactors was thought to be unlikely. In CANDU the fuel bundles are instead contained in much smaller metal tubes about 10 cm diameter; the tubes are contained in a larger vessel containing additional heavy water acting purely as a moderator. This vessel, known as a cala
Heavy water is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium, rather than the common hydrogen-1 isotope that makes up most of the hydrogen in normal water. The presence of deuterium gives the water different nuclear properties, the increase of mass gives it different physical and chemical properties when compared to normal water. Deuterium is a hydrogen isotope with a nucleus containing a proton; the additional neutron makes a deuterium atom twice as heavy as a protium atom. A molecule of heavy water has two deuterium atoms in place of the two protium atoms of ordinary "light" water; the weight of a heavy water molecule, however, is not different from that of a normal water molecule, because about 89% of the molecular weight of water comes from the single oxygen atom rather than the two hydrogen atoms. The colloquial term'heavy water' refers to a enriched water mixture that contains deuterium oxide D2O, but some hydrogen-deuterium oxide and a smaller amount of ordinary hydrogen oxide H2O.
For instance, the heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction—meaning that 99.75% of the hydrogen atoms are of the heavy type. For comparison, ordinary water contains only about 156 deuterium atoms per million hydrogen atoms, meaning that 0.0156% of the hydrogen atoms are of the heavy type. Heavy water is not radioactive. In its pure form, it has a density about 11% greater than water, but is otherwise physically and chemically similar; the various differences in deuterium-containing water are larger than in any other occurring isotope-substituted compound because deuterium is unique among heavy stable isotopes in being twice as heavy as the lightest isotope. This difference increases the strength of water's hydrogen-oxygen bonds, this in turn is enough to cause differences that are important to some biochemical reactions; the human body contains deuterium equivalent to about five grams of heavy water, harmless. When a large fraction of water in higher organisms is replaced by heavy water, the result is cell dysfunction and death.
Heavy water was first produced in a few months after the discovery of deuterium. With the discovery of nuclear fission in late 1938, the need for a neutron moderator that captured few neutrons, heavy water became a component of early nuclear energy research. Since heavy water has been an essential component in some types of reactors, both those that generate power and those designed to produce isotopes for nuclear weapons; these heavy water reactors have the advantage of being able to run on natural uranium without using graphite moderators that pose radiological and dust explosion hazards in the decommissioning phase. Most modern reactors use enriched uranium with ordinary water as the moderator. Semiheavy water, HDO, exists whenever there is water with light deuterium in the mix; this is because hydrogen atoms are exchanged between water molecules. Water containing 50% H and 50% D in its hydrogen contains about 50% HDO and 25% each of H2O and D2O, in dynamic equilibrium. In normal water, about 1 molecule in 3,200 is HDO, heavy water molecules only occur in a proportion of about 1 molecule in 41 million.
Thus semiheavy water molecules are far more common than "pure" heavy water molecules. Water enriched in the heavier oxygen isotopes 17O and 18O is commercially available, e.g. for use as a non-radioactive isotopic tracer. It is "heavy water" as it is denser than normal water —but is called heavy water, since it does not contain the deuterium that gives D2O its unusual nuclear and biological properties, it is more expensive than D2O due to the more difficult separation of 17O and 18O. H218O is used for production of fluorine-18 for radiopharmaceuticals and radiotracers and for positron emission tomography. Tritiated water contains tritium in place of protium or deuterium, therefore it is radioactive; the physical properties of water and heavy water differ in several respects. Heavy water is less dissociated than light water at given temperature, the true concentration of D+ ions is less than H+ ions would be for a light water sample at the same temperature; the same is true of OD OH − ions. For heavy water Kw D2O = 1.35 × 10−15, must equal for neutral water.
Thus pKw D2O = p + p = 7.44 + 7.44 = 14.87, the p of neutral heavy water at 25.0 °C is 7.44. The pD of heavy water is measured using pH electrodes giving a pH value, or pHa, at various temperatures a true acidic pD can be estimated from the directly pH meter measured pHa, such that pD+ = pHa + 0.41. The electrode correction for alkaline conditions is 0.456 for heavy water. The alkaline correction is pD+ = pHa + 0.456. These corrections are different from the differences in p and p of 0.44 from the corresponding ones in heavy water. Heavy water is 10.6% denser than ordinary water, heavy water's physically different properties can be seen without equipment if a frozen sample is dropped into normal water, as it will sink. If the water is ice-cold the higher melting tem
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has low absorption cross-section of thermal neutrons, high hardness and corrosion resistance. One of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of tin, iron, chromium and other metals, which are added to improve mechanical properties and corrosion resistance; the water cooling of reactor zirconium alloys elevates requirement for their resistance to oxidation-related nodular corrosion. Furthermore, oxidative reaction of zirconium with water releases hydrogen gas, which diffuses into the alloy and forms zirconium hydrides; the hydrides are weaker mechanically than the alloy. Commercial non-nuclear grade zirconium contains 1–5% of hafnium, whose neutron absorption cross-section is 600x that of zirconium.
Hafnium must therefore be entirely removed for reactor applications. Nuclear-grade zirconium alloys contain more than 95% Zr, therefore most of their properties are similar to those of pure zirconium; the absorption cross section for thermal neutrons is 0.18 barn for zirconium, much lower than that for such common metals as iron and nickel. The composition and the main applications of common reactor-grade alloys are summarized below; these alloys contain 0.1 -- 0.14 % oxygen. *ZIRLO stands for zirconium low oxidation. At temperatures below 1100 K, zirconium alloys belong to the hexagonal crystal family, its microstructure, revealed by chemical attack, shows needle-like grains typical of a Widmanstätten pattern. Upon annealing below the phase transition temperature the grains are equiaxed with sizes varying from 3 to 5 μm. Zircaloy 1 was developed as a replacement for existing tube bundles in submarine reactors in the 1950s, owing to a combination of strength, low neutron cross section and corrosion resistance.
Zircaloy-2 was inadvertently developed, by melting Zircaloy-1 in a crucible used for stainless steel. Newer alloys are Ni-free, including Zircaloy-4, ZIRLO and M5. Zirconium alloys react with oxygen, forming a nanometer-thin passivation layer; the corrosion resistance of the alloys may degrade when some impurities are present. Corrosion resistance of zirconium alloys is enhanced by intentional development of thicker passivation layer of black lustrous zirconium oxide. Nitride coatings might be used. Whereas there is no consensus on whether zirconium and zirconium alloy have the same oxidation rate, Zircaloys 2 and 4 do behave similarly in this respect. Oxidation occurs at the same rate in air or in water and proceeds in ambient condition or in high vacuum. A sub-micrometer thin layer of zirconium dioxide is formed in the surface and stops the further diffusion of oxygen to the bulk and the subsequent oxidation; the dependence of oxidation rate R on temperature and pressure can be expressed as R = 13.9·P1/6·expThe oxidation rate R is here expressed in gram/.
Thus the oxidation rate R is 10−20 g per 1 m2 area per second at 0 °C, 6×10−8 g m−2 s−1 at 300 °C, 5.4 mg m−2 s−1 at 700 °C and 300 mg m−2 s−1 at 1000 °C. Whereas there is no clear threshold of oxidation, it becomes noticeable at macroscopic scales at temperatures of several hundred °C. One disadvantage of metallic zirconium is that in the case of a loss-of-coolant accident in a nuclear reactor, zirconium cladding reacts with water steam at high temperature. Oxidation of zirconium by water is accompanied by release of hydrogen gas; this oxidation is accelerated at high temperatures, e.g. inside a reactor core if the fuel assemblies are no longer covered by liquid water and insufficiently cooled. Metallic zirconium is oxidized by the protons of water to form hydrogen gas according to the following redox reaction: Zr + 2 H2O → ZrO2 + 2 H2Zirconium cladding in the presence of D2O deuterium oxide used as the moderator and coolant in next gen pressurized heavy water reactors that CANDU designed nuclear reactors use would express the same oxidation on exposure to deuterium oxide steam as follows: Zr + 2 D2O → ZrO2 + 2 D2This exothermic reaction, although only occurring at high temperature, is similar to that of alkali metals with water.
It closely resembles the anaerobic oxidation of iron by water. This reaction was responsible for a small hydrogen explosion accident first observed inside the reactor building of Three Mile Island Nuclear Generating Station in 1979 that did not damage the containment building; this same reaction occurred in boiling water reactors 1, 2 and 3 of the Fukushima Daiichi Nuclear Power Plant after reactor cooling was interrupted by related earthquake and tsunami events during the disaster of March 11, 2011, leading to the Fukushima Daiichi nuclear disaster. Hydrogen gas was vented into the reactor maintenance halls and the resulting explosive mixture of hydrogen with air oxygen detonated; the explosions damaged external buildings and at least one con
NRX was a heavy-water-moderated, light-water-cooled, nuclear research reactor at the Canadian Chalk River Laboratories, which came into operation in 1947 at a design power rating of 10 MW, increasing to 42 MW by 1954. At the time of its construction it was Canada's most expensive science facility and the world's most powerful nuclear research reactor. NRX was remarkable both in terms of its heat output and the number of free neutrons it generated; when a nuclear reactor is operating its nuclear chain reaction generates many free neutrons, in the late 1940s NRX was the most intense neutron source in the world. NRX experienced one of the world's first major reactor accidents on 12 December 1952; the reactor began operation on 22 July 1947 under the National Research Council of Canada, was taken over by Atomic Energy of Canada Limited shortly before the 1952 accident. The accident was cleaned up and the reactor restarted within two years. NRX operated for 45 years, being shut down permanently on 30 March 1993.
It is undergoing decommissioning at the Chalk River Laboratories site. NRX was the successor to Canada's first reactor, ZEEP; because the operating life of a research reactor was not expected to be long, in 1948 planning started for construction of a successor facility, the National Research Universal reactor, which went critical in 1957. A heavy water moderated reactor is governed by two main processes. First, the water slows down the neutrons which are produced by nuclear fission, increasing the chances of the high energy neutrons causing further fission reactions. Second, control rods absorb neutrons and adjust the power level or shut down the reactor in the course of normal operation. Either inserting the control rods or removing the heavy water moderator can stop the reaction; the NRX reactor incorporated a calandria, a sealed vertical aluminium cylindrical vessel with a diameter of 8 m and height of 3 m. The core vessel held about 175 six-centimetre-diameter vertical tubes in a hexagonal lattice, 14,000 litres of heavy water and helium gas to displace air and prevent corrosion.
The level of water in the reactor could be adjusted to help set the power level. Sitting in the vertical tubes and surrounded by air were fuel elements or experimental items; this design was a forerunner of the CANDU reactors. The fuel elements contained fuel rods 3.1 m long, 31 mm in diameter and weighing 55 kg, containing uranium fuel and sheathed in aluminium. Surrounding the fuel element was an aluminium coolant tube with up to 250 litres per second of cooling water from the Ottawa River flowing through it. Between the coolant sheath and the calandria an air flow of 8 kg/second was maintained. Twelve of the vertical tubes contained; these could be raised and lowered to control the reaction, with any seven inserted being enough to absorb sufficient neutrons that no chain reaction could happen. The rods were held up by electromagnets, so that a power failure would cause them to fall into the tubes and terminate the reaction. A pneumatic system could use air pressure from above to force them into the reactor core or from below to raise them from it.
Four of these were called the safeguard bank while the other eight were controlled in an automatic sequence. Two pushbuttons on the main panel in the control room activated magnets to seal the rods to the pneumatic system, the pushbutton to cause the pneumatic insertion of the rods into the core was located a few feet away. NRX was for a time the world's most powerful research reactor, vaulting Canada into the forefront of physics research. Emerging from a World War II cooperative effort between Britain, the United States, Canada, NRX was a multipurpose research reactor used to develop new isotopes, test materials and fuels, produce neutron radiation beams, that became an indispensable tool in the blossoming field of condensed matter physics; the nuclear physics design of NRX emerged from the "Montreal Laboratory" of Canada's National Research Council, established at the University of Montreal during WWII to engage a team of Canadian and other European scientists in top-secret heavy-water reactor research.
When the decision was made to build the NRX at what is now known as Chalk River Laboratories, the detailed engineering design was contracted to Canada's Defense Industries Ltd. who subcontracted construction to Fraser Brace Ltd. In 1994 Dr. Bertram Brockhouse shared the Nobel Prize in Physics for his work in the 1950s at NRX, which advanced the detection and analysis techniques used in the field of neutron scattering for condensed matter research; the CIRUS reactor, based on this design, was built in India. It was used to produce plutonium for India's Operation Smiling Buddha nuclear test, it is claimed that the term "crud" stood for "Chalk River Unidentified Deposit", used to describe the radioactive scaling that builds up on internal reactor components, first observed in the NRX facility. Crud has since become common parlance for "Corrosion Related Unidentified Deposit" and similar expressions and is used with no relation to the Chalk River plant. On December 12, 1952, the NRX reactor suffered a partial meltdown due to operator error and mechanical problems in the shut-off systems.
For test purposes, some of the tubes were disconnected from high pressure water cooling and connected by hoses to a temporary cooling system and one was cooled only by airflow. During tests on low power, with low coolant flux through the core, the supervisor noticed several control rods being pulled from the core, found an operator in the basement opening pneumatic valves. Wrongly opened valves were closed, but some of the control ro
France the French Republic, is a country whose territory consists of metropolitan France in Western Europe and several overseas regions and territories. The metropolitan area of France extends from the Mediterranean Sea to the English Channel and the North Sea, from the Rhine to the Atlantic Ocean, it is bordered by Belgium and Germany to the northeast and Italy to the east, Andorra and Spain to the south. The overseas territories include French Guiana in South America and several islands in the Atlantic and Indian oceans; the country's 18 integral regions span a combined area of 643,801 square kilometres and a total population of 67.3 million. France, a sovereign state, is a unitary semi-presidential republic with its capital in Paris, the country's largest city and main cultural and commercial centre. Other major urban areas include Lyon, Toulouse, Bordeaux and Nice. During the Iron Age, what is now metropolitan France was inhabited by a Celtic people. Rome annexed the area in 51 BC, holding it until the arrival of Germanic Franks in 476, who formed the Kingdom of Francia.
The Treaty of Verdun of 843 partitioned Francia into Middle Francia and West Francia. West Francia which became the Kingdom of France in 987 emerged as a major European power in the Late Middle Ages following its victory in the Hundred Years' War. During the Renaissance, French culture flourished and a global colonial empire was established, which by the 20th century would become the second largest in the world; the 16th century was dominated by religious civil wars between Protestants. France became Europe's dominant cultural and military power in the 17th century under Louis XIV. In the late 18th century, the French Revolution overthrew the absolute monarchy, established one of modern history's earliest republics, saw the drafting of the Declaration of the Rights of Man and of the Citizen, which expresses the nation's ideals to this day. In the 19th century, Napoleon established the First French Empire, his subsequent Napoleonic Wars shaped the course of continental Europe. Following the collapse of the Empire, France endured a tumultuous succession of governments culminating with the establishment of the French Third Republic in 1870.
France was a major participant in World War I, from which it emerged victorious, was one of the Allies in World War II, but came under occupation by the Axis powers in 1940. Following liberation in 1944, a Fourth Republic was established and dissolved in the course of the Algerian War; the Fifth Republic, led by Charles de Gaulle, remains today. Algeria and nearly all the other colonies became independent in the 1960s and retained close economic and military connections with France. France has long been a global centre of art and philosophy, it hosts the world's fourth-largest number of UNESCO World Heritage Sites and is the leading tourist destination, receiving around 83 million foreign visitors annually. France is a developed country with the world's sixth-largest economy by nominal GDP, tenth-largest by purchasing power parity. In terms of aggregate household wealth, it ranks fourth in the world. France performs well in international rankings of education, health care, life expectancy, human development.
France is considered a great power in global affairs, being one of the five permanent members of the United Nations Security Council with the power to veto and an official nuclear-weapon state. It is a leading member state of the European Union and the Eurozone, a member of the Group of 7, North Atlantic Treaty Organization, Organisation for Economic Co-operation and Development, the World Trade Organization, La Francophonie. Applied to the whole Frankish Empire, the name "France" comes from the Latin "Francia", or "country of the Franks". Modern France is still named today "Francia" in Italian and Spanish, "Frankreich" in German and "Frankrijk" in Dutch, all of which have more or less the same historical meaning. There are various theories as to the origin of the name Frank. Following the precedents of Edward Gibbon and Jacob Grimm, the name of the Franks has been linked with the word frank in English, it has been suggested that the meaning of "free" was adopted because, after the conquest of Gaul, only Franks were free of taxation.
Another theory is that it is derived from the Proto-Germanic word frankon, which translates as javelin or lance as the throwing axe of the Franks was known as a francisca. However, it has been determined that these weapons were named because of their use by the Franks, not the other way around; the oldest traces of human life in what is now France date from 1.8 million years ago. Over the ensuing millennia, Humans were confronted by a harsh and variable climate, marked by several glacial eras. Early hominids led a nomadic hunter-gatherer life. France has a large number of decorated caves from the upper Palaeolithic era, including one of the most famous and best preserved, Lascaux. At the end of the last glacial period, the climate became milder. After strong demographic and agricultural development between the 4th and 3rd millennia, metallurgy appeared at the end of the 3rd millennium working gold and bronze, iron. France has numerous megalithic sites from the Neolithic period, including the exceptiona
The BN-600 reactor is a sodium-cooled fast breeder reactor, built at the Beloyarsk Nuclear Power Station, in Zarechny, Sverdlovsk Oblast, Russia. Designed to generate electrical power of 600 MW in total, the plant dispatches 560 MW to the Middle Urals power grid, it represents an evolution on the preceding BN-350 reactor. In 2014, its larger sister reactor, the BN-800 reactor began operation; the plant is a pool-type reactor, where the reactor, coolant pumps, intermediate heat exchangers and associated piping are all located in a common liquid sodium pool. The reactor system is housed in a concrete rectilinear building, provided with filtration and gas containment features. In the 1st 15 years of operation, there have been 12 incidents involving sodium/water interactions from tube breaks in the steam generators, a sodium-air oxidation/"fire" from a leak in an auxiliary system, a sodium "fire" from a leak in a secondary coolant loop while shut down. All these incidents were classified at the lowest level on the International Nuclear Event Scale, none of the events prevented restarting operation of the facility after repairs.
As of 1997, there had been 27 sodium leaks, 14 of which resulted in sodium-air oxidations/"fires". The steam generators are separated in modules so they can be repaired without shutting down the reactor; as of 2013, the cumulative "energy Availability factor" recorded by the IAEA was 74.6%. The reactor core is 1.03 meters tall with a diameter of 2.05 meters. It has 369 fuel assemblies, mounted vertically, each consisting of 127 fuel rods enriched to between 17–26% 235U. In comparison, normal enrichment in other Russian reactors is between 3–4% 235U; the control and scram system comprises 27 reactivity control elements including 19 shimming rods, two automatic control rods, six automatic emergency shut-down rods. On-power refueling equipment allows for charging the core with fresh fuel assemblies and turning the fuel assemblies within the reactor, changing control and scram system elements remotely; the unit employs a three-circuit coolant arrangement. Water and steam flow in the third circuit; the sodium is heated to a maximum of 550 °C in the reactor during normal operations.
This heat is transferred from the reactor core via three independent circulation loops. Each comprises a primary sodium pump, two intermediate heat exchangers, a secondary sodium pump with an expansion tank located upstream, an emergency pressure discharge tank; these feed a steam generator. The reactor has worked until 2012 as a breeder reactor, since as a burner reactor using weapon-grade plutonium. There is a lot of international interest in the fast-breeder reactor at Beloyarsk. Japan has its own prototype fast-breeder reactors. Japan paid 1 billion for the technical documentation of the BN-600; the operation of the reactor is an international study in progress. The reactor has been licensed to operate up to 2025. Generation IV reactor BN-Reactor BN-350 reactor BN-800 reactor BN-1200 reactor Rosenergoatom the Reactor BN-600 Overview of Fast Reactors in Russia and the Former Soviet Union BN-600 Hybrid Core Benchmark Analyses BN-600 Fuel Liquid Metal Cooled Reactors: Experience in Design and Operation Operating experience from the BN600 sodium fast reactor, IAEA Assessment of changes to the BN-600 to operate with a plutonium burner core
Graphite, archaically referred to as plumbago, is a crystalline form of the element carbon with its atoms arranged in a hexagonal structure. It occurs in this form and is the most stable form of carbon under standard conditions. Under high pressures and temperatures it converts to diamond. Graphite is used in lubricants, its high conductivity makes it useful in electronic products such as electrodes and solar panels. The principal types of natural graphite, each occurring in different types of ore deposits, are Crystalline small flakes of graphite occurs as isolated, plate-like particles with hexagonal edges if unbroken; when broken the edges can be angular. Ordered pyrolytic graphite refers to graphite with an angular spread between the graphite sheets of less than 1°; the name "graphite fiber" is sometimes used to refer to carbon fibers or carbon fiber-reinforced polymer. Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism, it occurs in igneous rocks and in meteorites.
Minerals associated with graphite include quartz, calcite and tourmaline. The principal export sources of mined graphite are in order of tonnage: China, Canada and Madagascar. In meteorites, graphite occurs with silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite; some microscopic grains have distinctive isotopic compositions, indicating that they were formed before the Solar system. They are one of about 12 known types of mineral that predate the Solar System and have been detected in molecular clouds; these minerals were formed in the ejecta when supernovae exploded or low- to intermediate-sized stars expelled their outer envelopes late in their lives. Graphite may be the third oldest mineral in the Universe. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond; the two most common are graphite. In diamond the bonds are sp3 and the atoms form tetrahedra with each bound to four nearest neighbors. In graphite they are sp2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart.
The individual layers are called graphene. In each layer, the carbon atoms are arranged in a honeycomb lattice with separation of 0.142 nm, the distance between planes is 0.335 nm. Atoms in the plane are bonded covalently, with only three of the four potential bonding sites satisfied; the fourth electron is free to migrate in the plane. However, it does not conduct in a direction at right angles to the plane. Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be separated, or to slide past each other; the two known forms of graphite and beta, have similar physical properties, except that the graphene layers stack differently. The alpha graphite may be either buckled; the alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1300 °C. The equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally.
The pressure changes linearly between 1.7 GPa at 0 K and 12 GPa at 5000 K. However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 °C and 1 standard atmosphere, the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible. However, at temperatures above about 4500 K, diamond converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed; the acoustic and thermal properties of graphite are anisotropic, since phonons propagate along the bound planes, but are slower to travel from one plane to another. Graphite's high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite oxidizes to form carbon dioxide at temperatures of 700 °C and above.
Graphite is hence useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers; these valence electrons are free to move. However, the electricity is conducted within the plane of the layers; the conductive properties of powdered graphite allow its use as pressure sensor in carbon microphones. Graphite and graphite powder are valued in industrial applications for their self-lubricating and dry lubricating properties. There is a common belief that graphite's lubricating properties are due to the loose interlamellar coupling between sheets in the structure. However, it has been shown that in a vacuum environment, graphite degrades as a lubricant, due to the hypoxic conditions; this observation led to the hypothesis that the lubrication is due to the presence of fluids between the layers, such as air and water, which are adsorbed from the