The United States of America known as the United States or America, is a country composed of 50 states, a federal district, five major self-governing territories, various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U. S. is the third most populous country. The capital is Washington, D. C. and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico; the State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean; the U. S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The diverse geography and wildlife of the United States make it one of the world's 17 megadiverse countries.
Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago. European colonization began in the 16th century; the United States emerged from the thirteen British colonies established along the East Coast. Numerous disputes between Great Britain and the colonies following the French and Indian War led to the American Revolution, which began in 1775, the subsequent Declaration of Independence in 1776; the war ended in 1783 with the United States becoming the first country to gain independence from a European power. The current constitution was adopted in 1788, with the first ten amendments, collectively named the Bill of Rights, being ratified in 1791 to guarantee many fundamental civil liberties; the United States embarked on a vigorous expansion across North America throughout the 19th century, acquiring new territories, displacing Native American tribes, admitting new states until it spanned the continent by 1848. During the second half of the 19th century, the Civil War led to the abolition of slavery.
By the end of the century, the United States had extended into the Pacific Ocean, its economy, driven in large part by the Industrial Revolution, began to soar. The Spanish–American War and World War I confirmed the country's status as a global military power; the United States emerged from World War II as a global superpower, the first country to develop nuclear weapons, the only country to use them in warfare, a permanent member of the United Nations Security Council. Sweeping civil rights legislation, notably the Civil Rights Act of 1964, the Voting Rights Act of 1965 and the Fair Housing Act of 1968, outlawed discrimination based on race or color. During the Cold War, the United States and the Soviet Union competed in the Space Race, culminating with the 1969 U. S. Moon landing; the end of the Cold War and the collapse of the Soviet Union in 1991 left the United States as the world's sole superpower. The United States is the world's oldest surviving federation, it is a representative democracy.
The United States is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, other international organizations. The United States is a developed country, with the world's largest economy by nominal GDP and second-largest economy by PPP, accounting for a quarter of global GDP; the U. S. economy is post-industrial, characterized by the dominance of services and knowledge-based activities, although the manufacturing sector remains the second-largest in the world. The United States is the world's largest importer and the second largest exporter of goods, by value. Although its population is only 4.3% of the world total, the U. S. holds 31% of the total wealth in the world, the largest share of global wealth concentrated in a single country. Despite wide income and wealth disparities, the United States continues to rank high in measures of socioeconomic performance, including average wage, human development, per capita GDP, worker productivity.
The United States is the foremost military power in the world, making up a third of global military spending, is a leading political and scientific force internationally. In 1507, the German cartographer Martin Waldseemüller produced a world map on which he named the lands of the Western Hemisphere America in honor of the Italian explorer and cartographer Amerigo Vespucci; the first documentary evidence of the phrase "United States of America" is from a letter dated January 2, 1776, written by Stephen Moylan, Esq. to George Washington's aide-de-camp and Muster-Master General of the Continental Army, Lt. Col. Joseph Reed. Moylan expressed his wish to go "with full and ample powers from the United States of America to Spain" to seek assistance in the revolutionary war effort; the first known publication of the phrase "United States of America" was in an anonymous essay in The Virginia Gazette newspaper in Williamsburg, Virginia, on April 6, 1776. The second draft of the Articles of Confederation, prepared by John Dickinson and completed by June 17, 1776, at the latest, declared "The name of this Confederation shall be the'United States of America'".
The final version of the Articles sent to the states for ratification in late 1777 contains the sentence "The Stile of this Confederacy shall be'The United States of America'". In June 1776, Thomas Jefferson wrote the phrase "UNITED STATES OF AMERICA" in all capitalized letters in the headline of his "original Rough draught" of the Declaration of Independence; this draft of the document did not surface unti
Caesium-137, or radiocaesium, is a radioactive isotope of caesium, formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons. It is among the most problematic of the short-to-medium-lifetime fission products because it moves and spreads in nature due to the high water solubility of caesium's most common chemical compounds, which are salts. Caesium-137 has a half-life of about 30.17 years. About 94.6 percent decays by beta emission to a metastable nuclear isomer of barium: barium-137m. The remainder directly populates the ground state of barium-137, stable. Ba-137m has a half-life of about 153 seconds, is responsible for all of the emissions of gamma rays in samples of caesium-137. 85.1% of metastable barium decays to ground state by emission of gamma rays having energy 0.6617 MeV. One gram of caesium-137 has an activity of 3.215 terabecquerel. The main photon peak of Ba-137m is 662 keV. Caesium-137 has a number of practical uses.
In small amounts, it is used to calibrate radiation-detection equipment. In medicine, it is used in radiation therapy. In industry, it is used in flow meters, thickness gauges, moisture-density gauges, in gamma ray well logging devices. Caesium-137 is not used for industrial radiography because it is quite chemically reactive, hence difficult to handle; the salts of caesium are soluble in water, this complicates the safe handling of caesium. Cobalt-60, 6027Co, is preferred for radiography, since it is chemically a rather nonreactive metal and produces higher energy gamma-ray photons; as a purely man-made isotope, caesium-137 has been used to date wine and detect counterfeits and as a relative-dating material for assessing the age of sedimentation occurring after 1954. Caesium-137 is used as a radioactive tracer in geologic research to measure soil erosion and deposition. Caesium-137 reacts with water; the biological behavior of caesium is similar to that of rubidium. After entering the body, caesium gets more or less uniformly distributed throughout the body, with the highest concentrations in soft tissue.
The biological half-life of caesium is rather short, at about 70 days. A 1972 experiment showed that when dogs are subjected to a whole body burden of 3800 μCi/kg of caesium-137, they die within 33 days, while animals with half of that burden all survived for a year. Accidental ingestion of caesium-137 can be treated with Prussian blue, which binds to it chemically and reduces the biological half-life to 30 days. Caesium-134 and caesium-137 were released into the environment during nearly all nuclear weapon tests and some nuclear accidents, most notably the Chernobyl disaster and the Fukushima Daiichi disaster; as of 2005 and for the next few hundred years, caesium-137 is the principal source of radiation in the zone of alienation around the Chernobyl nuclear power plant. Together with caesium-134, iodine-131, strontium-90, caesium-137 was among the isotopes distributed by the reactor explosion that constitute the greatest risk to health; the mean contamination of caesium-137 in Germany following the Chernobyl disaster was 2000 to 4000 Bq/m2.
This corresponds to a contamination of 1 mg/km2 of caesium-137, totaling about 500 grams deposited over all of Germany. In Scandinavia, some reindeer and sheep exceeded the Norwegian legal limit 26 years after Chernobyl; as of 2016 the Chernobyl caesium-137 has decayed by half, but could have been locally concentrated by much larger factors. In April 2011, elevated levels of caesium-137 were being found in the environment after the Fukushima Daiichi nuclear disasters in Japan. In July 2011, meat from 11 cows shipped to Tokyo from Fukushima Prefecture was found to have 1,530 to 3,200 becquerels per kilogram of Cs-137 exceeding the Japanese legal limit of 500 becquerels per kilogram at that time. In March 2013, a fish caught near the plant had a record 740,000 becquerels per kilogram of radioactive caesium, above the 100 becquerels per kilogram government limit. A 2013 paper in Scientific Reports found that for a forest site 50 km from the stricken plant, Cs-137 concentrations were high in leaf litter and detritivores, but low in herbivores.
By the end of 2014, "Fukushima-derived radiocesium had spread into the whole western North Pacific Ocean", transported by the North Pacific current from Japan to the Gulf of Alaska. It has been measured in the surface layer down to 200 meters and south of the current area down to 400 meters. Caesium-137 is reported to be the major health concern in Fukushima; the government is under pressure to clean up radioactivity from Fukushima from as much land as possible so that some of the 110,000 people can return. A number of techniques are being considered that will be able to strip out 80% to 95% of the caesium from contaminated soil and other materials efficiently and without destroying the organic material in the soil; these include hydrothermal blasting. The caesium precipitated with ferric ferricyanide would be the only waste requiring special burial sites; the aim is to get annual exposure from the contaminated environment down to 1 millisievert above background. The most contaminated area where radiation doses are greater than 50 mSv/year must remain off limits, but some areas that are less than 5 mSv/year may be decontaminated, allowing 22,000 residents to return.
Caesium-137 in the environment is anthropogenic. Caesium-137 is produced from the nuclear fission of plutonium and uran
Sweden the Kingdom of Sweden, is a Scandinavian Nordic country in Northern Europe. It borders Norway to the west and north and Finland to the east, is connected to Denmark in the southwest by a bridge-tunnel across the Öresund, a strait at the Swedish-Danish border. At 450,295 square kilometres, Sweden is the largest country in Northern Europe, the third-largest country in the European Union and the fifth largest country in Europe by area. Sweden has a total population of 10.2 million. It has a low population density of 22 inhabitants per square kilometre; the highest concentration is in the southern half of the country. Germanic peoples have inhabited Sweden since prehistoric times, emerging into history as the Geats and Swedes and constituting the sea peoples known as the Norsemen. Southern Sweden is predominantly agricultural, while the north is forested. Sweden is part of the geographical area of Fennoscandia; the climate is in general mild for its northerly latitude due to significant maritime influence, that in spite of this still retains warm continental summers.
Today, the sovereign state of Sweden is a constitutional monarchy and parliamentary democracy, with a monarch as head of state, like its neighbour Norway. The capital city is Stockholm, the most populous city in the country. Legislative power is vested in the 349-member unicameral Riksdag. Executive power is exercised by the government chaired by the prime minister. Sweden is a unitary state divided into 21 counties and 290 municipalities. An independent Swedish state emerged during the early 12th century. After the Black Death in the middle of the 14th century killed about a third of the Scandinavian population, the Hanseatic League threatened Scandinavia's culture and languages; this led to the forming of the Scandinavian Kalmar Union in 1397, which Sweden left in 1523. When Sweden became involved in the Thirty Years War on the Reformist side, an expansion of its territories began and the Swedish Empire was formed; this became one of the great powers of Europe until the early 18th century. Swedish territories outside the Scandinavian Peninsula were lost during the 18th and 19th centuries, ending with the annexation of present-day Finland by Russia in 1809.
The last war in which Sweden was directly involved was in 1814, when Norway was militarily forced into personal union. Since Sweden has been at peace, maintaining an official policy of neutrality in foreign affairs; the union with Norway was peacefully dissolved in 1905. Sweden was formally neutral through both world wars and the Cold War, albeit Sweden has since 2009 moved towards cooperation with NATO. After the end of the Cold War, Sweden joined the European Union on 1 January 1995, but declined NATO membership, as well as Eurozone membership following a referendum, it is a member of the United Nations, the Nordic Council, the Council of Europe, the World Trade Organization and the Organisation for Economic Co-operation and Development. Sweden maintains a Nordic social welfare system that provides universal health care and tertiary education for its citizens, it has the world's eleventh-highest per capita income and ranks in numerous metrics of national performance, including quality of life, education, protection of civil liberties, economic competitiveness, equality and human development.
The name Sweden was loaned from Dutch in the 17th century to refer to Sweden as an emerging great power. Before Sweden's imperial expansion, Early Modern English used Swedeland. Sweden is derived through back-formation from Old English Swēoþēod, which meant "people of the Swedes"; this word is derived from Sweon/Sweonas. The Swedish name Sverige means "realm of the Swedes", excluding the Geats in Götaland. Variations of the name Sweden are used in most languages, with the exception of Danish and Norwegian using Sverige, Faroese Svøríki, Icelandic Svíþjóð, the more notable exception of some Finnic languages where Ruotsi and Rootsi are used, names considered as referring to the people from the coastal areas of Roslagen, who were known as the Rus', through them etymologically related to the English name for Russia; the etymology of Swedes, thus Sweden, is not agreed upon but may derive from Proto-Germanic Swihoniz meaning "one's own", referring to one's own Germanic tribe. Sweden's prehistory begins in the Allerød oscillation, a warm period around 12,000 BC, with Late Palaeolithic reindeer-hunting camps of the Bromme culture at the edge of the ice in what is now the country's southernmost province, Scania.
This period was characterised by small bands of hunter-gatherer-fishers using flint technology. Sweden is first described in a written source in Germania by Tacitus in 98 AD. In Germania 44 and 45 he mentions the Swedes as a powerful tribe with ships that had a prow at each end. Which kings ruled these Suiones is unknown, but Norse mythology presents a long line of legendary and semi-legendary kings going back to the last centuries BC; as for literacy in Sweden itself, the runic script was in use among the south Scandinavian elite by at least the 2nd century AD, but all that has come down to the present from the Roman Period is curt inscriptions on artefacts of male names, demonstrating th
Plutonium is a radioactive chemical element with symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forms a dull coating when oxidized; the element exhibits six allotropes and four oxidation states. It reacts with carbon, nitrogen and hydrogen; when exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder, pyrophoric. It can accumulate in bones, which makes the handling of plutonium dangerous. Plutonium was first produced and isolated on December 14, 1940, by a deuteron bombardment of uranium-238 in the 1.5 metre cyclotron at the University of California, Berkeley. First neptunium-238 was synthesized which subsequently beta-decayed to form this new element with atomic number 94 and atomic weight 238. Since uranium had been named after the planet Uranus and neptunium after the planet Neptune, element 94 was named after Pluto, which at the time was considered to be a planet as well.
Wartime secrecy prevented its discovery being announced until 1948. Plutonium is the element with the highest atomic number to occur in nature. Trace quantities arise in natural uranium-238 deposits when U-238 captures neutrons emitted by decay of other U-238 atoms. Plutonium is much more common on Earth since 1945 as a product of neutron capture and beta decay, where some of the neutrons released by the fission process convert uranium-238 nuclei into plutonium-239. Both plutonium-239 and plutonium-241 are fissile, meaning that they can sustain a nuclear chain reaction, leading to applications in nuclear weapons and nuclear reactors. Plutonium-240 exhibits a high rate of spontaneous fission, raising the neutron flux of any sample containing it; the presence of plutonium-240 limits a plutonium sample's usability for weapons or its quality as reactor fuel, the percentage of plutonium-240 determines its grade. Plutonium-238 has a half-life of 88 years and emits alpha particles, it is a heat source in radioisotope thermoelectric generators, which are used to power some spacecraft.
Plutonium isotopes are expensive and inconvenient to separate, so particular isotopes are manufactured in specialized reactors. Producing plutonium in useful quantities for the first time was a major part of the Manhattan Project during World War II that developed the first atomic bombs; the Fat Man bombs used in the Trinity nuclear test in July 1945, in the bombing of Nagasaki in August 1945, had plutonium cores. Human radiation experiments studying plutonium were conducted without informed consent, several criticality accidents, some lethal, occurred after the war. Disposal of plutonium waste from nuclear power plants and dismantled nuclear weapons built during the Cold War is a nuclear-proliferation and environmental concern. Other sources of plutonium in the environment are fallout from numerous above-ground nuclear tests, now banned. Plutonium, like most metals, has a bright silvery appearance at first, much like nickel, but it oxidizes quickly to a dull gray, although yellow and olive green are reported.
At room temperature plutonium is in its α form. This, the most common structural form of the element, is about as hard and brittle as gray cast iron unless it is alloyed with other metals to make it soft and ductile. Unlike most metals, it is not a good conductor of electricity, it has an unusually high boiling point. Alpha decay, the release of a high-energy helium nucleus, is the most common form of radioactive decay for plutonium. A 5 kg mass of 239Pu contains about 12.5×1024 atoms. With a half-life of 24,100 years, about 11.5×1012 of its atoms decay each second by emitting a 5.157 MeV alpha particle. This amounts to 9.68 watts of power. Heat produced by the deceleration of these alpha particles makes it warm to the touch. Resistivity is a measure of how a material opposes the flow of electric current; the resistivity of plutonium at room temperature is high for a metal, it gets higher with lower temperatures, unusual for metals. This trend continues down to 100 K, below which resistivity decreases for fresh samples.
Resistivity begins to increase with time at around 20 K due to radiation damage, with the rate dictated by the isotopic composition of the sample. Because of self-irradiation, a sample of plutonium fatigues throughout its crystal structure, meaning the ordered arrangement of its atoms becomes disrupted by radiation with time. Self-irradiation can lead to annealing which counteracts some of the fatigue effects as temperature increases above 100 K. Unlike most materials, plutonium increases in density when it melts, by 2.5%, but the liquid metal exhibits a linear decrease in density with temperature. Near the melting point, the liquid plutonium has high viscosity and surface tension compared to other metals. Plutonium has six allotropes and forms a seventh at high temperature within a limited pressure range; these allotropes, which are different structural modifications or forms of an element, have similar internal energies but varying densities and crystal structures. This makes plutonium sensitive to changes in temperature, pressure, or chemistry, allows for dramatic volume changes following phase transitions from one allotropic form to another.
The densities of the different allotropes vary from 16.00 g/cm3 to 19.86 g/cm3. The presence of these many allotropes makes machining plutonium difficult, as it changes state readily. For example, the α form exists at room temperature in unalloyed plutonium, it has machinin
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 Hanford Project, Hanford Works, Hanford Engineer Works and Hanford Nuclear Reservation. Established in 1943 as part of the Manhattan Project in Hanford, south-central Washington, the site was home to the B Reactor, the first full-scale plutonium production reactor in the world. Plutonium manufactured at the site was used in the first nuclear bomb, tested at the Trinity site, in Fat Man, the bomb detonated over Nagasaki, Japan. During the Cold War, the project expanded to include nine nuclear reactors and five large plutonium processing complexes, which produced plutonium for most of the more than 60,000 weapons built for the U. S. nuclear arsenal. Nuclear technology developed during this period, Hanford scientists produced major technological achievements. Many early safety procedures and waste disposal practices were inadequate, government documents have confirmed that Hanford's operations released significant amounts of radioactive materials into the air and the Columbia River.
In 1989, the State of Washington, US Environmental Protection Agency, the US Department of Energy entered into the Tri-Party Agreement which sets targets, or milestones, for cleanup. EPA and Ecology share regulatory oversight based on CERCLA and RCRA; the weapons production reactors were decommissioned at the end of the Cold War, decades of manufacturing left behind 53 million US gallons of high-level radioactive waste stored within 177 storage tanks, an additional 25 million cubic feet of solid radioactive waste, areas of heavy Technetium-99 and uranium contaminated groundwater beneath three tank farms on the site as well as the potential for future groundwater contamination beneath contaminated soils. In 2011, DOE the federal agency charged with overseeing the site, "interim stabilized" 149 single-shell tanks by pumping nearly all of the liquid waste out into 28 newer double-shell tanks. Solids, known as salt cake and sludge, remained. DOE found water intruding into at least 14 single-shell tanks and that one of them had been leaking about 640 US gallons per year into the ground since about 2010.
In 2012, DOE discovered a leak from a double-shell tank caused by construction flaws and corrosion in the bottom, that 12 double-shell tanks have similar construction flaws. Since the DOE changed to monitoring single-shell tanks monthly and double-shell tanks every three years, changed monitoring methods. In March 2014, the DOE announced further delays in the construction of the Waste Treatment Plant, which will affect the schedule for removing waste from the tanks. Intermittent discoveries of undocumented contamination have slowed the pace and raised the cost of cleanup. In 2007, the Hanford site represented 60% of high-level radioactive waste by volume managed by the US Department of Energy and 7-9% of all nuclear waste in the United States. Hanford is the most contaminated nuclear site in the United States and is the focus of the nation's largest environmental cleanup. Besides the cleanup project, Hanford hosts a commercial nuclear power plant, the Columbia Generating Station, various centers for scientific research and development, such as the Pacific Northwest National Laboratory and the LIGO Hanford Observatory.
On November 10, 2015, it was designated as part of the Manhattan Project National Historical Park alongside other sites in Oak Ridge and Los Alamos. The Hanford Site occupies 586 square miles —roughly equivalent to half of the total area of Rhode Island—within Benton County, Washington; this land is closed to the general public. It is a desert environment receiving under 10 inches of annual precipitation, covered by shrub-steppe vegetation; the Columbia River flows along the site for 50 miles, forming its northern and eastern boundary. The original site was 670 square miles and included buffer areas across the river in Grant and Franklin counties; some of this land has been returned to private use and is now covered with orchards and irrigated fields. In 2000, large portions of the site were turned over to the Hanford Reach National Monument; the site is divided by function into three main areas. The nuclear reactors were located along the river in an area designated as the 100 Area; the site is bordered on the southeast by the Tri-Cities, a metropolitan area composed of Richland, Kennewick and smaller communities, home to over 230,000 residents.
Hanford is a primary economic base for these cities. The confluence of the Yakima and Columbia rivers has been a meeting place for native peoples for centuries; the archaeological record of Native American habitation of this area stretches back over ten thousand years. Tribes and nations including the Yakama, Nez Perce, Umatilla used the area for hunting and gathering plant foods. Hanford archaeologists have identified numerous Native American sites, including "pit house villages, open campsites, fish farming sites, hunting/kill sites, game drive complexes and spirit quest sites", two archaeological sites were listed on the National Register of Historic Places in 1976. Native American use of the area continued into the 20th
In physics, the electronvolt is a unit of energy equal to 1.6×10−19 joules in SI units. The electronvolt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences, because a particle with electric charge q has an energy E = qV after passing through the potential V. Like the elementary charge on which it is based, it is not an independent quantity but is equal to 1 J/C √2hα / μ0c0, it is a common unit of energy within physics used in solid state, atomic and particle physics. It is used with the metric prefixes milli-, kilo-, mega-, giga-, tera-, peta- or exa-. In some older documents, in the name Bevatron, the symbol BeV is used, which stands for billion electronvolts. An electronvolt is the amount of kinetic energy gained or lost by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. Hence, it has a value of one volt, 1 J/C, multiplied by the electron's elementary charge e, 1.6021766208×10−19 C.
Therefore, one electronvolt is equal to 1.6021766208×10−19 J. The electronvolt, as opposed to volt, is not an SI unit, its derivation is empirical, which means its value in SI units must be obtained by experiment and is therefore not known unlike the litre, the light-year and such other non-SI units. Electronvolt is a unit of energy; the SI unit for energy is joule. 1 eV is equal to 1.6021766208×10−19 J. By mass–energy equivalence, the electronvolt is a unit of mass, it is common in particle physics, where units of mass and energy are interchanged, to express mass in units of eV/c2, where c is the speed of light in vacuum. It is common to express mass in terms of "eV" as a unit of mass using a system of natural units with c set to 1; the mass equivalent of 1 eV/c2 is 1 eV / c 2 = ⋅ 1 V 2 = 1.783 × 10 − 36 kg. For example, an electron and a positron, each with a mass of 0.511 MeV/c2, can annihilate to yield 1.022 MeV of energy. The proton has a mass of 0.938 GeV/c2. In general, the masses of all hadrons are of the order of 1 GeV/c2, which makes the GeV a convenient unit of mass for particle physics: 1 GeV/c2 = 1.783×10−27 kg.
The unified atomic mass unit, 1 gram divided by Avogadro's number, is the mass of a hydrogen atom, the mass of the proton. To convert to megaelectronvolts, use the formula: 1 u = 931.4941 MeV/c2 = 0.9314941 GeV/c2. In high-energy physics, the electronvolt is used as a unit of momentum. A potential difference of 1 volt causes an electron to gain an amount of energy; this gives rise to usage of eV as units of momentum, for the energy supplied results in acceleration of the particle. The dimensions of momentum units are LMT−1; the dimensions of energy units are L2MT−2. Dividing the units of energy by a fundamental constant that has units of velocity, facilitates the required conversion of using energy units to describe momentum. In the field of high-energy particle physics, the fundamental velocity unit is the speed of light in vacuum c. By dividing energy in eV by the speed of light, one can describe the momentum of an electron in units of eV/c; the fundamental velocity constant c is dropped from the units of momentum by way of defining units of length such that the value of c is unity.
For example, if the momentum p of an electron is said to be 1 GeV the conversion to MKS can be achieved by: p = 1 GeV / c = ⋅ ⋅ = 5.344286 × 10 − 19 kg ⋅ m / s. In particle physics, a system of "natural units" in which the speed of light in vacuum c and the reduced Planck constant ħ are dimensionless and equal to unity is used: c = ħ = 1. In these units, both distances and times are expressed in inverse energy units (while energy and mass are expressed in the same units, see mas
Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms. Decay heat occurs from decay of long-lived radioisotopes that are primordially present from the Earth's formation. In nuclear reactor engineering, decay heat continues to be generated after the reactor has been shut down, nuclear chain reactions have been suspended; the decay of the short-lived radioisotopes created in fission continues at high power, for a time after shut down. The major source of heat production in a newly shut down reactor is due to the beta decay of new radioactive elements produced from fission fragments in the fission process. Quantitatively, at the moment of reactor shutdown, decay heat from these radioactive sources is still 6.5% of the previous core power, if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power.
After a day, the decay heat falls to 0.4%, after a week it will be only 0.2%. Because radioisotopes of all half life lengths are present in nuclear waste, enough decay heat continues to be produced in spent fuel rods to require them to spend a minimum of one year, more 10 to 20 years, in a spent fuel pool of water, before being further processed. However, the heat produced during this time is still only a small fraction of the heat produced in the first week after shutdown. If no cooling system is working to remove the decay heat from a crippled and newly shut down reactor, the decay heat may cause the core of the reactor to reach unsafe temperatures within a few hours or days, depending upon the type of core; these extreme temperatures can lead to minor fuel damage or major core structural damage in a light water reactor or liquid metal fast reactor. Chemical species released from the damaged core material may lead to further explosive reactions which may further damage the reactor. Occurring decay heat is a significant source of the heat in the interior of the Earth.
Radioactive isotopes of uranium and potassium are the primary contributors to this decay heat, this radioactive decay is the primary source of heat from which geothermal energy derives. In a typical nuclear fission reaction, 187 MeV of energy are released instantaneously in the form of kinetic energy from the fission products, kinetic energy from the fission neutrons, instantaneous gamma rays, or gamma rays from the capture of neutrons. An additional 23 MeV of energy are released at some time after fission from the beta decay of fission products. About 10 MeV of the energy released from the beta decay of fission products is in the form of neutrinos, since neutrinos are weakly interacting, this 10 MeV of energy will not be deposited in the reactor core; this results in 13 MeV being deposited in the reactor core from delayed beta decay of fission products, at some time after any given fission reaction has occurred. In a steady state, this heat from delayed fission product beta decay contributes 6.5% of the normal reactor heat output.
When a nuclear reactor has been shut down, nuclear fission is not occurring at a large scale, the major source of heat production will be due to the delayed beta decay of these fission products. For this reason, at the moment of reactor shutdown, decay heat will be about 6.5% of the previous core power if the reactor has had a long and steady power history. About 1 hour after shutdown, the decay heat will be about 1.5% of the previous core power. After a day, the decay heat falls to 0.4%, after a week it will be only 0.2%. The decay heat production rate will continue to decrease over time. An approximation for the decay heat curve valid from 10 seconds to 100 days after shutdown is P P 0 = 0.066 where P is the decay power, P 0 is the reactor power before shutdown, τ is the time since reactor startup and τ s is the time of reactor shutdown measured from the time of startup. For an approach with a more direct physical basis, some models use the fundamental concept of radioactive decay. Used nuclear fuel contains a large number of different isotopes that contribute to decay heat, which are all subject to the radioactive decay law, so some models consider decay heat to be a sum of exponential functions with different decay constants and initial contribution to the heat rate.
A more accurate model would consider the effects of precursors, since many isotopes follow several steps in their radioactive decay chain, the decay of daughter products will have a greater effect longer after shutdown. P P 0 = ∑ i = 1 11 P i e − λ t