Reactors nuclear

The first nuclear chain reaction occurred naturally about two billion years ago in Gabon, Africa, where a uranium deposit moderated by water spontaneously became critical. In 1942 as a result of the war efforts, a sustained chain reaction was achieved by E. Fermi in Chicago working on the Manhattan Project which eventually led to the atomic bomb. 

Of the naturally occurring isotopes, only is fissionable with a neutron absorption cross section (cr) of 582 bams with thermal neutrons at 0.025 eV. This cross section decreases as the energy of the neutrons increase (cr oc E ), and in the MeV range, cr is about 2 bams. reacts with neutrons (cr 0—1/4 for E 0.9 MeV) to form plutonium (Pu) by the reaction sequence  

Terrestrial exposure due to radium and other isotopes in 40 mRem 

Cosmic radiation during 10,000-km flight at 10-km altitude 4 mRem 

Plutonium is radioactive with U/, = 2.4 x lO years and is fissionable like It is also possible to convert to fissionable which has a half-life of 1.63 x 10 years by the reaction sequence. 

The principal nuclear constituents in the core of a homogeneous reactor are intimately mixed. The fuel-bearing substance will appear, in general, as some molecular compound, dissolved in a suitable solvent to form a solution, suspended in a fluid carrier to form a slurry, or fused at some appropriate temperature (with or without a carrier) to form a homogeneous fluid. The purpose and operating conditions of the reactor complex determine which of these possibilities is the most attractive for a given engineering application. 

The classification of the neutron-energy spectrum of a given reactor is determined principally by the neutron-moderating materials which it contains. If the nuclear masses of the nonfuel components of the reactor are relatively low, then the neutron spectrum will correspond to that of a thermal reactor (cf. Fig. 1.4c) if they are large, a fast spectrum will result (cf. Fig. 1.4o). The spectrums of intermediate reactors may be due to a number of nuclear characteristics, the presence of nuclear masses of moderate magnitudes being one cause. 

The first of these applications, power production, may be further classified according to three specific functions central-station power, package power, and mobile power. Central-station-power applications of nuclear reactors refers to the production of power for large municipal or industrial areas. Package-power applications, however, refer to power production for limited facilities. In this classification, one might include power plants for areas in unusual climatic conditions, advanced bases, or small, isolated establishments. The application of nuclear reactors to power mobile units can conceivably include any device which is designed for terrestrial operation or, for that matter, space craft as well. The possibilities in this area of application are as yet hardly known. 

The application of nuclear reactors for research purposes includes the production of high-intensity neutron and radiation fields and the production of radioisotopes. The use of radiation and neutron fields for experimental research in physical and biological sciences is well established, and the construction of nuclear reactors for use in university and research laboratories is already under way. The production of radioisotopes has, likewise, become an important application of nuclear reactors, no doubt stimulated by the considerable success of radiotracer techniques. 

A schematic diagram of a nuclear power plant. The energy from the fission process is used to boil water, producing steam for use in a turbine-driven generator. Cooling water from a lake or river is used to condense the steam after it leaves the turbine. 

Uranium was first discovered in 1789 as a component of pitchblende. Its name came from the planet Uranus, which had been discovered only shortly before. Uranium, which is not a rare element in the earth s crust (it is more abundant than tin), is widely scattered on earth. The most easily accessible deposits of uranium are found in the United States, Canada, South Africa, and Australia. Although uranium is used in small amounts as a coloring agent for glass and ceramics, its major use is as a nuclear fuel in fission reactors. 

Uranium consists principally of mixed with smaller amounts of Both isotopes are radioactive, although U decays more rapidly than U by about a factor of six. The U isotope splits into two smaller nuclei (fission) when it is bombarded by neutrons, which makes it useful for producing energy in nuclear reactions. 

Because fission of U requires carefully controlled conditions, scientists were shocked to find that a natural nuclear reactor existed millions of years ago 

A nuclear reactor in which fissionable fuel is produced while the reactor runs 

Water chemistry of hghtwater reactors. Isotope separation. 

The nuclear reactor is a device in which a controlled chain reaction takes place involving neutrons and a heavy element such as uranium. Neutrons are typically absorbed in uranium-235 [15117-96-17, or plutonium-239 [15117 8-5], Pu, nuclei. These nuclei spHt, releasing two fission fragment nuclei 

A variety of nuclear reactor designs is possible using different combinations of components and process features for different purposes (see Nuclear REACTORS, reactor types). Two versions of the lightwater reactors were favored the pressurized water reactor (PWR) and the boiling water reactor (BWR). Each requites enrichment of uranium in U. To assure safety, careful control of coolant conditions is requited (see Nuclearreactors, water CHEMISTRY OF LIGHTWATER REACTORS NuCLEAR REACTORS, SAFETY IN NUCLEAR FACILITIES). 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

I represents coal, El hydro power and other energy sources. Data from Reference 2. 

With nuclear fusion energy at the stage of research only, all the nuclear power referred to above was generated by fission reactors. Such reactors produce heat energy by causing neutrons to split both uranium and plutonium nuclei in a chain reaction that is sustained at a constant rate by the next-generation neutrons that are emitted at and soon after fission. The heat produced by the nuclear reactor is then used to raise steam in a conventional steam plant. 

A more advanced design is the plutonium-fuelled fast reactor, where the higher neutron speed permitted by avoiding thermalization means that slightly more neutrons are produced at each fission event. 

Nuclear power plants use nuclear fission to generate energy. The core of a typical nuclear reactor consists of four principal components fuel elements, control rods, a moderator, and a primary coolant ( FIGURE 21.18). The fuel is a fissionable substance, such as uranium-235. The natural isotopic abundance of uranium-235 is only 0.7%, too low to sustain a chain reaction in most reactors. Therefore, the content of 

The control rods are composed of materials that absorb neutrons, such as cadmium or boron. These rods regulate the flux of neutrons to keep the reaction chain self-sustaining and also prevent the reactor core from overheating.  

The probability that a neutron will trigger fission of a nucleus depends on the 

Why are nuclear power plants usually located near a large body of water  

The steam drives an electric generator, creating electricity 

The probability that a neutron will trigger fission of a nucleus depends on the speed of the neutron. The neutrons produced by fission have high speeds (typically in excess of 10,000 km/s). The function of the moderator is to slow down the neutrons (to speeds of a few kilometers per second) so that they can be captured more readily by the fissionable nuclei. The moderator is typically either water or graphite. 

The primary coolant is a substance that transports the heat generated by the nuclear chain reaction away fi om the reactor core. In a pressurized water reactor, which is the most 

Representation of a fission process in which each event produces two neutrons that can go on to spiit other nuciei, ieading to a seif-sustaining chain reaction. 

During World War II, the United States carried out an intense research effort called the Manhattan Project to build a bomb based on the principles of nuclear fission. This program produced the fission bomb, which was used with devastating effect on the cities of Hiroshima and Nagasaki in 1945. Basically, a fission bomb operates by suddenly combining two sub-critical masses, which results in rapidly escalating fission events that produce an explosion of incredible intensity. 

Mile Island facility in Pennsylvania in 1979 and the one at Chernobyl in the Soviet Union in 1986 have led many people to question the wisdom of continuing to build fission-based power plants. 

One potential problem facing the nuclear power industry is the limited supply of Some scientists believe that we have nearly depleted the uranium deposits that are rich enough in to make the production of fissionable fuel economically feasible. Because of this possibility, reactors have been developed in which fissionable fuel is actually produced while the reactor runs. In these breeder reactors, the major component of natural uranium, non-fissionable IfU, is changed to fissionable Pu. The reaction involves absorption of a neutron, followed by production of two /3 particles. 

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Results of uranium weight determination in nuclear reactor fuel elements. 

Powder diffraction studies with neutrons are perfonned both at nuclear reactors and at spallation sources. In both cases a cylindrical sample is observed by multiple detectors or, in some cases, by a curved, position-sensitive detector. In a powder diffractometer at a reactor, collimators and detectors at many different 20 angles are scaimed over small angular ranges to fill in the pattern. At a spallation source, pulses of neutrons of different wavelengdis strike the sample at different times and detectors at different angles see the entire powder pattern, also at different times. These slightly displaced patterns are then time focused , either by electronic hardware or by software in the subsequent data analysis. 

Beryllium is added to copper to produce an alloy with greatly increased wear resistance it is used for current-carrying springs and non-sparking safety tools. It is also used as a neutron moderator and reflector in nuclear reactors. Much magnesium is used to prepare light nieial allo>s. other uses include the extraction of titanium (p. 370) and in the removal of oxygen and sulphur from steels calcium finds a similar use. 

This is a radioactive element. It occurs in minute traces in barium and thorium minerals, but it can be produced by irradiation of bismuth in a nuclear reactor. (The study of its chemistry presents great difficulty because of its intense a radiation). 

On a larger scale, deuterium oxide has been used as a moderator in nuclear reactors, having some advantages over graphite. 

Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen bomb. 

Deuterium is used as a moderator to slow down neutrons. Tritium atoms are also present but in much smaller proportions. Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen (fusion) bomb. It is also used as a radioactive agent in making luminous paints, and as a tracer. 

IK as a cooling medium for nuclear reactors, and as a gas for supersonic wind tunnels. 

Beryllium is used in nuclear reactors as a reflector or moderator for it has a low thermal neutron absorption cross section. 

The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal. 

The metal is very effective as a sound absorber, is used as a radiation shield around X-ray equipment and nuclear reactors, and is used to absorb vibration. White lead, the basic carbonate, sublimed white lead, chrome yellow, and other lead compounds are used extensively in paints, although in recent years the use of lead in paints has been drastically curtailed to eliminate or reduce health hazards. 

In 1934, scientists discovered that when they bombarded natural bismuth (209Bi) with neutrons, 210Bi, the parent of polonium, was obtained. Milligram amounts of polonium may now be prepared this way, by using the high neutron fluxes of nuclear reactors. 

By far of greatest importance is the isotope Pu2sy with a half-life of 24,100 years, produced in extensive quantities in nuclear reactors from natural uranium 23su(n, gamma) —> 239U—(beta) —> 239Np—(beta) —> 239pu. Fifteen isotopes of plutonium are known. 

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

When irradiation is complete, the sample is removed from the nuclear reactor, allowed to cool while any short-lived interferences that might be present decay to the background, and the rate of gamma-ray emission is measured. 

The concentration of Mn in steel can be determined by a neutron activation analysis using the method of external standards. A 1.000-g sample of an unknown steel sample and a 0.950-g sample of a standard steel known to contain 0.463% w/w Mn, are irradiated with neutrons in a nuclear reactor for 10 h. After a 40-min cooling period, the activities for gamma-ray emission were found to be 2542 cpm (counts per minute) for the unknown and 1984 cpm for the standard. What is the %w/w Mn in the unknown steel sample ... 

Plutonium (Pu) is an artificial element of atomic number 94 that has its main radioactive isotopes at 2 °Pu and Pu. The major sources of this element arise from the manufacture and detonation of nuclear weapons and from nuclear reactors. The fallout from detonations and discharges of nuclear waste are the major sources of plutonium contamination of the environment, where it is trapped in soils and plant or animal life. Since the contamination levels are generally very low, a sensitive technique is needed to estimate its concentration. However, not only the total amount can be estimated. Measurement of the isotope ratio provides information about its likely... 

Chain reactions do not go on forever. The fog may clear and the improved visibility ends the succession of accidents. Neutron-scavenging control rods may be inserted to shut down a nuclear reactor. The chemical reactions which terminate polymer chain reactions are also an important part of the polymerization mechanism. Killing off the reactive intermediate that keeps the chain going is the essence of these termination reactions. Some unusual polymers can be formed without this termination these are called living polymers. 

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