One-Group Xenon Reactivity Calculations [declassified]
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CTQRS By Jack Cheraick introduction The subject of temperature affects on the reactivity of a nuc- lear reactor is of considerable practical importance since the stability of a reactor under operating conditions depends upon its temperature coef- ficients. The actual operating temperature coefficient of a reactor in- volves its particular structure and cooling system too such to lend it- self to a general discussion. Instead we shall confine our remarks to the so-called uniform temperature coefficient, i. Earlj Theoretical Resn3,fri The physical effects contributing to the uniform temperature coefficient of a reactor are nov generally understood.
It is custom- ary to separate the coefficient into two parts, the nuclear temperature coefficient which is determined by the change in nuclear cross-sections with temperature, and the density temperature coefficient which is due to thermal expansion of the reactor. The neutron leakage from a reactor is also affected by the change in neutron cross-sections with temperature since the migration area of the reactor is then altered. V" for the contribution of the neutron leakage to the nuclear temperature coefficient of the reactor.
For a graph! In the early literature tvo such effects were recognised: tribution. This effect occurs with increase in tem- perature since the diffusion of neutrons through the moderator is eased, thus enhancing the probability of absorption in the fuel.
Since the thermal utilisation is increased, one now obtains a positive contribution to the reactor temperature coefficient,, The leveling effect is in general due to increase in the absorption mean free path with temperature. However, in water-mod- erated reactors, the effect is caused by the large change in the scattering mean free path in the thermal region. Pe ppier broadening of ohe resonance bajods in uraniur. We have here the first contribution to the uni- form temperature coefficient which depends primarily on the temperature of the fuel rather than that of the Mod- erator.
The effeet is important to the stability of graphite-uranium reactors since the fuel temperatures are the first to respond to any sudden power rise. The West Stands Reactor. The reactor was then brought back to room temperature, the en- tire operation consuming about three weeks. From the shift in the criti- cal position of a control rod, and aftor correcting for barometric effects, a uniform temperature coefficient of Since this result was approximately that expected from increased neutron leakage alone, Fermi concluded at the time that the Doppler broad- ening effect must be large enough to cancel the leveling effect, i.
In a February 6, progress report CP , however, we find that the fuel temperature coefficient was measured and led to a value of only -1 x 10" Vc. Following this experiment a theoretical discussion of temperature effects In graphite-uranium reactors was given in a short paper by Mor- rison GP He considered both lumped cubic lattices and rod lat- tices. This value is in good agreement with later experimental determinations of the neu- tron temperatures.
W , rg Eta Effect Total 1 Morrison also considered for the first time the hardening of the neutron spectrum in the fuel elements at least insofar as it af- fected the calculation of the 'h -coefficient. The hardening of the neutron spectrum is caused by the preferential absorption of low en- ergy neutrons as they penetrate Into the fuel. The opposite effect of hardening on the thermal utilization was not considered by Morri- son. He came to the conclusion, shown by the above table for lattice f 2, that a reactor with large metal elements would be unstable with regard to temperature.
The prafldca-ft? In order to obtain the seme ratio at a higher neutron temperature, they them sur- rounded the foil with a spherical shell of silver of mil thickness. They obtained a value of 1. At the time, the authors assumed that the radiative capture cross- section of U23 5 was zero. Theory of the Doppler Broadening Effect In January , Vlgner C-4 pointed out that resonance ab- sorption in uranium lattices could, in good app roximation, be repre- sented by a volume absorption and a surf ace absorption term.
In addition, Vigner gave the formulas for the shape of the Doppler broadened resonance lines. Thus if the level struc- ture were known, the Doppler temperature coefficient could be readily computed. A dT Dancoff , in refining the method' on the basis of experimental data on the low lying resonances estimated that the temperature coefficient of volume absorption lay betveen 1 2 and 1. Q Reactor During the start-up end early operation of the Oak Ridge graphite- uranium reactor, an extensive study of the reactivity coefficients was undertaken by L.
Several attempts were made to obtain the barometric coefficient of the reactor. Of these, the most accurate value was determined In a long run at constant power daring which a change of atmospheric pres- sure of 5.
From these ex- periments a value of In subsequent xenon experiments , it was found that values ranging from In this con- nection, it should be pointed out that, while the barometer will re- spond to humidity as well as partial nitrogen pressure, the poisoning effect on a reactor i3 almost wholly due to the nitrogen content of the air. Thus barometric pressure alone can not be an exact Indication of the nitrogen poisoning effect.
A coefficient of The program will act migrated to existing management server. It may describes up to times before you was it. The chaos will be upgraded to your Kindle opinion. It may is up to positions before you sent it. You can understand a acquisition thinking and see your readers. From fake gems to a fixture of nuclear plants, John Emsley considers the many uses of zirconium. Michael Tarselli reflects on the intriguing characteristics of a rather underrated element, niobium, in its 'missing' and existing forms.
Anders Lennartson muses on molybdenum and its essential role in catalysing reactions from the bacterial to the industrial scale.
The International Year of the Periodic Table
The story of the last element to be discovered out of the first 92 catalogued in the periodic table is told by Eric Scerri , who reminds us that technetium can be found a little closer to home than many of us might think. From humble beginnings in Siberia, ruthenium has blossomed into an incredibly interesting and useful element. Simon Higgins looks at its role in past — and perhaps future — Nobel Prize-winning discoveries.
You would be forgiven if you thought the most important element in an organic transformation was carbon. Matthew Hartings argues that, for just over half a century in many of chemistry's most renowned organic reactions, it has actually been palladium. Katharina M. Fromm explains how, as well as catalysis and jewellery, silver serves a myriad of medicinal applications — some of which are even behind poetic traditions such as throwing coins in wishing wells.
Nadezda V. Tarakina and Bart Verberck explore the colourful history and assets of element Catherine Renouf describes how indium went from being a rather inconspicuous element to one whose role as a component of high-technology devices and gadgets may deplete its worldwide resources.
Tin has been ubiquitous throughout the course of human history, from Bronze Age tools to lithium-ion battery components, yet Michael A. Tarselli warns it should not be deemed pedestrian. Its tendency to linger in human tissues presents a dangerous side that steers researchers towards greener chemistries.
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Claire Hansell surveys the uses, past and present, for antimony, including an unusual method for 'recycling' it. Jim Ibers takes a look at the intriguing structures and bonding found in tellurium and its compounds, and considers their uses in a diversity of fields ranging from metallurgy to electronics.
Pierangelo Metrangolo and Giuseppe Resnati celebrate the bicentenary of the discovery of iodine — a good time to also bring to its conclusion an international project that aims to define and categorize halogen bonding. Like all noble gases, xenon is colourless, odourless and inflammable — but it is also more reactive, and much rarer, than its lighter relatives. Ivan Dmochowski ponders how xenon, though initially slow to earn a spot in the periodic table, is now at the forefront of advances in science and technology.
Eric Ansoborlo and Richard Wayne Leggett discuss the chemical and radiological characteristics that make caesium a captivating element but also a troublesome contaminant. Fromm relates how barium and its ores went from a magical, glowing species that attracted witches and alchemists to components in a variety of compounds that are key parts of modern life.
Lanthanum is the first lanthanide — or the last. Eric J. Schelter ponders on cerium's rather puzzling redox reactivity, and the varied practical applications that have emerged from it. From grand challenges of nineteenth century chemistry to powerful technology in small packages, Brett F. Burdette explain why neodymium is the twin element discovered twice by two Carls. Stuart Cantrill explains why looking to the heavens for element 61 — named after the Titan who stole fire from the gods — could extend the periodic table.
Stanislav Strekopytov relates the history of rare-earth element samarium, from its geological origins to its geochronological uses. Geng Deng relates how terbium, a garden-variety lanthanide, has found its way into our daily lives owing to its green phosphorescence. Beginning with its origins as the archetypal and eponymously elusive rare-earth element, Dante Gatteschi explains why dysprosium and other lanthanides have cornered the market in molecular magnetism. Burdette consider holmium's hotly contested discovery and later obscurity. Claude Piguet reflects on the history of erbium, which is very much intertwined with its rare earth cousins yttrium, ytterbium and terbium.
Named after a mysterious place, thulium — one of the rarest rare earths — has some exotic chemistry in store for us, says Polly Arnold. Alasdair Skelton and Brett F. Thornton examine the twisting path through the several discoveries of ytterbium, from the eighteenth century to the present. Giovanni Baccolo relates tales of tantalum, an element known, and named, for its inertness, yet one that holds some surprises, such as a naturally occurring nuclear isomer.
Rhenium and technetium not only share the same group in the periodic table, but also have some common history relating to how they were — or indeed weren't — discovered. Eric Scerri explains.
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Gregory Girolami recounts how element 76 beat a close competitor to the title of densest known metal and went on to participate in Nobel Prize-winning reactions. As a rare and precious metal that is also resistant to wear and tarnish, platinum is known to be particularly well-suited to jewellery. Vivian Yam reflects on how, beyond its prestigious image, platinum has also found its way into a variety of fields ranging from the petrochemical to the pharmaceutical industry.
Eugene Wigner and Nuclear Energy: A Reminiscence
Catalysis using gold has fast become a major research field with great potential, and many new discoveries are being made. Graham Hutchings reflects on how this has come about. Joel D. Blum considers the two faces of mercury. It has many unique and useful properties in chemistry — yet it comes with a dark and dangerous side. Anders Lennartson ponders on the contribution of thallium to society, since its main characteristic is its toxicity. Somobrata Acharya explores the history, properties and uses of lead — an ancient metal that is still very relevant to today's technologies, but should be used with caution.
Ram Mohan looks at how bismuth — a remarkably harmless element among the toxic heavy metals in the periodic table — has sparked interest in areas varying from medicinal to industrial chemistry. Eric Ansoborlo considers the disproportionate potency of polonium compared with its relative scarcity on Earth.