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[en] The role of prominent Soviet physicist B I Davydov in the development of our understanding of diffusion is briefly reviewed, with emphasis on the ideas he put forward in the 1930s: introducing additional partial derivatives into diffusion equations and extending diffusion concepts to phase space. (from the history of physics)
[en] The equations describing diffusion on a heterogeneous lattice for low concentrations are considered taking into account lattice site blocking. It is shown that lattice site blocking cannot be disregarded in the case of a strongly heterogeneous lattice even for low concentrations. It is established that the equation with a fractional time derivative holds only in a bounded time interval. Anomalous diffusion, which is described by the equation with a fractional time derivative at the initial stage, must be described over long time periods by an ordinary diffusion equation with a concentration-dependent diffusion coefficient
[en] An increasing number of natural phenomena do not fit into the relatively simple description of diffusion developed by Einstein a century ago. As all of us are no doubt aware, this year has been declared 'world year of physics' to celebrate the three remarkable breakthroughs made by Albert Einstein in 1905. However, it is not so well known that Einstein's work on Brownian motion - the random motion of tiny particles first observed and investigated by the botanist Robert Brown in 1827 - has been cited more times in the scientific literature than his more famous papers on special relativity and the quantum nature of light. In a series of publications that included his doctoral thesis, Einstein derived an equation for Brownian motion from microscopic principles - a feat that ultimately enabled Jean Perrin and others to prove the existence of atoms (see 'Einstein's random walk' Physics World January pp19-22). Einstein was not the only person thinking about this type of problem. The 27 July 1905 issue of Nature contained a letter with the title 'The problem of the random walk' by the British statistician Karl Pearson, who was interested in the way that mosquitoes spread malaria, which he showed was described by the well-known diffusion equation. As such, the displacement of a mosquito from its initial position is proportional to the square root of time, and the distribution of the positions of many such 'random walkers' starting from the same origin is Gaussian in form. The random walk has since turned out to be intimately linked to Einstein's work on Brownian motion, and has become a major tool for understanding diffusive processes in nature. (U.K.)
[en] Here we consider an unsteady detonation with diffusion included. This introduces an interaction between the reaction length scales and diffusion length scales. Detailed kinetics introduce multiple length scales as shown though the spatial eigenvalue analysis of hydrogen-oxygen system; the smallest length scale is ∼ 107 m and the largest ∼ 10-2 m; away from equilibrium, the breadth can be larger. In this paper, we consider a simpler set of model equations, similar to the inviscid reactive compressible fluid equations, but include diffusion (in the form of thermal/energy, momentum, and mass diffusion). We will seek to reveal how the complex dynamics already discovered in one-step systems in the inviscid limit changes with the addition of diffusion.
[en] This work is devoted to investigate explicit solutions of the time-fractional diffusion equations with external forces by considering various diffusion coefficients and an absorbent rate. Besides, the 2nth moment related to such an equation is also discussed. Consequently, the diffusion type can be determined from the mean-square displacement. In addition, a rich class of diffusive processes, including normal and anomalous ones, can be obtained
[en] Modern notions in the field of theory and practice of the obtaining of complex diffusion coatings are presented. Rational procedures carbo-chromizing, chromo-nitridation, chromo-boriding and titaniding to obtain the surfaces with special physicochemical properties for the use under complex conditions of exploitaion are presented. The results of metallographical, physical, chemical, mechanical and other methods of investigation of complex coatings are given. Examples of the application of such coatings and modern equipment for their obtaining are described
[en] Description is given of a treatment for porous metallic filters used in isotope separation of UF6 with a view to improve their mechanical and corrosion resistance. The filters are introducted in presence of a Al base cement into a heat and oxidation resisting receptacle. The closed receptacle is filled up with an inert gas and heated in a furnace between 800 and 10000C during one hour
[fr]On decrit un procede de traitement des filtres metalliques poreux utilises dans le separation isotopique de l'hexafluorure d'uranium en vue d'ameliorer leur resistance mecanique et leur resistance a la corrosion. Selon ce procede on place les filtres en presence d'un cement a base d'aluminium dans un recipient ferme resistant a la chaleur et a l'oxydation, et on place l'interieur du recipient en atmosphere de gaz inerte et on le chauffe dans un four entre 8000C et 10000C pendant environ une heure
[en] We study the voter model and related random copying processes on arbitrarily complex network structures. Through a representation of the dynamics as a particle reaction process, we show that a quantity measuring the degree of order in a finite system is, under certain conditions, exactly governed by a universal diffusion equation. Whenever this reduction occurs, the details of the network structure and random copying process affect only a single parameter in the diffusion equation. The validity of the reduction can be established with considerably less information than one might expect: it suffices to know just two characteristic timescales within the dynamics of a single pair of reacting particles. We develop methods to identify these timescales and apply them to deterministic and random network structures. We focus in particular on how the ordering time is affected by degree correlations, since such effects are difficult to access by existing theoretical approaches.
[en] The Portsmouth gaseous diffusion plant (PORTS) is a uranium enrichment facility that was historically used to enrich uranium to levels that range from 2% to greater than 97%. The feed material for PORTS was obtained from the Paducah Gaseous Diffusion Plant (PGDP) that produced uranium in the form of UF6 that was enriched to about 1 to 2%. The enrichment process involves a multistage process by which gaseous UF6 passed through a diffusion barrier in each stage. The porous diffusion barrier in each stage retards the rate of the diffusion of the heavier 238U atoms relative to the diffusion of the lighter 235U atoms. By this process the product stream is slightly enriched by each stage of the process. Each stage consists of a compressor, converter and a motor. There are more than 4000 stages that are linked together with piping of various diameters to form the PORTS cascade. The cascade spans three interconnected buildings and comprises miles of piping, thousands of seals, converters, valves, motors, and compressors. During operation, PORTS process equipment contained UF6 gas with uranium enrichment that increased in the process stream from the first to the last stage in a known manner. Gaseous UF6 moving through the PORTS process equipment had potential to form deposits within the process equipment by several mechanisms, including solidification due to incorrect temperature and pressure conditions during the process, inleakage of atmospheric moisture that chemically reacts with UF6 to form hydrated uranyl fluoride solids, reduction reactions of UF6 with cascade metals, and UF6 condensation on the internal equipment surfaces. As a result, the process equipment of the PORTS contains a variable and unknown quantity of uranium with variable enrichment that has been deposited within the equipment during plant operations. The exact chemical form of this uranium is variable, although it is expected that the bulk of the material is of the form of uranyl fluoride that will become hydrated on exposure to moisture in air when the systems are no longer buffered. The deposit geometry and thickness is uncertain and variable. However, a reasonable assessment of the level of material holdup in this equipment is necessary to support decommissioning efforts. The assessment of nuclear material holdup in process equipment is a complex process that requires integration of process knowledge, nondestructive assay (NDA) measurements, and computer modeling to maximize capabilities and minimize uncertainty. The current report is focused on the use of computer modeling and simulation of NDA measurements.