Nuclear diffusion

NMR Nuclear magnetic resonance [223, 224] Chemical shift of splitting of nuclear spin states in a magnetic field H [225], C [226, 227], N [228], F [229], 2 Xe [230] Other Techniques Chemical state diffusion of adsorbed species... 

Carr H Y and Purcell E M 1954 Effects of diffusion on free precession in nuclear magnetic resonance experiments Rhys. Rev. 94 630-8... 

Ahn C B and Cho Z H 1989 A generalized formulation of diffusion effects in pm resolution nuclear magnetic-resonance imaging Med. Rhys. 16 22-8... 

Uranium-235 can be concentrated by gaseous diffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive. 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Uranium hexafluoride is used in the gaseous diffusion process for the separation and enrichment of uranium-235, which exists in low concentration in natural uranium. The enriched UF is converted back into an oxide and used as fuel for the nuclear power industry. 

The UFg is then isotopicaHy enriched by gaseous diffusion or gas centrifuge processes for nuclear appHcations. 

The rationale for the development of such fibers is demonstrated by their appHcation in the medical field, notably hemoperfusion, where cartridges loaded with activated charcoal-filled hoUow fiber contact blood. Low molecular weight body wastes diffuse through the fiber walls and are absorbed in the fiber core. In such processes, the blood does not contact the active sorbent direcdy, but faces the nontoxic, blood compatible membrane (see Controlled RELEASE TECHNOLOGY, pharmaceutical). Other uses include waste industrial appHcations as general as chromates and phosphates and as specific as radioactive/nuclear materials. 

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

P. A. Abelson, N. Rosen, and J. 1. Hoovei, Eiquid Thermal Diffusion, Oak Ridge, Teimessee, National Nuclear Energy Series, Div. IX, Vol. 1, TlD-5229, U.S. AEG Technical Information Service, 1951. 

A number of special processes have been developed for difficult separations, such as the separation of the stable isotopes of uranium and those of other elements (see Nuclear reactors Uraniumand uranium compounds). Two of these processes, gaseous diffusion and gas centrifugation, are used by several nations on a multibillion doUar scale to separate partially the uranium isotopes and to produce a much more valuable fuel for nuclear power reactors. Because separation in these special processes depends upon the different rates of diffusion of the components, the processes are often referred to collectively as diffusion separation methods. There is also a thermal diffusion process used on a modest scale for the separation of heflum-group gases (qv) and on a laboratory scale for the separation of various other materials. Thermal diffusion is not discussed herein. 

In 1985, owiag to the declining demand by the nuclear power industry for enriched uranium, the Oak Ridge gaseous diffusion plant was taken out of operation and, subsequently, was shut down. The U.S. gaseous diffusion plants at Portsmouth, Ohio and Paducah, Kentucky remain ia operation and have a separative capacity of 19.6 million SWU (separative work unit) per year which as of this writing is not fully utilized. 

G. F. Mailing and E. Von H.a]le,Merocfnamic Isotope Separation Processes for Cranium Enrichment Process Requirement, paper presented at the Symposium on New Advances ia Isotope Separation, Div. of Nuclear Chemistry and Technology, American Chemical Society, San Francisco, Calif., Aug. 1976 CCC-ND Report K/OM-2872, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Term., Oct. 7, 1976. 

PasquiU Atmo.spheric Diffusion, Van Nostrand, 1962) recast Eq, (26-60) in terms of the dispersion coefficients and developed a number of useful solutions based on either continuous (plume) or instantaneous (puff) releases, Gifford Nuclear Safety, vol, 2, no, 4, 1961, p, 47) developed a set of correlations for the dispersion coefficients based on available data (see Table 26-29 and Figs, 26-54 to 26-57), The resulting model has become known as the Pasquill-Gifford model. 

Further support for this approach is provided by modern computer studies of molecular dynamics, which show that much smaller translations than the average inter-nuclear distance play an important role in liquid state atom movement. These observations have conhrmed Swalin s approach to liquid state diffusion as being very similar to the calculation of the Brownian motion of suspended particles in a liquid. The classical analysis for this phenomenon was based on the assumption that the resistance to movement of suspended particles in a liquid could be calculated by using the viscosity as the frictional force in the Stokes equation... 

Figure 4 Spectrum of diffusion in the mineral olivine ((Mg, Fe)2 SiO ) taken using nonresonant profiling technique with the reaction (p, a) Both the a particles resulting from the nuclear reaction and backscattered protons are collected. Inset shows expanded region of the spectrum, where a yield indicates diffusion of into the material.

The neutron activation technique mentioned in the preceding paragraph is only one of a range of nuclear methods used in the study of solids - methods which depend on the response of atomic nuclei to radiation or to the emission of radiation by the nuclei. Radioactive isotopes ( tracers ) of course have been used in research ever since von Hevesy s pioneering measurements of diffusion (Section 4.2.2). These techniques have become a field of study in their own right and a number of physics laboratories, as for instance the Second Physical Institute at the University of Gottingen, focus on the development of such techniques. This family of techniques, as applied to the study of condensed matter, is well surveyed in a specialised text... 

An example of a journal hovering between broad and narrow spectrum is Journal of Alloys and Compounds, subtitled an interdiciplinary journal of materials science and solid-state chemistry and physics. One which is more restrictively focused is Journal of Nuclear Materials (which I edited for its first 25 years). Ceramics has a range of journals, of which the most substantial is Journal of the American Ceramic Society. Ceramics International is an example of an international journal in the field, while Journal of the European Ceramic Society is a rather unusual instance of a periodical with a continental remit. More specialised journals include Solid State Ionics Diffusion and Reactions, and a new Journal of Electroceramics, started in 1997. 

Johnson, W. B. et al., 1975, Gas Tracer Study of Roof-Vent Effluent Diffusion at Millstone Nuclear Power Station, Atomic Industrial Forum AIF/NESP 0076. 

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