Axial heterogeneity

In the other limiting case, tRo tc or Pe I/R, radial diffusion occurs very quickly and it quickly washes out the heterogeneity of concentration in the radial direction. If hn or Pcd R/L, then axial diffusion occurs much faster than convective transfer, and the axial heterogeneity of concentration spreads out very... 

Two design options are considered for the reactor core a conventional homogoteous core and an axial heterogeneous core with an internal fertile slice just below core midplane. Both concepts (see Figio) have three core zones with different plutonium contents and are fully compatible with each other. The core is surrounded by one row of breeder subassemblies and an axial breeder blanket located above and below core. 

One of the main requirements for advanced core development is ensuring of a zero sodium void effect of reactivity. For compensation of an additional positive component of reactivity arising due to higher burn-up a transition to axial heterogeneous core concept was made. Herewith the inner blanket of depleted uranium dioxide is placed in the central part of the core. A size of this blanket is 2/3 of the core radius. 

In some applications diffusion theory may be too approximate, especially if a pronounced angular dependence of the neutron flux exists in a reactor region. This situation becomes almost usual when analysing some new concepts, such as the axially heterogeneous design [4.55] with an axial uranimn layer inside the core and a sodium plenum above the core. 

Another potential remedy could be the use of the axially heterogeneous (AX-HET) fuel concept which is under development in Europe [7.38,7.39] in this concept, a breeder slice is inserted into the central part of the fuel column. 

Figure 5.1 Typical homogeneous and heterogeneous sodium-cooled fast reactor core configurations (a) homogeneous core, (h) axial heterogeneous core, and (c) radial heterogeneous core. All rights reserved hy Japan Atomic Energy Agency.

To enhance safety, France made a decision to adopt an innovative core concept with low sodium void reactivity, called Coeur a Faible Vidange (CFV Sciora et al., 2011). The CFV is an axially heterogeneous core with a stepwise core height and a sodium plenum. This configuration exhibits the multiplier effect on the significant reduction... 

Sohd Catalysts Processes with solid catalysts are affected by diffusion of heat and mass (1) within the pores of the pellet, (2) between the fluid and the particle, and (3) axially and radially within the packed bed. Criteria in terms of various dimensionless groups have been developed to tell when these effects are appreciable. They are discussed by Mears (Ind. Eng. Chem. Proc. Des. Devel., 10, 541-547 [1971] Jnd. Eng. Chem. Fund., 15, 20-23 [1976]) and Satterfield (Heterogeneous Cataly.sls in Practice, McGraw-Hill, 1991, p. 491). 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

The multifrequency EPR and Mdssbauer properties of the [FesSJ in C. vinosum NiFe-hydrogenase are particularly interesting since they provide evidence of magnetic interactions with nearby paramagnetic species (151, 154, 155). The magnetically isolated form exhibits a well-resolved, almost axial EPR signal, g = 2.018, 2.016, 2.002, indicative of minimal conformational heterogeneity. However, a com-... 

The characteristic derivative-shaped feature at g 1.94 first observed in mitochondrial membranes has long been considered as the sole EPR fingerprint of iron-sulfur centers. The EPR spectrum exhibited by [4Fe-4S] centers generally reflects a ground state with S = I and is characterized by g values and a spectral shape similar to those displayed by [2Fe-2S] centers (Fig. 6c). Proteins containing [4Fe-4S] centers, which are sometimes called HIPIP, essentially act as electron carriers in the photoinduced cyclic electron transfer of purple bacteria (106), although they have also been discovered in nonphotosynthetic bacteria (107). Their EPR spectrum exhibits an axial shape that varies little from one protein to another with g// 2.11-2.14 and gi 2.03-2.04 (106-108), plus extra features indicative of some heterogeneous characteristics (Pig. 6d). 

The transmission electron microscope is now well established as a useful tool for the characterization of supported heterogeneous catalysts(l). Axial bright-field imaging in the conventional transmission electron microscope (CTEM) is routinely used to provide the catalyst chemist with details concerning particle size distributions, 3), particle disposition over the support material(2-6) as well as particle morphology(7). Internal crystal structure(8-10), and elemental compositions(ll) may be inferred by direct structure imaging. 

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