Voids, localized

Toughening results from the formation of voids in the material. These voids locally disturb stress field in the material in such a way that damage (or plastic deformation) occurs. This leads to energy dissipation. 

Reactivity worths of major reactor materials were measured in this 600-llter, nickel-reflected core. Average worths found were U-235, 36.5 Ih/kg U-238, -1.3 iVhs 0, 21.7 Ih/kg Na, 13.7 Wkg and steel, 2.2 Ih/kg (relative to void). Local sodium worths were extensively investigated since this was the first large volume, high sodium content core constructed in ZPR-ni. Sodium worths were found to decrease from 18.8 Ih/kg at the core center to 12 Ih/kg at the radial edge of the core in the center plane. Experiments to determine the effect of sodium density on its worth showed little correlation. 

Localization of void sliding ducts in the concrete of a storage silo... 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Local average values of concentrations and partial pressures, rather chan point values, are also used in Che flux relations. However, in this case it is convenient to use interstitial local averages, based on the void volume rather than the total volume, since these are. ore closely related to the properties of bulk gases. 

Plastic strain localization and mixing due to void collapse in porous materials works in the same way, with perhaps an even greater degree of actual mixing due to jetting, and other extreme conditions that can occur at internal free surfaces in shock-loaded solids. 

Tensile stresses stimulate void growth, and subsequent loss in local strength of the material, hence simulating spall in ductile materials. 

In 1984, an explosion in a water pumping station at Abbeystead, UK, killed 16 people, most of them local residents who were visiting the plant. Water was pumped from one river to another through a tunnel. When pumping was stopped, some water was allowed to drain out of the tunnel and leave a void. Methane from the rocks below accumulated in the void and, when pumping was restarted, was pushed through vent valves into a valvehouse, where it exploded [18J. 

Unless beams are present at the entry the stubs should be close to the stmctural ceiling. Beam fairing may be used where the void has to be shallow and this can reduce the risk of re-entrainment due to local high-velocity pressure. Zone barriers are fitted within the void where separate rooms are to be fed. The ceiling need not cover the whole room. 

Formation of vapor bubbles in rapidly flowing or turbulent water causing risk of pumping failure and erosion and/or corrosion. Due to an increase in velocity at the pump head resulting in a localized pressure reduction and the subsequent collapse of the vapor into voids or cavities. Where FW temperatures are high (over perhaps 195-205 °F) the pump velocity can reduce FW vapor pressure below that corresponding to the temperature of the liquid and cavitation can occur accompanied by some noise. Warning of severe pump cavitation is often indicated by a heavy noise. 

In Table 6.7, C is the Martinelli-Chisholm constant, / is the friction factor, /f is the friction factor based on local liquid flow rate, / is the friction factor based on total flow rate as a liquid, G is the mass velocity in the micro-channel, L is the length of micro-channel, P is the pressure, AP is the pressure drop, Ptp,a is the acceleration component of two-phase pressure drop, APtp f is the frictional component of two-phase pressure drop, v is the specific volume, JCe is the thermodynamic equilibrium quality, Xvt is the Martinelli parameter based on laminar liquid-turbulent vapor flow, Xvv is the Martinelli parameter based on laminar liquid-laminar vapor flow, a is the void fraction, ji is the viscosity, p is the density, is the two-phase frictional... 

In the past two decades, 129Xe NMR has been employed as a useful technique for the characterization of the internal void space of nanoporous materials. In particular, the xenon chemical shift has been demonstrated to be very sensitive to the local environment of the nuclei and to depend strongly on the pore size and also on the pressure [4—6], Assuming a macroscopic inhomogeneity resulting from a distribution of adsorption site concentrations, 129Xe NMR spectra of xenon in zeolites have been calculated, and properties such as line widths, shapes as well as their dependence on xenon pressure can be reproduced qualitatively. A fully quantitative analysis, however, remains difficult due to the different contributions to the xenon line shift. (See Chapter 5.3 for a more detailed description of Xe spectroscopy for the characterization of porous media.)... 

As in the data of Figure 4.26, it is seen that the local decrease in F y-ray yield relate to an increase in Li y-ray yield, due to voids in the spinel cathode that relate to local accumulations of gel-polymer. The authors conclude that PIGE and PIXE... 

In the following sections, the flow patterns, void fraction and slip ratio, and local phase, velocity, and shear distributions in various flow patterns, along with measuring instruments and available flow models, will be discussed. They will be followed by the pressure drop of two-phase flow in tubes, in rod bundles, and in flow restrictions. The final section deals with the critical flow and unsteady two-phase flow that are essential in reactor loss-of-coolant accident analyses. 

The basic mechanism for transition from bubble to slug flow appears to be the same as in vertical pipe flow. That is, as the gas flow rate is increased for a given liquid flow rate, the bubble density increases, many collisions occur and cell-type Taylor bubbles are formed, and the transition to slug flow takes place. As shown in the case of vertical pipe upflow, Taitel et al. (1980) assumed that this transition takes place when ac = 0.25. This criterion is also applicable here. However, because of the preferable geometry in the rod bundle, where the bubbles are observed to exist, instead of in the space between any two rods, this void fraction of 0.25 applies to the local preferable area only, a.L. The local voids, aL, can be related to the average void by (Venkateswararao et al., 1982)... 

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