Passive stability

Significant advances have also been made in reactor safety. Earlier reactors rely on a series of active measures, such as water pumps, that come into play to keep the reactor core cool in the event of an accident. A major drawback is that these safety devices are subject to failure, thereby requiring backups and, in some cases, backups to the backups The Generation IV reactor designs provide for what is called passive stability, in which natural processes, such as evaporation, are used to keep the reactor core cool. Furthermore, the core has a negative temperature coefficient, which means the reactor shuts itself down as its temperature rises owing to a number of physical effects, such as any swelling of the control rods. 

A survey of reactor developments of ihc three aforementioned types reveals a number of common generic technical features. These include passive stability, simplification, ruggedness, ease of operation, and modularity. 

Passive Stability. Passive design characteristics ensure core stability by eliminating the potential for a runaway chain reaction. In the innovative program, this has been a hallmark from the outset of the program, Passive characteristics are internal governors—that is, physical laws ensure that the reaction rate decreases instantaneously as the temperature of die coolant or fuel or the power of the reactor increases, without the need for external conlrol devices. 

Designing a micromechanical flyer, a device with maximum dimensions of 25 mm and a mass of 100 mg, for example, is a challenge on many fronts— aerodynamics, actuation, transmission, power supply, sensing, control algorithms, compact low-power electronics, and flight behavior. As far as I know, no flyer smaller than the Caltech microbat has flown freely. Only the 10-gram Caltech microbat omithopter (Figure 1) has flown under its own power with passive stabilization. 

Piles, piers, buttress, or walls are passive stabilization systems that is, further movements of the slope increase pressure on them, and the system reaction forces put into the slide mass lead to stabilization. 

What is meant by passive stability in nuclear reactor design ... 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

From polarization curves the protectiveness of a passive film in a certain environment can be estimated from the passive current density in figure C2.8.4 which reflects the layer s resistance to ion transport tlirough the film, and chemical dissolution of the film. It is clear that a variety of factors can influence ion transport tlirough the film, such as the film s chemical composition, stmcture, number of grain boundaries and the extent of flaws and pores. The protectiveness and stability of passive films has, for instance, been based on percolation arguments [67, 681, stmctural arguments [69], ion/defect mobility [56, 57] and charge distribution [70, 71]. 

To illustrate some of the different approaches, let us consider passive films grown on Fe-Cr alloys. It has been established since 1911 [72] that an increase of Cr in the alloy increases the stability of the oxide film against dissolution. 

This example illustrates that exact infonnation on the chemistry and stmcture of the passive film is necessary to clarify the mechanisms relevant to stability and protectiveness of passive films. 

Flarfenist S A and Wang Z L 1999 Fligh-temperature stability of passivated silver nanocrystal superlattices J. Phys. Chem. B 103 4342... 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is we are not comparing the stability of the two to each other Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic Cycloheptatrienyl cation although aromatic is still a carbocation and reasonably reac tive toward nucleophiles Its special stability does not imply a rock like passivity but rather a much greater ease of formation than expected on the basis of the Lewis struc ture drawn for it A number of observations indicate that cycloheptatrienyl cation is far more stable than most other carbocations To emphasize its aromatic nature chemists often write the structure of cycloheptatrienyl cation m the Robinson circle m a ring style... 

The decomposition of aqueous hydrogen peroxide is minimized by various purification steps during manufacture, use of clean passive equipment, control of contaminants, and the addition of stabilizers. The decomposition is zero-order with respect to hydrogen peroxide concentration. 

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

Surface films are formed by corrosion on practically all commercial metals and consist of solid corrosion products (see area II in Fig. 2-2). It is essential for the protective action of these surface films that they be sufficiently thick and homogeneous to sustain the transport of the reaction products between metal and medium. With ferrous materials and many other metals, the surface films have a considerably higher conductivity for electrons than for ions. Thus the cathodic redox reaction according to Eq. (2-9) is considerably less restricted than it is by the transport of metal ions. The location of the cathodic partial reaction is not only the interface between the metal and the medium but also the interface between the film and medium, in which the reaction product OH is formed on the surface film and raises the pH. With most metals this reduces the solubility of the surface film (i.e., the passive state is stabilized). 

Erosion and Corrosion combined require special consideration. Most of the stainless steels and related corrosion-resistant alloys ow e their surface stability and low rate of corrosion to passive films that develop on the surface either prior to or during exposure to reactive fluids. If conditions change from passive to active, or if the passive film is removed and not promptly reinstated, much higher rates of corrosion may be expected. 

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

The effect of anions on the zones of corrosion and passivation can be exemplified by a comparison of the Pb-H20 and Pb-H20-S04 equilibrium diagrams (see Section 4.3, Figs. 4.13 and 4.14) and it can be seen that in the presence of SOl the corrosion zone corresponding with stability of... 

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