Production barrier

The retarding influence of the product barrier in many solid—solid interactions is a rate-controlling factor that is not usually apparent in the decompositions of single solids. However, even where diffusion control operates, this is often in addition to, and in conjunction with, geometric factors (i.e. changes in reaction interfacial area with a) and kinetic equations based on contributions from both sources are discussed in Chap. 3, Sect. 3.3. As in the decompositions of single solids, reaction rate coefficients (and the shapes of a—time curves) for solid + solid reactions are sensitive to sizes, shapes and, here, also on the relative dispositions of the components of the reactant mixture. Inevitably as the number of different crystalline components present initially is increased, the number of variables requiring specification to define the reactant completely rises the parameters concerned are mentioned in Table 17. 

An important characteristic feature, common to all these reactions, is the formation of a single product (barrier) phase. In addition, the lattice structures of both reactants and products are relatively simple and information on appropriate physical and chemical properties of these substances is available. Complex iodide formation is of particular interest because of the exceptionally large cation mobilities in these phases. Experimental methods have been described in Sect. 1 and Chap. 2. 

The rate of product formation can be predicted from a knowledge of the conductivity and transport numbers of cations in the product barrier phase and appropriate thermodynamic data. This interpretation is based on the Wagner model (Sect. 2.3). 

Two product barrier layers are formed and the continuation of reaction requires that A is transported across CB and C across AD, assuming that the (usually smaller) cations are the mobile species. The interface reactions involved and the mechanisms of ion migration are similar to those already described for other systems. (It is also possible that solid solutions will be formed.) As Welch [111] has pointed out, reaction between solids, however complex they may be, can (usually) be resolved into a series of interactions between two phases. In complicated processes an increased number of phases, interfaces, and migrant entities must be characterized and this requires an appropriate increase in the number of variables measured, with all the attendant difficulties and limitations. However, the careful selection of components of the reactant mixture (e.g. the use of a common ion) or the imaginative design of reactant disposition can sometimes result in a significant simplification of the problems of interpretation, as is seen in some of the examples cited below. 

SOLUBILITY PRODUCT Barriers to chemical reactivity, CHEMICAL KINETICS B a ryes,... 

Sweeney, M. Davenport, S. Edwards, L. Validation Issues for a Product Barrier/Isolator Sterile Liquid Filling System in a Controlled Environment, Proceedings of PDA/ISPE Conference on Advanced Barrier Technology, 1995. 

The simplest rate equation, which applies when the reaction zone has a constant area and the rate of product formation decreases in direct proportion to the thickness of the product barrier layer, is ... 

With the growing demand for coextruded products, barrier plastics have shown significant growth in the last several years. Historically, the high barrier resins market has been dominated by three leading materials — vinylidene chloride (VDC) copolymers, ethylene vinyl alcohol (EVOH) copolymers, and nitrile resins. Since 1985, however, there has been a lot of interest worldwide in the development of moderate to intermediate barrier resins, as apparent from the introduction of a number of such resins, notably, aromatic nylon MXD-6 from Mitsubishi Gas Chemical Company, amorphous nylons SELAR PA by Du Pont and NovamidX21 by Mitsubishi Chemical Industries, polyacrylic-imide copolymer EXL (introduced earlier as XHTA) by Rohm and Haas and copolyester B010 by Mitsui/Owens-Illinois. 

In some cases, the barriers necessary to develop the desired resistance to corrosion can be formed on structural alloys by appropriate composition modihcation. In many practical applications for structural alloys, however, the required compositional changes are not compatible with the required physical properties of the alloys. In such cases, the necessary compositional modifications are developed through the use of coatings on the surfaces of the structural alloys and the desired reaction-product barriers are developed on the surfaces of the coatings. 

II Protection by Multiple Fission Product Barriers Cr.l4 Reactor coolant pressure boundary... 

Cr.20 Protection system functions Cr.21 Protection system reliability and testability II Protection by Multiple Fission Product Barriers Cr.l9 Control room II Protection by Multiple Fission Product Barriers Cr.l9 Control room... 

Fission Product Barriers Reactor Inherent Protection 11... 

The reactor vessel houses the reactor core, which is the heat source for steam generation. The vessel contains this heat, transfers the heat to the IHX, and serves as one of the fission product barriers during normal operation. 

Inadequate understanding of product barrier requirements poses a package-design problem in predicting adequate shelf life from gas-transmission data. 

Maintain the fission product barriers of the fuel clad, the reactor vessel and coolant system, and the containment vessel. Maintaining the fuel clad by transfer of decay heat out of the core using natural, unpumped mechanisms like natural circulation, evaporation, conduction, convection, and condensation. The containment vessel is the final barrier against radioactive releases to the environment. 

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