Reactors types

Several components are required in the practical appHcation of nuclear reactors (1 5). The first and most vital component of a nuclear reactor is the fuel, which is usually uranium slightly enriched in uranium-235 [15117-96-1] to approximately 3%, in contrast to natural uranium which has 0.72% Less commonly, reactors are fueled with plutonium produced by neutron absorption in uranium-238 [24678-82-8]. Even more rare are reactors fueled with uranium-233 [13968-55-3] produced by neutron absorption in thorium-232 (see Nuclear reactors, nuclear fuel reserves). The chemical form of the reactor fuel typically is uranium dioxide, UO2, but uranium metal and other compounds have been used, including sulfates, siUcides, nitrates, carbides, and molten salts. 

The second important component is the cooling agent or reactor coolant which extracts the heat of fission for some usefiil purpose and prevents melting of the reactor materials. The most common coolant is ordinary water at high temperature and high pressure to limit the extent of boiling. Other coolants that have been used are Hquid sodium, sodium—potassium alloy, helium, air, and carbon dioxide (qv). Surface cooling by air is limited to unreflected test reactors or experimental reactors operated at very low power. 

The fourth component is the set of control rods, which serve to adjust the power level and, when needed, to shut down the reactor. These are also viewed as safety rods. Control rods are composed of strong neutron absorbers such as boron, cadmium, silver, indium, or hafnium, or an alloy of two or more metals. 

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

The sixth component of the system is the shield, which protects materials and workers from radiation, especially neutrons and gamma rays. 

The interested reader is referred to the literature [7]. The chapter on reactor design, while somewhat dated in its presentation, offers an outstanding background to the chemical engineering principles necessary for successful achievement of a working CVD system. 

In CVD, all reactants enter the reactor in the vapor phase. In the region of the substrate, they decompose, forming a solid reaction product (deposited film) and vapor phase reaction co-products (residual gas). 

Solid catalysts can be used in all of the major reactor types, batch, semibatch, continuous stirred tank and tubular. In the first three cases particulate (powder) catalysts would be appropriate, whereas with the tubular reactor the catalyst would often need to be formed into pellets.8,9 

Batch reactors using particulate catalysts need to be well stirred in order to give uniform compositions and to minimise mass transport limitations. They are likely to be preferred for small-scale production of high-priced products or 

Adsorbent Solid material on which adsorption occurs 

Adsorption isotherm The relation at constant temperature between the amount adsorbed and equilibrium pressure or concentration 

Monolayer Amount required to cover the entire surface 

On the basis of the Hatta number, the transformations carried out in biphasic systems can be described as slow (Ha 0.3), intermediate (with a kinetic-diffusion regime 0.3 Ha 3.0), and fast (Ha 3). These are diffusion limited and take place near the interface (within the diffusion layer). Slow transformations are under kinetic control and occur mostly in a bulk phase, so that the amount of substrate transformed in the boundary layer in negligible. When diffusion and reaction rate are of similar magnitude, the reaction takes place mostly in the diffusion layer, although extracted substrate is also present in the continuous phase, where it is transformed at a rate depending on its concentration [38, 50, 54]. 

Membrane reactors can be considered passive or active according to whether the membrane plays the role of a simple physical barrier that retains the free enzyme molecules solubilized in the aqueous phase, or it acts as an immobilization matrix binding physically or chemically the enzyme molecules. Polymer- and ceramic-based micro- and ultrafiltration membranes are used, and particular attention has to be paid to the chemical compatibility between the solvent and the polymeric membranes. Careful, fine control of the transmembrane pressure during operation is also required in order to avoid phase breakthrough, a task that may sometimes prove difficult to perform, particularly when surface active materials are present or formed during biotransformahon. Sihcone-based dense-phase membranes have also been evaluated in whole-cell processes [55, 56], but 

The first step in downstream processing is the separation of the product-rich phase from the second phase and the biocatalyst. This may be simplified if the enzyme is immobilized or if a membrane module is included in the experimental set-up. In the case of emulsion reactors, centrifugation for liquid phase separation is a likely separation process [58], although the small size of droplets, the possibility of stable emulsion formation during the reaction, particularly if surface-active 

1 Oak Ridge National Laboratory, operated by Union Carbide Corporation for the U.S. Atomic Energy Commission. 

2 In general v E) and f E) will vary from point to point in the reactor if the ratio of concentrations of fissionable species is not the same throughout the reactor—as in 

Reactor theory, in highest approximation, concerns itself with the angular neutron flux, ( x,E,Sl,t)—i.e., the sum of the speeds of all the neutrons which, at time t, are in unit volume at x and unit energy interval at E and whose velocity vectors lie on unit solid angle at SI. The integral of over all directions 

We suppose that there are no extraneous sources, and that fission neutrons are produced isotropically. Then the number of fission neutrons produced in unit interval around (x,E,Sl,t) is 

Equation (1) is the general kerjiel form of the reactor equation for the angular flux under the restriction of no extraneous sources, constant ratio of fissionable species, and isotropic emission of fission neutrons. 

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Having made a choice of the reaction path, we need to choose a reactor type and make some assessment of the conditions in the reactor. This allows assessment of reactor performance for the chosen reaction path in order for the design to proceed. 

Figure 2.3 Choice of reactor type for mixed parallel and series reactions when the parallel reaction has a higher order than the primary reaction.

In the preceding section, the choice of reactor type was made on the basis of which gave the most appropriate concentration profile as the reaction progressed in order to minimize volume for single reactions or maximize selectivity for multiple reactions for a given conversion. However, after making the decision to choose one type of reactor or another, there are still important concentration effects to be considered. 

Multiple reactions in parallel producing byproducts. Once the reactor type is chosen to maximize selectivity, we are in a position to alter selectivity further in parallel reaction systems. Consider the parallel reaction system from Eq. (2.20). To maximize selectivity for this system, we minimize the ratio given by Eq. (2.21) ... 

Having discussed the choice of reactor type and operating conditions at length, let us try two examples. 

It should be emphasized that these recommendations for the initial settings of the reactor conversion will almost certainly change at a later stage, since reactor conversion is an extremely important optimization variable. When dealing with multiple reactions, selectivity is maximized for the chosen conversion. Thus a reactor type, temperature, pressure, and catalyst are chosen to this end. Figure 2.10 summarizes the basic decisions which must be made to maximize selectivity. ... 

Heat transfer. Once the basic reactor type and conditions have been chosen, heat transfer can be a major problem. Figure 2.11 summarizes the basic decisions which must be made regarding heat transfer. If the reactor product is to be cooled by direct contact with a cold fluid, then use of extraneous materials should be avoided. 

Reactor conversion. In Chap. 2 an initial choice was made of reactor type, operating conditions, and conversion. Only in extreme cases would the reactor be operated close to complete conversion. The initial setting for the conversion varies according to whether there are single reactions or multiple reactions producing byproducts and whether reactions are reversible. 

Choice of reactor. The first and usually most important decisions to be made are those for the reactor type and its operating conditions. In choosing the reactor, the overriding consideration is usually raw materials efficiency (bearing in mind materials of construction, safety, etc.). Raw materials costs are usually the most important costs in the whole process. Also, any inefficiency in raw materials use is likely to create waste streams that become an environmental problem. 

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