Lead-walled reactors

The reactor used for this study was the horizontal atmospheric-pressure hot-wall reactor (reactor A in Fig. 6.11). The susceptor accommodated three substrates side by side at an angle of 15.5 ° above horizontal. The leading edge of the film experiences a more reactant-rich gas stream and is closer to the... 

Plasma oxide can be grown from a number of oxidizers plus SiH4. Among these are N20, 02, C02 and even TEOS (tetraethoxysilane). Generally, 02 is not used as it too often leads to homogeneous nucleation. The preferred reactants have proven to be SiH4 and N20, so we will restrict our discussion to these. Films grown in both cold-wall and hot-wall reactors will be considered. 

As shown in Figure 5.27 the I-CVI process is performed in a hot-wall reactor, in which both the heat and mass are transferred from the outer region to the inner region of the preform. As a result it leads to a severe density gradient and forms an impermeable skin which impedes the further densification of the inner part of the preform. However, thermal gradient CVI (TG-CVI) is a cold-wall technique, although the densification is also dependent on the mass diffusion. A temperature gradient can be deliberately established across the fibre preform. Unlike the I-CVI model, TG-CVI has a mobile infiltration front within the preform, caused by a... 

This process may lead to the deposition of free carbon, but it can also be controlled by the proper adjustment of the reaction parameters. Atmosphere (reducing or inert), gas flow, and temperature can be used to control the particle size of deposited materials. Different methods can also be used for heating of the system (cold- and hot-wall reactors). 

Reactors for conventional thermally activated CVD are of two types cold-wall and hot-wall reactors, respectively internally and externally heated. The disadvantage of a hot-wall reactor is deposition on the wall and partial depletion of reactants leading to nonuniform coatings. A correct reactor geometry and gas inlet manifold can compensate for gas depletion in hot-wall reactors. There is no limit to the form of the objects to be coated, but sizes are restricted. In a cold-wall reactor the substrates to be coated are heated by a graphite susceptor that is inductively heated by an rf generator. Only the hot parts are coated and not the reactor walls, which remain relatively cold. 

From an industrial viewpoint, coalescence is undesirable for some processes and desirable for others. For example, coalescence during suspension polymerization is undesirable and leads to reactor setup, or buildup of polymer on vessel walls and agitation equipment. On the other hand, mass transfer processes, such as extraction, centrifugation, and decantation, depend on coalescence to achieve desirable rates of operation. Coalescence between drops leads to intunate mixing in the newly formed larger drop. 

Pressure-tubes allow the separate, low-pressure, heavy-water moderator to act as a backup hesit sink even if there is no water in the fuel channels. Should this fail, the calandria shell ilsdf can contain the debris, with the decay heat being transferred to the water-filled shield tank around the core. Should the severe core damage sequence progress further, the shield tank and the concrete reactor vault significantly delay the challenge to containment. Furthermore, should core melt lead to containment overpressure, the concrete containment wall will leak and reduce the possibility of catastrophic structural failure (Snell, 1990). 

The nuclear reactor also must be shielded against the emission of radioactive material to the external environment. Suitable radiation controls include both thermal and biological shielding systems. Radiation from alpha particles (a rays) and beta particles ((3 rays) has little penetrating power, but gamma rays have deep penetration properties. Neutron radiation is, however, the primary area of risk. Typically, extremely thick concrete walls are used as a neutron absorber, but lead-lined concrete and special concretes are also used. 

In 1976 the Swedish government stipulated that no new nuclear reactors should be charged until it had been shown how the radioactive waste products could be taken care of in an "absolutely safe manner" (8). Consequently, the nuclear power industry (through their joint Nuclear Fuel Supply Co, SKBF) embarked on a program referred to as the Nuclear Fuel Safety (KBS) Project (8). In one of the schemes (9) a repository for spent nuclear fuel elements in envisaged at a depth of 500 m in granitic bedrock. The repository will ultimately contain 6000 tonnes of uranium and 45 tonnes of plutonium. The spent fuel elements will be stored in copper cylinders (0.8 m in diameter and 4.7 m in length) with a wall thickness of 200 mm the void will be filled with lead. 

Naiijj Na apor exists. Above this T, the sodium pressure is no longer sufficient to prevent the thermal dissociation of NaB, and syntheses lead to a second phase with a lower Na content, Na Bu. However, Na,(B 5 can be prepared at < 1100°C, as long as the sodium pressure in the vapor phase is kept low by having a cold wall in the reactor or, e.g., by substituting a Na-K alloy for sodium. ... 

An interesting way to retard catalyst deactivation is to expose the reaction mixture to ultrasound. Ultrasound treatment of the mixture creates local hot spots, which lead to the formation of cavitation bubbles. These cavitation bubbles bombard the solid, dirty surface leading to the removal of carbonaceous deposits [38]. The ultrasound source can be inside the reactor vessel (ultrasound stick) or ultrasound generators can be placed in contact with the wall of the reactor. Both designs work in practice, and the catalyst lifetime can be essentially prolonged, leading to process intensification. The effects of ultrasound are discussed in detail in a review article [39]. 

Generally, whenever fluids are processed in a confined space, two different types of phenomena are observed surface and volume effects. An example of a surface effect is a heterogeneously catalyzed reaction occurring at the walls of the vessel, whereas the motion of a fluid due to gravitational forces would be described as a volume effect. In brief, it can be stated that the surface effects gain in importance compared with the volume effects when the size of a reactor decreases. In particular, the reduction of length scale leads to a... 

Hot reactor walls are sometimes used as a means to increase the density of the films that are deposited on the walls. This reduces the amount of adsorbed contaminants on the walls, and leads to lower outgassing rates. A hot wall is particularly of interest for single-chamber systems without a load-lock chamber. Material quality is similar to the quality obtained with a cold reactor wall [145],... 

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