Cores, silicon carbides

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

Two fibers are presently produced by CVD on a commercial scale boron and silicon carbide. The production of these two fibers requires a monofilament starter core capable of being heated resistively such as a tungsten or graphite fiber. I l The deposition apparatus is shown schematically in Fig. 19.1. 

Coated spherical Th02- or U02-particles are increasingly utilized in the fuel of gas-cooled high temperature reactors. Their 50 to 1500 pm core of uranium(IV) oxide is manufactured using conventional sintering techniques. This is then pyrolytically coated with many layers of carbon and silicon carbide (see Section 5.7.5.1). 

The electric furnaces, in which this reaction is carried out, are ca. 15 3 3 m- in size and are lined with refractory material. Electrodes at opposite ends are connected to a graphite core. The furnace is filled round this core with the reaction mixture and electrically heated to 2200 to 2400°C. The heating up time is ca. 18 h and the reaction time a further ca. 18 h. After cooling, the sides of the furnaces are removed and the unreacted material on the edges removed. The silicon carbide, which has formed round the graphite core, is broken up and separated into different qualities. 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

The fuel pins used in the core are a new type for OCR s. The vast majority of OCR reactors use a TRISO type fuel embedded in a graphite matrix and are thermal neutron spectrum reactors. TRISO fuels are small UC spheres coated with layers of silicon carbide and pyrolitic carbon. While this results in fuels that can be used to extremely high bumup, the uranium density of the fuel is low. A coolant hole is then drilled through the blocks of fuel and these blocks are put in a prismatic array. 

The primary barrier to fission product release from the core is the silicon carbide and/or pyrocarbon coatings of each fuel particle. Both the SiC and outer pyrocarbon (OPyC) coatings provide a barrier to the release of fission gases. The SiC coating acts as the primary barrier to the release of... 

After the current is switched of, the furnace is allowed to cool for several days. Then the side walls are removed, and the excess reaction mixture is carried off. The roll of silicon carbide contains not only the original resistance core, but also graphite formed by decomposition of silicon carbide. The boundary between the graphite and the SiC is very sharply defined. The innermost SiC zone is of the highest quality, as the purest and largest crystals can form in this region. Moving... 

The other method consists in decomposing halides on an electrically heated metal filament, which then forms the core of the inorganic fiber. Silicon carbide fibers produced in this way (from the decomposition of SiCU with, for example, toluene or acetone) possess tensile strengths of up to 280,000 N/cm. ... 

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