Ceramic fuels discussion

Descriptions will be given below of the methods available for determining the defect structure in nonstoichiometric compositions of nuclear ceramic fuels, but the discussions on the defect structure will be limited mainly to binary system. 

As we have discussed, the strategies to realize optimized ceramic fuel cells are essentially materials search strategies. 

The well-established ceramic fuel cell concepts discussed above comprise oxide ion conducting oxides as solid electrolyte separator material, distinct electrocat-alytically active electrodes made from metals or mixed conducting oxides and well separated gas chambers. Alternative approaches are based on electrolytes... 

The next two chapters discuss two supporting components of the fuel cell stack —specifically, interconnects and sealants. The interconnect conducts the electrical current between the two electrodes through the external circuit and is thus simultaneously exposed to both high oxygen partial pressure (air) and low oxygen partial pressure (fuel), which places stringent requirements on the materials stability. Ceramic interconnects have been used, but metallic interconnects offer promise... 

The PEM cell is the cleanest fuel cell since the fuel is hydrogen, the oxidant oxygen and the product water. Although it clearly falls outside the scope of a text focused on electroceramics there are good reasons for prefacing the present discussion with a brief outline of those elements of the science and technology basic to it and common to the ceramics-based fuel cells. Also, for an intelligent... 

The interconnect normally links the anode of one cell to the cathode of the next. It must, of course, be an electronic conductor and also a gas barrier preventing the direct meeting of fuel and oxidant gases. Fig. 4.27 illustrates how the interconnection is achieved in the case of the so-called planar fuel cell stack. In the later discussion of the ceramics-based cells a tubular configuration is described, but the principles are the same. 

Porous membranes, especially ceramic and carbon compositions, are the focus of intense development efforts. Perhaps, the least studied of the group, at least for hydrogen separations, are the ion-conducting membranes (despite the fact that many fuel cells incorporate a proton-conducting membrane as the electrolyte), and this class of membranes will not be discussed further in this chapter. 

The second technique is based on a filter to capture the soot particulates. Common filters are wall flow monoliths or ceramic foams. Cordierite wall flow monoliths are probably currently the most used particulate traps. They can capture diesel particulates with an efficiency of 99%. At normal diesel engine exhaust gas temperatures, the captured soot is not reactive enough to prevent build up on the filter, with an intolerable high pressure drop over the exhaust system as a result. The oxidation rate of the soot should, therefore, be increased which can be achieved by increasing the temperature of the filter, resulting in higher fuel consumption and thus making this solution unfavourable. The other possibility is catalytic oxidation of the collected soot. Several catalytic systems will be discussed. 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental... 

This whole process should remind you of our discussion of phase boundaries (PBs) in Chapter 15 and reactions in Chapter 25. The solar part of this has many similarities to processes that are now being explored for producing lime (the endothermic calcinations of CaCOs) and other ceramics. Currently we use fossil fuels to produce cement and lime, which account for 5% and 1%, respectively, of the global human-made CO2 emissions— up to 40% of this is from burning the fossil fuels. 

The objective of this paper is to discuss the safety issues associated with the immobilization of excess weapons plutonium in ceramic form in the United States. The U.S. government has recommended a dual-track approach to dispose of excess weapons plutonium. According to this approach, about 33 metric tons of pure Pu will be fabricated into mixed oxide (MOX) fuels which will be burned in commercial nuclear light water reactors and up to 17 metric tons of impure Pu will be immobilized into ceramic form which will be permanently disposed of in a geologic repository. It should be noted that a portion of the 33 metric tons of pure Pu may also be immobilized into ceramic form depending on the future decision of the U.S. government. 

The ceramic process described above is similar to the MOX fuel process. Actually, as far as safety is concern, several conditions and parameters of the ceramic process appear to be less stringent than those of the MOX fuel process. These similarities and differences are summarized in Table 2 and discussed below ... 

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