Ceramic fuels properties

Some properties of the ceramic fuels UO2 and UC are summarized in Table 11.5. UO2 is preferably used as nuclear fuel in all modem light-water reactors (LWR) of the boiling-water (BWR) as well as of the pressurized-water (PWR) type. The main advantages of UO2 are the high melting point and the resistance to H2, H2O, CO2 and radiation. The main disadvantage is the low thermal conductivity, which has to be compensated by application of thin fuel rods. 

Solid-state electrochemistry, as a subsection of electrochemistry, emphasizes phenomena in which the properties of sohds play a dominant role. This includes phenomena involving ionically and/or electronically conducting phases (e.g., in potentiometric or conductometric chemical sensors). As far as classical electrochemical cells are concerned, one refers not only to all-solid-state cells with sohd electrolytes (e.g., ceramic fuel cells), but also to cells with hquid electrolytes, such as modern Li-based batteries in which the storage within the sohd electrode is crucial [1-3]. 

Nuclear ceramics Nuclear properties Absorbers, fuel rods, shields... 

Similar to other nuclear installations, the defence-in-depth concept incorporated in the MARS provides for multiple barriers to radioactivity release from the fuel and for measures to maintain the integrity of these barriers. Such a barrier structure largely leans upon the known properties of the fuel (spherical fuel elements with coated particles), i.e., the retention of a large amount of radionuclides in a ceramic fuel kernel and the prevention of radionuclide release to the coolant by the fuel particle coatings. The graphite matrix of fuel elements that has an ability to absorb certain radionuclides facilitates a reduction of radioactivity release to the coolant. A two-circuit plant scheme provides an additional barrier to radioactivity release to the environment. 

Considering chemical structures of ceramic fuels, these fuels can be categorized as oxide fuels, carbide fuels, and nitride fuels. Oxide fuels such as UO2, mixed oxide (MOX), and thorium dioxide (Th02) have low thermal conductivities compared to carbide and nitride fuels. Hence, from the heat transfer point of view, oxide fuels can also be identified as low thermal conductivity fuels. On the other hand, carbide (eg, UC and UC2) and nitride (eg, UN) fuels are identified as high thermal conductivity fuels. Table 18.3 lists basic properties of these fuels at 0.1 MPa and 25°C. 

As a ceramic fuel, UO2 is a hard and brittle material due to its ionic or covalent interatomic bonding. In spite of that, UO2 is currently used in PWRs, BWRs, CANDU reactors, and others due to its properties. Oxygen has a very low thermal neutron absorption cross section, which does not result in a serious loss of neutrons. UO2 is chemically stable and does not react with water within the operating temperatures of these reactors. UO2 is stmcturally very stable. The crystal structure of the UO2 fuel... 

As already mentioned in the previous section that the traditional LaCr03-based interconnect has difficulties such as machining, failure due to mechanical properties during cell operation, availability of matching sealants and the cost of chromium. Because of these limitations of ceramic interconnects, metal-based interconnects have been introduced in the anode supported thin film electrolyte structure which operates in the temperature range 700-800°C. The necessity to operate the ceramic fuel cell much below 1000°C (preferably below 750°C) has pushed the researchers to search for suitable metals or alloys. The metals have a number of advantages over ceramic interconnects. 

E. Stephens, J. Vetrano, B. Koeppel, Y. Chou, X. Sun, M. Khaleel, Experimental characterization of glass-ceramic seal properties and their constitutive implementation in solid oxide fuel cell stack models. J. Power Sources 193(2), 625-631 (2009)... 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

Primdahl S, Sprensen BF, and Mogensen M. Effect of nickel oxide/yttria-stabilized zirconia anode precursos sintering temperature on the properties of solid oxide fuel cells. J Am Ceram Soc 2000 83 489 -94. 

Nishiyama H, Aizawa M, Sakai N, Yokokawa H, Kawada T, and Dokiya M. Property of (La,Ca)Cr03 for interconnect in solid oxide fuel cell (part 2). Durability. J. Ceram. Soc. Japan 2001 109 527-534. 

Molten Carbonate Fuel Cell The electrolyte in the MCFC is a mixture of lithium/potassium or lithium/sodium carbonates, retained in a ceramic matrix of lithium aluminate. The carbonate salts melt at about 773 K (932°F), allowing the cell to be operated in the 873 to 973 K (1112 to 1292°F) range. Platinum is no longer needed as an electrocatalyst because the reactions are fast at these temperatures. The anode in MCFCs is porous nickel metal with a few percent of chromium or aluminum to improve the mechanical properties. The cathode material is hthium-doped nickel oxide. 

These early studies were carried out on metals of typically 90-99% purity, which sufficed to determine at least their gross properties. During the 1960s, interest diminished somewhat in actinide metallurgy due in part to the increasing use of ceramic rather than metallic fuel elements in nuclear reactors. The bulk of actinide metal research was for secret military purposes and only a fraction of the fundamental research was published. 

Thorium is used to make ceramics, lantern mantles, and metals used in the aerospace industry and in nuclear reactions. Thorium can also be used as a fuel for generating nuclear energy. More than 30 years ago thorium oxides were used in hospitals to make certain kinds of diagnostic x-ray photographs. Further information on the properties and uses of thorium can be found in Chapters 3 and 4 of this profile. 

The ceramic properties of EU2O3 were investigated by Curtis and Tharp [305]. The electric conductivities of rare earth oxides including EU2O3 between 600—1300° C were reported [675]. The selective oxidation of Ci to C5 olefins and Ci to C5 alcohols by direct fuel cells employing noble metal anodes and aqueous H2SO4 electrolytes was found to be enhanced [676] by small additions of soluble salts of Ce, Eu and Yb. 

The high-temperature gas-cooled reactor (HTGR) is a thermal reactor that produces desired steam conditions. Helium is used as the coolam. Graphite, with its superior high temperature properties, is used as the moderator and structural material. The fuel is a mixture of enriched uranium and thorium in the form of carbide particles clad with ceramic coatings. 

Atkinson, A. and Selquk, A., Mechanical properties of ceramic materials for solid oxide fuel cells, in Proceedings of Solid Oxide Fuel Cells V, U. Stimming, S.C. Singhal, H. Tagawa and W. Lehnet (Eds.), The Electrochemical Society, Pennington, NJ, 1997, p. 671. 

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