Carbidization pyrolytic coatings

The increase in sensitivity from the pyrolytically coated tube has been noted by others [86] who cautioned against impregnation of the graphite tube with salts of elements which form stable carbides (Ta, Si, Nb, Zr, W, La). Decreased sensitivities for V have been obtained following such treatment which was attributed to the possible formation of ternary compounds between the impregnating element, vanadium and graphite, e.g. V—Ta—C. 

Advanced waste form work is also being carried out in the Ceramics and Graphite Section at PNL, where high temperature gas-cooled reactor fuel technology is applied to waste solidification. Waste particles are coated with pyrolytic carbon followed by a cover coat of silicon carbide. These coated particles would then be placed in a matrix of inert material contained in a canister of yet another material. 

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). 

In some circumstances it is found advantageous to coat graphite rods (or tubes) with a layer of pyrolytic graphite this leads to improved sensitivity with elements such as vanadium and titanium which are prone to carbide formation. 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

The apparent slow atomization of some elements may be caused by carbide formation. Rapid heating and a reproducible surface (e.g. a pyrolytic surface) help reduce this problem, as does coating of the tube (e.g. with lanthanum, using lanthanum nitrate solution), and the use of metallic tubes or boats. 

The fuel particles used in these studies were typical pyrolytic carbon-coated thorium-uranium dicarbide, (Th,U)C2, microspheres. The kernels, — 200/i in diameter, were prepared from Th02, U02, and C and converted to the carbide at temperatures below 2200°C., followed by a spheroidization above the melting point, 2450°-2500°C. The bare kernels were coated with a 30-50fi layer of low density (— 1.0 gram/cm.3) buffer pyrolytic carbon, followed by a 40-70/a layer of high density... 

Pyrolytic carbon and silicon carbide coatings are applied to the fuel particles by a fluidized bed, vapor phase coating technique in which the particles are levitated in a flowing gas stream within a vertieal heated tube. 

Here the use of pyrolytic graphite coated graphite tubes is helpful, as the diffusion of the analyte solution into the graphite and thus the risk of the carbide formation are decreased. Alternatively, flushing the furnace with nitrogen can be helpful. Indeed, in the case of Ti a nitride is then formed which in contrast to the carbide can be dissociated easier. Other thermochemical means to decrease interferences, as discussed earlier, are known as matrix modification. The addition of a number of substances, such as Pd-compounds or Mg(N03)2, has been shown to be successful for the realization of a matrix-free vapor doud formation (see e.g. Ref. [278]). The mechanisms involved also relate to surface effects in the furnace (see e.g. Ref. [279]) and are in themselves a spedfic field of research. 

Preliminary results show it should be possible to operate the MHR with a coolant outlet temperature of up to 1 000°C using nuclear-grade graphite fuel blocks, carbon-carbon composite materials for control rods and other internal reactor components, and existing coated-particle fuel technology with silicon carbide (SiC) and pyrolytic carbon coatings. 

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