Abstract: New aspects in the characterization of pyrocarbon coatings of reactor fuel particles

Abstract

Pyrocarbon coatings of nuclear fuel kernels are spherical, with coating thicknesses of some 10 μm. For special cases coatings can be produced up to 200-μm thick. In the past, these coatings have been thought to be homogeneous with respect to the material as well as to the values of the material properties. Following this assumption, the characterization procedures applied to the pyrocarbon coatings yielded integral, mean values of the examined property. Such characterization procedures are measurements of pyrocarbon density and the x-ray procedures, e.g., measurements of crystalline anisotropy, apparent crystallite size, and mean layer spacing. Apart from the problems which arise from these methods themselves, it was shown that in spite of the small layer thickness the pyrocarbon coatings were not homogeneous. By x-ray techniques, it could be shown that the crystalline anisotropy varied across the coating in such a way that it generally had smaller values in the inner region of the coating and higher ones on the outside. Recently, careful examination of these bulk coatings proved that several other parameters were not constant throughout the coating. Neutron irradiation experiments demonstrated that it was just these property gradients that strongly influenced the irradiation behavior of the entire coated particle. Moreover, after such irradiation experiments it was found that not only the parameter variations were relevant to irradiation behavior, but also the relative amount and the distribution of the two carbon components forming the bulk pyrocarbon coating. This abstract describes the latest advances in the development of new characterization methods with high local resolution. O p t i c a l measurement of crystalline anisotropy: This method is based on the property of graphite (and also pyrocarbon) to be pleocroitic or bireflecting. A ceramographic section of the pyrocarbon sample to be examined was illuminated by plane polarized light of vertical incidence. The illuminated spot was circular, with a minimum diameter of 3 μm. The intensity of the light reflected by the sample is measured by a photomultiplier in two different positions of the sample: (1) with the direction of polarization and the direction of preferred orientation of crystallographic c-axes perpendicular; and (2) with both directions parallel. The ratio of these two intensities is directly proportional to the crystalline anisotropy. Using this technique, very marked deviations from the medium value of anisotropy in pyrocarbon coatings were found. Density gradients: Density variations in the pyrocarbon coating on a fuel kernel also influence the neutron irradiation behavior. In general, coatings with a homogeneous density result in the best behavior. Presently, a grinding technique for a large number of fuel particles, e.g., 100, is used to calculate average density gradient profiles. By measuring weight and geometrical data, the density of 5-μm layers just ground off can be calculated. A new technique is presently being developed to do this on individual particles to obtain local densities. Also, some work is being done on correlating microhardness with density. D i s t r i b u t i o n of different carbon components: Past work has shown that these variations in crystalline optical anisotropy and density can be due to the presence of two types of pyrocarbon. One type is well ordered and crystalline with high density, while the other type is disordered or amorphous carbon. Procedures to ’’etch’’ pyrocarbon to reveal these structures have been developed. The experimental work was done on ceramographic sections of coated fuel particles. The polished surface of such a section was treated with either an oxygen plasma (in the case of the plasma—or cold oxidation) or with suitable oxidizing agents (wet oxidation). The polished surface of the section of the spherical pyrocarbon coatings became rough and profiled. The lower the degree of crystallinity, the more intense is the erosion of the material. Thus, a surface profile was produced which was higher in areas with crystalline pyrocarbon and lower where amorphous carbon existed. The surface structure was visible and was photographed by classical microscopic techniques with a resolution of 2 μm. For a more detailed description of some of this work, see H. Nickel, J. Vac. Sci. Technol. 11, 687 (l974).