Abstract
Properties affecting the irradiation performance of outer pyrolytic carbon (PyC) layers on Triso- and Biso-coated fuel particles were studied. Irradiation temperatures were 1000 to 1500/sup 0/C (1273 to 1773 K). Fast-neutron fluences reached 12.4 x 10/sup 25/ n/m/sup 2/ (E greater than 29 fJ)/sub HTGR/, which is 55 percent beyond the large high-temperature gas-cooled reactor peak design exposure of 8.0 x 10/sup 25/ n/m/sup 2/. Coatings with densities between 1.85 and 1.95 Mg/m/sup 3/ and mean optical anisotropy values of equal to or less than 1.03 (BAF/sub 0/ units) exhibited the best irradiation performance on Triso particles. For Biso particles, it is necessary to deposit the outer layer at coating rates between 3 and 8 ..mu..m/min and with densities equal to or greater than 1.84 Mg/m/sup 3/ to produce coatings impermeable to fission gases after irradiation. Data from fuel rod tests show that it is important to limit the degree of surface-connected porosity of the outer PyC layer and the amount of binder phase in the matrix to prevent coating failures resulting from coating-matrix interactions.
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