The initial crystalline anisotropy and density of the outer pyrocarbon coating of a two-layer Biso fuel particle have a very large effect on calculated diametral changes and coating failure probabilities during irradiation. Of the two, the anisotropy is by far the most important parameter affecting the coating failure probability. These effects arise because of variations in the irradiation-induced dimensional changes, irradiation-induced changes in crystalline anisotropy, and mechanical properties of pyrolytic carbons with these two structural parameters. In support of the stress-analysis model used for these calculations, predicted diametral changes of Biso-coated particles agree well with those observed in irradiation experiments.

Representative production fuel fabricated for the Fort St. Vrain (FSV) high-temperature gas-cooled reactor (HTGR) was tested in capsule F-30. The irradiation conditions experienced by the fuel encompassed the FSV service conditions designed for a 6-yr fuel cycle. Fuel specimens were irradiated at temperatures ranging from 825 to 1250°C (1098 to 1523 K) and to a peak fast-neutron exposure of 9.4 × 1025 n/m2 (E > 29 fJ)HTGR, which is 18% beyond the design FSV peak fast-neutron exposure. In-pile fission gas release measurements and postirradiation examination indicated good irradiation stability of the fuel specimens. The 13 bonded fuel rods were intact, and their irradiation-induced dimensional changes were in good agreement with dimensional change curves used in the FSV core design. Total fuel particle failure fractions determined by visual examination, metallography, and fission gas release measurements were consistent with the criterion of <1% failure at peak exposure conditions assumed in FSV design and licensing evaluations. Fuel performance in the FSV reactor was evaluated using the capsule F-30 irradiation results. The good irradiation behavior of production fuel in this test gives a high degree of confidence in the performance of the FSV core throughout its lifetime and demonstrates the conservative nature of the FSV fuel particle design.

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°C (1273 to 1773 K). Fast-neutron fluences reached 12.4 × 1025 n/m2 (E > 29 fJ)HTGR, which is 55% beyond the large high-temperature gas-cooled reactor peak design exposure of 8.0 × 1025 n/m2. Coatings with densities between 1.85 and 1.95 Mg/m3 and mean optical anisotropy values of ≤1.03 (BAF0 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 µm/min and with densities ≥1.84 Mg/m3 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.

Capsules P13R and P13S were the seventh and eighth in a series of irradiation tests conducted under the ERDA-sponsored HTGR Fuels and Core Development Program. Reference type LHTGR fuel fabricated with a broad spectrum of property and process variables was irradiated to extreme temperature and fluence conditions. Postirradiation examination revealed that the bonded fuel rods exhibited good stability after irradiation to fast neutron fluences of 12.4 x 10/sup 21/ n/cm/sup 2/ (E greater than 0.18 MeV), which is 55 percent beyond the LHTGR peak design fast neutron fluence of 8.0 x 10/sup 21/ n/cm/sup 2/. Thermal cycling to high temperatures did not adversely affect fuel rod integrity. Particle batches with coating designs representative of the design requirements envisioned for the LHTGR exhibited excellent irradiation performance. Ten batches of fissile and fertile particles were irradiated without coating failure to fast neutron exposures which exceeded the LHTGR peak design exposure by 35 to 52 percent. Capsules P13R and P13S were considered to be very successful qualification tests of LHTGR fuel components. These results provided a substantial data base for the LHTGR Fuel Product Specification and Performance Models used in HTGR core design studies, and demonstrated the excellent irradiation performance of reference LHTGR fuel to well beyond peak design exposures.

The performance of TRISO-coated carbide fissile particles, of the type to be used in the large HTGR, correlates well with statistically based calculations of stresses in the SiC coating. Three coated particle batches, containing a total of nearly 104 individual coated particles, showed insignificant coating failure (≤0.2%) after exposure to essentially the most severe combined conditions of fast neutron exposure, burnup, and temperature to be experienced by fuel in a large HTGR. This high reliability derives from the fact that less than 1% of the particles in each batch had SiC tensile stresses greater than 30 000 psi, while the SiC layer in about 80% of the coated particles in each batch remained in compression throughout life. Two additional experimental batches of TRISO-coated carbide fissile particles had thinner coatings that resulted in higher mean SiC stresses in each batch and in probabilities of SiC coating stresses greater than 30 000 psi of 3.5 and 8.5%. This compares with the observed incidence of coating failure during irradiation to full design exposures of about 4% in both cases. These results provide further confirmation of the value of analytical stress models in interpreting the results of coated particle irradiation experiments, and emphasize the importance of a statistical approach to coated particle design.