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Release behavior of metallic fission products from the Triso-coated UO 2 particles was studied by postirradiation heating tests in the temperature range 1600 to 1900°C (1873 to 2173 K) and subsequent postheating examinations. The fission gas release monitoring and the postheating examinations revealed that no pressure vessel failure occurred in the tests. Ceramographic observations showed no palladium attack and thermal decomposition of SiC, 137 Cs, 134 Cs, 110m Ag, 154 Eu and 155 Eu were released from the coated particles through the coating layers during postirradiation heating. The diffusion coefficient of 137 Cs in the SiC layer was evaluated from the release curves based on a simple diffusion model assuming a one-layer coated particle. Fractional release measurements suggested that the diffusion coefficient of 110m Ag in SiC be larger than that of 137 Cs.

This paper reports on the ZrC-coated UO[sub 2] particle which is a promising fuel for high-temperature gas-cooled reactors. Particle fuels with multiple layers of pyrolytic carbon and ZrC have been irradiation-tested to a maximum fast-neutron fluence exceeding 2 [times] 10[sup 25]/m[sup 2] (E [gt] 29 fJ). In-reactor fission-gas release measured at a burnup of 1.5 at.% was minimal. The failure fraction by postirradiation examination was null for all samples. The ZrC-coated particles at 4 at.% burnup were postirradiation heated to 2400[degrees]C/min without failure until after 6000 s at the maximum temperature. It was found that the ZrC layer could sustain a large strain at such high temperatures. The behavior is in strong contrast with that of SiC of standard Triso coating, which is brittle to very high temperatures.

In this paper a model to predict the ultimate failure of TRISO-coated fuel particles under hypothetical core heatup events is proposed. Features of the model include the ability to treat the statistical variation of the number of coated fuel particles and to make a thermodynamic estimation of the stoichiometry of irradiated UO{sub 2} kernels and the equilibrium CO pressures. The model predictions agree well with the results of postirradiation heating tests. The thermal creep of pyrolytic carbon, however, must be taken into account to further improve the accuracy of the prediction.