[en] Zircaloy sheathed ThO₂ - 2 wt% UO₂ pellets, which had pre-formed cracks and holes, were irradiated for 69 days at a power rating of ∫λdθ = 61 W/cm. Central temperatures were 2400 ± 200^oC, based on an estimated λ = 0.031 W/cm deg C. Where circumferential crack widths had been large the diametral expansion of the sheath was low, indicating that the hot fuel was plastic and could move into the voidage. Cracks and axial holes were eliminated above an estimated temperature of 1700 ± 150^oC, compared with 1400 ± 100^oC for UO₂ : both temperatures are 0.55 of the absolute melting point. There was no sublimation across any crack; other possible crack healing mechanisms, e,g. creep, surface diffusion, grain boundary sliding have been considered. Compared with UO₂ irradiated a t the same power per unit length, the thoria fuel had less diametral expansion, less grain growth: autoradiographs showed essentially no fission product relocation. The presence of a circumferential crack near the periphery throughout the irradiation did not significantly impede heat transfer. (author)