Stereological Evolution of the RIM Structure in PWR Fuels at Prolonged Irradiation: Dependencies with Burn-up and Temperature

The stereology of the rim-structure was studied for PWR-fuels up to the ninth irradiation cycle, achieving maximum local burn-ups of 240 GWd/tM and beyond. At intermediate radial positions (0.55 < r/r 0 < 0.7), a small increase of the pore and grain size of recrystallized areas was found, which is attributed to the increase of the irradiation temperatures in the outer half-pellet-radius due to deterioration of the thermal conductivity. In the rim-zone marked pore coarsening and pore-density-drop occur on surpassing the local burn-up of 100 GWd/tM, associated with cavity fractions of ≈0.1. Above this threshold the porosity growth rate drops and stabilizes at a value nearing the matrix-gas swelling-rate (≈0.6%/10 GWd/ tM). The rim-cavity coarsening shows ingredients of both Ostwald-ripening and coalescence mechanisms. Despite individual pore-contact events, no clusters of interconnected pores were observed up to maximum pore fractions checked (≈0.24). The rim-pore-structure is found to be well represented in its lower bound by the model system of random penetrable spheres, with percolation threshold at Φ c = 0.29. Rim-cavities are expected to remain closed at least up to this limit.

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