Abstract. The actual tendency all over the world is to manufacture fuel bundles capable to resist high bumup. In literature are presented theoretical and experimental data concerning confining factors of burn-up extinction for LWR [1] and CANDU fuel [2], [3], [4]. The factors affecting the burn-up increase are: the pellet-cladding mechanical interaction (PCM1), the oxidation and hydriding of the Zircaloy-4 sheath, the increase of internal pressure, stress corrosion cracking, Zircaloy-4 irradiation growth, fuel swelling...

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