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With the aim of optimizing the basic properties of stress-relieved Zircaloy-4 cladding tubes, particularly those that make it possible to push back the initial technological limits that may be encountered, and of reducing the scatter of those properties and enhancing tube quality, the role of the main parameters involved in manufacturing the ingot, Trex, and cladding tube has been evaluated on an industrial scale. A series of large-sized tube lots were produced under controlled manufacturing conditions, then characterized by out-of-pile test results (short-and long-term corrosion, stress corrosion cracking (SCC), creep, mechanical, and structural properties) on finished tubes. For the investigated parameters (chemical composition, number of melt, quench rate, accumulated annealing parameter, the ΣA factor, surface condition (outside and inside diameters), and finished tube quality), this role is indeed important but complex due to the highly interactive nature of the variables investigated.
Adjustment of the chemical composition within ASTM limits enables generalized corrosion resistance to be enhanced and irradiation growth to be minimized. A significant decrease of the observed scatter in corrosion and mechanical properties is obtained by optimization of the ΣA range, the quenching rate, and the final heat treatment. The optimum seems to be reached for a final treatment at the highest possible temperature compatible with the stress-relieved state, corresponding to an average precipitate size and ΣA. Moreover, by adding anneals upstream in the process, a further increase in this ΣA no longer seems to have a significant effect on generalized corrosion.
Finally, extensive efforts have been employed in the pilgering, surface preparation (outside diameter polishing, flush-pickling), and examination method (UT, EC) leading to a sizable improvement in SCC resistance and to a reduction in scatter for finished tubes.
The result of these optimizations has been implemented in the current AFA-2G, that shows that under irradiation a 30% corrosion gain is reached after three cycles, without degrading creep strength or growth. The intrinsic effect of tin on generalized corrosion resistance under irradiation was also confirmed on this occasion.
zirconium, zirconium alloys, cladding tubes, process outling, uniform corrosion, quenching, annealing parameter, precipitates, nondestructive testing, in-reactor behavior, nuclear materials, nuclear applications, radiation effects
Consulting engineer, FRAGEMA, Lyon,
Senior scientist, CEZUS, Centre de Recherche d'Ugine, Ugine,
Manager, Research and Development, ZIRCOTUBE, Paimboeuf,