Within the scope of the optimization of the M5 cladding tubes made of ternary alloy (Zr, Nb, O), an extensive program of investigation and industrial development has been undertaken. The various possible factors and potential causes of variability have been thoroughly analyzed with the aid of industrial-scale or laboratory ingots from the point of view of their impact on the finished product properties.
In this way, all the chemical composition variabilities of the alloying elements (Nb, O) and impurities (Fe, S, C) have been studied through variable-composition ingots. Also, a number of manufacturing process variants (number of melts, quench, extrusion, heat treatments, pilgering …) have been studied. In some cases, it was possible to investigate the combined effect of two types of parameters (sulfur-process and iron-process interactions). For each of the products manufactured in this way, systematic characterization of: creep, microstructure (optical microscopy, TEM), corrosion (autoclave) tests was accomplished. In each case, the influence of each variability parameter was tested, and in many cases correlations with the out-of-pile characteristics of the finished tubes were established.
Lastly, for some variables (process, S content, …) the effect of irradiation was more specifically analyzed. These investigations pointed to a new and very important factor, the effect of sulfur concentration on the in-pile operating properties, especially creep and growth.
This set of results constitutes a database covering the whole industrial variability range of this alloy, allowing Framatome to embark upon its industrial development phase and to offer M5 cladding tube on the market.
This product has been irradiated over a wide range of PWR service and environmental conditions in Europe and the U.S. The improvements in corrosion (Factors 3 to 4), hydriding (Factors 5 to 6), and creep and growth (Factors 2 to 3) data after five cycles (55 GWd/tU) show impressive gains over optimized low-tin Zircaloy-4.
