Irradiation Creep and Growth Behavior, and Microstructural Evolution of Advanced Zr-Base Alloys Available to Purchase

This paper deals with the irradiation-induced changes in the microstructure of FRAMATOME advanced Zr-base alloys and the correlations with their irradiation creep and growth behavior. The first part is dedicated to experimental irradiations performed at 280 and 350°C in a CEA metallurgical test reactor (Siloé, 1 to 10¹⁸ n/m² s, E > 1 MeV) on stress-relieved (SRA) and recrystallized (RXA) low-tin Zircaloy-4 and two advanced RXA materials (Alloy 4 (M4): Zr-SnFeV, and Alloy 5 (M5): Zr-NbO), which are proposed for fuel rod cladding applications in PWRs. The irradiation creep results confirm the improved behavior of RXA Zy-4, M4, and M5 in comparison to that of SRA Zy-4. Similar trends are observed for irradiation growth, the SRA Zy-4 exhibiting a quasi-linear behavior with increasing fluence while RXA alloys undergo an early saturation phenomenon. Among RXA materials, M5 has the higher irradiation growth resistance. These creep and growth results at moderate neutron fluences (<5 × 10²⁵ n/m²) are corroborated by the total elongation measurements of SRA Zy-4 and RXA M4 and M5 claddings irradiated up to 5 cycles (≈10 X 10²⁵ n/m²) in a PWR.

The second part relates to the microstructural evolution of these Zr-base alloys irradiated as growth specimens in MTR or claddings (SRA Zy-4, M4, and M5) and guide tubes (RXA Zy-4) in PWRs. TEM examinations were performed with special emphasis on the evolution of dislocation loop microstructures, Fe- and Nb-rich phases, and alloying element redistribution with irradiation temperature and neutron fluence. Moreover, the comparison between samples taken from the higher part of a RXA Zy-4 guide tube irradiated five cycles and the lower parts of M4 and M5 claddings irradiated four cycles provides relevant information for similar irradiation temperatures (≈320 and 330°C) and fluences (≈9.5 and 7.2 × 10²⁵ n/m², respectively). Thus, in M4 the Zr(Fe,V)₂ Laves phases undergo a partial crystal-to-amorphous transformation that is slower and coupled to a significantly lower Fe-redissolution than the Zr(Fe,Cr)₂ Laves phases in Zy-4. Consequently, basal ⟨c⟩-component loops are detected only in the vicinity of intermetallic particles in M4, while they are distributed uniformly throughout the matrix in RXA Zy-4. As for M5, the β-Nb particles undergo neither radiation-induced amorphization nor dissolution, and the Fe content is too low to stabilize a significant density of basal ⟨c⟩-component loops. This excludes the possibility of reaching an accelerated growth regime for the fuel rod burnup achievable in PWR.