High-Fluence Irradiation Growth of Cold-Worked Zr-2.5Nb

Abstract

Irradiation growth specimens manufactured from cold-worked Zr-2.5Nb pressure tube material have been irradiated in Osiris at a fast flux of ∼1.8 × 1018 n · m-2 · s-1 E > 1 MeV, at nominal temperatures of 553 and 583 K, to growth strains of 1%. The pressure tubes have a pronounced crystallographic texture, with ∼95% of the basal plane normals in the radial/transverse plane, predominantly in the transverse direction. Both longitudinal specimens, which generally exhibit positive growth strains, and transverse specimens, which generally exhibit negative strains with approximately 50% of the magnitude of the axial strains, show nonlinear growth, the rate increasing gradually with fluence up to 1.3 x 1026 n · m-2, E > 1 MeV (580 K) and 1.7 × 1026 n · m-2, E > 1 MeV (550 K). The previously reported temperature dependence of the axial growth strains (decreasing with increasing temperature for fluences up to 4 to 5 × 1025 n · m-2, E > 1 MeV) is maintained to the higher fluences now achieved, as is the effect of Fe (axial growth strains decreasing with increased Fe concentration in the range 380–1090 ppm by wt.). The effects of temperature and Fe in the transverse direction are approximately the reverse of those in the longitudinal direction. Examination by transmission electron microscopy and X-ray diffraction of Zr-2.5Nb pressure tubes irradiated to 1 × 1026 n · m-2, E > 1 MeV, at 520 to 570 K in power reactors, and fracture toughness specimens machined from pressure tubes and irradiated, unstressed, in Osiris to 1.5 × 1026 n · m-2, E > 1 MeV, at 530 K, shows a multiplication of the c-component dislocations with increasing fast fluence. This contrasts with a rapid saturation of the density of a-type dislocations after about 0.1 × 1026 n · m-2, E > 1 MeV. The multiplication of the c-component dislocations occurs mainly by nucleation of vacancy loops on screw dislocations; this mostly occurs in crystals with their c-axes close to the radial direction because they tend to contain many c-component screw dislocations. Crystals with their c-axes close to the transverse direction contain fewer screw dislocations and exhibit less dislocation multiplication over the fluence range studied. Thus, the gradual evolution of the growth rate with fluence appears to be related to the gradual increase in the c-dislocation density with time as in the case of Zircaloy-2.