Effect of microstructural evolution on in-reactor creep of Zr–2.5Nb tubes

Point defect properties in hcp and bcc Zr with trace solute Nb revealed by ab initio calculations

“…These results suggested that Nb atoms tend to migrate to the beta phase Zr from alpha phase Zr if some beta Zr grains are formed in the Zr-Nb alloy under irradiation, and that Nb atoms are more likely to agglomerate in beta Zr than in alpha Zr, supposing the Nb atoms could get enough 'activate energy' under irradiation. This is consistent with the experiment analysis [12] that the thermal diffusion effect facilitates the supersaturated Nb atoms in the a-Zr grains to move to the b-Zr, transforming it to the b-Nb. It is also seen from Table 3 that the formation energy of vacancy in beta Zr is 0.36 eV, while it is 1.94 eV in alpha Zr.…”

“…Another mechanism was that some grains of alpha Zr transformed into beta Zr under irradiation and niobium atoms took the position of the beta Zr atoms to form a beta Nb structure. Experimental results have proved the existence of [8][9][10][11][12], lending a support for the possibility of the second mechanism. Nevertheless, no direct experimental evidence has been found to support or deny the first mechanism.…”

“…This results in annihilation of the Frenkel pair with alien atom to form the substitutional Nb atom, accompanying with the decrease of the system energy of 3.82 eV as shown as the binding energy in Table 2. It would be interesting to know the formation and binding energies of simple defects in a beta phase Zr, as the formation of beta phase Zr with about 20-88% Nb under irradiation conditions was observed in the experiments [8][9][10][11][12]. The calculated values in beta phase Zr are given in Table 3.…”

Anisotropic delayed hydride cracking velocity of CANDU Zr–2.5Nb pressure tubes

“…To understand the decomposition behavior of the b-Zr with the operating temperature and fast neutron fluence, the Nb concentration in the b-Zr grains, as a measure of the b-Zr decomposition [17], were determined at the following three locations of the irradiated Zr-2.5Nb tube that was discharged after a 10 year operation in a reactor: the inlet, the middle and the outlet regions. As shown in Table 1 [18], the outlet and inlet regions were exposed to the highest and lowest temperatures, respectively, while the middle region had the highest neutron fluence (E > 1 MeV). To examine the Nb concentrations in the b-Zr grains with a higher confidence, a carbon replica method was applied to extract the b-Zr grains only, as shown in Fig.…”

“…In other words, the outlet region exposed to the highest temperature had the highest Nb concentration of 69 at.% Nb in the b-Zr due to a thermal decomposition, possibly leading to a fully discontinuous distribution of the b-Zr as observed on the unirradiated tubes by thermal annealing [9,16]. In contrast, the inlet region had little thermal decomposition of the bZr but it rather had the a-Zr grains supersaturated with the Nb atoms due to radiation enhanced dissolution [18,19]. Thus, it seems that the distribution of the b-Zr grains is maintained as semi-continuous at the inlet region, and becomes more discontinuous from the inlet towards the outlet region.…”