Evolution of Microstructure in Zirconium Alloys During Irradiation

Abstract

X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to characterize microstructural and microchemical changes produced by neutron irradiation in zirconium and zirconium alloys. Zircaloy-2, Zircaloy-4, and Zr-2.5Nb alloys with differing metallurgical states have been analyzed after irradiation for neutron fluences up to 25 × 1025 n.m-2 (E > 1 MeV) for a range of temperatures between 330 and 580 K. Irradiation modifies the dislocation structure through nucleation and growth of dislocation loops and, for cold-worked materials in particular, climb of existing network dislocations. In general, the a-type dislocation structure tends to saturate at low fluences (< 1 × 1025 n.m-2). The c-component dislocation structure, however, may evolve over long periods of irradiation (for fluences >10 × 1025 n.m-2 in some cases). The phase structure is also modified by irradiation. The common alloying/impurity elements, Fe, Cr, and Ni, are relatively insoluble in the α-phase but are dispersed into the α-phase during irradiation irrespective of the state of the phase initially containing these elements, i.e., metastable β-phase or stable intermetallic precipitate. The stable intermetallic particles may undergo structural changes dependent on their composition and the temperature. For the metastable dual-phase α/β-alloys (Zr-2.5Nb alloy), the β-phase structure is modified during irradiation, but the change is complex, being a combination of thermal decomposition and radiation-induced mixing.

Nano-scale chemical evolution in a proton-and neutron-irradiated Zr alloy

“…These samples were prepared from cladding and channel material of a BWR (280-330 °C [27]), irradiated to neutron fluences between 8.7 and 14.7 x10 25 n m -2 (E > 1 MeV) ~14.5 to 24.5 dpa, assuming a conversion of 0.6 x10 25 n m -2 dpa -1 [28]. As the damage rate in light water reactors is ~6-1 x10 17 n m -2 s -1 [28][29][30], the BWR damage rate is ~1 x10 -7 dpa s -1 . As such, the proton irradiations in the present work incurred a damage rate higher than that of the neutronirradiated material by a factor of ~70.…”

Performance of Pressure Tubes in Candu Reactors

“…Irradiation of hexagonal close-packed grains in Zr-alloys results in the formation of a high density of a-type dislocation loops [15,42]. In general, the a-type dislocation density increases rapidly at low fluences (<1 × 10 25 n·m −2 ), and there is little further change in the a-type dislocation for higher fluences.…”

“…Dislocation density and decomposition of the beta-phase can affect hydrogen diffusion and DHC growth [10,14], so a measure of the state of these variables is required. The dislocation density can be correlated with X-ray line broadening profiles [15]. Using the same instrumentation, the lattice parameter of the body-centered cubic beta-phase (a measure of the betaphase decomposition) is also indicative of the effect of irradiation on the microstructure.…”

“…Figure 7 (upper) shows the correlation between dislocation density (represented by integral breadth) and axial DHC growth rate for CANDU reactor pressure tube 1790. Figure 7 (lower) shows %Nb in the beta-phase (one derived measure of the beta-phase decomposition [15]) and its correlation with axial DHC growth rate. For personal use only.…”

“…In general, the a-type dislocation density increases rapidly at low fluences (<1 × 10 25 n·m −2 ), and there is little further change in the a-type dislocation for higher fluences. The c-component dislocation structure, however, evolves at a slower rate, over longer periods of the irradiation [8,9,15]. The integral breadth can be used to show trends in line broadening behavior that represent changes in dislocation densities.…”