Effects of High Neutron Fluences on Microstructure and Growth of Zircaloy-4

Abstract

Irradiation of Zircaloy affects its microstructure and macroscopical properties, for example, influencing its irradiation growth. To gain more insight into these phenomena, experimental fuel rods and growth specimens with various fabrication parameters were irradiated in a pressurized water reactor (PWR) to high fluences. Some of the growth specimens were exposed to a fast neutron fluence of up to 2.3 × 1022 cm-2 (⩾0.82 MeV) over a period of 10 years. Following exposure, the irradiation-induced alterations of the microstructure and the intermetallic precipitates were studied by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). At a temperature of 300°C during irradiation to fluences up to 7 × 1021 cm-2, growth increases with increasing yield strength. Recrystallized material, which has a low yield strength, exhibits an increased growth rate at very high fluences (⩾1 × 1022 cm-2). Postirradiation annealing studies indicate that the early irradiation growth of the recrystallized material can be recovered, whereas the later accelerated growth does not seem to be recoverable. At temperatures in the range of 330 to 350°C, the growth depends on the grain size, especially below 2 μm, and on the carbon-content. The effect of yield strength on growth was less at 330 to 350°C than at 300°C, probably because of an irradiation-induced recovery. Moreover, TEM showed that an irradiation-induced formation of dislocations with a c-component occurs at neutron fluences ⩾9 × 1021 cm-2, whereas dislocation loops or fine precipitates or both form at lower fluences. The intermetallic precipitates observed in the microstructure of the unirradiated, initial material exhibit two types of intermetallics. One type contains (Fe + Cr + Zr) and the other type contains only (Fe + Zr). The effect of irradiation on those intermetallics depends on temperature. At ⩽300°C the (Fe + Zr)-type intermetallic precipitate dissolves at neutron fluences above 5 × 1021 cm-2. The (Fe + Cr + Zr)-type precipitates become more and more amorphous and release iron to the matrix resulting in a decreasing Fe/Cr ratio. The diameter and the number of the precipitates decrease with increasing neutron fluences at this temperature. Only a few small precipitates can still be observed after a neutron fluence of 1.5 × 1022 cm-2. At temperatures above 340°C the size of intermetallics increases because of irradiation enhanced ripening.

Radiation Effects in Zirconium Alloys

“…It can be noticed that the stationary growth strain of the polycrystal is higher than that for the Zr single-crystal [299] suggesting an effect of the grain boundaries on the growth mechanisms. For higher fluence, higher than 5× 10 25 n m -2 , a growth breakaway is observed, yielding a high growth rate, similar to that of SRA zirconium alloys (Figure 36) [304] [298]. This phenomenon is also observed for recrystallized Zr1%Nb and Zr-Nb-Sn-Fe alloys as reported by Kobylyansky et al [115] [118].…”

“…The stress free growth is the actual mechanism of interest here. Whereas the fuel rod growth is not only due to irradiation growth (stress free growth), but anisotropic creepdown and, at higher fluences, the pellet cladding mechanical interaction, also contribute [304] [309] [310].…”

Radiation Effects in Zirconium Alloys

“…It can be noticed that the stationary growth strain of the polycrystal is higher than that for the Zr single-crystal [299] suggesting an effect of the grain boundaries on the growth mechanisms. For higher fluence, higher than 5× 10 25 n m -2 , a growth breakaway is observed, yielding a high growth rate, similar to that of SRA zirconium alloys (Figure 36) [304] [298]. This phenomenon is also observed for recrystallized Zr1%Nb and Zr-Nb-Sn-Fe alloys as reported by Kobylyansky et al [115] [118].…”

Polycrystalline simulations of in-reactor deformation of recrystallized Zircaloy-4 tubes: Fast Fourier Transform computations and mean-field self-consistent model

“…Rogerson (Rogerson, 1988) provides growth data for recrystallized Zy-2 (with grain size of 20 µm and axial Kearns factor of 0.1) at 280°C under a neutron flux between 4.1 and 10.8×10 17 n.m -2 .s -1 in the DIDO reactor. Garzarolli et al (Garzarolli et al, 1989) provide growth data for recrystallized Zy-4 (grain size 7 µm, axial Kearns factor 0.046) at 295°C in a PWR under a typical neutron flux of 6 to 10×10 17 n.m -2 .s -1 . Finally, Holt and Gilbert (Holt and Gilbert, 1986) and Griffiths et al (Griffiths et al, 1995) provide other data obtained on guide-tubes made of recrystallized Zy-4 at temperatures ranging from 287°C to 311°C under a typical flux of 6 to 10×10 17 n.m -2 .s -1 in PWR reactors, with similar grain size and Kearns factors.…”