Precipitate behavior in zirconium-based alloys in BWRs

The metallic elements Cr, Fe, Ni, Nb, and Mo have low solubilities in zirconium, so that zirconium-based alloys made with these elements contain numerous small precipitates. Some of these undergo substantial changes in composition, crystalline structure, and size during in-reactor exposure. Data on these phenomenon have been obtained using the scanning transmission electron microscope (STEM) to study precipitates before and after reactor exposure. Results on the following alloys are reported here: Zircaloy-2 (Zr-1.5Sn-0.15Fe-0.10Cr-0.05Ni), Zry-0.2Nb (Zircaloy-2 with 0.2Nb), NoCr (Zr-1.5Sn-0.17Fe-0.17Ni), NSF-2 (Zr-1Nb-1Sn-0.2Fe), and XLL (Zr-1.5Sn-0.3Nb-0.3Mo). The dissolution of Fe from various precipitates is apparent from direct compositional measurements. The dissolution of Cr can be inferred in Zry-0.2Nb from Cr:Nb gradients in amorphous Cr-Fe-Zr-Nb precipitates and from the formation of intergranular, Cr,Fe-rich precipitates during postirradiation annealing. Two types of phase change have been seen. First, crystalline phases can become amorphous, as with hexagonal Laves Zr(Fe,Cr)2 in Zircaloy-2 and cubic Fe(Zr,Nb)2 in NSF-2. In the second type, long-range order is eliminated, as with Laves (Mo,Nb)2Zr precipitates in XLL which become BCC. The precipitate size distributions can change in reactor. The greatest changes were measured in small, Cr,Fe-rich precipitates. (orig.)