PRECIPITATE BEHAVIOR OF 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 con tain numerous small precipitates. Some of these undergo substantial ch anges in composition, crystalline structure, and size during in-reacto r exposure. Data on these phenomenon have been obtained using the scan ning transmission electron microscope (STEM) to study precipitates bef ore and after reactor exposure. Results on the following alloys are re ported here: Zircaloy-2 (Zr-1.5Sn-0.15Fe-0.10Cr-0.05Ni), Zry-0.2Nb (Zi rcaloy-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 va rious precipitates is apparent from direct compositional measurements. The dissolution of Cr can be inferred in Zry-0.2Nb from Cr: Nb gradie nts in amorphous Cr-Fe-Zr-Nb precipitates and from the formation of in tergranular, Cr,Fe-rich precipitates during postirradiation annealing. Two types of phase change have been seen. First, crystalline phases c an 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 gr eatest changes were measured in small, Cr,Fe-rich precipitates.