Volume 3, Issue 1 (January 2006)
Effect of Alloying Elements and Impurities on in-BWR Corrosion of Zirconium Alloys
The data base on the corrosion behavior of Zr alloy materials under BWR conditions was evaluated with respect to the burnup target of 70 MWd/kgU. At high burnups, corrosion rate and the rate of hydrogen pickup (HPU) may increase. This onset of increase obviously depends on the material, but also seems to be significantly affected by the coolant water chemistry. Because small differences in corrosion behavior at lower burnup might become more and more important with increasing burnup, Framatome ANP has performed several studies on the separate and combined effects of (1) alloying content of the claddings, (2) cladding material condition, (3) impurity content of the cladding, and (4) the coolant chemistry. This paper focuses on the effects the concentration of alloying elements and of impurities (including microstructural differences imposed by the annealing treatment) have on corrosion. The corrosion effects were evaluated in material test irradiation programs in two BWRs. Zircaloy type materials processed at low temperatures (LTP), defined by a low particle growth parameter (PGP) value, exhibit a maximum corrosion resistance between 1.2 and 1.5 % Sn. Impurities, such as C, O, and P can increase the corrosion of Zircaloy in BWRs at high burnup. The higher the corrosion resistance of the base material, the more pronounced is the increase seen at high burnup. Above a critical PGP value, in-pile corrosion increases. At high burnups, Zry-4 shows a higher increase with increasing PGP than Zry-2, whereas at lower burnups both behave similarly. The critical PGP value varies with the chemical composition, such as Fe, Cr, and Ni content and the distribution of second phase particles (SPP). The effect of Si is more complex. Si increases in-pile corrosion at contents in excess of 140 ppm. Contents at 80 to 140 ppm can be beneficial, when the β-quench rate applied during fabrication is not high enough to ensure a uniform distribution of the SPP, and the alloying composition and the concentration of impurities is in a beneficial range. The hydrogen pickup fraction (HPUF) of Zircaloy type samples in BWRs decreases with decreasing corrosion resistance but differs from plant to plant. There are indications that the difference can partially be attributed to the Fe content in the coolant.
The results are in agreement with the irradiation experience with Zry-2 LTP cladding extending up to 73 MWd/kgU in different BWRs.