Effect of Alloying Elements and Impurities on in-BWR Corrosion of Zirconium Alloys

Abstract

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.

Optimization of a fast INAA method for determination of trace concentrations of chlorine in zirconium based alloy and oxide samples

“…If there is any excess or less concentration with respect to required specifications, it adversely affects its properties as well as durability of the alloys in the power reactors. The trace impurities which are undesired in zircaloys are O, C, Si, P and Cl [1]. Chlorine is considered a detrimental impurity in the zircaloys, because even at trace level it can cause the local depassivation of the oxide film on the surface of the zircaloy clads leading to corrosion and eventual failing of the cladding [2].…”

Modeling of H(n,n) Recoil Proton Injection into LWR Fuel Cladding with Sequential Use of MCNP and SRIM Codes

“…By out of pile tests, correlation has been found between thickness of oxide layer on outer surface of the fuel rods and the hydrogen content in the Zircaloy claddings. Accordingly, they consider major process of the H absorption is (1) in reactor coolant water (moderator) and sequential absorption of the extra H atoms in the Zircaloy. Radical species yielded by ionization of H 2 O by γ rays and neutrons are also considered to enhance the H absorption process described above.…”