Effect of gamma radiolysis on pit initiation of zircaloy-2 in water containing sea salt

Abstract

In spent fuel pools at the Fukushima Daiichi Nuclear Power Station (1F), seawater was injected for cooling purposes after the tsunami disaster in March 2011. It is well known that the chloride in the seawater has the potential to cause localized corrosion (e.g., pitting corrosion) in metals. In this study, we evaluated the pitting potentials of zircaloy-2, the material used in the fuel cladding tubes in 1F, as a function of chloride concentration. To accomplish this, we used artificial seawater under gamma-ray irradiation and investigated the effect of radiolysis on pit initiation of zircaloy-2 in water containing sea salt. Changes in the composition of water containing sea salt were analyzed as well, both before and after gamma-ray irradiation. The characteristics of the resultant oxide films formed on zircaloy-2 were evaluated by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The experimental results showed that the pitting potential under irradiation was slightly higher than that under conditions in which no radiation was present, and that the pitting potential decreased with increasing chloride concentration in the presence as well as the absence of radiation. Solution analyses for water containing sea salt showed that hydrogen peroxide was generated by irradiation. The oxide film was composed of zirconium oxide and was made thicker during the irradiation. The higher pitting potential could thus be explained by the capacity of hydrogen peroxide to oxidize the surface and enhance oxide film formation. Under gamma-ray irradiation, the zircaloy-2 surface with an oxide film formed by radiolysis products was found to be resistant to pitting in the presence of chloride.

Effects of seawater components on radiolysis of water at elevated temperature and subsequent integrity of fuel materials

“…According to a previous study, at several mol·dm −3 Cl − and high absorbed dose, the only long-lived product originating from NaCl is ClO 3 − (concentration in the order of 10 −3 mol·dm −3 ) and HClO and ClO 2 − concentrations were near the detection limit (10 −5 mol·dm −3 ) [4]. A simulated seawater experiment at room temperature also showed that no Cl − and Br − induced products were found after a 100 h irradiation at 5 kGy h −1 [11]. In this study, since the concentration of Cl − was much lower than in this report, radiolytic products originating from seawater components in the autoclave were negligible compared to the considerable amounts of Cl − which are present in seawater.…”

“…Venkateswaran [10] reported that galvanically decoupled zircaloy-2 without irradiation will not undergo pitting attack. Also, Motooka et al [11] reported that Cl − was the main aggressive species and oxide formation was promoted by irradiation in the pitting test for zircaloy-2 in simulated seawater at room temperature. However, both these studies were conducted at lower temperatures and without the galvanic coupling which actual fuel assemblies have.…”