Investigating the Corrosion Behaviour of Zirconium Materials in Simulated Groundwater

Abstract

The internationally accepted solution for the long-term storage of used nuclear fuel is to isolate the fuel bundles underground in a deep geological repository (DGR). In the Canadian multi-barrier approach, the Zry-based fuel bundles will be sealed in copper-coated carbon steel used fuel containers (UFC), and these will be placed underground and encased in highly compacted bentonite clay [1]. In the unlikely event of a UFC failure, the groundwater from the host rock has the potential to fill the UFC, leading to the fuel bundle being exposed to potentially corrosive groundwater. While extensive literature investigates the long-term performance of materials used in the multi-barrier system, there is limited knowledge of the corrosion rate and mechanism of the Zr fuel cladding under DGR conditions [2]. Previous literature exploring the corrosion behaviour of Zr materials under reactor coolant conditions concluded that these materials become more corrosion-resistant with increasing oxide film thickness [3]. However, groundwater contains high concentrations of Cl¯, which has the potential to cause localized corrosion of the passive Zr fuel cladding [4]. This work investigates the corrosion properties of three Zr materials under conditions relevant to a DGR. Each Zr material was exposed to a naturally aerated simulated groundwater solution, representative of groundwater found in crystalline rock in Canada, at room temperature for three to seven days.

The influence of each material's composition on its corrosion properties was investigated using electrochemical measurements coupled with surface analysis. It was determined that all three zirconium- based materials tested (pure zirconium, Zircaloy-2 and Zircaloy-4) exhibited passive behaviour when exposed to an aerated simulated groundwater solution. Zircaloy materials had higher corrosion resistance and were less susceptible to localized corrosion than pure zirconium. The results from this work provide insight into the potential barrier effect of the Zr cladding, potentially preventing the release of radionuclides into groundwater.