Shadow corrosion: Experiments and modeling

Abstract Shadow corrosion has long been an issue in BWRs (Boiling Water Reactors): the use of Ni-based grids in close connection to Zry-2 cladding induces locally an accelerated corrosion which can in some instance lead to low remaining metal thickness at the contact point. It is understood since at least 20 years that it is a galvanic coupling phenomenon. However, some important aspects are still debated, due in particular to the fact that it cannot be reproduced without irradiation. Under (neutron) irradiation and at reactor temperature, several parameters are not known accurately (for example the electrical conductivity of the coolant and of the oxide, or the Open Circuit Voltage and current of the metal parts). The role of the contact point between the grid and the cladding is also poorly understood. To improve our understanding, a photoelectrochemical cell, simulating the conditions of Light Water Reactors was designed, developed and validated. Electrochemical methods, performed in-situ, were used to study the electrochemical behavior of Zr-based and Ni-based alloys. Out of pile experiments with or without UV light were performed to measure the OCV (Open Circuit Voltage), the exchange current densities as well as the Tafel slopes (transfer coefficients) of Ni- and Zr-electrodes. The influence of the irradiation flux allowed extrapolations in order to attempt to assess what could happen under neutron irradiation. A finite element modeling based on the Butler-Volmer equation allowed to take simultaneously into account the thermal and galvanic corrosion. Thermal feedback due to the thermal insulating property of the oxide is also included. Parametric studies allowed deriving order of magnitudes of the unknown parameters. To our knowledge, this is currently the most advanced shadow corrosion modeling.