Copper canisters for the final disposal of spent nuclear fuel are expected to be subject to general (uniform) corrosion and a limited degree of localised attack. Although the canister is expected to be exposed to predominantly active conditions, it is nevertheless prudent to assess the probability of pitting corrosion in passive conditions. Pitting is an inherently stochastic process which makes a probabilistic approach appropriate. In addition, there are other sources of uncertainty and variability to take into account, such as the variability in the canister surface environment from one deposition hole to another and uncertainty in the conditions that produce a passive surface. Here we describe the development of a probabilistic pitting model for aerobic conditions, in a water saturated bentonite buffer. The aims of the work are to develop the methodology for probabilistic pitting assessments and to apply that methodology to this particular set of environmental conditions. In doing so, we acknowledge that aerobic, saturated conditions may be unlikely to occur if saturation is slow or O2 consumption is fast, but we have chosen these conditions as a starting point and because of the availability of input data for the model. We intend to apply a similar methodology to the assessment of localised corrosion under aerobic, unsaturated conditions in the future.
The probabilistic pitting model is based on a Monte Carlo approach where the pH, chloride, sulphate and carbonate concentrations at the canister surface are selected from defined distributions. Each Monte Carlo simulation represents the time evolution of an individual deposition hole with the selected environmental parameters. The evolution of the near-field environment is simulated based on the time-dependent canister surface temperature (which also determines the solubility-limited concentrations of sulphate and carbonate) and an assumed oxygen depletion rate. A single run or realization represents the evolution of the pitting behaviour of a single canister over a period of 100 years, the presumed maximum duration of the aerobic phase. One million Monte Carlo realizations were run to capture the variability and uncertainty of the disposal environment and the pitting characteristics.
The criteria for pit initiation are that the canister surface is passive due to the oxide film, and that the value of the corrosion potential ECORR exceeds the breakdown potential EB. If a pit initiates, propagation continues for the period that ECORR exceeds the re-passivation potential ERP. Qin et al. (2017) conducted a comprehensive survey of the effects of temperature, pH, and of chloride, sulphate, and carbonate concentrations on the active/passive behaviour of copper. Using the Qin et al. (2017) data set for defined environmental conditions, a classification algorithm was trained for predicting active/passive behaviour of environmental conditions expected in the Swedish deep geological repository. Training for breakdown potential and repassivation potentials used a statistical analysis on the Qin et al (2017) dataset which also included these values for the subset of the experiments with a passive surface. The corrosion potential is calculated from King et al. (1995) including dependence on temperature, oxygen and chloride concentrations. Pit depths are calculated on the basis of empirical pit-growth rates and are tracked over all Monte Carlo simulations to determine pit depth distribution and other statistical properties. Similarly, the number of active/passive simulations are tracked and the results show that approximately 90% of Monte Carlo realizations are active with the rest being passive and an even smaller subset resulting in pitting.
References:
King, F., Litke, C. D., Quinn, M. J., and LeNeveu, D. M. (1995). The measurement and prediction of the corrosion potential of copper in chloride solutions as a function of oxygen concentration and mass-transfer coefficient. Corrosion Science, 37(5), 833-851.
Qin, Z., Daljeet, R., Ai, M., Farhangi, N., Noël, J. J., Ramamurthy, S., Shoesmith, D., King, F., and Keech, P. (2017). The active/passive conditions for copper corrosion under nuclear waste repository environment. Corrosion Engineering, Science and Technology, 52(sup1), 45-49.
active/passive corrosion, copper pitting, Monte Carlo, bentonite