A model has been developed to predict the corrosion behaviour of copper nuclear waste containers in a Canadian disposal vault. The model is based on a finite-difference solution of a series of mass-balance equations that describe the effects of mass transport and various chemical and electrochemical reactions between the container material and the surrounding environment. Each of these reactions is discussed briefly. The corrosion behaviour of the containers is expected to change over time as the environmental conditions within the disposal vault evolve. For copper, the most important environmental parameters affecting the corrosion behaviour are the limited amount of oxidants in the vault and the restrictive mass-transport conditions in the compacted clay:sand buffer material in which the container will be emplaced.
The mathematical formulation of the model is briefly described and the results from preliminary runs are given. As expected, uniform corrosion of the containers effectively stops once all the initially trapped oxygen in the disposal vault is consumed. Dissolved copper from the container is predominantly in the form of Cu(II) species under oxidizing conditions, and slowly transforms into Cu(I) by reaction with Fe(II) assumed to be present in the vault. Predictions of the variation of the corrosion potential of the container with time are also made and, following a criterion for pitting of copper based on a mechanism proposed in the literature, the extent of pitting of the containers is estimated. Lifetimes of 25-mm-thick copper containers subject to uniform corrosion and pitting are predicted to exceed 106 a in a Canadian nuclear fuel waste disposal vault.
