Predicting the Permeability and Relative Permeability of Concrete

Abstract

France has today reassessed the safety level of its power plants due to the Fukushima incident. The duration of the cooling system breakdown urged the industry to consider a much longer worst case scenario than was done before. The time frame for the ’severe’ accident subsequently went up from 24h to two weeks. This, in turn, called for an extensive study of creep and vapour transport issues that had previously been left aside. This was the objective of the MACENA project, which aims at evaluating the tightness of a containment vessel under extreme conditions, namely a 5 bar water vapour pressure and 180C temperature during two weeks. Among the scientific issues involved in this problem, the intrinsic permeability of concrete, its relative permeability to vapour and to water and their subsequent evolution upon damage are the subject of this contribution. Our analysis starts from the work of Khaddour et al. on the impact of damage on both intrinsic and apparent permeability with the help of a random hierarchical model based on the pore size distribution of the material. This model yielded good results in terms of the evolution of permeability with damage. Here, we extend this model to the description of multiphase fluid flow. Modifications of the above-mentioned random hierarchical model involve a set of rules that capture the effect of condensation as described by Kelvin’s law. In addition, the capillary assembling process is modified in order to better account for the influence of capillaries of very small diameter, which had no importance on the prediction of the intrinsic permeability but have a strong influence on the relative permeability to vapour. Segments of capillaries of small diameter are redistributed in downstream ends of the bundle according to a probability distribution that has the same shape as the pore size distribution. The extended model is shown to provide rather accurate prediction of relative permeability for a wide range of geomaterials (from mortar to tight and permeable rocks).