[en] A new set of model functions have been developed for activity build-up on system surfaces in BWRs. The model work has resulted in (A) a combination of traditional water phase equilibriums and a surface complexation sub-model for cation adsorption on oxide surfaces and (B) a sub-model for diffusion of corrosion product metals and activated nuclides in the passive oxide layers. The model principles laid down in this work should make it possible to explain several of the chemical influences on activity buildup through basic chemistry and physics, e.g. the influence of pH, iron to nickel ratio, zinc and ECP. In addition, a procedure is outlined for extracting values of surface complexation constants by applying the model to water chemistry and surface activity data from operating BWRs. (authors)