Modelling the transport in the porous layer of oxide films formed on material surfaces in nuclear power plants: Model extension to more general conditions

In this study, the role of the porous oxide film, deposited on top of the primary passive (compact) film, to control activity incorporation on the primary circuit surfaces of nuclear power plants (NPPs) is investigated. To serve this purpose, a computerized steady-state transport model, previously developed for a simplified BWR case, has been extended to take into consideration more general high-temperature aqueous and oxide compositions. In practical terms, computationally laborious multi-species reactions for ions, aqueous complexes, precipitates and dissolved gases as well as three types of boundary conditions at the compact film/porous film interface can now be included in the revised model. The model is rendered flexible enough to enable comparison of the effects of various NPP operating conditions, including those prevailing in PWRs. In this report, results calculated with the revised transport model for a BWR system are presented and discussed. The results allow, among other things, a qualitative evaluation of the fate of soluble, radioactive minority species within the pore fluid of the porous film à priori. A further elaboration of this system was highly warranted in that it serves as a template when moving to computationally more demanding PWR conditions. An outline of a technique to incorporate adsorption by way of mechanistic surface complexation into the transport model is also given.