In the context of the stress tests that have to be applied to nuclear power plants, this work presents the study of the flooding processes of the service galleries of a nuclear plant caused by a hypothetical failure of some of the pipes of the Essential Services Water System (ESWS). To assess the flood propagation along the galleries, two-dimensional hydraulic modelling tools, based on the solution of the shallow water equations with the finite volume method, were used. Due to the complexity and special features of the geometry and hydraulic processes, when compared with more standard urban flood assessment works, several specific modules were developed. A relevant one is a new module to consider the effect of the occupation of the galleries on the advance of the waterfront. This module was developed and verified prior to being applied to a case study. The results show the suitability of the proposed methodology to be used as part of the stress tests to ensure high security standards of nuclear power plants.