In multitubular fixed bed reactors for heterogeneously catalyzed, highly exothermic gas phase reactions the catalyst bed is usually diluted with inert particles for improving the heat management. To help balancing the heat generation and heat removal out of the reactor by catalyst bed dilution, the active and inert particles have to be mixed as well as possible. Otherwise, the thermal stability of the reactor cannot be guaranteed and, e.g., catalyst deactivation or thermal runaway may take place because of the formation of local hotspots. Since loading a mixture of active and inert particles into a reactor tube inherently is a statistical process, fluctuations in local activity cannot be avoided. We have analyzed this phenomenon in detail based on numerically generated 3-dimensional randomly packed beds of spheres. The statistical process of loading a mixture of particles into a reactor tube was mimicked by a random number based approach. The simulations show that activity variations are evenly distributed along the axial direction, whereas in radial direction the fluctuations become larger towards the tube center. The synthetic packed beds provide detailed information about the packed bed structure, which has been used to identify accumulations of active particles in local clusters. This evaluation reveals significant non-idealities compared to the assumption of a uniform activity profile, e.g., clusters of up to 20 active particles could be observed. Especially for highly diluted beds and/or low D/d ratios typical for highly exothermic reactions this aspect should be kept in mind during process development and design. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.