Right now it wouldn’t be possible to do this during depletion itself. What you’d have to do is to run a critical search on the boron concentration at each depletion time point as a post-processing step. You can recreate a model with the correct material compositions by using openmc.deplete.ResultsList.export_to_materials (new in version 0.12.1) and then use openmc.search_for_keff to find the critical boron concentration. However, note that this is not the same as doing a critical boron search during each depletion time point (which itself would affect the reaction rates and evolution of the material compositions).