International audience ; The purpose of the OSCAR code, developed by the CEA in collaboration with EDF and AREVA NP, is to simulate the contamination of the PWR (Pressurized Water Reactor) primary system. The OSCAR code, coupled to its chemistry module PHREEQCEA, allows researchers to analyse the corrosion product behaviour and to calculate the volume and surface activities of a primary system. It synthesizes knowledge and allows us to progress in understanding the phenomena involved in the contamination caused by activated corrosion products.The OSCAR V1.3 code has been used to simulate the effect of zinc on the primary circuit contamination of a Belgian PWR operated by ENGIE. This paper highlights the physicochemical phenomena involved during a zinc injection. The OSCAR simulation shows thatZinc precipitates on the inner oxide (Cr-rich layer) instead of cobalt showing that zinc has a better affinity than cobalt with chromites (change in the Co equilibrium concentration).Cobalt concentration and $^