1. A process for estimating the concentration (C) of a chemical element in the primary coolant of a nuclear reactor, the reactor comprising means for injecting a solution for diluting the concentration of said chemical element below a predetermined limit into the primary coolant, means for injecting a concentrated solution of said chemical element having a predetermined concentration (C*) into the primary coolant, and a sensor able to measure a quantity (C m ) representing the concentration of said chemical element in the primary coolant, wherein the process is an iterative process comprising repeatedly in each time step k: a stage of acquisition of a quantity (q dk ) representing the injected flow of the dilution solution in step k, a quantity (q k ) representative of the injected flow of concentrated solution in step k, and a quantity (C mk ) representing the concentration of said chemical element measured by the sensor in the primary coolant; a stage of calculating an estimated value (C ek+1 ) for the concentration of said chemical element in the primary coolant in step k+1 based on the representative quantities (q dk , q ck , C mk ) acquired in step k.