In a process for estimating the concentration (C) of a chemical element in the primary coolant of a nuclear reactor, a dilution solution or a concentrated solution of said chemical element in a predetermined concentration (C*) is injected into the primary coolant within the reactor, and the reactor includes a sensor capable of measuring a quantity (Cm) representing the concentration of said chemical element. The process is an iterative process in which repeatedly in each time step k: a stage of acquiring quantities (qdk) and (qck) representing the injected flows of dilution solution and concentrated solution in step k, and a quantity (Cmk) representing the concentration measured by the sensor; a stage of calculating an estimated value (Cek+1) of the concentration of said chemical element in the primary coolant in step k+1 on the basis of representative quantities (qdk, qck, Cmk) acquired in stage k.