Method for treating, during an operating cycle, an internal surface of at least one metal part of a primary circuit of a water-cooled nuclear reactor, the metal part being made from a nickel-based or cobalt-based alloy selected from a steam generator tube, a pump or a fitting, the method comprising: - an estimation of a quantity of nickel liable to be discharged into the primary circuit by all the metal parts of the primary circuit in contact with the water of the primary circuit, during an operating cycle of the reactor; - a calculation of a quantity of magnetite necessary for fixing the whole of the quantity of nickel estimated; - formation of a treated water obtained by dissolving the quantity of magnetite calculated in the water of the primary circuit when the water has a temperature of between 80°C and 180°C; and - putting this treated water in circulation in the primary circuit so as to form, on the internal surface of at least one metal part, a layer of a protective oxide able to fix all or part of the quantity of nickel estimated; and wherein, a portion of the treated water circulating in the primary circuit being purified continuously, during the operating cycle, while being diverted into an auxiliary circuit comprising purification means and then reinjected into the primary circuit, the method furthermore comprises an injection, into this auxiliary circuit and downstream of the purification means, of an aqueous iron solution (Fe(II)), so as to compensate for the quantity of iron removed from the purified treated water by the purification means. the method comprising: - an estimation of a quantity of nickel liable to be discharged into the primary circuit by all the metal parts of the primary circuit in contact with the water of the primary circuit, during an operating cycle of the reactor; - a calculation of a quantity of magnetite necessary for fixing the whole of the quantity of nickel estimated; - formation of a treated water obtained by dissolving the quantity of magnetite calculated in the water of the primary circuit when the water has a temperature of between 80°C and 180°C; and - putting this treated water in circulation in the primary circuit so as to form, on the internal surface of at least one metal part, a layer of a protective oxide able to fix all or part of the quantity of nickel estimated; - an estimation of a quantity of nickel liable to be discharged into the primary circuit by all the metal parts of the primary circuit in contact with the water of the primary circuit, during an operating cycle of the reactor; - a calculation of a quantity of magnetite necessary for fixing the whole of the quantity of nickel estimated; - formation of a treated water obtained by dissolving the quantity of magnetite calculated in the water of the primary circuit when the water has a temperature of between 80°C and 180°C; and - putting this treated water in circulation in the primary circuit so as to form, on the internal surface of at least one metal part, a layer of a protective oxide able to fix all or part of the quantity of nickel estimated; and wherein, a portion of the treated water circulating in the primary circuit being purified continuously, during the operating cycle, while being diverted into an auxiliary circuit comprising purification means and then reinjected into the primary circuit, the method furthermore comprises an injection, into this auxiliary circuit and downstream of the purification means, of an aqueous iron solution (Fe(II)), so as to compensate for the quantity of iron removed from the purified treated water by the purification means. <EOS>