Method of decontaminating metal surfaces in a cooling system of a nuclear reactor

A method of decontaminating metal surfaces in a cooling system of a nuclear reactor, wherein the metal surfaces are coated with metal oxides including containing radioisotopes, and wherein the cooling system comprises one or more primary loops including at least one reactor coolant pump, and a residual heat removal system, the method comprises conducting a plurality of treatment cycles, with each of the treatment cycles comprising:

• a) an oxidation step wherein the metal oxides containing radioisotopes are contacted with an aqueous solution of a permanganate oxidant;
• b) a decontamination step wherein the metal oxides subjected to the oxidation step are contacted with an aqueous solution of an organic acid selected from the group consisting of oxalic acid, formic acid, citric acid, tartaric acid, picolinic acid, gluconic acid, glyoxylic acid and mixtures thereof, so as to dissolve at least part of the metal oxides and the radioisotopes;
• and
• c) a cleaning step wherein at least the radioisotopes are immobilized on an ion exchange resin;
• wherein the oxidation step comprises at least one acidic oxidation step and at least one alkaline oxidation step carried out one after another in either the same or different treatment cycles, and wherein the plurality of treatment cycles comprises at least one treatment cycle including a high temperature oxidation step, during which high temperature oxidation step the permanganate oxidant solution is kept at a temperature of at least 100° C. and wherein the at least one reactor coolant pump is used to circulate and heat the oxidation solution inside the one or more primary loops and the residual heat removal system is used to control the temperature of the oxidant solution during the high temperature oxidation step.