Method of suppressing cobalt leaching from feedwater heater tube

Purpose: To effectively perform preliminary oxidizing treatment for suppressing cobalt from leaching out of heater tubes of a feedwater heater made of stainless steels in a BWR type power plant. Method: Oxidizing treatment is applied at the plant start up test, to a tube of a feedwater heater installed in the nuclear power plant. At first, prior to the oxidizing treatment, existent oxide layers deposited on the surface of the tube are eliminated by electrolytic polishing. Then, oxidizing treatment is carried out while controlling the oxidizing temperature for 200 hours during reactor start up test of about 1 % thermal power. Specifically, the high pressure feedwater heater is heated by high temperature main steams to increase the temperature of water flowing through the tube to higher than 240 0C to thereby oxidize the inside of the tube. The oxidizing treatment at a temperature higher than 240 0C protective oxide films are formed on the inner surface of the tube. (Ikeda, J.)