Advanced Boron Soaking Procedure for Steam Generators

Abstract

An experimental study on boric acid penetration into the crevices between tube and tube-support-plate, and intergranular-attack (hereinafter called "IGA") cracks in crevices has been performed to obtain the optimum boron soaking procedure in operating steam generators with IGA. IGA is a corrosion mechanism involving the dissolution of the metal grain boundary. It consists of several cracks which propagate along metal grain boundaries from the outer surface to the inside of the tube. The term "IGA crack" refers to these cracks. Two experimental crevice models were set up. One was of the packed-crevice type where the crevice gap is completely packed by sludge, and the other was of the open-crevice type where the crevice gap is not packed, but reduced by sludge. The boron in IGA cracks was investigated using ion microanalysis in order to confirm the existence of anticorrosive film in IGA propagation. The optimum reactor power for effective boric acid penetration into the tube surface and into the IGA cracks within the crevice was found to be a power level of about 5% and 30%, which are applicable to both the packed-and open-crevice types.