Hydrazine and hydrogen co-injection to mitigate stress corrosion cracking of structural materials in boiling water reactors (5). Effects of hydrazine and dissolved oxygen on flow accelerated corrosion of carbon steel

Hydrazine and hydrogen co-injection into reactor water via the feed water line can mitigate stress corrosion cracking in BWRs. The effects of water quality variation due to hydrazine injection on flow accelerated corrosion (FAC) of carbon steel used in the BWR feed water line should be considered from the viewpoints of radioactive exposure, radioactive waste generation and plant integrity. Optimization of oxygen injection into feed water was considered to suppress FAC during hydrazine and hydrogen co-injection. In this report, corrosion tests of carbon steel were made in hydrazine and/or dissolved oxygen injected solutions (conditions: no pH adjustment by ammonia; temperature=215degC; flow rate=3.5m·s-1) to study the effects of dissolved oxygen and hydrazine on FAC. Hydrazine and oxygen co-injection suppressed the total corrosion rate by about 1/5 compared with the case of 0μmol·kg-1 hydrazine and <0.06μmol·kg-1 oxygen injection. With hydrazine and oxygen co-injection, the inner layer, which is protective oxide, was close packed. Thickness of the inner layer also became larger compared with the case for only hydrazine injection or only oxygen injection. α-Fe2O3, which is thermodynamically more stable than Fe3O4, was formed in addition to Fe3O4 which was the single species formed for the case of only hydrazine injection. From these results, it was concluded FAC will be suppressed by keeping the oxygen concentration at the current level in BWRs (about 1μmol·kg-1) if hydrazine and hydrogen co-injection is applied. (author)