Root cause analysis for cracked pipe in auxiliary system in nuclear power plant

Abstract

Penetrated cracks were found in succession in 316LN stainless steel elbows in auxiliary system in a Chinese nuclear power plant after about 19 years’ service. In order to clarify the root cause of the cracking, the cracked elbow with nominal diameter of 12 in. was cut and its chemical composition, hardness, microstructure and residual strain were investigated. The results show that this elbow was heavily cold worked during push-bending and the degree of cold work was within 25∼40%. The manufacturer did not obey RCC-M M3317 to solution treat this elbow before delivery. The heavily cold-worked elbow suffered stress corrosion cracking in occluded primary water.