Evaluation of the vent header crack at Edwin I. Hatch Unit number2 Nuclear Power Station

A metallurgical failure analysis was performed on pieces of the cracked vent header pipe from the Edwin I. Hatch Unit 2 Nuclear Power Plant. The analysis consisted of optical microscopy, chemical analysis, mechanical charpy impact testing and fractography. The general conclusions drawn from this analysis were: 1) the material of the vent header met the mechanical and chemical properties of ASTM A516 Gr. 70 material and that the microstructures were consistent with this material; 2) the fracture faces of the cracked pipe were predominantly brittle in appearance with no evidence of fatigue contribution; 3) the NDTT (Nil Ductility Transition Temperature) for this material is approximately -600F (-510C); and 4) the fact that the material's NDTT is significantly out of the normal operating range of the pipe suggests that an impingement of low temperature nitrogen (caused by a faulty torus inerting system) induced a thermal shock in the pipe which, when cooled below its NDTT, cracked in a brittle manner