Reactor Vessel Nozzle Inner Radius Fracture Analyses Using Elastic-Plastic Fracture Mechanics

The American Society of Mechanical Engineers (ASME) published Section XI Code Case N-648-1 [1] in order to provide alternative examinations of reactor vessel nozzle inner radii. The Code Case was created because ultrasonic examination of the inner radius regions of reactor vessels nozzles is not practical within the operating fleet and the likelihood of flaws developing within these locations is extremely low. Justification for using alternative visual examinations was provided in a paper published at the 2001 Pressure Vessel and Piping (PVP) Technology Conference [2]. This 2001 PVP paper used linear elastic fracture mechanics (LEFM) to demonstrate tolerance for flaws significantly larger than would be detected using nondestructive examination techniques. However, the Code Case [1] and PVP paper [2] were only applicable to operating plants in the United States. Thus, there was a need to provide a similar fracture analysis considering the AP1000® design to support elimination of volumetric examinations of the nozzle inner radius regions. It was also important to consider improvements in facture mechanics techniques that have been recently published in the ASME Code. The ductile behavior of the material at operating temperatures allow for the use of elastic plastic fracture mechanics (EPFM) methods which provides significantly improved flaw tolerance results. This paper compares results from analyses using LEFM and the EPFM methods for the AP1000 reactor vessel nozzle inner radii region and demonstrates tolerance for large flaws within these regions in order to support a basis for elimination of volumetric inspection during in-service and pre-service examination for the AP1000 design.