Abstract
ASME III Appendix G provides a non-mandatory approach for obtaining the allowable loadings for ferritic pressure-retaining components based on the principles of Linear Elastic Fracture Mechanics (LEFM). Its purpose is to derive a relationship between Temperature and allowable Pressure of a ferritic vessel in order to prevent it from being subjected to high pressure (and therefore stress) when the metal temperature is low and potentially susceptible to non-ductile failure. The purpose of this paper is to present operational experience using ASME III Appendix G to form operational boundaries for specific irradiated pressure vessels. An overview of the historic and recent use of ASME III Appendix G is presented alongside observations on improvements and relaxations of conservatisms inherent to this method. A discussion is also presented regarding alternative methods which link the calculation of maximum allowable pressure during low temperature operation to the defect tolerance assessments produced in support of safety cases, whilst maintaining appropriate levels of conservatism in the assessment. The conclusion of the paper is that whilst ASME III Appendix G generally supports the provision of reasonable operational boundaries for irradiated ferritic pressure vessels there are a number of alterations and revisions required for specific components, and indeed specific regions of components, where standard methods result in pessimistic limitations to performance.
