Effect of cladding on stress intensity factors during normal plant operation

The method for determining the allowable pressure-temperature operating limits is contained in Section XI, Appendix G of the ASME Boiler and Pressure Vessel Code. The Section XI Appendix G approach for calculating these limits uses a fracture mechanics method with conservative assumptions and defined margins to protect the vessel against brittle crack initiation. However, in applying this method, stresses due to the cladding have generally been ignored and are believed to be bounded by the margin on pressure and the assumption of a large quarter-thickness reference flaw. This method is considered to be conservative and adequate for protecting against vessel failure. The Appendix G methodology is currently being reviewed by the Section XI Working Group on Operating Plant Criteria to quantify the safety margins and to improve the overall method. As part of this review, a benchmark problem was recently performed examining the contribution of cladding stresses to the crack tip stress intensity factor for a range of hypothetical reference flaw sizes. This paper describes the insults of this benchmark problem and the effects of cladding on reactor vessel integrity