Some applications related to the structural integrity analyses of CANDU 6 pressure tubes

The flaws found during in-service inspection of Zr-2.5%Nb CANDU pressure tubes include fuel bundle bearing pad fretting flaws and debrise fretting flaws. In-service flaws are evaluated using fitness-for-service procedures to justify continued operation of pressure tube containing the flaw. The flaw evaluation procedures address crack initiation due to Delayed Hydride Cracking (DHC) under constant loading and also address fracture initiation and plastic collapse. The paper presents some applications related to the influence of the residual hoop stresses at roll-expanded joint region into stainless steel fittings at both ends on the structural integrity evaluation in the presence of blunt flaws. Two cases of blunt flaws were considered for evaluation: fuel bundle bearing pad fretting flaws and debrise fretting flaws. The blunt flaw geometry modeling and stress-strain analyses were performed by means finite element method (FEM) with FEA-Flaw computer code. The allowable peak flaw-tip stress and the Failure Assessment Diagram (FAD) for DHC initiation criterion were used for integrity assessment for the mentioned blunt flaws. Applications are performed as part of the research program address to evaluation of the in-service inspection results of the fuel channels from Cernavoda NPP. (author)