Mechanical behaviour of HTR materials: Developments in support of defect assessment, structural integrity and lifetime evaluation

Abstract

Mod 9Cr–1Mo steel (T91) is a candidate material for pressure vessels and for some internal structures of GCR (Gas Cooled Reactors). In order to validate this choice, it is necessary, firstly to verify that it is able to withstand the planned environmental and operating conditions, and secondly to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr–1Mo steel has been undertaken at CEA in order to characterize the behaviour of this material and of its welded junctions. In this program, the role of the Laboratory for structural Integrity and Standards (LISN) is to develop high temperature defect assessment procedures under fatigue and creep loadings. Concerning the GCR, complementary studies are conducted in order to validate the existing methods (developed for the fast reactors) and to get new experimental data on Mod 9Cr–1Mo steel. Moreover, if the geometry and the loadings of a standard CT specimen allow performing a 2D analysis, the case of industrial loadings appears much more complicated, notably because of surface defects which propagate and present shapes that can be considered as half ellipse. Therefore, in the frame of the defect assessment methods validation, the LISN undertakes both standard tests on CT specimens to determine the propagation laws and bending tests on large plates under high temperature fatigue and creep loadings. These components present an initial semi-elliptical surface notch normal to the loading direction and its initiation and propagation are studied.