EXPERIENCES WITH FRACTURE MECHANICS LEAK-BEFORE-BREAK EVALUATIONS WITHIN THE GERMAN INTEGRITY CONCEPT FOR NPPS

摘要

The Integrity Concept for mechanical components and systems practiced in German nuclear power plants (NPPs) is a concept to ensure break preclusion including the application of the Basic Safety Concept. In the German Basic Safety Concept the fundamental requirements of materials, design and manufacturing of the safety relevant components and pipe systems are defined, completed by independent redundancies keeping in mind unit specific conditions. Only if break preclusion has been verified for safety relevant pipe systems, reduced leak postulates can be used: In general a leak of 0.1 x pipe section or smaller leaks based on a fracture mechanics foundation.Therefore the evidence of break preclusion contains the ensurance of a design and manufacturing following the Basic Safety Concept as well as of knowing and controlling the causes of possible in-operation degradation mechanisms combined with a numerical leak-before-break fracture mechanics analysis.The paper presents the outlines of a fracture mechanics evaluations procedure within a break preclusion evidence methodology. The procedure is agreed by the several German TUEN/ inspection organisations for NPP and takes into consideration recent national and international developments. The attention is in particular turned to the well-founded determining of safety margins as integral parts of the fracture mechanics analysis