Evaluation of methods for assessment of cracks in piping systems

Non-destructive examinations performed on piping of nuclear power plants in Germany had sometimes revealed cracks that have been expected neither with respect to their nature nor to their extent (e.g., cracks in austentic piping of boiling water reactors). At present the assessment of findings at the occasion of such examinations is based on foreign design codes (e.g. ASME-Code) since in German codes (KTA-Regelwerk) no quickly usable directions are provided for such cases. In both Germany and foreign countries several experiments and theoretical analyses exist that can be used for the assessment of findings at non-destructive examinations. On the basis of these data the foundations of procedures in the following spheres of work have been reviewed and assessed: a. Fracture mechanics experiments on cracked piping elements, b. the nature of cracks detected and their causes, c. practicable simplified methods for the assessment of cracks, d. occurrences of cracks that have been left after inspection, e. crack growth models used at present. On the basis of these investigations recommendations are given that allow a standardised assessment procedure of findings of non-destructive examinations. These recommendations can be submitted to corresponding bodies, e.g. RSK, KTA for further discussion and to give recommendations to the regulators. From the experience gained with the unexpected findings of non-destructive examinations, some recommendations were given by the corresponding institutions for the practice of future examinations practice, that in part have been implemented already into the KTA safety standards. (orig.)