Integrity Concept for Piping Systems With Corresponding Leak and Break Postulates in German Nuclear Power Plants

In order to exclude the possibility of catastrophic failure of safety relevant pressure-retaining components in nuclear power plants during operation, the “integrity concept” is applied in Germany. It has been developed over the past 30 years on the basis of the safety criteria of the guidelines for damage precautions, as set by the German Advisory Committee on Reactor Safeguards (RSK-LL) and the basic safety concept. The integrity concept is based on the requirements of proven basic safety characteristics: design, construction, material, and manufacturing. Complementary elements (so-called redundancies) also have to be considered: the principle of inspections by multiple parties, the worst-case principle, the principle of plant monitoring and documentation, as well as the principle of verification. This includes consideration of possible operational damage mechanisms in terms of interaction between causes and consequences as well as consideration of any new knowledge, if necessary in the framework of additional safety analyses. The integrity concept is applied in German PWR plants, i.e. the main coolant lines and their connecting lines, as well as in German BWR plants with large main steam and feedwater lines within the pressure-retaining boundary (primary system) up to the outer containment. A fracture mechanics safety analysis with postulated defect sizes as well as the experimental basis of load behavior are essential parts of the integrity concept. The measures determined and verified in this way guarantee that no major deviations from design values occur. This is confirmed by periodic in-service inspections. The advantage of this concept is the application of reduced leakage assumptions for important safety-related pipe systems, i.e. either through application of a conservative postulate of 0.1 F according to RSK-LL or reduced leak sizes. The operational experience gained with this concept has been positive in all German PWR and BWR plants. This paper demonstrates that the German integrity concept has been proven to be successful over the past 30 years and reflects the state of the art. Its implementation in plants and its incorporation in the German Nuclear Codes (KTA rules) contribute to the safety of German nuclear power plants in terms of precautionary damage prevention.Copyright © 2011 by ASME