On the Role of Fracture Mechanics Assessment Within the Integrity Concept

Abstract

In the EnBW nuclear power plant Neckarwestheim, the “Integrity Concept” is applied to important piping systems. As basis of this concept is an existing quality according to the requirements this has to be proven (e.g. during design and manufacturing) or verified (after a period of operation). This means for example that the material characteristics are as demanded, the construction is appropriate in detail and there is a fault free status. Possible causes of damage mechanisms in operation (loads, environment, degradation of material behavior) are either excluded or controlled, i.e. analyzed and assessed according to the requirements (standards). Within the scope of the “Integrity Concept” fracture mechanics analysis is a redundant measure. In the state of design, already, the assessment of a postulated crack is necessary to produce a “fault-tolerant” component and to determine effective non destructive test regions, methods and test intervals, so that possible consequences of damage mechanisms like cracks can be detected in an early stage. The size of the postulated crack envelopes the detectable crack depending on the resolution of the non destructive test method used during the manufacturing process and in operation. Using the operation loads the growth of the postulated crack is analyzed and assessed. Additionally, it is shown that the size of a through wall crack is safe below the critical crack size. Furthermore, it is demonstrated that the maximum loads in operation as well as in emergency and faulted conditions are below the collapse load. The high quality of the systems as produced has to be guaranteed in operation using redundant measures. Ageing phenomena have to be controlled concentrating on the relevant causes of possible damage. In most of the cases, damage mechanisms are caused by local effects; consequently, these local effects (like loads, water chemistry etc.) have to be monitored, evaluated and assessed regularly. Thus the knowledge about the system is permanently increased. Inservice inspection and testing are redundant measures to safeguard integrity. The efficiency of the entire procedure has to be assessed, periodically, taking into account the actual state of knowledge. Following this procedure the safety margin of the systems and components can be verified (and demonstrated) at every stage of operation. If the “Integrity Concept” is applied consequently there is no reason to restrict the application e.g. to certain systems or to nominal diameters > 200 mm. Furthermore, as fracture mechanics analysis is a tool to control a postulated extreme case (that can not occur if the analysis is correct) it is obvious that a special leak detection system is obsolete because it does not increase safety. Therefore it makes no sense to analyze leak rates etc.