Experience with piping in German NPPs with respect to ageing-related aspects
Introduction
Thirteen NPPs with PWRs and six plants with BWRs are currently in operation in Germany (Fig. 1). For the 13 plants with PWRs, the operating times rank from 10 to 31 years and for the six plants with BWRs, from 15 to 23 years. Because of the turnkey approach chosen by the utilities for the design and construction, there is a considerable consistency in the overall engineering concepts applied, even though continuous evolution took place in the design of the different generations of reactors. This resulted in numerous differences in technical solutions and details.
Based on the German regulatory framework, the general objective is to sustain a high level of safety in a coherent way with the developments in the state of the art. This includes that any kind of degradation which could impair the reliability of systems, structures and components has to be followed closely in order to take timely actions. Degradation mechanisms, which have an impact on the reliability, are frequently summarized in the international literature as ageing and the actions necessary to limit the impact as ageing management.
For almost 25 years, GRS has been systematically evaluating the operating experience of German NPPs (Kotthoff, 1997, Jungclaus et al., 1998). During the last years, GRS has been dealing with ageing in more detail (Michel and Schulz, 1996, Michel, 1997, Liemersdorf and Michel, 1998). In the following, starting with a short description of the general approach addressing technical issues for long-term operation in German NPPs, the operating experience with piping in safety-relevant systems of German NPPs is reviewed and the experience with actions taken against piping degradation is illustrated by examples.
Section snippets
Approach addressing long-term operation of German NPPs
The general approach followed in Germany to address relevant technical issues for long-term operation contains as major elements:
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Continuous evaluation of operating experience to identify changes in the reliability of systems, structures and components.
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Extended plant monitoring to enhance the understanding of system behaviour and load conditions of the components.
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Evaluation of safety margins for lower bound conditions by experimental and/or analytical R&D programmes.
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Generic studies to identify
Methodology
The operating experience with piping in German NPPs with LWRs is evaluated by GRS based on the analysis of the incidents reported by the plant operators (Bieniussa and Reck, 1996). This information has been centrally collected at GRS since 1974 and is recorded in a database. Regarding piping degradation, e.g. a report is required if there has been leakage in a safety-relevant pipe following a wall-penetrating crack or if indications of initial cracks have been detected during in-service
Results
The overall number of piping incidents in German NPPs is small. The evaluation of the operating experience with piping of J- and K-systems shows 118 incident reports for plants with PWRs and 83 reports for plants with BWRs for a period of 25 years. It can be derived from Fig. 2 that despite an increase in the number of plants in operation, the number of piping incidents per year in German NPPs has almost remained constant.
Pipe ruptures have only occurred in very small piping (NB ≤25) (Fig. 3).
Discussion of selected technical issues
The following technical issues are discussed in more detail with regard to the efficiency of ageing management:
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Intergranular stress corrosion cracking (IGSCC) in the weld region of piping of larger diameter manufactured from stabilized austenitic steel in BWR plants;
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Strain-induced corrosion cracking (SICC) in ferritic piping of BWR plants manufactured from high-strength steel;
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Flow-accelerated corrosion (FAC) in piping of the water/steam cycle system;
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Chloride-induced transgranular stress
Conclusions
The total number of cases of observed piping damage in German NPPs is small. Despite an increase in the number of plants in operation and their operating time, the number of piping incidents per year in German NPPs has almost remained constant. No piping incident occurred that would have led to an actuation of the safety systems.
Mainly pipes with small diameters, as used for example for instrumentation or actuation of valves, were involved in incidents in plants with PWRs. Almost all of these
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