Assessment of the impact of ageing on the performance of CANDU special safety and safety related systems; Safety analysis perspective

An abstract of a tecnical paper presented at:
The 8th International Topical Meeting on Nuclear Thermal-Hydraulics, Operation and Safety (NUTHOS-8) Shanghai, China,
October 10–14, 2010

Prepared by:
Haldun-O. Tezel, Nicolas Christodoulou, Noreddine Mesmous
Canadian Nuclear Safety Commission
Ottawa, Ontario, Canada

Abstract

In nuclear power plants (NPPs), the principle of defence-in-depth is implemented by means of a series of physical barriers to the release of radioactive materials. The reliability of those physical barriers is enhanced by applying the concept of defence in depth through the deployment of several different levels of provisions. As part of overall NPP safety, the design has to consider the effects of ageing on systems, structures and components (SSCs) for all levels of defence-in-depth provisions.

Techniques have been developed for monitoring, testing, sampling and inspecting SSCs to assess ageing mechanisms, verify predictions and identify unanticipated behaviour and degradation that may occur during operation as a result of ageing.

Based on the experience gained in the mid to late 1990s through the ageing assessment of two CANDU 600 designs and later on similar assessments of other CANDU plants, in this paper we present a regulatory perspective and expectations for a process to demonstrate plant safety considering the affect of plant ageing on level three of defence-in-depth provisions (shutdown systems) – specifically, on the performance of the shutdown systems.

Given that it requires the contribution of various disciplines, the assessment of the impact of plant ageing on shutdown system effectiveness is complex. It is further complicated by the fact that the impact of individual ageing mechanisms may not be always easily identifiable because all ageing mechanisms are happening together at the same time. Various ageing mechanisms may synergistically affect one another, while some may obscure their detection and/or recognition of others.

In CANDU reactors, there are specific ageing mechanisms. Major ageing mechanisms involve ageing of the pressure tubes (diametral expansion and axial elongation), pressure and calandria tube sag, change of material properties and mechanical damage (scratching, manufacturing defects and fretting marks); ageing of steam generators (due to fouling of tubes) and leaks between the primary and secondary sides (erosion and vibration induced cracks); wall thinning of feeders exhibited in the outlet feeders due to flow assisted corrosion and increase in roughness of the primary heat transport system (PHTS) piping due to the selective removal and deposition of magnetite.

The impact of the integrated effects of the PHTS ageing tends to reduce margins‑to‑failure. Furthermore, it is plausible that the overall impact of ageing on plant safety may be larger than the sum of the impact of the individual ageing mechanisms.

Margin-to-failure is defined as the sum of two separate margins; analysis-margin and the safety-margin. Analysis-margin is defined as the margin between the safety-limit (acceptance criteria) and the analysis results, and the safety-margin is defined as the difference, in physical units, between the safety-limit (acceptance criteria) and the physical failure point of a system or component. Safety analysis is the cornerstone to analysis-margin and hence margin-to-failure. In addition to demonstration of analysis-margin (or margin-to-failure), safety analysis plays an important role in the establishment of compliance procedures for safe operation (during normal operating conditions).

This paper emphasizes the safety analysis aspect of the plant ageing issues from a regulatory perspective. The demonstration of margin-to-failure is accomplished by safety analysis. Therefore, special emphasis is placed on the safety analysis methodology, assumptions, models, correlations, initial and boundary conditions, acceptance criteria and prescribed safety margins. The choice of safety analysis methodology and the safety impact of these choices are discussed.

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