Enhancing French nuclear fleet’s safety to help secure energy supply

The French nuclear operator, EDF, commissioned Tractebel to carry out studies on piping systems of several reactors that are facing stress corrosion cracking problems. Our engineers have made thermo-mechanical calculations on the basis of the 3D modelling of piping portions of certain secondary circuit systems on several units of the French nuclear fleet.

Mechanical integrity

At the end of 2021, EDF was confronted with a rare anomaly: a stress corrosion cracking phenomenon observed on stainless steel piping in the safety injection system of several reactors. Inspections and expert assessments have confirmed the existence of crack-type flaws due to stress corrosion in some specific locations. As part of EDF strategy to fix the issue, mechanical analyses are required to define repair priorities and demonstrate that the structural integrity of the piping systems can be guaranteed at least for a cycle as required by the French law in the locations where cracks are discovered. The challenge is to justify the mechanical integrity of piping portions affected by the faults cracks in order to ensure the continuous operation of the unit.

“Cracked 3D” calculations

Tractebel's tasks consist of creating models used to calculate and analyse the effect of transient thermal and mechanical loads on the cracked assessed pipes. The data used, provided by EDF, is the result of initial calculations (not carried out by Tractebel) and corresponds to the most severe situations to make sure the system can withstand the worst case conditions. The regulatory framework for the studies is the RSE-M, Règles de Surveillance en Exploitation des Matériels Mécaniques des Ilots Nucléaires REP. The "cracked 3D calculations" involve modeling a fictitious crack with penalizing assumptions and analysing its stability when submitted to certain thermal and mechanical conditions. The aim is to assess the maximal admissible depth of cracks on the studied piping systems under the most severe loads, and confirm the integrity of these systems to resume reactors’ operation in compliance with the highest level of safety standards.

Examples of “cracked 3D” calculations

“We are proud to contribute to the up-coming restart of several units in France that are crucial to securing electricity supply in the current geopolitical context. These studies are an opportunity for Tractebel to provide EDF with cutting-edge expertise in the field of mechanical and structural integrity, and to position Tractebel as a reliable, efficient and responsive partner.”

Romain Vénier, Project Manager   

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