During the 2012 outage at Doel 3 and Tihange 2 Nuclear Power Plants, a large number of quasi-laminar indications were detected, mainly in the lower and upper core shells of the Reactor Pressure Vessels (RPVs). The observed indications could subsequently be attributed to hydrogen flaking induced during the component manufacturing process. As a consequence, both units remained core unloaded pending the elaboration of an extensive evaluation demonstrating that they can be safely operated.

The Structural Integrity Assessment of the RPVs, through the Flaw Acceptability Assessment, aimed at demonstrating that the identified indications do not jeopardize the integrity of the reactor vessel in all operating modes, transients and accident conditions. This demonstration has been done on the basis of a specific methodology inspired by the ASME B&PV Code Section XI procedure but adapted to the nature and the number of indications found in the Doel 3 and Tihange 2 RPVs.

As requested by Article IWB-3610(d)(2) of ASME B&PV Code Section XI, one of the parts that have to be addressed through the Flaw Acceptability Assessment is the Primary Stress Re-Evaluation assuming local area reductions of the pressure retaining membrane i.e., of the core shells, due to presence of the flaws. This is performed using the limit analysis provided by Article NB-3228.1 of the ASME B&PV Code Section III. Results are compared to those using the plastic analysis of Article NB-3228.3. The acceptance criterion that needs to be verified is that the calculated collapse pressure should be more than 1.5 times the design pressure.

The paper presents the 2D conservative approach developed in order to carry out this analysis dealing with large number and high density of flaws. Furthermore, the paper validates this 2D conservative methodology through detailed 3D XFEM elastic-plastic calculations.

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