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 RPVs.

In the frame of the Structural Integrity demonstration of these RPVs according to ASME XI principles, ASME XI IWB-3300 article requires combining closely spaced flaws in order to account for their mechanical interactions. However, it appeared early that the characterization rules were adapted neither to quasi-laminar flaws nor to such densities of flaws.

Therefore, an alternative methodology to derive characterization rules for quasi-laminar flaws has been developed, implemented and validated.

This work, based on 2D eXtended Finite Element Method (X-FEM) calculations and presented during ASME PVP 2014, has led to the proposed ASME Code Case N-848 “Alternative characterization rules for quasi-laminar flaws – Section XI, Division I”.

This 2D approach, even though better suited to quasi-laminar flaws, results however in very conservative proximity rules.

Therefore, it appeared that more realistic — although still conservative — proximity rules based on 3D X-FEM calculations could be developed.

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