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. As a consequence, both units remained core unloaded pending the elaboration of an extensive Safety Case demonstrating the Structural Integrity of the RPVs in all operating modes, transients and accident conditions.

A large part of this demonstration consists of the Flaw Acceptability Assessment inspired by the ASME XI procedure but adapted to the nature and number of indications found in the Doel 3 and Tihange 2 RPVs. In particular, ASME XI IWB-3300 article requires combining closely spaced flaws in order to account for their mechanical interactions. However, it appeared early that the strict application of the current ASME XI proximity criteria for laminar flaws to the actual flaw indications found at Doel 3 led to unrealistic results and conclusions. Therefore, an alternative methodology to derive suitable characterization rules applicable to specific flaws observed at Doel 3 and Tihange 2 RPVs has been successfully developed, implemented and validated.

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