Abstract

During the 2012 outage at Doel 3 and Tihange 2 Nuclear Power Plants, a large number of quasi-laminar indications were detected 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 Safety Case demonstrating that they can be safely operated. One of the most challenging parts of this demonstration was the Flaw Acceptability Assessment, aiming at demonstrating that the identified indications do not jeopardize the integrity of the reactor vessel in all operating modes, transients and accident conditions. This analysis was done by using a methodology: innovative, in line with existing ASME Code Section XI requirements, specific, sufficiently wide to be accepted and, first and foremost, conservative. Through a brief reminder of the Flaw Acceptability Assessment methodology, the paper presents the main hypotheses done for the calculation and quantifies the conservatism related to each of them. This quantification clearly highlights the reliability of final result i.e., the demonstration of the Fitness-for-Service for continued operation of both Doel 3 and Tihange 2 RPVs.

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