In the frame of a Fracture Mechanics Assessement in the perforated part of the steam-generator tubesheet, a defect relying two holes is postulated, which represent a singular configuration compared to conventional FMA. Indeed the defect is relatively small in comparison to the plate thickness and the crack length is limited by the distance between two holes.

To take benefit of the geometry effect of this configuration, an application of the two parameters J-Q approach is performed on tubesheet component.

At first, the methodology was applied to define and validate the methodology on an experimental data base with different constraint levels. This data base contains numerous fracture tests performed by both EDF and Framatome on various specimen geometries extracted from a 18MND5 low alloy ferritic steel plate, in the brittle-to-ductile transition regime. C (T), SEN (T), SEN (B), and non-standard specimens [1].

Secondly this methodology is applied to predict the benefits of this approached for tubesheet configuration. Different configurations are tested: underclad defect, surface defect, mechanical loading, thermal shock, in the middle of the perforated zone or near the border of the perforated zone.

These applications show some significant constraint effect for underclad defect and some moderate effect for surface effect.

A methodology is proposed to correct the classical Engineering Fracture Mechanics Assessment to gain margins without considering any probability of failure or statical approach.

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