Alternative Characterization Rules for Quasi-Laminar Flaws

Abstract

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.

Combination Criterion for Multiple Laminar Flaws in Steel Components

“…To this end, since laminar flaws are a subset of quasi-laminar flaws, a revision of the combination rules for laminar flaws has been proposed on the basis of the combination rules developed for quasi-laminar flaws using Fracture Mechanics analyses [4,5,6].…”

“…Firstly, 2D Extended Finite Element Method (XFEM) calculations were used [4]. They led to proximity rules that are were implemented in the revision 0 of the ASME Code Case N-848 [5] which is now part of the ASME B&PV Code Section XI.…”

“…After presenting the current treatment of the laminar flaws by the ASME B&PV Code Section XI, this paper proposes a revision of the combination rules for laminar flaws based on the characterization rules developed for quasi-laminar flaws [4,5,6]. As this revision is thus also based on mechanical interaction concepts, the paper allows making uniform the combination rules regardless of the kind of flaws which we would have to deal with.…”

ORNL Evaluation of Electrabel Safety Cases for Doel 3 / Tihange 2: Final Report

“…EBL developed specific grouping rules for closely spaced flaws using 3-D finite element modeling and experimental data [8]. These grouping rules were applied to all possible pairs of flaws to determine whether or not two or more flaws satisfy established interaction rules [9][10] that require they be grouped into one single flaw. In Fig.…”

“…The effects of proximity/flaw interaction and flaw orientation were examined in two recent papers [9][10] and translated into the ASME Code Case N-848 [19]. Those documents provide suitable rules to incorporate flaw interaction and orientation effects for quasi-laminar and planar flaws based on their alignment and distance between them.…”