Since the establishment of the Connors criterion, the in-plane (IP) stability of steam generator U-bend tubes has been treated as secondary to the out-of-plane (OOP) direction, an assumption that the nuclear industry re-examined following the 2012 tube-to-tube wear event at the San Onofre Nuclear Generating Station. This paper reports an experimental determination of the IP stability limit of the hybrid U-bend tube bundle of the APR1400 steam generator (SG), in which the array changes from a rotated triangular layout in the vertical legs to a rotated square layout in the horizontal section. A full-scale test section, reproducing the bundle geometry and the nominal tube to support gap, was subjected to air-water cross-flow admitted only to the U-bend region, thereby reproducing the partial admission condition of the operating SG, over homogeneous void fractions of 40% to 97%. Three support configurations were examined: the nominal fully supported design; a degraded configuration with the upper vertical and batwing supports removed; and an IP unsupported configuration with the egg-crate supports removed, used to force instability. Modal damping was measured independently over void fractions of 0% to 97% using a single flexible tube configuration. The fully supported and degraded configurations both remained IP stable over the entire velocity and void fraction range tested. Removal of the upper supports raised the RMS response by a factor of 2.5 to 3.2 at a void fraction of 90% without producing instability, showing that these supports contribute damping rather than IP stiffness. IP instability was obtained only in the fully unsupported configuration, at void fractions of 80% and 90%, where the measured stability constants wereK=16.3 and K=16.8. At the remaining void fractions the array stayed stable to the highest velocity attainable, giving minimum constants of 14.8, 15.7 and 16.1, so that the lower bound envelope of the whole campaign is K=14.7. OOP instability occurred at a lower constant, K=11.3 at a void fraction of 97%, confirming that the OOP direction remains governing and that IP instability is unlikely at APR1400 operating conditions. Compiling the available IP instability data from the literature gives a lower envelope of K=6.5, proposed as a lower-bound value for IP stability assessment in place of values inherited from OOP practice.
To enhance the corrosion resistance of structural materials in lead–bismuth eutectic (LBE) cooled systems, two AlCrFeMoTi-based high-entropy alloy coatings—an equiatomic (EA) coating and a Ti reduced low-Ti (LTi) coating—were deposited on ferritic/martensitic steel by magnetron sputtering. Their performance was evaluated under 450 °C static LBE corrosion conditions. Although the EA coating exhibited a slightly higher scratch critical load, the LTi coating showed better resistance to Pb/Bi penetration under oxygen-saturated LBE. Microstructural characterization reveals that the LTi coating develops a continuous, multi-layered oxide scale in which Pb/Bi enrichment is mainly confined to the outer porous oxide layers, whereas the EA coating forms a columnar oxide scale with Pb enrichment along columnar grain boundaries. This study proposes a coating-design strategy that tailors oxide morphology through elemental control, offering a new avenue for developing LBE compatible coatings.
