Influence of a periodic latency time on the impact/sliding wear damage of two PWR control rods and guide cards specimens

Abstract

In nuclear power plants (PWR), the rod cluster control assemblies (RCCAs) can be damaged by impact-sliding wear due to flow-induced vibrations which generate contacts with their guidance devices (RCC guide tube) inside the reactor pressure vessel. Two localized impact/sliding wear tests have been performed at conditions close to the PWR primary system on stainless steel claddings (316L) using wear autoclave simulators operating without water flow. The influence of the excitation frequency and type of motion signal on the wear process and damage has been studied (other experimental conditions have been fixed for the tests). Some extensive non-destructive examinations have been performed on the worn specimens (rod and guide card), using weighing, scanning electron microscopy, EDX and 2D profilometry. The analyses of the wear scars show that a small frequency associated with a latency time between two-rod movements lead to a wear damage more severe than those generated at higher frequency even if the cumulative sliding distance is 300 times lower. Moreover, the linear weight loss (per slid kilometer) of the slow test specimen (∼5500 μg/km) is much higher than the fast test one (∼2 μg/km). So our observations brought us to the conclusion that the wear mechanism related to the small frequency damage is a chemical dissolution (corrosion) of the austenitic steel associated with a mechanical depassivation of the oxidized surface layers. This first stage is followed by an oxides compaction or detachment stage due to the impact at the end on each motion. The guide cards are less damaged than the rods but both exhibit a typical cup-shaped pattern with many ellipsoidal-like marks (also called dimples or scallop/hammering marks) aligned in the direction of the tube motion.