Mechanistic Study of Post-Lay-Up Debris Formation in Upper Nozzles of Open-Recirculating Cooling Water Systems

This study investigates the formation mechanism of corrosion debris observed in an open-recirculating cooling water system after lay-up period. To elucidate the underlying mechanism, a two-step simulating experiment was designed to reproduce the actual operating cycle of the system, considering the alternating lay-up and operating conditions. The products generated through this procedure exhibited similar structure and composition with debris collected from the actual cooling water system, supporting the validity of the simulating experiment. Additionally, quantitative X-ray diffraction analysis revealed progressive phase transformation during lay-up period. In parallel, corrosion simulation for lay-up period was conducted to quantify iron dissolution and the subsequent formation of corrosion products, demonstrating that the volume of corrosion product was approximately three times greater than the volume of dissolved iron. This volumetric expansion promotes cracking, spallation, and detachment of corrosion products, which are transported and accumulated at cooling water nozzles upon system reoperation. Based on these results, it was determined that debris formed as corrosion products during operation precipitated and subsequently underwent phase transformation and volume expansion when exposed to wet-dry cycles.