Multiscaled Experimental Investigations of Corrosion and Precipitation Processes After Loss-of-Coolant Accidents in Pressurized Water Reactors

In a joint research project of the Zittau/Goerlitz University of Applied Sciences, the Technische Universitat Dresden, and the Helmholtz-Zentrum Dresden-Rossendorf, the main emphasis is the time-related assignment of simultaneous and interacting mechanisms at zinc sources and zinc sinks at boundary conditions of a loss-of-coolant accident (LOCA) in German pressurized water reactors (PWRs). The required experiments are carried out at semitechnical and laboratory scales. Zinc is used as a protective coating, e.g., for gratings in the containment, showing high corrosion resistance due to a gradual formation of passivating layers. In contrast, its long-term behavior during LOCA changes significantly under the influence of the coolant chemistry of German PWRs. As a consequence, installations in the containment act as zinc sources. Released zinc ions change the chemical properties of the coolant and could, e.g., lead to layer-forming depositions of zinc borates in the core, which increases the possibility of a hindered heat dissipation. For experimental and methodical investigations of these phenomena, the test rig Zittau flow tray, a scaled sump model of a German PWR, was equipped with a full-length 3 x 3 fuel assembly dummy acting as core model, a preheater, and a cooler component. Nine 4.4-m-long fuel rod dummies simulate the decay heat by internal heating cartridges. This rig design enables experimental investigation of physicochemical mechanisms considering coolant containing boric acid and zinc and their influence on the thermohydraulic processes in the reactor core at post-LOCA boundary conditions. Additional zinc corrosion and zinc borate precipitation studies to elucidate chemical zinc corrosion mechanisms and dependencies of those processes on typical LOCA parameters were carried out using lab-scale corrosion/precipitation test facilities. The time-dependent zinc release at hot-dip galvanized gratings (HGGs) was investigated regarding their position (e.g., inside or near the leaking jet, freely suspended, or submerged in the coolant) and their surface area as well as the temperature and flow rate of the coolant. The experimental database allows the approximation of corrosion rates in dependence of HGG position and the accident-specific coolant leakage rate as well as first mathematical approaches for the modeling of zinc sources.