LOCA scenario-related zinc borate precipitation studies at lab scale

During the sump recirculation operation after a postulated loss-of-coolant accident (LOCA) in a pressurized water reactor (PWR), coolant spilling out of the leak in the primary cooling circuit is collected in the reactor sump and recirculated to the reactor core by residual-heat removal pumps. The long-term contact of the boric acid containing coolant with hot-dip galvanized containment internals (e.g. grating treads, strainers, support grids) may cause corrosion of the corresponding materials forming zinc borates (ZnB) dissolved in the cooling water.
Investigations regarding such zinc corrosion processes, changes of the coolant chemistry and possible resulting in-core effects are subject of joint research projects of the Helmholtz-Zentrum Dresden - Rossendorf (HZDR), TU Dresden (TUD) and Zittau/Görlitz University of Applied Sciences (HSZG). Lab-scale experiments at HZDR and TUD are focused on elucidation of physico-chemical corrosion and precipitation processes as well as resulting fouling effects at hot surfaces.
Long-term experiments of up to three weeks in a lab scale facility were conducted to simulate the simultaneous zinc dissolution (in sump) and ZnB precipitation (in hot core regions) during sump recirculation operation under boundary conditions of selected PWR LOCA scenarios. This includes LOCA-related zinc dissolution (corrosion) rates as well as experimental simulation of previously calculated scenario-related temperature courses of the coolant in the sump and area-related decay heat power courses of the reactor core. Results indicate significant precipitations of different solid ZnB products during the experiments. It turned out that the period between the start of the sump recirculation operation and the start of the ZnB precipitation as well as the precipitation rate essentially depend on the specific LOCA scenario (e.g. leak size). The ZnB precipitates usually formed dense layers on hot surfaces of electrically heated PWR cladding tubes of the lab scale facility. Additionally, flocculation or formation of solid ZnB particles inside the fluid has been observed. In most experiments, the different types of precipitates (layers, flocs or particles) were quantified and in certain cases the chemical compositions of the solid ZnB species were determined using different chemical analysis methods.
Since an influence of the ZnB precipitates on the thermal hydraulics inside the core cannot be ruled out, the results obtained at lab-scale were complemented by corresponding experiments in semi-technical test facilities of the project partner HSZG.
The investigations are supported by the German Federal Ministry for Economic Affairs and Energy under contract nos. 1501491 and 1501496.