Interactions of coolants with hot-dip galvanized materials after loss-of-coolant accidents in pressurized water reactors

During the sump recirculation phase after loss-of-coolant accidents (LOCA) in pressurized water reactors, 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 as part of the emergency core cooling system. The long-term contact of the boric acid containing coolant with hot-dip galvanized containment internals (e.g. grating treads, supporting grids of sump strainers) may cause corrosion of the corresponding materials.
Generic investigations regarding the influence of such corrosion processes on the coolant chemistry and possible resulting effects in the reactor core 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 [1].
Results of generic experiments in a lab-scale corrosion test facility suggest that there is a multi-stage corrosion process. The first stage comprises dissolution of the zinc layer in the coolant forming zinc ions and in turn affecting the coolant chemistry. During the second stage, the base material (steel) corrodes forming insoluble corrosion particles. The main influences on corrosion were identified as impact of the coolant leak jet onto the corroding surface, the coolant chemistry and the zinc surface / coolant volume ratio.
Furthermore, retrograde solubility of zinc corrosion products in boric acid containing coolants with increasing temperature was observed. Thus, formation and deposition of solid corrosion products cannot be ruled out if zinc containing coolant is heated up during its recirculation into hot downstream components (e.g. hot-spots in core). Corrosion experiments, which included formation of corrosion products at heated zircaloy cladding tubes, proved that zinc, dissolved in the coolant at low sump temperatures, turns into solid deposits of zinc borates when contacting heated zircaloy surfaces. Due to alternating heating and cooling of the coolant during sump recirculation operation, a cycle of zinc corrosion and zinc borate precipitation may be initiated.
Based on the experimental results, water chemical measures were tested to reduce corrosion and zinc borate precipitation effects [1]. Additionally, joint research projects have been established by the TUD and the HSZG dealing with local effects of corrosion, corrosion product precipitation and the interplay thereof at LOCA-specific conditions [1-2].
The investigations have been supported by the German Federal Ministry for Economic Affairs and Energy under contract nos. 1501363, 1501430, 1501467 and 1501496.

References
[1] Kryk, H. , Harm, U., Hampel, U.: Reducing in-core zink borate precipitation after loss-of-coolant accidents in pressurized water reactors, Proceedings of the Annual Meeting on Nuclear Technology (AMNT), Hamburg, 2016
[2] Seeliger, A.; Alt, S.; Kästner, W., Renger, S., Kryk, H., Harm, U. : Zinc corrosion after loss-of-coolant accidents in pressurized water reactors - thermo- and fluid- dynamic effects. Nuclear Engineering and Design, 2016, 305, 489-502