Corrosion of hot-dip galvanized containment installations – A potential cause for thermal-hydraulic effects after LOCA in PWR?


Corrosion of hot-dip galvanized containment installations – A potential cause for thermal-hydraulic effects after LOCA in PWR?

Kryk, H.; Harm, U.; Hampel, U.

During the sump recirculation phase after loss-of-coolant accidents (LOCA) in pressurized water reactors (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 as part of the emergency core cooling system (ECCS). The contact of the coolant with several forms of debris may influence the sump strainer clogging behavior as well as the cooling water chemistry. Damage to fibrous insulation materials located near to the leak may compromise the operation of the ECCS, if insulation fibers are transported to the strainers. Furthermore, 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 strainer clogging as well as on 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 clogging effects.
Results of generic experiments in a lab-scale corrosion test facility, representing the ECCS operation in a simplified manner, 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, which can subsequently lead to accelerated clogging of fiber-laden strainers within a few hours. The main influences on corrosion were identified as impact of the coolant jet onto the corroding surface, water chemistry and 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 a heated cladding tube, 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, which may eventually influence the thermal hydraulics in downstream components during the post-LOCA stage. The results obtained at lab-scale were confirmed by corresponding experiments in semi-technical test facilities of the project partner HSZG.
Based on the experimental results, water chemical measures were tested to reduce corrosion and/or zinc borate precipitation effects. 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.
The investigations have been supported by the German Federal Ministry for Economic Affairs and Energy under contract nos. 1501363, 1501430, 1501467 and 1501496.

Keywords: loss-of-coolant accident; LOCA; pressurized water reactor; PWR; sump strainer clogging; chemical effects; corrosion; zinc borate

  • Lecture (Conference)
    ICONE 24 – International Conference on Nuclear Engineering, 26.-30.06.2016, Charlotte, USA
  • Contribution to proceedings
    ICONE 24 – International Conference on Nuclear Engineering, 26.-30.06.2016, Charlotte, USA
    ICONE 24 – Conference proceedings; Volume 3: Thermal-Hydraulics: ASME - Digital Collection, 978-0-7918-5003-9
    DOI: 10.1115/ICONE24-60273
    Cited 1 times in Scopus

Permalink: https://www.hzdr.de/publications/Publ-23918
Publ.-Id: 23918