Chemical Processes of Galvanized Steel Corrosion in the Post-LOCA Phase of a PWR and the Prevention of Sump Screen Clogging


Chemical Processes of Galvanized Steel Corrosion in the Post-LOCA Phase of a PWR and the Prevention of Sump Screen Clogging

Hoffmann, W.; Kryk, H.

The Emergency Core Coolant System has to remove the decay heat in case of a Loss of Coolant Accident (LOCA). Sump strainers are mounted at the pump inlets to retain particles and fibrous insulation material during recirculation. However, fiber fragments, debris or corrosion products could initiate a critical head loss on strainers. Problems of insulation materials NUKON® (fiberglas) or CalSil and aluminum may appear if containment spray systems using alkaline additives are installed. In such cases, dissolution / precipitation reactions resulting from insulation materials were observed, which increase the risk of sump screen blockage. In German NPPs, there are no containments spray systems, and insulation consists of more resistant materials like mineral wool (rock wool) and stainless steel. However, large scale experiments from AREVA have shown that sump screen clogging may be initiated by boric acid containing leakage jets directed towards galvanized containment internals.
The down-scaled test facility KorrVA was designed for generic corrosion investigations of galvanized steel under post-LOCA conditions. About 90 experiments were carried out with galvanized steel gratings and galvanized steel coupons in boric acid media in order to determine corrosion mechanisms depending on different experimental conditions like temperature, water chemistry and hydrodynamic conditions (flow impact, simulated by different nozzles). Changes of the chemical composition of the circulating media were determined by chemical analysis and general parameters such as conductivity and pH were measured. Galvanized samples and fiber beds were examined after each experiment by means of photographic methods, light-microscopy and different kinds of chemical analysis.
The chemical analyses of the deposits on fiber beds showed that the clogging is predominantly caused by the corrosion products of iron and lower amounts of zinc compounds. Thus, the corrosion of galvanized steel in boric acid is explained by a mechanism starting at the surface with fast Zn dissolution but without formation of solid corrosion products. The Zn corrosion is mainly influenced by pH and concentration of zinc ions in the coolant. Since boric acid/borate acts as a buffer system, the pH value increases faster at the beginning and reaches up to 6.8 in case of sufficient solved Zn, generated by corrosion. A local (flow induced) corrosion occurs if a fast liquid flow strikes the top face of a horizontal galvanized coupon. Precondition for this process is a sufficiently low pH of solution in connection with a high hydrodynamic impact of the liquid flow on the corroding surface.
For a limited period, the risk of strainer clogging due to formation of corrosion products of galvanized steel may be reduced by an additional amount of submerged Zn or changing the coolant chemistry by alkaline additions. These two possibilities were investigated by test series using galvanized steel coupons. The addition of borax seems to be the most effective method to reduce the corrosion rate and the risk of sump screen clogging.
The results were validated with galvanized gratings in a further test series since the flow conditions of a liquid jet on flat coupons significantly differ from those on gratings. Three different regions of corrosion attack were noticed on these real samples contributing to the increase of solved Zn. But the Zn dissolution should be limited since it may lead to undesired secondary effects by deposition of sparingly soluble borate salts depending on coolant temperature.

Keywords: LOCA; zinc; corrosion; sump screen clogging; PWR

  • Poster
    NPC2012, Nuclear Plant Chemistry Conference, 24.-27.09.2012, Paris, Frankreich
  • Contribution to proceedings
    NPC2012, Nuclear Plant Chemistry Conference, 24.-27.09.2012, Paris, Frankreich
    CD-ROM

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Publ.-Id: 17564