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The NPP Borssele was designed and built in the late sixties, early seventies. At that time it was common to use low and unalloyed carbon steels for the secondary systems in conjunction with phosphate chemistry. Due to the amount of copper in the secondary systems other chemistry was not obvious.
In the early years the corrosion that was experienced was called erosion corrosion.
When more knowledge was established about the cause of this form of corrosion, it became known as Flow-Accelerated Corrosion (FAC) in the nineties.
In the early years FAC was experienced in Borssele in about 80% of the secondary system. Much had to be learned about this kind of corrosion. Complete systems had to be replaced after a few years of operation.
All kinds of measures were taken.
- Accessible inner pipes and components were spray metal coated with Metrology.
- Heat exchanger tubes were replaced by steels with higher chromium content.
- Piping parts with high flows (velocity) or direction changes had to be replaced.
- Stainless steel skirts were mounted in the outlet site of valves where the velocity was high.
- Baffle plates were periodically replaced.
- Differential pressure over tube support plates was reduced.
- Etc.
It was clear that the chemistry at that time in combination with the used materials was affecting the safety and reliability of the plant. Together with the designer of the plant (KWU) a program was developed to monitor and take mitigating action to "fight" FAC. This program took into account:
- material content of alloys
- pH
- oxygen content
- temperature
- Flow, geometry and velocity
A simple system was developed to determine the most affected systems and components. In the most affected systems the geometry was looked at to determine the most affected spots this was partly done with the isometric drawings and partly by plant walk downs. Special attention is given to flow changes like orifices, elbows, baffle and impingement plates, branches, elbows, Isotope nozzles, tube support plates, etc.
Measures were taken to change the chemistry in the secondary system. Important was the reduction of the experienced FAC rate and to reduce the transport of iron to the steam generators. The copper alloy condenser tubing was replaced by Titanium tubing. By this reduction of copper its was possible to slowly increase the pH from 9.1 to about 9.9 by changing the chemistry from Phosphate chemistry to All Volatile treatment chemistry (AVT). The FAC rate reduced rapidly with approx. a factor 100.
The Surry and Mihama incidents were moments for Borssele to look extra into the applied program. Borssele is still following the events and experiences in the world about FAC and tries to be pro active in its program to manage FAC for at least 60 years of lifetime of the majority of the systems and components.
In addition to the originally developed program, systems are inspected where the FAC rate may be affected by the lack of (small amounts of) oxygen to maintain a protective layer in for example drain lines.
We have to be aware of the effect of system changes.
FAC demands a multi disciplinary approach to combine knowledge about amongst others corrosion, chemistry, operation, design and the particular lay-out of the plant.
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