Control of corrosion product ingress into the steam generators

5.3.1.1 Reduction of the flow accelerated corrosion

Evaluation of field experience shows that an important source of the corrosion products entering the steam generators is the flow accelerated corrosion (FAC) at the wet steam areas of the steam water cycle. It is a process assisted by fluid dynamic mechanisms, which results in wall thinning of piping, vessels and equipment made of carbon steel and occurs only under certain conditions of flow, temperature, chemistry, geometry and material. The influence factors are discussed in Section 4.1.5.1 and 4.5.4.

Basic measures:

 Increase pH in the system. Target: to achieve a pH_Tws > 6.5 where T_ws is the temperature of wet steam at HP turbine outlet- cross under- moisture separator.

 By increasing ammonia concentrations until achieving a pH25°C close to 10.0 in final feed water.

Hydrazine can supply partly or totally the ammonia required. Copper elimination from BOP systems is required.

 By applying advanced amines, if ammonia are not sufficient or not applicable.

 Pipe replacement with low alloyed materials having a composition with more than 2.5% Cr content.

5.3.1.2 Removal of the copper materials from the secondary coolant system The reasons for copper replacement are described in Section 4.1.7.

Copper bearing materials corrode under the presence of ammonia and oxygen. This will happen especially at (but not limited to) the air exhaust sections of the main condenser at condensers tubed with copper bearing materials. Therefore, the ammonia concentration has to be maintained low in systems having Cu alloy surfaces in contact with the fluid. Therefore, the presence of copper bearing materials in the secondary side limits the operational pH-value in the final feed water, which adversely affects the corrosion product control, leading to a continuous increased ingress of corrosion products into the steam generators, promoting impurity enrichment and corrosion.

Besides, copper limits indirectly the hydrazine concentration, which produces ammonia by thermal decomposition, adversely affecting the reducing conditions necessary in steam generators.

Copper can also act as oxidant, promoting corrosion mechanisms like pitting or TSP/tube sheet denting.

As a consequence, the long term integrity of the SG tubing cannot be guaranteed unless frequent steam generator cleaning such as sludge lancing of the tube sheet area and/or chemical cleaning of the tube bundle is practiced, in order to keep the steam generators clean.

Less generation of corrosion products due to erosion corrosion in the steam systems will also beneficially influence the operational performance of the secondary systems (suppression of erosion corrosion) and will reduce inspection and repair work in this area.

The replacement of copper bearing materials from the secondary side is not mandatory. However, it is recommended because in this way the safety margin against possible corrosion phenomena can be additionally increased and steam generator fouling performance can be improved.

5.3.1.3 Minimization of corrosion product and impurity ingress during startup transients See fundamentals in Section 4.1.5.2.

After the outage at the beginning of the startup process before the heat-up starts, efforts should be made to minimize the inventory of corrosion products in the feedwater system, which could be transported to the SGs together with impurities, which may have been introduced into the feedwater train and various parts of the secondary system during annual outages. Consequently, it is important to carefully eliminate them before startup.

 Otherwise, the corrosion product input can represent a significant part of the SG inventory.

 Impurities that may be harmful for SG tubing must be eliminated before they hide out during power operation (concentration of impurities), which becomes significant above about 25% of nominal power.

It is important to purify the secondary system:

 By feed and bleed of the feedwater train before startup, if recirculation line with condensate polishing systems or mechanical filters are not considered in the design.

 Afterwards by the steam generator blowdown system.

This can be done by recirculation of water from the effluent of the high pressure heater to the condenser and purification. Sufficient startup time should be allocated to accomplish SG and feedwater clean-up. Corrosion products transported during startup are generally more oxidizing than those transported during normal operation, because of potential exposure to oxidizing conditions during lay-up, particularly for those plants, which do not use nitrogen blanket during lay-up. For these

reasons, it is important to pay attention to minimize the transport of corrosion products into the SGs during startup and to make efforts to reduce the oxidation state of the corrosion products that enter the SGs.

The feed and bleed of feedwater train is a good way for early elimination of ionic impurities before they enter the SG, if condensate and feedwater cannot be purified by Condensate Polishing System (CPS) using recirculation line. Then SG blowdown will be used before increasing the power. The objectives of chemistry during shutdown and startup operations are to keep a good water quality in order:

 To decrease the quantity of demineralised water that will be used to rinse the feedwater train during startup for reaching the specified values.

 Thus to decrease the volume of liquid effluents containing chemicals.

 To decrease time spent during startup for reaching the specified values.

5.3.1.4 Follow-up of corrosion product transport

By a follow-up of the system pH an indirect control of the corrosion product is performed.

This control is carried out measuring the pH25°C directly, and/or the amine concentration. The pH measurement is not very reliable and requires a careful calibration. The specific conductivity (SC) is almost exclusively function of the amine concentration, providing a very reliable way to follow up the alkalinity in the system, especially when only ammonia is used the SC measurement can replace the pH measurement. A comparative pH determination is shown in Figure 5.9. When Advanced Amines are used, the only alternative monitoring way is the direct measurement of the amines.

Figure 5.9. Accuracy of feed water pH measurements.

The estimation of the corrosion product inventory is important to assess the SG condition and decide about remedial actions such as chemical cleaning. This issue has been often disregarded, when the importance of corrosion product transport is not recognized.

A reliable measurement of the corrosion product at least in feedwater is necessary, and whenever possible in all relevant streams transporting corrosion products (main condensate, heater drains) especially in plants having a high CP input (low AVT)

9,2 9,4 9,6 9,8 10,0 10,2 10,4 10,6 10,8

0 50 100 150 200 250 300

Operation days

pH (25°C)

pH FW

pH FW calc. from SC pH FW calc. from NH3

The sampling of suspended solids represents a problem requiring well designed sampling nozzles, sampling lines, sampling point and sampling (filtration) equipment as well as specific sampling procedures.

The factors influencing the behaviour of particles submerged in a fluid are widely described in the literature [104, 105]. The main influencing factors are inertial forces (flow direction changes, sedimentation), thermophoresis, electrostatic precipitation, impaction (impingement), Brownian diffusion [106]. From these, the inertial forces are of major concern, followed by the electrostatic precipitation, which plays a role in case of small oxide particles (colloidal forms).

Sampling nozzles: isokinetic probes are sometimes used. However they are not required under the feedwater sampling expected conditions [13].

Making an analysis of the drag forces⁵ to which a particle submerged in a moving fluid and considering the terminal velocities of the particles under the sampling conditions (temperature, particle diameter, density of the liquid and the particle, Reynolds number) [104, 105]. It results evident that if the flow velocity of the sampling stream is 1 to 2 m/s (linear flow velocity recommended by ASTM:

1.8 m/s [107, 108]), particles of ~ 3 µm in diameter will tend to drift out of the flow line at a terminal velocity of only about 1% of the fluid velocity at 300°C, and even lower at 50°C. That means, if the fluid changes direction, the particle will ‘follow’ well the fluid stream and a representative sampling of such particles is possible.

Anyway, the nozzle must have a design avoiding wall effects, like that shown in Figure 5.10.

Figure 5.10. Sampling nozzle of ‘inclined face’ type for corrosion product sampling (DIN).

Sampling lines: turbulent flow with (Re = Dvp/µ > 8000) is required. This corresponds with a linear flow rate of 1 m/s in a DN6 line. A common mistake is to use too big lines at low flow rates. The lines must be short (<10 m) to avoid perturbations. This makes necessary to implement local sampling points, not centralized. Horizontal lines must be avoided to prevent deposition.

Sampling point should be chosen sufficiently far away from flow perturbations like elbows, valves.

Sampling devices must ensure a continuous uniform sample flow. The sample analysis is performed in many different ways, not being the limiting factor.

5 The drag force represents the ‘resistance-to-movement’ of the particle relative to the surrounding fluid. A high value of Fd means that the particle will tend to have a high resistance to inertial forces, like for example those derived from the change of direction of the fluid. The drag force (‘friction’ force) will be bigger for higher fluid density and viscosity, keeping al the other parameters constant.

5.3.2 Redox potential conditions in secondary circuit 5.3.2.1 Oxygen control

Besides the minimization of deposits, it is imperative to ensure reducing conditions in the SG environment. This prerequisite is achieved by adding hydrazine to secondary circuit⁶. Fundamentals about the system redox potential condition requirements are given in Section 4.1.5.1. Nevertheless, due to difference in design of the BOP of several OEMs, some characteristics have to be considered.

See also Section 4.1.7. Two main design concepts can be recognized being the major difference in the feedwater supply.

The first one has no feedwater deaerator in its design due to cost reduction reasons and supplies the feedwater during plant startup operations either from the ‘auxiliary feedwater tank’ (AFT) or from the

‘condensate storage tank’ (CST). This concept is used typically in USA PWRs but also in many other countries like in some EDF plants in France. In this concept the oxygen control is performed by hydrazine injection for all operation modes including plant startup operations by airtight sealing of the AFT and CST.

The second design concept considers the use of feedwater deaerator tank or feedwater tank. This concept has the advantage of supplying the SGs with oxygen free hot feedwater during all operation modes including the startup operations. All Siemens-KWU PWRs, Japanese and the new EPR™ are using this secondary side concept. Feedwater dearators are also used in other PWRs, like all Japanese PWRs, many CANDU plants and several other PWRs like several EdF plants in Europe.

The rate of the reaction between hydrazine and oxygen is very slow at low condensate temperatures and starts to accelerate at temperatures above 150°C. The rate of the reaction above 150°C is more than 300 times faster than at temperatures below 100°C [91]. The reaction is also a catalytic reaction with corrosion products. Therefore the reaction rate is accelerated especially in the tube bundle area of the feedwater heat exchangers, where huge amount of surface area covered with corrosion products is exposed to hydrazine and oxygen containing water (Figure 5.11).

In the feedwater line, the surface area is relatively smaller, accordingly, the reaction rate is also remarkably slower than at the tube bundle area. This fast oxygen removal by hydrazine in the tube bundle of the HP feedwater heat exchangers can also be confirmed by visual inspection of the surface colours at the inlet und outlet of the heat exchangers. Red colours indicate still presence of oxygen, whereas the black colours confirm the oxygen free reducing conditions. The amount of hydrazine to be injected for the oxygen control is plant specific depending on the air leak tightness of the secondary side.

It has to be noted, that hydrazine is used to ensure reducing conditions in the steam generator not in the feedwater line. As mentioned before in Section 4.1.5.1, red oxide colour indicates hematite (Fe₂O₃) whereas the black one is magnetite (Fe₃O₄). Hematite is less soluble than Magnetite, which leads to a less corrosion product ingress into the steam generators. Several organizations, like Siemens and Japan Atomic Power Company (JAPC), have started to investigate several years ago the possibility of establishing local oxidizing conditions by oxygen injections into secondary side without affecting the reducing conditions in the SGs. These investigations confirmed that one of the most important issues is the monitoring of the oxygen in the effected system areas and especially in the final feedwater up-stream SGs, in order to ensure the reducing conditions for SGs.

6 The replacement of hydrazine by other components is being searched, owing to the carcinogenic nature of this reagent, but there is still no equivalent, qualified alternative.

Figure 5.11. Measured oxygen profile along feed water train (Initial levels of O2 = 5 ppb and N2H4 = 100 ppb, pH 9.8) [109].

The formation of hematite can be achieved by only a small increase of the redox potential by 100 mV, which yields to a decrease of the iron solubility by factor 10. Figure 5.12 shows the calculated iron solubility as a function of the redox potential at constant pH of 6.39 at 280°C. Therefore it is rather recommended to obtain ‘red surfaces’ in the feedwater system. Only small amount of oxygen is needed to achieve this (Figure 5.13).

Figure 5.12. Iron solubility as a function of redox potential at a constant pH (280°C) = 6.4 [110].

Figure 5.13. Pourbaix-Diagram for the system Fe-H2O (280°C) [110].

Oxygen injection Oxygen injection

0,0000001 0,000001 0,00001 0,0001 0,001 0,01 0,1 1 10

-0,8 -0,6 -0,4 -0,2 0 0,2 0,4

Potential [V]

cFesol [µg/kg]

Fe – H2O – System at 280.000 ºC

The first known published feedwater oxygen treatment in a PWR was performed in 1984, at the Biblis-B plant in Germany [111]. At that time, the plant was operating under low pH AVT conditions with a feedwater pH_25°C < 9.5, due to existing copper bearing tubes in the condensers. In this plant, two separate feedwater lines are feeding two SGs separately from the feedwater tank. During the annual inspection, it was discovered that one of these feedwater lines was black coloured and the other was red coloured. The black coloured line had significant FAC damage on the carbon steel flow deflecting grids in the HP feedwater heater, whereas the red one had no damage, see Figure 4.23.

Obviously, the feedwater oxygen removal was not homogenously in the large sized feedwater tank (500 m³ longitudinal tank). As a temporary measure to counteract FAC, the utility had decided to inject oxygen with aerated make-up water into the feedwater line up-stream feedwater pump. This oxygen treatment was successful to stop the FAC in the HP feedwater heater in the black feedwater line.

In Japan oxygen injection treatment has been also considered at the Tsuruga-2 PWR unit due to carbon steel FAC damage at a limited location. Before introduction of the so-called oxygenated water chemistry (OWC) treatment, JAPC has performed detailed preparatory work and a demonstration test at Tsuruga-2 [109, 112, 113]. The objective of these investigations was to confirm that the reducing conditions in the SG would not be affected. After finalizing the preparatory work, JAPC performed a demonstration test at Tsuruga-2 PWR.

During this demonstration test, oxygen was injected up-stream the feedwater booster pump with an injection rate to achieve < 5 μg/kg O₂ in the feedwater. The hydrazine injection was adjusted to maintain a feedwater pH_25°C of 9.8 and a hydrazine concentration of 100 μg/kg. The test was monitored by oxygen, ECP, and iron concentration measurements up-stream and down-stream HP feedwater heaters. The ECP measurements at Tsuruga 2 plant confirmed the protection of carbon steel surfaces against FAC by oxygen injection. Based on these results, JAPC is planning in the next future to implement oxygen injections (OWC treatment) in combination with high AVT. The concept of this chemistry is illustrated in Figure 5.14.

As described the operational experience showed that even if oxygen is dosed in the secondary circuit the prerequisite of reducing conditions in SGs is achievable. Nevertheless, the plant specific behaviour has to be considered.

Figure 5.14. Full scale OWC concept planned for Tsuruga-2 [109].

5.3.2.2 Follow up

O2 measurements represent also a problem. Long sampling lines transporting sample at high temperature will result in a lower oxygen concentration than that actually existing in the feedwater line (see Figure 5.15).

In principle, as for corrosion products, it would be better having a short line, or at least having the sample cooler close to the sampling point and not in the central sampling panel as commonly found.

A better alternative to a direct O2 measurement is an on-line redox/corrosion potential measurement [114, 115].

The electrochemical potential on-line measurements started to be performed in the 1980s and since then it has been adopted as control parameter of reducing conditions. The Figure 5.16 clearly shows the lack of sensitivity of the oxygen on-line measurement in feedwater using the centralized plant sampling system (curve 6), showing almost no response even if the concentration in main condensate achieved the order of 40 ppb caused by air in-leakage (curve 5). The redox potential measurement directly in feedwater (curve 2) shows a variation following the oxygen concentration in feed water, as well as the corrosion and redox potentials measured inside the SG using provisory electrodes adapted to a SG inspection hole. Redox potential should be seriously considered as the most suitable method for follow up of reducing conditions.

Figure 5.15. Influence of the sampling line length on the O2 measurement.

Figure 5.16. Redox and corrosion potential measurements in the SGs and feed water of a PWR.

Comparison with oxygen measurements.

Main condensate

Feed water

5

6

Time Potentials:1: Redox potential in steam generator

2 : Corrosion potential in steam generator 3: Redox potential in feed water 4: Corrosion potential in feed water

O2 measurements:5: Main condensate 6: Feed water

Potential VSHE