HIGH TEMPERATURE WATER CHEMISTRY MONITORING

P. AALTONEN Metals Laboratory,

Technical Research Centre of Finland, Espoo, Finland

Abstract

Almost all corrosion phenomena in nuclear power plants can be prevented or at least damped by water chemistry control or by the change of water chemistry control or by the change of water chemistry.

Successful water chemistry control needs regular and continuous monitoring of such water chemistry parameters like dissolved oxygen content, pH, conductivity and impurity contents. Conventionally the monitoring is carried out at low pressures and temperatures, which method, however, has some shortcomings. Recently electrodes have been developed which enables the direct monitoring at operating pressures and temperatures.

Fig. 1. Interaction of environment monitoring and the deterioration model to give an assessment of plant integrity and life extension /Ford et al. 1987/.

1. INTRODUCTION

do

In order to extend the operating life of the nuclear power plants and to avoid corrosion related degradation it is important to estab-lish and maintain appropriate water chemistry conditions. This in-cludes at least continuous monitoring of pH, conductivity and impurity levels, oxygen content and related corrosion potentials of construc-tion materials at operating temperatures. Due to the complexity of the corrosion phenomena and the wide variations between plants, it is difficult to define one appropriate environment for all plants.

Therefore water chemistry monitoring should incorporate the field experiments and the laboratory test results to extend the operating time of components.

The purpose of this paper is to describe the water chemistry monitor-ing system developed at the Technical Research Centre of Finland

(VTT) .

2. NEED FOR WATER CHEMISTRY MONITORING IN PREVENTION OF CORROSION

The reliable operation of components in nuclear power plants needs evaluation and understanding of long-term deterioration mecha-nisms of potentially critical components. Since the environment has an influence on the degradation of materials through corrosion it is necessary to use and further develope environment monitoring systems.

Fig. 1.

In power plants extensive instrumentation and laboratory analysis programmes are applied to provide rapid and reliable diagnosis of water chemistry. However, at the moment chemical monitoring is applied mainly at low temperature, low pressure conditions or by using grab

samples. More relevant information concerning the chemical environment could be obtained by using a flow-through cell for high temperature, high pressure measurements of pH, conductivity and electrochemical potentials, which indicate the presence of oxidizing elements in the water. Fig. 2.

2.1. pH-Measurements at High Temperatures

pH i.e. the activity of hydrogen ions is highly influenced by the temperature. Due to the nature of pure water, pH usually shifts towards neutral values at higher temperatures. Generally the pH of base solutions is decreased and pH of acid solutions is increased when temperature increases. This neutralizing effect is caused by the interaction between the water and the dissolved species. The resulting pH of the solution is highly dependent on the buffering capacity of the dissolved species. However, the pH-shift and its direction is difficult to predict or calculate in complicated, dilute solutions at higher temperatures. Therefore it is particularly useful to be able to measure pH.

2.2. Conductivity at High Temperatures

The conductivity of pure water increases with temperature. The reason for this is partly due to the dissociation of pure water itself and partly due to the increase in impurities dissolved at higher temperatures. Additionally the deposition of dissolved ions changes the conductivity of the solutions to some extent if the temperature decreases. These are the reasons why the conductivity in pure water at high temperatures cannot be evaluated reliably by using low tempera-ture conductivity measurements.

Temperature

Conductivity electrode pH electrode

Fig. 2. Flow-through cell and electrodes for water chemistry moni-toring.

2.3. Electrochemical Potentials at High Temperature

The electrochemical potential of a metal is a measure of the equilibrium reaction obtained between the surface and the environment.

Because most metals form some kind of oxide layer on its surface in water containing solutions, the electrochemical potentials are mainly controlled by the oxygen content and the temperature of the environ-ment.

3. HIGH TEMPERATURE MONITORING SYSTEMS

The major area of interest in material water reactions monitoring systems for high temperature and pressure environments is corrosion reactions. Most metals are unstable with respect to water and their utilization in water containing environments depends on the kinetics of their corrosion reactions. In many cases the reaction rates are so small that they permit practical application of the materials. How-ever, corrosion reactions are dominating when the long term stability and integrity of energy and process plants are considered. In order to better predict the reliability of materials, the real service condi-tions should be known.

For the most common metals and alloys critical potentials for different corrosion processes has been determined by experimental laboratory tests. Thus pitting, crevice corrosion, stress corrosion and hydrogen embrittlement can be avoided if the metals potential is higher or lower than this critical potential in the specific environ-ment. The methods to control corrosion potentials are twofold; either the chemical environment i.e. the redox-potential can be controlled, or the corrosion potential of the material can be controlled by external current supply i.e. anodic or cathodic protection.

External reference electrode systems, in which the electrodes itselves are housed in separate compartments maintained at ambient temperature but in operation pressure via solution bridges, have made the measurement of corrosion potentials in high temperature aqueous environments possible. However, the temperature gradient between the high temperature environment and the reference electrode at ambient temperature gives rise to a thermal liquid junction potential/ which can be numerically corrected /MacDonald et al. 1979/.

Technical Research Centre of Finland (VTT) and Imatran Voima Oy (IVO) have jointly developed the on-line monitoring system for the power plant water chemistry monitoring under actual operating condi-tions, without pressure reduction or cooling of the sample flow. These flow-through measurement cells with electrodes have been in operation in the OECD Halden reactor since March 1987, Fig. 3, and in Loviisa PWR plants since June 1988, Fig. 4. At the end of 1989 monitoring of the water chemistry parameters in HDR-test reactor in Germany was started.

3.1. Structure of the Monitoring System High temperature pH electrode

The body of the pH sensor is made of stainless steel and is de-signed to withstand high operation pressures and temperatures. A

9|——T

§ 8

-a

0

__o- —— -°———'

D pHj measured

O pH at room temperature 11

- 10 JQ a

1.4 1.6 1.8 2.0 2.2 2.1» 2.6 ppm Li

Fig. 3. Measurements at OECD Halden ^reactor. The effect of Li concentration on the pH at 300°C and at 25 °C.

0.2 0.0

-0.40

Xo

-0.6 g ne -0.8

oo

2Z6.88 23.6. 24.6. 25.6. 26.6. 27.6.

DATE

1 Ammonia supply stopped, the removal of cations with the primary water clean-up system and the degasification of the primary circuit water is started.

2 An equilibrium of 5 ml/kg H2 is established.

3 The degasification of the primary circuit water is stopped and small amounts of ammonia is supplied to the primary circuit every three hours.

4 The reactor is subcritical and the boration of the primary circuit is started.

5 The cooling down of the primary circuit is started.

Fig. 4. The data from the monitoring system during shutdown at Loviisa 1.

classic pH-electrode, the glass electrode, is suitable for pHj detec-tion up to 120 °C. The use of stabilized zirconium oxide as a sensing membrane instead of the glass allows the pH detection up to 300 °C. The pHT sensing with zirconia membrane is based on the selective diffusion of oxygen ions through the membrane and this limits the lowest opera-tion temperature to about 150 °C. The pHT measurement technique also needs, in addition the pHT electrode, a stable, high temperature reference electrode.

High temperature reference electrode

The body of the silver/silver chloride reference electrode is made of stainless steel and is designed to withstand high operation pressures and temperatures. The salt bridge connecting the sensor with

the test solution is made of oxide powder saturated with the internal electrolyte. The internal electrolyte is usually a potassium chloride solution with a defined concentration. The entry of the internal electrolyte to the test solution is prevented by using porous ceramic plug, which separates the test solution from the salt bridge.

Conductivity electrode

The body of the conductivity electrode is made of stainless steel, and it can be used in high temperatures and pressures. The electrolytic conductivity is measured by the electrodes which are in contact with the test solution in such a way that the measured elec-trical conductance between the platinum plates can be related to the conductivity of the test solution. The cell constant of the electrode system can be changed allowing measurements in various test solutions.

3.2 Data Logging System

The data logging system consists of the following main parts seen in Fig. 5.

- measurement amplifiers

- personal computer with AD-converter - hardcopy device (printer, plotter) - software.

The pH, redox, conductivity, cell temperature and external temperature transmitters are included in the measurement electronics.

The measurement amplifiers usually are located near the flow-through cell, and the signals are transferred via triaxial cables from the electrodes to the electronics. The standard signals 4 - 20 mA are transferred from the measurement electronics via coaxial cables to the AD-converter, which is situated in the extension slot of the personal computer.

The software gathers and analyzes the data coming from the measurement electronics. The software runs on AT or compatible micro-computers under MS DOS -operation system and user interface. The software measures pHT, redox, conductivity and temperature continuously and displays measured signals graphically on the screen, stores data on the hard disc, and, if necessary, outputs data to the printer.

4. CONCLUSIONS

The on-line water chemistry monitoring system has proven to give reliable and useful information over a very long measurement periods.

The results obtained in PWR environments are used to study the differ-ences in the water chemistry conditions during the steady state operation and transients like shutdowns. The water chemistry has shown to be important in order to control the activity buildup, especially during the chemical changes in the primary circuit water. In BWR

oo OS

Measurement amplifiers

Fig. 5a.

Hardcopy device Data logging system.

Fig. 5b. The on-line monitoring system in operation in the sample line of the primary circuit in the Loviisa Power plant.

REFERENCES

environments the continuous monitoring of the redox potential and the conductivity of the coolant are important to avoid the stress corro-sion incidents of sensitized materials.

Material testing in simulated reactor environments is necessary for further improvement of nuclear materials. Simulation of reactor environments can be difficult, especially when tests are carried out in small scale laboratory test loops where the coolant volume to the internal metal surface area ratio is different from that in real power reactors. In order to be able to simulate the reactor conditions, the local water chemistry conditions in specific locations of the reactor should be estimated or experimentally measured. Based on these parame-ters the simulation can be carried out.

FORD, F. P., TAYLOR, D. F., ANDRESEN, P. L. & BALLINGER, R. G., Corrosion assisted cracking of stainless and low-alloy steels in LWR environments, EPRI, NP-5064 M, Palo Alto, CA (1987) . 100 p.

MACDONALD, D. D., SCOTT, A. C. & WENTRECK, P. J., Electrochera. Soc.

126(1979), p. 908.

COOLANT CHEMISTRY IN PRESSURIZED HEAVY