Introduction to Electrochemical Techniques Used in Nuclear Energy Development: Coolant Chemistry Monitoring

In the nuclear energy area, most chemical reaction takes place in very severe conditions compared with those in other fields. In reactor coolant system, the operating temperature is over 300 .deg. C, much higher than the temperature we are familiar with. For recycling of fuels, fissile elements including uranium in spent fuels are electrochemically reduced and refined in molten salts. Especially, in the next generation reactor systems called GEN-IV, they need lots of chemical information under severe conditions such as high temperature steam, molten metal, supercritical water and so on. In such severe media, generally two kinds of techniques have been used for the measurement and control of the chemical conditions and reactions. These are the spectroscopy and the electrochemistry techniques. However, the electrochemical technique is only effective method to control chemically the systems. Therefore, lots of electrochemical techniques have been developed and used in nuclear energy areas. The chemistry of the reactor coolant is one of the most important areas to improve the safety and efficiency of plants operation by electrochemical techniques. Among the chemical factors, the coolant pH in the important one to control the corrosion process of the metal structure. However, the current pH measurement by sampling at room temperature has a limitation for directly monitoring the chemical condition of the coolant at high temperature. Therefore, real-time pH monitoring techniques have developed for last several decades. Macdonald et al. have developed many YSZ (yttria stabilized zirconia) pH electrodes. In spite of their endeavor, there are still some difficulties associated with the electrode accuracy and stability. In order to improve the performance of pH measurement at high temperature, we added an enhanced function to the system for the calibration of the measurement devices. Another chemical factor is a redox potential. Instead of the redox potential, the concentration of dissolved H2 (DH) has been used to evaluate the redox condition of the coolant. In addition, the electrical conductivity, a chemical factor, depicts the degree of impurities in the coolant. Until now, these two methods, DH and conductivity, have been also used for measuring chemical condition in nuclear reactor coolant. However, these two methods also have the limitation for applying at high temperature, because they have been developed to be used at room temperature. In order to overcome the current limitation, we developed a real-time chemical measurement system, which can be used in high temperature water, by adding some of electrochemical techniques to the measurement system. This measurement system could be a promising method for directly monitoring the proton (pH) and impurity ions in high temperature coolant