Method and apparatus for the quantitative analysis of solutions and nuclear water quality control system

Abstract: This invention comprises electrochemical analytical devices (2) capable of performing electrolysis in multiple pulse modes. It relates to a method of quantitative analysis and equipment for performing quantitative analysis which uses a combination of these pulse modes to realize the measurement of concentrations of multiple substances to be measured, occurring in the test solution to be measured and also a nuclear reactor(1) water quality control system employing such a quantitative analysis method.

Description: This invention relates to a method and an apparatus for the quantitative analysis of solutions according to the preambles of the independent claims 1 and 8. Such a method and such an apparatus have been known from EP-A-0 122 511. The invention also relates to a nuclear reactor water quality control system intended for water quality control for nuclear reactors. This invention is suitably applicable to water quality testing and control in boiling water reactors (BWR) and pressurized water reactors (PWR). The water used in nuclear reactors contains dissolved H₂, O₂, and H₂O₂, The amounts of these dissolved gases change as the gases circulate in the nuclear reactor cooling system For each gas, there is a maximum tolerable limit, beyond which the gas can corrode the reactor pipes, posing a potential danger of water leak or even pipe breakage. Therefore, an important requirement is to sample the high-temperature, high-pressure water circulating in the cooling system of a nuclear reactor, to perform quantitative analyses of H₂, O₂, and H₂O₂, and to ensure an adequate control of the water quality based on the results of the analyses. Among the means of simultaneously detecting dissolved O₂ and H₂O₂ contained in the reactor water in a nuclear reactor is a method of simultaneously measuring dissolved oxygen and hydrogen peroxide by using a dissolved oxygen meter and a hydrogen peroxide decomposition activated charcoal column, published in JP-A-55-142437. The above prior art, however, provides no specific information on performing a quantitative analysis of dissolved oxygen in the reactor water. It is unclear whether or not this prior art is applicable to the analysis of high-temperature, high-pressure water in the primary cooling system of nuclear reactors. EP-A-0 122 511 discloses a method and an apparatus for measuring dissolved gas concentrations. In this apparatus, two dissolved gas components in sample water, e.g., oxygen and hydrogen in core water, are readily quantitatively determined in an oxygen meter by using a membrane producing an overlapped potential region where a potential region for an oxidation limiting current plateau is overlapped with that for a reduction limiting current plateau between two dissolved gas components. Between an electrode and a counterelectrode a more positive potential than the equilibrium potential and a potential more negative than the equilibrium potential, respectively, is applied, and the current generated between the electrodes is measured. It is the object of the invention to provide a method and an apparatus allowing an improved detection of chemical species present in a solution, and a system for nuclear reactor water quality control. This object is achieved in accordance with the features of the independent claims 1, 8 and 17 ; the dependent claims are directed on preferred embodiments of the invention. The method of the present invention for the quantitative analysis of solutions comprises the steps of contacting a working electrode, a counterelectrode and a reference electrode in an electrolytic cell with a solution to be tested containing a plurality of chemical species, electrolyzing the solution by applying different voltages for the chemical species to be analyzed to the electrolytic cell; determining current values, according to the kind of voltammetry employed, from the electrolysis current resulting from the above electrolysis step, and calculating the concentrations of the plurality of chemical species to be analyzed, based on the current values thus determined; it is characterized in that in the electrolyzing step different voltage-time waveform signals for the chemical species to be analyzed are applied, in the determining step limiting current or peak current values and the voltage required to produce the electrolysis currents are determined, and in the computing step the concentrations are computed based on the limiting current or peak current values. The apparatus of the present invention for the quantitative analysis of solutions, particularly for carrying out the above-defined method comprises an electrolytic cell for electrolyzing the solution to be tested, the cell being provided with a working electrode, a counterelectrode and a reference electrode; voltage application means for applying different voltage signals having specified parameters determined by the kind of voltammetry employed, to the working electrode in said electrolytic cell as reference electrode standard voltage, current determination means for determining currents obtained by electrolyzing the sample, and concentration calculation means for calculating the concentrations of the chemical species to be analyzed, based on the current values thus obtained; it is characterized in that the voltage application means apply different voltage-time waveform signals for the chemical species to be analyzed, the current determination means determine limiting current or peak current values based on the currents obtained by electrolyzing the sample, and the concentration calculation means (16) calculate the concentrations based on the limiting current or peak current values. The voltage-time waveform signal applied to the working electrode in the electrolytic cell has certain parameters which are determined by the kind of voltammetry employed. When a solution containing H₂ and H₂O₂ or a solution containing O₂ and H₂O₂ is electrolyzed by simple electrochemical procedures, the observed electrolysis current contains current components of both H₂ and H₂O₂ or both O₂ and H₂O₂, and it is impossible to determine the individual concentrations of H₂, O₂, and H₂O₂. According to the present invention, on a solution containing two chemical species, H₂O₂ and either H₂ or O₂, two kinds of voltammetry are performed; similarly, on a solution containing three chemical species, H₂, O₂, and H₂O₂, three kinds of voltammetry are performed. Each voltammetry is performed by changing the parameters for the voltage-time waveform signals which are applied to the electrolytic cell, thus allowing simultaneous determination of the individual concentrations of H₂, O₂, and H₂O₂. Electrolytic analysis is well suited to in situ quantitative analysis of high-temperature, high-pressure water in the primary cooling system of a nuclear reactor. Further, its fast concentration determination capability lends itself to rapid control of water quality of the reactor water. A favored method of voltammetry for the purpose of this invention is the pulse-mode pulse voltammetry. In the embodiments of this invention, to be described later, favorable results have been obtained by using normal pulse voltammetry and differential pulse voltammetry. The types of pulse voltammetry that can be used for the purpose of this invention include: normal pulse voltammetry, reverse pulse voltammetry, differential voltammetry, differential normal pulse voltammetry, and square-wave pulse voltammetry. A discussion of the pulse mode in computerized pulse voltammetry may be found in Analytical Chemistry, Vol. 43, No. 3, March 1971, pp 342-348. A detailed description of the differential normal pulse voltammetry may be found in the Journal of Electroanalytical Chemistry, 110 (1980) pp 1-18. The application of the square-wave voltammetry to the quantitative analysis is discussed in Analytical Chemistry, 49, 13 (1977) pp 1904-1908, and in Analytical Chemistry 36 (1987) pp 420-424. Discussions on normal pulse voltammetry, differential pulse voltammetry, and reverse pulse voltammetry, and a comprehensive theory on pulse voltammetry, may be found in the Bulletin of the Chemical Society of Japan 53 (1980) pp 3439-3446, and the Journal of Electroanalytical Chemistry 205 (1986) pp 21-34. However, in the above publications pulse voltammetry is discussed from a purely electrochemical standpoint. Until now, most of the discussions concerning the applicability of pulse voltammetry to quantitative analysis were centered on the merits of analytical methods using a single-pulse mode, the reason being that pulse voltammetry employed in the single-pulse mode offers the highest accuracy among the current voltammetry methods, and that for most quantitative analysis purposes the attainable accuracy of the single-pulse mode is sufficient. However, there has been little discussion on quantitative analytical methods employing a combination of several types of pulse modes. In the quantitative analysis apparatus for the purpose of this invention, a potentiostat should be provided as voltage application means for applying voltage-time waveform signals to the working electrode in the electrolytic cell, and a CPU should be provided for supplying the waveform signals to the potentiostat. The CPU should provide the following features: a signal input unit for entering into the potentiostat voltage-time waveform signals having specified parameters according to the kind of voltammetry employed; a storage unit for storing in memory the results of voltammetry analyses, for each chemical species in the sample solution to be analyzed and treated by the potentiostat, as a relationship between the electrolysis current and the voltage required to produce the electrolysis current; current determination means which process the data stored in the storage unit and which determine the limiting current or peak current values according to the kind of voltammetry employed; and concentration calculation means which calculate the concentrations of the desired chemical species, based on the determined limiting current or the peak current values. The function of the potentiostat is to accurately apply the voltage-time waveform signals, supplied by the CPU with function generators, as reference electrode standard voltage to the working electrode. In the above calculation step of the method according to the present invention, an equation in which the concentrations of the desired substances are treated as unknown variables is set up for each voltammetry, and the concentrations of the desired substances are determined by solving these equations as a set of simultaneous equations. The nuclear reactor water quality control system of the present invention comprises analysis means for performing a quantitative analysis of the reactor water in the primary cooling system of a nuclear reactor based on electrolysis, and reactor water control means for controlling the water quality of the reactor water based on the results obtained from the analysis means; it is characterized in that the analysis means are an apparatus according to the invention as defined above, and the reactor water control means control the water quality of the reactor water based on the concentrations calculated in the analysis means. In accordance with a preferred embodiment, the water quality control system comprises an apparatus as defined above which is characterized in that the voltage application means comprise a potentiostat and a CPU comprising a signal discharge unit for applying voltage-time waveform signals which are put in from the CPU to the working electrode in the electrolytic cell, the CPU being provided with a signal input unit for entering voltage-time waveform signals, having specified parameters which are determined by the kind of voltammetry employed, in the potentiostat; and it further comprises a storage unit which stores in a memory the results of the voltammetry performed by the potentiostat, for each chemical species to be analyzed and present in the test solution, as a relationship between the resulting electrolysis current and the voltage required to produce it, wherein the current determination means process the data stored in the storage unit. This nuclear reactor water quality control system comprises an electrode housing unit in the electrolytic cell, which is installed in at least one location in the primary cooling system so that the cell comes into contact with reactor water; a signal discharge unit which applies voltage-time waveform signals having specified parameters to the working electrode in said electrolytic cell as reference electrode standard voltage; a signal input unit which inputs to the signal discharge unit voltage-time waveform signals having parameters which are determined by the kind of voltammetry utilized, for each chemical species contained in the primary cooling system, and by changing the parameter for each chemical species involved; a storage unit which stores in a memory the results of the voltammetry performed according to the signals supplied by the signal discharge unit, in terms of a relationship between electrolysis current and the voltage required to produce the electrolysis current; a current value determination unit as current determination means which processes the data stored in the storage unit and which determines the limiting current or peak current, according to the kind of voltammetry employed; a concentration calculation unit as concentration calculation means which calculates the concentrations of the desired chemical species in the reactor water, based on the limiting current or peak current values thus determined; and reactor water control means which regulate the water quality of the reactor water, based on the information produced by the concentration computation unit. The electrode housing unit for the electrolytic cell should be installed in the reactor core, in the reactor core outlet, or in the water vapor separator in the reactor. Particularly, it should be installed in the reactor core. If it is installed in the reactor core, it should be installed in an instrumentation column, such as a neutron instrumentation column. Further, in the electrolytic cell, a hydrophobic membrane permeable to gases should be provided; and the dissolved gases in the reactor water should be allowed to come into contact with the electrodes through the gas-permeable membrane, to prevent leakage to the outside world of electrolytic substances, such as potassium chloride, potassium nitrate, and sodium sulfate, which are provided in the electrolytic cell for the purpose of increasing the electrical conductivity of the solution. An example of a hydrophobic gas-permeable membrane that might be used is a membrane made of polytetrafluoroethylene. Examples of water quality parameters to be controlled by performing a quantitative analysis of the reactor water are H₂ and O₂, and the amount of dissolved hydrogen (DH) which is to be injected into the vaporizer in the reactor. In a nuclear reactor, it is possible to control the amount of H₂ in the reactor water in terms of H₂ data, O₂ data, H₂O₂ data, or by a combination of these 2 or 3 types of data, obtained by quantitative analysis. The amount of N₂H₂ to be injected into the vaporizer should be controlled in terms of the O₂ concentration data obtained by the quantitative analysis. In a system designed to control the water quality of reactor water, there should be a concentration comparison unit for comparing the results produced by the concentration calculation unit of the CPU with standard values. Such a comparison unit should be provided in the CPU. In the water quality control system of the present invention, the water quality should be regulated by measuring at least one among the variables of the reactor water conductivity, the pH, or the electric potential at the sampling point during the quantitative analysis of the reactor water, and by taking this information and the dissolved gas information into consideration. The nuclear reactor water quality control system as defined above may preferably comprise means of setting off an alarm when the results obtained by the quantitative analysis means exceed a predefined value. Such a system comprises the following elements: An electrode housing unit for the electrolytic cell, which is installed in at least one location in the primary cooling system so that the cell will come in to contact with reactor water; a signal discharge component which applies to the working electrode of the electrolytic cell voltage-time waveform signals having specified parameters, as reference electrode standard voltage; a signal input unit, which supplies to the signal discharge unit voltage-time waveform signals having parameters according to the kind of voltammetry employed, for each chemical species contained in the primary cooling system, the signal input unit being capable of changing the parameter for each chemical species; a storage unit which stores in a memory the results of the voltammetry performed according to the signals supplied by the signal discharge unit, and storing the data in terms of a relationship between the electrolysis current and the voltage required to obtain the electrolysis current; a current value determination unit which processes the data stored in the storage unit and which determines limiting current or peak current values, according to the kind of voltammetry employed; a concentration calculation unit as concentration calculation means which calculates the concentrations of the desired chemical species in the reactor water, based on the determined limiting current or peak current values; a concentration comparison unit which compares the results of the concentration calculation unit with standard values; and the aforementioned alarm generator which sets off an alarm when the results produced by the concentration calculation unit, based on the information supplied by the concentration comparison unit, exceed the standard value. It is even more desirable that the primary cooling system of the nuclear reactor be provided with means for measuring the ion concentrations of SO 4 2- , SO 3 - , Cl⁻, and Na⁺, which are the decomposition products of chemical resins which are provided for the purpose of cleaning the nuclear reactor water; and means for setting off an alarm when one or more of these ion concentrations exceed the standard value. In accordance with a preferred embodiment, the system is an accident forewarning system for the primary cooling system of a nuclear reactor which predicts a potential accident in the primary cooling system, based on the information supplied by the comparison unit. It is highly desirable that the above-described nuclear reactor water quality control system, reactor water accident prevention system, or the accident prediction system be provided with a monitoring system to ensure that the signal input unit, the signal supply unit, the concentration calculation unit, and the concentration comparison unit are working properly. As an additional feature, in the event of a malfunction detected by the monitoring system, the system should be able to set off an alarm. Any input of signals generated by the signal input unit to the signal discharge unit should be performed via a function generator. Among the prior art relating to water quality assurance for nuclear reactors is a method for reducing the concentration of radioactive species in the reactor water by controlling the concentrations of nickel and cobalt ions in the water supply in such a way that the concentrations will be higher at the beginning of the reactor operation, declining gradually with the passage of time, according to their concentration change curves (JP-A-61-148394). However, there is no mention in this document of a quantitative in situ analysis of chemical species such as H₂, O₂, and H₂O₂ found in the cooling water system, or a method of controlling the water quality by feeding the analytical information back to the control system. JP-A-60-238791 discloses a problem diagnosis system for nuclear power plants. This system compares with boundary values the deviation of data collected from the plant from the output values obtained by entering the data into a reference model, so as to automate the process of identifying the type of breakage that has occurred, based on codes generated by the model. Concerning plant emergency operation support equipment, JP-A-61-164194 describes a system consisting of a detector for measuring a process quantity of the plant, a monitoring device for checking the conditions of multiplexed pieces of equipment, three evaluation methods, and display units, for reducing the workload on the operating personnel. Adequate methods for quantitatively analyzing the concentrations of H₂O₂, O₂, and H₂ are necessary in many fields, especially for controlling the corrosive environment prevailing in nuclear reactor water cooling systems. Among the existing prior art regarding the measurement of the concentration of H₂O₂, JP-A-55-146035 describes a method of electrolyzing an aqueous solution of H₂O₂ under constant cathode voltage, and determining the concentration of H₂O₂ by measuring the resulting electrolysis current. However, in cases where both O₂ and H₂O₂ are present, the electrolysis current observed under this method will contain the current component due to O₂ as well that due to H₂O₂. Thus, this method does not allow a determination of the concentration of H₂O₂ alone. A device for determining the concentration of H₂O₂ in the presence of O₂ is described in JP-A-56-37549. This device removes the effect of oxygen by dipping both an oxygen detector and an auxiliary oxygen detector in the test solution. However, since this device contains an oxygen-permeable membrane and a hydrogen peroxide decomposition membrane, its analytical performance can be significantly affected by the surrounding environment. At any rate, it cannot be used in high-temperature, high-pressure nuclear reactor cooling water. Quantitative analysis devices for the analysis of O₂ or H₂, applicable under high-temperature, high-pressure conditions, have been developed, such as the devices described in JP-A-59-174748 and JP-A-59-3345. However, these devices do not allow a quantitative analysis of O₂ or H₂ in solutions in which H₂O₂ occurs together with O₂ or H₂. To achieve an in situ analysis of the water quality of nuclear reactor water and to control the water quality, a technique for the simultaneous quantitative analysis of O₂, H₂ and H₂O₂ is essential. A survey of existing reactor water monitoring techniques, even if the survey is narrowly focused on this requirement, indicates that no such techniques exist in the public domain, as discussed above. The electrolysis currents, measured by running several types of pulse voltammetry using different voltage-time waveform signals, are given in different function forms. Therefore, the diffusion limiting current values and peak current values of a current/ voltage curve, consisting of additive electrolysis current components of the several types of the substances to be measured, which have been measured in different pulse electrolysis modes, can be expressed for each pulse mode as independent equations whose unknown variables are the concentrations of the target substances. Therefore, the concentrations of these substances can be determined by solving these equations by treating them as a set of simultaneous equations. Through execution of multiple voltammetries, a voltammetry device, especially a pulse voltammetry device, which is connected to a computer is capable of performing simultaneous quantitative in situ analyses of multiple chemical species present in a reactor water system. Such a device can also print or display the analytical results to present the results of water quality tests in clear format to human operators, thus helping humans to monitor the water quality. This voltammetry instrument system can compare previously entered standard values for various chemical species with the results of simultaneous quantitative in situ analyses by using a computer, and based on this comparison, can feed gases and chemical species rapidly and accurately into the reactor in order to adjust its water quality. In this manner, the device performs water quality control. In the following, the invention will be explained with reference to the drawings. Figure 1 is a schematic configuration diagram showing a BWR plant water quality control system containing a tri-electrode system inside instrumentation columns. Figure 2 is a schematic configuration diagram showing a BWR plant water quality control system containing a tri-electrode system in an electrolytic cell which is provided with permeable membranes and which is installed in an instrumentation column. Figure 3 is a schematic configuration diagram of a BWR plant water quality control system equipped with a safety diagnosis system. Figure 4 is a schematic configuration diagram of a BWR plant water quality control system, in which water quality monitors are provided in various locations. Figure 5 (a) is a reduction current/voltage curve of O₂, using normal pulse voltammetry. Similarly, Figure 5 (b) is a reduction current/voltage curve of H₂O₂. Also similarly, Figure 5 (c) is a reduction current/voltage curve of O₂ and H₂O₂, whose concentrations are equal to those shown in Figures 5 (a) and (b). Figure 6 is an applied signal waveform diagram for normal pulse voltammetry. Figure 7 is a characteristic diagram showing the influence of impurities on sensitized SUS304 steel, as determined by slow strain rate tests (SSRT). Figure 8 is a characteristic diagram showing the influence of impurities on sensitized SUS316 steel, as determined by SSRT tests. Figure 9 is a correlational graph showing the IGSCC sensitivity of H₂O₂-O₂ at 150°C. Figure 10 is a characteristic diagram showing the influence of the pH and anions on the IGSCC of sensitized SUS304 steel in hot water containing oxygen. Figure 11 is a characteristic diagram showing the influence of the corrosion voltage on the speed of fracture in sensitized SUS304 steel. Figure 12 is a characteristic diagram showing the time dependencies of corrosion voltage and dissolved hydrogen concentration which are regulated by an H₂ injection control system. Figure 13 is a characteristic diagram showing the time dependencies of the dissolved O₂, dissolved H₂, dissolved H₂O₂, iron ion concentration, pH, conductivity, and corrosion voltage, which are regulated by a water quality control system. Figure 14 is an enlarged view of the electrode housing unit shown in Figure 1. Figure 15 is an enlarged view of the electrode housing unit shown in Figure 2. Figures 16 - 19 are applied signal waveforms of the various voltammetries. In the following, this invention will be further explained with reference to embodiments; however, this invention is not limited to the embodiments given below. Example 1 Example 1 illustrates the location of installation of a tri-electrode system in a BWR nuclear reactor pressure vessel, the configuration of the tri-electrode system, an analytical system, a water quality control system, a safety diagnosis system, analysis implementation examples, and the underlying principles of analyses. Figure 1 shows the peripheral structural constituent elements in a BWR nuclear reactor pressure vessel 1 of Example 1. Figure 14 shows an enlarged view of the electrode housing unit. The electrode housing unit 2 of the tri-electrode system is inserted into the reactor instrumentation column 14, and is in contact with the reactor water inside the reactor core 10. The reactor instrumentation column 14 into which the tri-electrode system is inserted has an opening inside the reactor core 10; the electrodes are inserted into the column in a double structure. After the insertion, the neutron instrumentation inner column 7 into which the tri-electrode system is inserted serves to shield the reactor water from the outside world. The electrode system comprises a counterelectrode 3, a working electrode 4, also known as a monitor electrode, a reference electrode 5 and represents an electrode support 6, which is an insulator. The monitor electrode 4 is operated by a potentiostat, an electro-chemical interface 15, and a CPU 16 (a computer CPU and an auto-control unit). The analytical results are transmitted to a remote control command system and an analytical results display unit 19. The analytical results are compared in the CPU 16 with previously entered standard values, and based on this comparison the CPU 16 sends instructions to the control system in a gas/chemical injection system 17 to cause valves to be opened and closed. By repeating this process, the CPU 16 regulates the water quality. The monitor electrode 4 can be either a plate electrode, a microelectrode or a wire-type needle electrode, which is on the same plane as the electrode support 6 is; the monitor electrode can be made in any shape and with any material. The reference electrode 5 may be a saturated calomel electrode (SCE), a standard hydrogen electrode (SHE), or a silver-silver chloride electrode (Ag-AgCl), and so on. In the drawing, the dashed lines indicate the connection conditions of electrical circuits and interface bus lines. In pressure vessels in nuclear reactors, H₂, O₂, and H₂O₂ gases are generated when radiation decomposes water. Most of such gas generation occurs in the nuclear core. Therefore, in controlling the gas formation reactions so as to control the water quality, the capability of a water quality control system can be improved by conducting water quality analysis directly inside the nuclear core. And for maintenance checks on electrodes, the electrodes can be removed from the nuclear core instrumentation columns. As with the case of the above in-reactor instrumentation column, the electrodes can be installed on the drier, separator 9, radiation test piece insertion instruments, and other removable equipment and parts, so that the electrodes can be installed inside the nuclear reactor pressure vessel. Also, they can be installed directly on the walls of the pressure vessel. As a variation of this embodiment, flangings can be installed directly inside the nuclear reactor recirculation pipe 18, or the reactor water can be routed to a sampling pipe, and the electrodes can be attached to the sampling line, thus ensuring the measurement of the water quality. Although electrodes attached at such locations take more time to detect the reaction products generated inside the reactor core, for the purpose of controlling the contents of impurities circulating in the primary cooling system they offer sufficient measurement accuracy. The most general application of these monitoring systems would be to attach them all over the components such as nuclear reactor water supply pipe 11, nuclear reactor cleaning system 12, and around the nuclear reactor cleaning and desalinization equipment 13, rather than attaching them in the reactor core or in flangings only, so as to manage and control the water quality and to detect potential leaks of the cooling water, thus providing a comprehensive, total water quality control and safety diagnosis system. Similarly, the electrodes can be installed in a boiling water reactor (BWR) or inside the secondary vaporizer of a pressurized water reactor (PWR), or in the boilers in thermal plants. In Figure 2, the electrode housing unit 2 consists of an electrolytic cell provided with a permeable membrane which is a hydrophobic gas-permeable membrane 20. Sealed in the cell is a dilute electrolyte solution 21. Figure 15 shows an enlargement of the electrode housing unit. Use of this cell facilitates electrochemical measurements. In Figure 3, the BWR reactor comprises the electrodes and the electrolytic cell used in that of Figures 1 and 2, to which a control monitoring system 22 for monitoring the water quality control system has been added. The control monitoring system 22 comprises at least one computer/CPU. These computers determine whether or not the CPU 16, for controlling the water quality control system, and the electrochemical interface 15 are operating properly, in order to ensure the reliability and stability of the water quality control system. For detecting an abnormal operation, the signals from the water quality control system consisting of the electrochemical interface 15, the CPU 16, and the gas/chemical injection system 17 are returned to a dummy circuit, and the criterion employed for detecting the presence of an abnormal operation is based on the determination whether or not the results of computation, analysis, and control are within specified values and specified precision. Figure 4 is a comprehensive water quality control system consisting of all the constituent elements depicted in Figures 1 through 3. Specifically, in this control system electrode housing units 2 are distributed in the reactor, the water supply system, the circulation system, and the cleaning system; the control system performs water quality controls based on the analysis of the water quality at these various locations. Especially, during a water leak accident it is useful to detect any abnormality in the concentration of dissolved O₂ at these various locations. Figure 5 (a) is a reduction current/voltage curve of O₂, measured by using the normal pulse voltammetry mode. Similarly, Figure 5 (b) is a reduction current/voltage curve of H₂O₂, measured by using the normal pulse voltammetry mode. Figure 5 (c) is a normal pulse voltammogram in the direction of reduction of the primary cooling water solution, containing O₂ and H₂O₂ of the same concentrations as the concentrations of O₂ and H₂O₂ measured in (a) and (b), where the horizontal axis represents the applied voltage. As shown in the normal pulse signal waveforms in Figure 6, the applied voltage is applied and sweeped in pulses, with an applied step width of Esh relative to the reference electrode, where tp denotes the pulse width. Each of the vertical axes in Figure 5 represent the electrolysis current which was observed relative to the different pulse-like applied voltages. The electrolysis current is sampled at the final step 23 of pulse width tp of the normal pulse voltammetry shown in Figure 6. i NP / O₂shown in Figure 5 (a) is the limiting current value for reducing O₂, while i NP / H₂O₂ shown in Figure 5 (b) is the limiting current value for reducing H₂O₂. I NP / (O₂+H₂O₂) in Figure 5 (c) is the limiting current for the simultaneous reduction of O₂ and H₂O₂. These 3 current/voltage curves were measured by using a platinum disc electrode as monitor electrode 4. Figure 5 (a) was measured using a solution containing O₂ but no H₂O₂. Figure 5 (b) was measured using a solution containing H₂O₂ but no O₂. Figure 5 (c) was measured using a solution containing both H₂O₂ and O₂. The limiting current value I NP / (O₂+H₂O₂) in Figure 5 (c) is given as the sum of the electrolysis current components i NP / O₂ and i NP / H₂O₂ of O₂ and H₂O₂, respectively: I NP O 2 +H 2 O 2 = i NP O 2 + i NP H 2 O 2 Generally, i NP / O₂ and i NP / H₂O₂ can be given by the following equations: i NP O 2 = f n O 2 ,D O 2 ,tp C O 2 i NP H 2 O 2 = f n H 2 O 2 ,D H 2 O 2 ,tp C H 2 O 2 where n j , Dj, and Cj (j denoting either O₂ or H₂O₂) represent the total number of reacting electrons involved in the reduction reaction, the diffusion coefficient, and the concentration of O₂ and H₂O₂ in the solution, respectively. tp represents the pulse width. f(nj, Dj, tp) is given by the following equation: f(nj, Dj, tp) = nj FA(Dj/πtp) 1/2 where F denotes the Faraday constant, and A is the electrode surface area of the monitor electrode. By inserting Equations (2) and (3) into Equation (1), the limiting current value is obtained: I NP O 2 +H 2 O 2 = f n O 2 ,D O 2 ,tp C O 2 + f n H 2 O 2 ,D H 2 O 2 ,tp C H 2 O 2 Therefore, the normal pulse voltammetry in the reducing direction of a solution containing both O₂ and H₂O₂ of unknown concentration gives the current/ voltage curve of the form shown in Figure 5 (c), and the observed diffusion limiting current can be expressed in Equation (5), where I NP / (O₂+H₂O₂) denotes the observed diffusion limiting current, and f(nj, Dj, tp) represents a function which can be defined when the experimental conditions employed are known. Therefore, Equation (5) turns out to be a two-dimensional first order equation containing two unknown variables, C O 2 and C H 2 O 2 . The peak current value I DP / (O₂+H₂O₂) which is measured by differential pulse voltammetry for the reduction of a solution containing both O₂ and H₂O₂ of unknown concentrations is given by the following equation: I DP O 2 +H 2 O 2 = g n O 2 ,D O 2 ,tp,..... C O 2 + g n H 2 O 2 ,D H 2 O 2 ,tp,..... C H 2 O 2 As discussed in Journal of Electroanalytical Chemistry, 175 (1984) pp 1-13, g(nj, Dj, tp, ...) represents a family of functions entirely different from equation (4) containing f(nj, Dj, tp). Thus, the measured peak current valve I DP / (O₂+H₂O₂) is clearly different from I NP / (O₂+H₂O₂) . Since g(nj, Dj, tp, ...) is determined by setting measurement parameters and experimental conditions, Equation (6) constitutes a two-dimensional first-order equation independent of Equations (5), containing the unknown variables C O 2 and C H 2 O 2 . Therefore, the solutions C O 2 and C H 2 O 2 of the set of linear equations (5) and (6) are given by the following equations: Consequently, if there is an overlap between the current/voltage curves of two kinds of measured substances of unknown concentrations, their concentrations can be determined by the above method by measuring the electrolysis current values by using two kinds of voltammetry, necessary for setting up two independent equations. It follows, then, that if 3 kinds, 4 kinds, or 5 kinds of substances to be measured are electrolyzed in approximately the same voltage range, and if their electrolysis current components are observed as overlapping values, by combined use of 3 to 5 kinds of voltammetries, the concentrations of the individual measured substances can be determined. These analytical processes are handled in the potentiostat-electrochemical interface 15 and the CPU 16 (computer CPU and auto-control unit), and water quality control is achieved by comparing these analytical results with the standard values and controlling the gas/chemical injection system 17. Of the various pulse voltammetries, those of which can be obtained limiting current values include reverse pulse voltammetry, in addition to normal pulse voltammetry. Those of which can be obtained peak current values include differential normal pulse voltammetry and square-wave pulse voltammetry, in addition to differential pulse voltammetry. Figures 16 - 19 show the voltage-time waveform signals for various pulse voltammetries. Example 2 Example 2 is designed to study appropriate reactor water quality management conditions by use of low distortion speed stretching tests, in order to study the limiting voltage and limiting electric conductivity at which intergranular stress corrosion cracking (IGSCC) ceases to occur; using the data thus obtained, in this embodiment the reactor water environment is controlled with a system shown in Figures 1 through 4 in order to inhibit the occurrence of SCC. Figure 7 shows the relationship between the sensitivity of SUS304 steel to IGSCC under the real reactor water environment, to which no hydrogen is supplied, the corrosion voltage, and the electric conductivity. In the figure, the solid line (limiting line) indicates the condition under which the occurrence of SCC was observed. As can be seen in Figure 7, the higher the corrosion voltage and the higher the electric conductivity, the greater is the likelihood of occurrence of an SCC. To prevent the occurrence of SCC, in this invention the water quality conditions of the reactor water and of the primary cooling water system, especially the H₂ gas concentrations and their relationship to water quality, were controlled by constant monitoring, so that the corrosion voltage and the conductivity did remain in the SCC-inhibiting region lying below the curve shown in Figure 7. Figure 8 shows the relationship between the sensitivity of SUS316 steel to IGSCC, the corrosion voltage, and the electric conductivity, under the same real reactor water environmental conditions as those described above. For comparison, the figure also provides the relationship between the limiting voltage and the limiting electric conductivity of SUS304 steel (solid line). As is evident from Figure 8, the limiting line at which SCC ceases to occur in SUS316 steel is in agreement with the limiting line at which SCC ceases to occur in SUS304 steel (Figure 7). SUS316 steel has the same SCC-inhibiting region as SUS304 region. Thus, it is demonstrated that the occurrence of SCC in SUS304 or SUS316 can be inhibited by using the same water quality control conditions. Example 3 For determining the best water quality conditions, this embodiment was intended to evaluate the region in which SCC is likely to occur, using the concentrations of oxygen and hydrogen peroxide in the reactor water as parameters. Figure 9 shows an H₂O₂ - O₂ correlation graph, indicating the sensitivity of structural materials to intergranular stress corrosion cracking (IGSCC) at 150°C. In Figure 9, "x" indicates the occurrence of IGSCC; "o" indicates the absence of IGSCC. As is evident from Figure 9, in regions in which the concentration of dissolved oxygen is higher than 0.1 ppm, the critical concentration of hydrogen peroxide necessary to inhibit the occurrence of IGSCC increases as the concentration of dissolved oxygen increases. Thus, it is clear that when a solution contains different chemical species dissolved there in, its SCC propensity cannot be evaluated on the basis of the corrosion voltage alone. In view of this fact, this embodiment, as shown in the preceding Example 2, sets the corrosion voltage and conductivity in a range which inhibits the occurrence of IGSCC, and uses a system of the configuration shown in Figures 1 through 4, to constantly monitor H₂O₂, O₂, and H₂ so that the concentrations of H₂O₂ and O₂ will be kept within a range wherein the occurrence of IGSCC is inhibited, in order to control the water quality to prevent the IGSCC of materials. Example 4 The purpose of Example 4 was to study the influence of the pH on SCC in order to identify optimal water quality conditions. Figure 10 shows the influence of the pH and anions on the IGSCC of sensitized SUS304 steel in hot water containing 40 ppm of oxygen. When the pH is adjusted using sulphuric acid and ammonia water, in the neighborhood of pH 5, the amount of time until a material shows cracking is exceedingly short. In low pH regions, anions such as of sulphuric acid and salts show the same degree of corrosiveness relative to IGSCC; by contrast, carbonic acid shows low corrosiveness. Therefore, in this invention, when judged solely on the basis of the relationship between the pH and IGSCC, the occurrence of IGSCC can be prevented by maintaining a high pH, as indicated in the figure. Example 4 demonstrates that the occurrence of IGSCC can be prevented by adjusting the pH of the solution within the range of 7 - 7.5, using the system of the present invention. Example 5 This Example was intended to study, using the techniques developed in this invention, the influence of the corrosion voltage on the speed of fracture, in order to control the water quality, based on the data thus obtained, so as to ensure a system having optimal corrosion voltage. Figure 11 shows the results of evaluating the influence of the corrosion voltage on the speed of fracture development in sensitized SUS304 steel. When the corrosion voltage is set below -230 mV vs.SHE, the speed of fracture development becomes exceedingly slow, virtually eliminating SCC. Figure 12 shows the results of controlling this system to maintain a corrosion voltage of less than -230 mV by supplying H₂ gas from the gas/chemical supply system 17. The corrosion voltage was controlled by controlling the amount of dissolved hydrogen, using the system of the present invention. Since the corrosion voltage falls below - 230 mV if the quantity of dissolved hydrogen under a steady state is maintained in the 150 - 200 ppb range, the quantity of hydrogen was maintained in this range by feeding back the observed concentration of dissolved H₂ to the system and by regulating the amount of hydrogen gas which was blown into the solution from the gas/chemical supply system 17. Example 6 In this Example 6, the water quality control system shown in Figures 1 through 4 was used to achieve comprehensive water quality control by monitoring the above-described dissolved O₂ concentration, H₂O₂ concentration, pH, corrosion voltage, conductivity, and dissolved H₂ concentration. By controlling the water quality parameters to maintain the optimal water quality conditions shown in Example 5 above, such as dissolved O₂ concentration: 10 - 50 ppb; H₂O₂ concentration: 10 - 50 ppb; pH: 7 - 8; corrosion voltage: -230 mV vs. SHE or lower; and dissolved H₂ concentration: 150 - 200 ppb, a system could be successfully constructed capable of achieving an overall reduction in the amount of corrosion and prevention of IGSCC. Figure 13 shows the results of monitoring and controlling the concentrations of dissolved O₂, H₂O₂, and H₂, pH, corrosion voltage, conductivity, and the concentration of iron ions, using the present water quality control system during the operation of a simulated nuclear power plant. All values are within the range of optimal conditions, namely, the water quality conditions satisfying the standard values, confirming that the system was operating properly. The concentration of iron ions detected by the simultaneous quantitative in situ analysis equipment system was below 5 ppb. EFFECTS OF THE INVENTION This invention allows simultaneous the quantitative in situ analysis of various chemical species found in the cooling water system and the reactor water in nuclear power plants, allowing on-the-spot water quality monitoring of the reactor water and cooling system. Further, the invention allows an overall evaluation of the water quality by correlating the water quality monitoring, analytical results, and the pH, voltage, and electric conductivity of the reactor water, e.g. at various locations of the water cooling system. Further, the above effects can be advantageously achieved with a computer and a calculation processing system, allowing a fast, accurate and automated water quality control.

Claims: Method for the quantitative analysis of solutions, comprising the steps of contacting a working electrode, a counterelectrode and a reference electrode in an electrolytic cell with a solution to be tested containing a plurality of chemical species, electrolyzing the solution by applying different voltages for the chemical species to be analyzed to the electrolytic cell; determining current values, according to the kind of voltammetry employed, from the electrolysis current resulting from the above electrolysis step, and calculating the concentrations of the plurality of chemical species to be analyzed, based on the current values thus determined; characterized in that in the electrolyzing step different voltage-time waveform signals for the chemical species to be analyzed are applied, in the determining step limiting current or peak current values and the voltage required to produce the electrolysis currents are determined, and in the computing step the concentrations are computed based on the limiting current or peak current values. Method according to claim 1, characterized in that the voltage-time waveform signals applied to the electrolytic cell are pulse-mode voltage-time waveform signals. Method according to claim 1 or 2, characterized in that the solution to be tested is a solution containing chemical species of at least one of H₂ and O₂ and also H₂O₂ which are to be analyzed simultaneously. Method according to one of the claims 1 to 3, characterized in that the waveform signals are normal pulse voltammetry voltage-time waveform signals and differential pulse voltammetry voltage-time waveform signals; and the concentration of H₂ or O₂ and the concentration of H₂O₂ are calculated based on the limiting current values resulting from the normal pulse voltammetry and the peak current values resulting from the differential pulse voltammetry, based on the electrolysis current resulting from the electrolysis step and the voltage required to produce the electrolysis current. Method according to one of the claims 1 to 4, characterized in that reactor water in a primary cooling system in a nuclear reactor is analyzed. Method according to claim 5, characterized in that the reactor water is brought into contact with the three electrodes in the electrolytic cell through a hydrophobic gas-permeable membrane. Method according to claim 5 or 6, characterized in that the three electrodes in the electrolytic cell are brought into contact with the reactor water in the reactor by installing them in an in-reactor instrumentation column in the nuclear reactor. Apparatus for the quantitative analysis of solutions, particularly for carrying out the method according to claims 1 to 7, comprising an electrolytic cell (2) for electrolyzing the solution to be tested, the cell being provided with a working electrode (4), a counter electrode (3) and a reference electrode (5); voltage application means (15) for applying different voltage signals having specified parameters determined by the kind of voltammetry employed, to the working electrode in said electrolytic cell as reference electrode standard voltage, current determination means for determining currents obtained by electrolyzing the sample, and concentration calculation means (16) for calculating the concentrations of the chemical species to be analyzed, based on the current values thus obtained; characterized in that the voltage application means (15) apply different voltage-time waveform signals for the chemical species to be analyzed, the current determination means determine limiting current or peak current values based on the currents obtained by electrolyzing the sample, and the concentration calculation means (16) calculate the concentrations based on the limiting current or peak current values. Apparatus according to claim 8, characterized in that the solution to be tested is a solution containing chemical species of H₂O₂ and either H₂ or O₂ which are to be analyzed simultaneously. Apparatus according to claim 8, characterized in that the solution to be tested is a solution containing chemical species of H₂, O₂ and H₂O₂ which are to be analyzed simultaneously. Apparatus according to one of the claims 8 to 10, characterized in that the waveform signals are pulse-mode voltage-time waveform signals. Apparatus according to one of the claims 8 to 11, characterized in that also H₂ is to be analyzed, and the voltage application means (15) for applying voltage-time waveform signals are means for applying two types of pulse-mode voltage-time waveform signals comprising normal pulse waveform signals and differential pulse waveform signals. Apparatus according to one of the claims 8 to 12, characterized in that also O₂ is to be analyzed, and the voltage application means (15) are means for applying two types of pulsemode voltage-time waveform signals comprising normal pulse waveform signals and differential pulse waveform signals, and the current determination means determine limiting current values, based on the electrolysis current obtained by applying the above normal pulse waveform signals and the voltage required to produce said electrolysis current, and peak current values, based on the electrolysis current obtained by applying the above differential pulse waveform signals and the voltage required to produce said electrolysis current. Apparatus according to one of the claims 8 to 13, characterized in that the voltage application means comprise a potentiostat and a CPU (16), comprising a signal discharge unit for applying voltage-time waveform signals which are put in from the CPU to the working electrode in the electrolytic cell, the CPU being provided with a signal input unit for entering voltage-time waveform signals, having specified parameters which are determined by the kind of voltammetry employed, in the potentiostat; and it further comprises a storage unit which stores in a memory the results of the voltammetry performed by the potentiostat, for each chemical species to be analyzed and present in the test solution, as a relationship between the resulting electrolysis current and the voltage required to produce it, wherein the current determination means are a current value determination unit which processes the data stored in the storage unit. Apparatus according to claim 14, characterized in that the solution to be tested is a solution containing either H₂ or O₂ and H₂O₂ which are to be analyzed simultaneously, the voltage-time waveform signals are voltage-time waveform signals for normal pulse voltammetry having input parameters, and those for differential pulse voltammetry, and the current value determination unit determines the limiting current of the normal pulse voltammetry and the peak current of the differential pulse voltammetry by treating the data stored in the storage unit. Apparatus according to claim 14 or 15, characterized in that three kinds of voltage-time waveform signals are applied to the electrolytic cell from the potentiostat, all comprising pulse-mode waveform signals. Nuclear reactor water quality control system, comprising analysis means for performing a quantitative analysis of the reactor water in the primary cooling system of a nuclear reactor (1) based on electrolysis, and reactor water control means (13, 17) for controlling the water quality of the reactor water based on the results obtained from the analysis means, characterized in that the analysis means are an apparatus according to one of the claims 8 to 16, and the reactor water control means (13, 17) control the water quality of the reactor water based on the concentrations calculated in the analysis means. System according to claim 17, characterized in that the electrolytic cell is installed in the reactor core (10) of the nuclear reactor. System according to claim 17, characterized in that the electrolytic cell is installed in a reactor instrumentation column (14) in the nuclear reactor. System according to claim 17, characterized in that the electrolytic cell is installed on the reactor core outlet side of the primary cooling system. System according to claim 17, characterized in that the electrolytic cell is installed in the water vapor separator unit in the reactor. System according to one of the claims 17 to 21, characterized in that the reactor water control means comprise means (17) for regulating the amount of H₂ gas that is injected into the primary cooling system. System according to one of the claims 17 to 21, characterized in that the reactor water control means comprise means for regulating the amount of N₂H₄ that is injected into the water vapor generator in the reactor. System according to claim 23, characterized in that the quantity of N₂H₄ that is injected into the water vapor generator is based on the quantitative analysis of dissolved O₂ gas in the primary cooling system, computed by the concentration calculation means. System according to one of the claims 17 to 24, characterized in that it further comprises a concentration comparison unit which compares the results from the concentration calculation means with standard values, and the reactor water control means control the water quality of the reactor water based on the information produced by the concentration comparison unit. System according to claim 25, characterized in that the concentration comparison unit compares the dissolved amount of at least one of H₂, O₂ and H₂O₂ in the primary cooling water with standard values, and means are provided for regulating the quantity of at least one of H₂, O₂ and H₂O₂, that is injected into the reactor water based on the information produced by the concentration comparison unit. System according to one of the claims 17 to 26, characterized in that the concentration calculation means form equations in which the concentrations of the substances measured, and present in the reactor water, are treated as unknown variables, based on the limiting current or peak current values determined by the current determination means, and calculate the concentrations of the substances by solving the set of equations. System according to one of the claims 17 to 27, characterized in that it further comprises measuring means for measuring at least one of reactor water conductivity, pH and electric potential at sampling points in the primary cooling system; a further comparison unit which compares the results of the measurements with standard values; and the reactor water control means regulate the water quality in the reactor water based on the information produced by the further comparison unit and the concentration comparison unit. System according to one of the claims 17 to 28, characterized in that it further comprises alarm generation means which generate an alarm when the results from the concentration calculation means exceed a standard value. System according to one of the claims 17 to 29, characterized in that it is adapted to measure the concentrations of SO 4 2- , SO₃ ⁻, Cl⁻ and Na⁺ ions which are decomposition products of the nuclear reactor cleaning chemical resins in the primary cooling system; and the alarm generating means generating an alarm when any one of the ionic concentrations exceeds a standard value. System according to one of the claims 25 to 30, characterized in that the concentration comparison unit converts concentration values produced by the concentration calculation means into electric conductivity values and compares them with standard values, and the reactor water control means regulate the water quality in the reactor water by maintaining said converted values below standard values. System according to one of the claims 17 to 31 characterized in that it comprises a function generator between the signal discharge unit and the signal input unit, and the waveform signals produced by the signal input unit are entered in the signal discharge unit through the function generator. System according to one of the claims 25 to 32, characterized in that it further comprises prediction means for predicting potential accidents in the primary cooling system based on the information produced by the concentration comparison unit, and the alarm generation means generate an alarm based on that prediction. System according to one of the claims 25 to 33, characterized in that the system comprises a monitoring system which monitors the proper operation of the voltage application means, the concentration calculation means and the concentration comparison unit. System according to claim 34, characterized in that the alarm generating means generate an alarm in the event that a malfunction is detected by the monitoring system. System according to one of the claims 17 to 35, characterized in that the nuclear reactor is a boiling water nuclear reactor or a pressurized water nuclear reactor.

Inventors: KIKUCHI EIJI, MABUCHI KATSUMI, OHNAKA NORIYUKI, OHSUMI KATSUMI, SAKAI MASANORI, TAKAHASHI TAKUYA, KIKUCHI, EIJI, MABUCHI, KATSUMI, OHNAKA, NORIYUKI, OHSUMI, KATSUMI, SAKAI, MASANORI, TAKAHASHI, TAKUYA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: G01N27
IPC codes: G01N27, G21C19, G21D3, G21C17