JPH0426439B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention directly generates steam using the heat of a nuclear reactor such as a boiling water reactor (hereinafter referred to as BWR) or an advanced converter reactor (hereinafter referred to as ATR), and drives a turbine to generate electric power. The present invention relates to a water quality control method and device for a direct cycle light water reactor. [Background of the Invention] Figure 1 shows the configuration of a conventional BWR primary system. In the BWR primary system, high-purity, neutral cooling water is sent to the reactor core 2 from the recirculation pump 16, and part of it becomes steam due to heat generation in the reactor core 2, passes through the main steam system 12, and is connected to the high-pressure turbine 10. It serves as a driving source for the low pressure turbine 11. low pressure turbine 1
A condenser 9 is connected to the reactor core 1, and the steam generated in the reactor core 2 is condensed here and becomes water. The condensed water passes through a condensate pump 8 and a condensate demineralizer 6, is pressurized by a water supply pump 7, is heated, and is then returned to the pressure vessel 1. In a BWR, the quality of the cooling water is made neutral in order to suppress corrosion of structural materials and to prevent deterioration of the reactor's nuclear thermal properties, and the purity is increased by removing impurities in the reactor purification system 4 and condensate demineralizer 6. I'm keeping it. Gas components in cooling water, especially oxygen, which has a large effect on material corrosion, are
In BWR, before increasing the reactor output,
The condenser 9 is operated in a vacuum degassing operation to lower the oxygen concentration in the water to around several tens of ppb in advance. In Sweden's BWR, before the output increase,
Nitrogen is injected from the emergency core cooling system to keep the oxygen concentration low. As a result of taking various measures, the dissolved oxygen and hydrogen concentrations during normal operation of the BWR are approximately 150 to 250 ppb and 5 to 10 ppb, respectively. The reason why it is higher than during the degassing operation shown above is because these oxygen and hydrogen are radiolysis products produced as a result of water being irradiated with neutrons and gamma rays in the reactor core 2. Most of the volatile components such as oxygen and hydrogen generated as a result of radiolysis in the reactor core 2 pass through the main steam system 12 and are then transferred to the air extractor 1.
3, the gas is sent to an off-gas system consisting of a recombiner 14, a stack 15, etc. A very small portion of the decomposition product species generated in the reactor core 2 remains in the liquid phase and is detected outside the reactor as the oxygen and hydrogen concentrations described above. BWR temperature and pressure conditions (285℃, 70MPa)
The oxygen concentration of 250 ppb is considered to be a sufficiently low value from the perspective of inhibiting corrosion of structural materials. Hydrogen injection into the system has been carried out and is attracting attention.
Hydrogen injection has the effect of suppressing the radiolysis of water itself by injecting hydrogen into cooling water. In an actual example, hydrogen is injected from the downstream side of the condensate demineralizer 6, and its effectiveness is monitored by actually measuring the oxygen and hydrogen concentrations in the off-gas and in the recirculation system. As a result, as shown in Figure 2, although there was no significant reduction effect on the amount of oxygen gas released, it was found that a relatively small amount of hydrogen injection had a large reduction effect on the oxygen concentration in the recirculation system. Served. However, these findings based on sampling values of the primary cooling system have several problems, as described below. () The values measured by manual analysis after sampling and cooling the cooling water are the results of many radiolysis products of water eventually becoming stable oxygen and hydrogen, so it is difficult to estimate the value at the sampling point. The concentration of the decomposition products itself is unknown. () The concentration of radiolysis product species of water, including oxygen, hydrogen, and hydrogen peroxide, generated in the reactor core depends on the radiation dose rate in each region of the primary system or the elapsed time from the reactor core. It's decided. Therefore, even if the sampling value correctly gives the concentration at the sampling point, the situation in other areas of the primary system cannot be known from it. In particular, in recent years, sites other than the recirculation system, e.g.
There are also requests to suppress the oxygen concentration in the riser pipe above the core, and the basic idea is to control the water quality of the entire primary system. However, as is clear from the above-mentioned problems, it is impossible to meet such demands with the technology conventionally used in actual plants. Therefore, in order to provide a means to meet these demands, the inventors have developed () a dissolved oxygen meter for high-temperature and high-pressure water, and () a theoretical evaluation model for radiolysis of BWR reactor water. An outline of the techniques () and () above will be explained below. () Dissolved oxygen meter for high temperature and high pressure water Figure 3 is a schematic structural diagram of a dissolved oxygen meter for high temperature and high pressure water. The sensor body 18 is immersed in the liquid to be measured 28 in the container 17. The liquid to be measured 28 is in contact with an internal electrolyte 20 in which KCl is dissolved at one end of the sensor via a Teflon membrane 23 that is selectively permeable to oxygen. In order to maintain the mechanical strength of the Teflon membrane, a metal, for example, stainless steel filter 24 is placed on the outside of the membrane, and a gold porous plate 22 that also serves as a cathode is placed on the inside of the membrane.
An anode 21 is disposed at one end within the internal electrolyte 20 . In order to have a heat-resistant structure, the sensor body 1
8 are all made of a heat-resistant material such as Teflon, and a bellows 19 is provided on the side surface to absorb thermal expansion of the electrolyte. With this kind of structure and material,
It can be used under BWR reactor water conditions (285℃, 70MPa). Oxygen in the reactor water permeates through the Teflon membrane and is converted into a ring by the following reaction on the gold electrode 22. O ₂ +2H ₂ O+4e ^- →4OH ^- ...(1) On the other hand, as a result of this reaction, the reaction Ag+Cl ^- →AgCl+e ^- ...(2) progresses on the Ag/AgCl electrode, and as a result, a current flows between the two electrodes. flows. The inflow rate of oxygen that permeates into the electrolyte 20 through the Teflon membrane 23 is approximately proportional to the oxygen concentration of the liquid to be measured, so the current value measured by the ammeter 26 determines the oxygen concentration of the liquid to be measured. You can know. () Theoretical evaluation model for radiolysis of BWR reactor water This method determines moment-to-moment changes in the concentration of radiolysis products in BWR reactor water by numerical analysis using fluid fragments of primary cooling water. Since the content of the model is detailed in the references listed at the end of this [Background of the Invention] chapter, only an outline of the model and results will be shown here. When water is irradiated with radiation such as gamma rays and neutron rays, the following decomposition product species are formed as primary products. The production rate of these primary products is called the g value (the number of products produced per 100 eV of absorbed energy), and the actually measured values shown in Table 1 have been obtained for high temperature water.

[Table] O ₂ , H ₂ O ₂ ,
Secondary products such as HO ₂ , O ₂ ^- and HO ₂ ^- are generated.
Including these production reactions, the chemical reactions shown in Table 2 are said to proceed between the primary and secondary products.

【table】

【table】

[Purpose of the invention]

An object of the present invention is to provide a simple and highly reliable water quality calculation system for the entire BWR primary system, and a water quality control method and device based thereon. [Summary of the Invention] The present invention is based on new findings discovered through hydrogen injection experiments and theoretical evaluation models conducted at BWRs in Sweden and the United States. First, the contents are shown below. (i) Knowledge obtained from experiments First, the effective injected hydrogen concentration is defined by the following equation. C _H = G _I /Q _C ×10 ⁹ ...(7) where, C _H : Effective injection hydrogen concentration (ppb) G _I : Hydrogen injection rate (g/s) Q _C : Core flow rate (g/s ) At this time, the reduction rate of the oxygen gas release rate and the oxygen concentration in the recirculation system of the BWR commercial reactor are determined by the following formula, as shown in Figures 5 and 6, to values that are in relatively good agreement with the actual measured values. is obtained. R = 1.00exp (-0.00985C _H ) ...(8) C _O = 191.3exp (-0.0483C _H ) ...(9) Here, R: Oxygen gas release rate (-) C _O : Recirculation system Oxygen concentration (ppb) (ii) Findings from the theoretical analysis model The theoretical analysis model can determine the concentration distribution of decomposition product species during normal operation or hydrogen injection for a specific plant. however,
In reality, the concentration distribution can vary from plant to plant, or even within the same plant, due to differences in parameters as shown below. (1) Core power density (2) Core flow rate (3) Feed water flow rate (4) Distribution of energy absorption rate in the region outside the core (5) Residence time of coolant in the primary system region (6) Presence or absence of jet pumps As a result of analyzing the parameters by changing the input values within a realistic range,
A number of linear relationships were found to exist between the concentrations of decomposition product species. An example is shown in FIG. Figure 7 shows the concentration correlation between decomposition product species in the reactor core, and the circles or triangles in the figure are
Calculated values under various conditions are shown. From the figure,
It can be seen that the concentrations of decomposition product species vary within a certain range depending on the conditions, but for the most part they vary while maintaining a constant relationship with each other. This means that if the concentration of a certain decomposition product species is known, the concentrations of other decomposition product species can also be automatically calculated with high probability.
That is, in Figure C, if the hydrogen peroxide concentration in the core is known, the oxygen and hydrogen concentrations in the core can also be calculated. The strength of such bivariate linear linear correlation is usually expressed using a correlation coefficient r. In the example shown in FIG. 7, all values are 0.95 or more. Such a correlation is determined between the concentrations of decomposition product species at all locations in the BWR primary system. FIG. 8 shows this as the upper triangular part of the correlation coefficient matrix. In Figure 8, in order to show the outline of the correlation coefficient matrix, elements whose absolute values exceed 0.9 are shown with black outlines (0.9 or more) or scattered dots, and each element is shown vertically and horizontally. There is a high correlation between the concentrations of decomposition product species at the positions corresponding to , indicating that they can be calculated from each other. The above are the findings that form the basis of the present invention. Based on the above knowledge, the present invention is based on the measured values of oxygen and hydrogen concentration in the main steam system and oxygen concentration in the recirculation system.
The main idea is to calculate the water quality of the entire BWR primary system, use this calculated value to determine the control amount for the hydrogen injection device, and control the BWR water quality. More specifically, in Figure 8, there is a particularly strong correlation between the concentration of oxygen and hydrogen in the main steam system and the concentration of radiolysis products in the core, separator, and mixing plenum, and that Focusing on the strong correlation between the radiolyzed species in the lower plenum, we calculated each concentration using the linear relationship established between these quantities, and determined that this calculated value was the target. The water quality is controlled by determining the rate of hydrogen injection into the system to achieve the desired concentration. On the other hand, specific control variables include (i) hydrogen injection rate (water supply system), and (ii) oxygen or air injection rate for recombination (off-gas system). and hydrogen injection equipment can be controlled economically and appropriately. [Embodiment of the invention] Currently, to completely prevent stress corrosion cracking (SCC) in stainless steel, it is necessary to reduce the dissolved oxygen concentration to 20~20°C at 285°C.
It is said that it is desirable to suppress it to 40ppb or less. According to the inventors' theoretical analysis, in the reactor water,
In addition to oxygen, it also contains hydrogen peroxide, O ₂ ^- , etc., and the effects of these decomposition products on material corrosion are as follows:
This has not necessarily been clarified, and as research progresses in the future, it is possible that new constraint conditions may be added or the target value of dissolved oxygen concentration may be relaxed. Therefore, although the control goals may vary,
In order to specifically show how to control water quality, here we set the following tentative goals. Control target: downcomer and riser (separator)
The oxygen concentration in the liquid phase at 40ppb (9.3×
10 ^-7 mol/), 200ppb (4.64×10 ^-6 mol/)
Each is controlled as follows. First, use the following procedure to calculate the oxygen concentration in the main steam system and recirculation system when the oxygen concentration in the downcomer and riser reaches 40 ppb. (i) Calculate the oxygen concentration in the liquid phase of the reactor core as follows. [O ₂ ] (rise pipe) = 0.94 x [O ₂ ] (core) + 1.04 x 10 ^-7 ... (10) In the example, it is 208.1 ppb (4.83 x 10 ^-6 mol/). (ii) Next, from this value, determine the oxygen concentration in the steam as follows. [O ₂ ] (core) = 0.402 x [O ₂ ] (steam) + 1.68 x 10 ^-6 ... (11) In the example, it is 7.1 ppm (7.82 x 10 ^-6 mol/). (iii) Multiply the value obtained above by the main steam flow rate to find the reduction rate R of the oxygen release rate, and use equation (8) to find the effective injected hydrogen concentration C _H. (iv) On the other hand, the oxygen concentration in the downcomer part is 40ppb
(9.3×10 ^-7 mol/) Find the oxygen concentration in the recirculation system as follows. [O ₂ ] (downcomer) = 1.05 x [O ₂ ] (recirculation system) ... (12) In the example, it is 38 ppb (8.9 x 10 ^-7 mol/). (v) Calculate the required effective injection hydrogen concentration using equation (9). In the example, it is 33.5ppb. (vi) Compare the C _H obtained in (iii) with the above value, choose the larger one, multiply it by the safety factor, and use this as the target value C _H of the effective hydrogen concentration. (vii) Multiply C _H by the core flow rate to calculate the hydrogen injection rate G _I and control the hydrogen injection device. (viii) From equation (8), find the oxygen gas release reduction rate under hydrogen injection, and multiply this by the main steam flow rate to find the oxygen gas volumetric release rate. (ix) Calculate the excess volume release rate of hydrogen using the following formula. (Hydrogen excess volume release rate) = (Hydrogen volume injection rate)
−2×(oxygen gas volumetric release rate)……(13) (x) Find the volumetric injection rate of oxygen or air to be injected into the off-gas system using the following formula, multiply by the safety factor,
Decide the oxygen injection rate to the off-gas system and control the injection device. (Oxygen volume injection rate) = (Hydrogen excess volume release rate)/2 ... (14) The procedure shown above is a method for setting the initial control amount. The procedure is as follows. (i) Calculate the oxygen concentration in the riser pipe from the measured values of oxygen and hydrogen concentrations in the main steam using equations (10) and (11). (ii) From the measured value of oxygen concentration in the recirculation system, calculate the oxygen tolerance in the downcomer section using equation (12). (iii) Increase or decrease the hydrogen injection rate so that each estimated value matches the target value. (iv) Determine the oxygen or air injection rate into the off-gas system from the oxygen and hydrogen gas concentrations in the main steam. In the example shown above, the actual measurement value of the recirculation system was used to calculate the concentration in the downcomer, and the concentration in the main steam was used to calculate the concentration in the riser. In addition, the behavior of radiolysis products in the BWR primary system is divided into two regions: (i) the region that integrates the main steam system, core, separator, and mixing plenum, and (ii) the region from the downcomer to the lower plenum. This is because there is a strong correlation within each of them. In the example, only oxygen is shown, but it is clear from FIG. 8 that the concentrations of hydrogen peroxide, hydrogen, and other decomposition product species in each region can also be calculated using the same procedure. FIG. 9 shows the system configuration for realizing the above calculation control method. Concentration, hydrogen injection rate, off-gas oxygen (air) injection rate, core flow rate, feed water flow rate, main steam flow rate, etc. are required. These signals are transmitted from a steam component measuring device 36 installed in the bleed system 29, etc., a dissolved oxygen meter 32 for high-temperature and high-pressure water installed upstream of the furnace purification system, a hydrogen injection device 33, an oxygen injection device 31, and general process equipment. From the instrumentation system, cable 34
guided by. FIG. 10 is a generalized block diagram of the algorithm for setting the initial hydrogen and oxygen injection rates of the calculation control device, and even if the target concentration values and types are different, the guiding algorithm can be used. FIG. 11 similarly shows a generalized block diagram of the control method during hydrogen injection. [Effects of the Invention] The present invention provides a concrete means for the first time to control the water quality of the BWR primary system, which has hitherto been impossible to calculate or actually measure.
The present invention provides a reactor water quality control method and device that can significantly improve the safety and soundness of a BWR, and efficiently and economically improve the water quality of a BWR through hydrogen injection.

[Brief explanation of drawings]

Figure 1 shows the schematic configuration of the BWR primary system, and Figure 2 shows the results of hydrogen injection experiments at the Oskarsjam No. 2 reactor in Sweden and the Dresden No. 2 reactor in the United States, showing the amount of oxygen and hydrogen released into the off-gas system, and the amount of hydrogen released into the off-gas system. Diagram showing oxygen and hydrogen concentrations in the circulation system (solid lines and broken lines are analytical values based on a theoretical model), Figure 3 is a schematic structural diagram of a conventional dissolved oxygen meter for high-temperature, high-pressure water, and Figure 4 is a primary BWR based on a theoretical model. A diagram showing the dissolved oxygen concentration distribution of the system as a function of hydrogen injection rate. Figure 5 shows the reduction rate of oxygen gas release rate under hydrogen injection in BWR. Figure 6 shows the dissolved oxygen concentration distribution in the recirculation system under hydrogen injection. Figure 7 shows the oxygen concentration; Figure 7 shows the correlation between the concentrations of radiolysis product species in the core; Figure 8
The figure shows an outline of the upper triangular part of the correlation coefficient matrix between decomposition product species concentrations in the entire area of the BWR primary system, Fig. 9 shows the system configuration of the present invention, and Fig. 10 shows the water quality estimation control device. FIG. 11 is a block diagram showing an algorithm for initial value setting, and FIG. 11 is a block diagram showing an algorithm for control during hydrogen injection. 1...BWR pressure vessel, 2...core, 3...recirculation system, 4...reactor purification system, 5...water supply system, 6...
Condensate demineralizer, 7...Water pump, 8...Condensate pump, 9...Condenser, 10...High pressure turbine,
11...Low pressure turbine, 12...Main steam piping, 1
3... Air extractor, 14... Acid/hydrogen recombination device, 15... Stack, 16... Recirculation pump,
17... Container, 18... Sensor body, 19... Bellows, 20... Electrolyte (KCl solution), 21...
Silver electrode, 22... Gold electrode, 23... Teflon membrane,
24... Stainless steel filter, 25... Infusion pump, 26... Ammeter, 27... DC power supply, 28...
...Liquid to be measured, 29...Breaking system piping, 31...Oxygen injection device, 32...Dissolved oxygen concentration meter for high temperature and high pressure water, 33...Hydrogen injection device, 34...Signal cable, 35...BWR primary system Water quality calculation control device, 3
6... Concentration measurement device in vapor.

Claims (1)

[Scope of Claims] 1. A water quality control method for a nuclear reactor in which the water quality of the reactor water is controlled by injecting hydrogen into the primary cooling system of the nuclear reactor, comprising:
From at least one of X ₁ and hydrogen concentration X ₂ , the formula
By Y ₁ = aX ₁ (or X ₂ ) + b (ab is a constant),
Calculate the concentration of water decomposition products _Y1 in the range downstream from the reactor core and upstream of the water supply confluence, and calculate the concentration of oxygen in water in the recirculation system that supplies cooling water to the reactor _X3 from The concentration Y ₂ of water decomposition products in the reactor water in the area downstream of the feed water confluence and upstream of the core inlet is expressed by the formula Y ₂ = cX ₃
+d (cd is a constant), and determining the hydrogen injection rate to the primary cooling system so that these calculated values Y ₁ and Y ₂ match a target range. 2. In a water quality control device for a nuclear reactor that has a hydrogen injection device that injects hydrogen into the primary cooling system of the nuclear reactor and controls the water quality of the reactor water by changing the hydrogen injection rate, the main steam system generated in the reactor and a means for measuring the oxygen concentration in each of the recirculation systems that supply cooling water to the nuclear reactor, using a heat-resistant sensor having an electrolytic solution inside through a permeable membrane selective to oxygen; Based on the oxygen concentration in the steam system, the reactor core,
Means for calculating the concentration of radiolysis product species in the region consisting of the separator and the mixing plenum, and calculating the concentration of radiolysis product species in the region from the downcomer to the lower plenum based on the oxygen concentration of the recirculation system; A primary system water quality calculation control means for adjusting the hydrogen injection device so that the calculated concentration of the radiolysis product species in the main steam system and the recirculation system becomes a target concentration. Reactor water quality control equipment.