JPH0431079B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to water quality control for reducing the surface dose rate of primary system piping in a nuclear power plant, and the present invention relates to water quality control for reducing the surface dose rate of primary system piping in a nuclear power plant. This invention relates to a nuclear power plant suitable for optimally controlling the amount of nonferrous metal ions and reducing the dose rate.

[Conventional technology]

Corrosion of piping, pumps, valves, etc. used in the water supply system of a nuclear power plant, such as a boiling water nuclear power plant, generates ionic or insoluble corrosion products that are carried into the reactor. The corrosion products brought in adhere to the fuel rods and become radioactive corrosion products when irradiated with neutrons. Some of the radioactive corrosion products are re-eluted or desorbed into the primary cooling water. For example, Mn, Ni, and Co are transformed by neutron irradiation into radioactive corrosion products with long half-lives such as ⁵⁴ Mn, ⁵⁸ Co, and ⁶⁰ Co, respectively. These radioactive corrosion products adhere to the surfaces of recirculation system piping and the like while circulating in the primary cooling water system. Therefore, the dose rate on the pipe surface increases, increasing the radiation exposure of workers when performing maintenance and inspection of the reactor.

As a method for reducing the surface dose rate of pipes, it has been proposed to suppress the generation of corrosion products in the water supply system and to suppress the adhesion of radioactive corrosion products to the pipe surface.

Conventionally, as a method for reducing the generation of corrosion products, particularly the elution of Co and Ni, Japanese Patent Application Laid-Open No. 59-89775 has proposed a method of forming an oxide film on the surface of stainless steel in advance.

However, recent results have shown that simply suppressing the generation of corrosion products is not sufficient.

Furthermore, in order to suppress the adhesion of radioactive ions to the structural materials of nuclear power plants, JP-A-58-79196 discloses that the amount of metal ions in the coolant is measured using a conductivity meter, and this measurement value is Based on Mg, Cr, Ni
A method has been proposed in which non-ferrous metal ions such as ions are injected into the coolant. This is intended to reduce the rate of attachment of radioactive ions to the structural material ferrite layer by increasing the concentration of metal ions in the coolant in which radioactive ions are present.

[Problem to be solved by the invention]

In the above conventional technology, when ions such as Ni are activated in a nuclear reactor, they generate radionuclides with long half-lives such as ⁵⁸ Co. Therefore, even if the deposition rate decreases, the concentration of radionuclides increases, so the amount of deposition increases and the surface dose rate of the structural material increases.

An object of the present invention is to provide a nuclear power plant equipped with a water quality control method and apparatus necessary for effectively reducing the primary system piping surface dose rate in the nuclear power plant.

[Means to solve the problem]

In order to achieve the above object, in the present invention, in a nuclear power plant equipped with a nuclear reactor, a turbine condenser, a low-pressure feedwater heater, a high-pressure feedwater heater, and water supply piping connecting these, a means for measuring the amount of iron cladding in the reactor feed water; a means for measuring the amount of nonferrous metal ions in the reactor water supply; and a means for measuring the amount of iron cladding in the iron cladding relative to the amount of nonferrous metal ions based on the measurement results by each of the means. A cladding filter for reducing the amount of iron cladding and an ion filter for reducing the amount of non-ferrous metal ions are provided so that the ratio of iron cladding becomes 2 to 8 (weight ratio).

[Effect]

Corrosion products produced by corrosion damage to structural materials consist of water-soluble ions and insoluble metal oxides, the latter commonly referred to as crud. The main component of the cladding is Fe, and in boiling water reactors with an oxidizing atmosphere, hematite (α-
(Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), nickel ferrite (NiFe ₂ O ₄ ), etc. in pressurized water reactors with a reducing atmosphere.

The present inventors investigated the characteristics of a high temperature filter using hematite particles. At 285℃,
FIG. 2 shows the relationship between the differential pressure that occurs when water containing hematite particles is passed through the tube at a flow rate of 1/h for 1 hour and the concentration of hematite particles in the flowing water. A silver membrane filter with a pore size of 0.45 μm was used as the membrane filter, and a glass filter was used as the fiber layer filter.

When a fluid is filtered through a membrane filter, all particles larger than the pore size of the filter are captured on the surface of the filter. In contrast, in fiber layer filters, particles are trapped not only on the surface but also on the interior.

In the case of a membrane filter, particles are captured on the surface, so the captured particles of the first layer will block the pore size of the filter. Therefore, the differential pressure across the filter increases rapidly. The differential pressure at this time is proportional to the particle concentration in the fluid and the flow rate, and does not depend on the shape of the particles as long as the particles are larger than the pore diameter of the filter. Since the trapped particles in the second and subsequent layers can be treated as a powder-filled bed, the differential pressure generated by the trapped particles in the second and subsequent layers depends on the shape of the particles. In this way, when a membrane filter is used, the relationship between differential pressure and particle concentration is in two stages as shown in FIG.

In the case of a fibrous layer filter, particles are trapped on the surface and inside the filter, so that the differential pressure across the filter caused by particle trapping is proportional to the particle concentration in the fluid and the flow rate.

From the above experimental results, when using a membrane filter with a pore size of 0.45 μm, particles with a particle size of 0.45 μm or more, that is, iron cladding, can be collected at a water flow rate of 1.
Iron cladding concentrations up to 10ppb can be measured with high sensitivity using filter differential pressure. Using a fiber layer filter, it is possible to quantify iron cladding above 10 ppb. At this time, if the water flow rate is increased, the differential pressure will also increase, so the measurement accuracy can be improved. The filter is selected depending on the iron clad concentration in the water supply.

To find the absolute iron concentration from the differential pressure, proceed as follows. First, a calibration curve showing the relationship between differential pressure and iron cladding concentration as shown in Fig. 2 is created. Now, the differential pressure generated at flow rate V (/h) and water flow time t is P
(Kg/cm ² ), the amount of iron cladding W (ppb) can be determined from the differential pressure P using a calibration curve as shown in FIG.
Assuming that the iron cladding is hematite (α-Fe ₂ O ₃ ), the iron content W _Fe (μg) in the iron cladding is determined by the following formula.

W _Fe = W × 56 × 2 / 56 × 2 + 16 × 3 × V × t ∴W _Fe = 7 / 10 WVt (μg) As ionic corrosion products in the water supply, Ni ²⁺ ,
There are Cu ²⁺ , Mn ²⁺ , Co ²⁺ , Mg ²⁺ , Zn ²⁺ etc.
These behave similarly in reactor water, and by neutron irradiation, ⁵⁸ Co, ⁶⁴ Cu, ⁶⁶ Cu, ⁵⁴ Mn, ⁶⁰ Co,
Generates radionuclides such as ²⁷ Mg and ⁶⁹ Zn. Cr is
It exists in the form of CrO ₄ ^2- .

The conductivity K can be expressed as the sum of the conductivities of various ions as shown in the following equation.

K=Λ _H ⁺ C _H ⁺ +Λ _0H ^- C _0H ^- + 〓 ⁱ ΛiC _1Where , Λ: Limiting equivalent ion conductivity C: Ion concentration The magnitude of the limiting equivalent ion conductivity varies depending on the ion and changes with temperature. There is dependence. The ultimate equivalent ionic conductivity for the corrosion products of the cations is 25
It is almost the same as (54±1) Ω ^-1 cm ² mol ^-1 at °C, and it is thought that it will have a similar size even at high temperatures. The anions are now converted into OH ^- ions by a high temperature anion filter. According to the law of conservation of charge, the same amount of OH ^- ions as corrosion product cations were produced. This means that the conductivity of the cations was sensitized by the conductivity of the OH ^- ions. That is, if the electrical conductivity of pure water is K _H20 , the electrical conductivity Ki due to cations and the same amount of OH ^- ions is determined by the following equation.

Ki＝K _0b −K _H20 = 〓 ⁱ ΛiCi＋ ^Λ _OH ^- C′ _OH -where, K _0b : Measured conductivity C′ _OH ^- : OH ^- ion concentration generated by the anion filter (= 〓 ⁱ
Ci) In this way, the nonferrous metal ion concentration can be determined from the conductivity K ₁ . Figure 3 shows Co(CH) ₂ and
Shows the results of measuring the electrical conductivity of Ni(OH) ₂ at 285°C. The ultimate equivalent ionic conductivity at high temperature is greater than the value at room temperature;
Due to the contribution of OH ^- ions, the conductivity due to nonferrous metal ions shown in Figure 3 is about one order of magnitude larger than the value at room temperature.

If the PH of the water supply is not neutral, correction is required accordingly. When the PH of the feed water is acidic, the anions are converted to OH ^- ions in the anion filter, so no correction is necessary, but when the PH of the feed water is alkaline, the ionic conductivity due to alkali metals is corrected. Must. Since the amount of alkali metal ions can be determined from the pH of the water supply, the nonferrous metal ion concentration can be determined more accurately by providing a pH meter and sending the signal to the controller 16 for correction.

FIG. 4 shows an example of the surface dose rate of piping with respect to the ratio (weight ratio) of Fe concentration to Ni ion concentration in the water supply, based on actual data. From the figure, it can be seen that the optimal range of the ratio (weight ratio) between Fe concentration and Ni ion concentration in the water supply is 2 to 8. The optimum range for the ratio (weight ratio) of the Fe concentration to the concentration of ionic corrosion products other than Ni is also the same. Therefore, in general, the surface dose rate of the piping can be most effectively reduced by controlling the ratio (weight ratio) between Fe concentration and nonferrous metal ion concentration in the water supply to 2 to 8. In the example shown in Figure 4, most of the nonferrous metal ions in the water supply are Ni ions and other ions can be ignored, so the control targets are Fe concentration and Ni.
There was no problem in determining the ratio of ion concentrations. However, if the concentration of non-ferrous metal ions other than Ni ions cannot be ignored, controlling only by the ratio of Fe concentration to Ni ion concentration may deviate from the optimal range of surface dose rate, and there is a risk that the surface dose rate may increase instead. Therefore, the ratio (weight ratio) between Fe concentration and non-ferrous metal ion concentration should be taken as the control target.

〔Example〕

Embodiments of the present invention will be described below with reference to FIG. FIG. 1 shows the basic system of the water supply system of a boiling water nuclear power plant to which the present invention is applied. In Figure 1, 1 is a turbine, 2 is a condenser, 3 is a condensate pump, 4 is a condensate demineralizer, 5 is a feed water pump, 6 is a low pressure feed water heater, 7 is a boost pump, 8 is a high pressure water feed heater, 9 is a nuclear pressure vessel,
10 and 11 are valves, 12 is a high temperature filter, 13 is a differential pressure gauge, 14 is a high temperature anion filter, 15 is a high temperature conductivity meter, 16 is a controller, 17, 18, 19
20 is a high temperature cladding filter, and 21 is a high temperature cation filter.

In FIG. 1, the difference from the conventional boiling water nuclear power plant is that the outlet of the high-pressure feed water heater 8 is equipped with a high-temperature filter 12, a differential pressure gauge 13, a high-temperature anion filter 14, a high-temperature conductivity meter 15, and a valve 1.
A high-temperature cladding filter 20, a high-temperature cation filter 21, and a valve 17 are attached between the point where one end is connected to the water supply line and the high-pressure water supply heater 8 and the branch line. ,1
8 and 19, and a controller 16.

In this embodiment, the water quality of the water supply system is controlled by the following procedure. First, the differential pressure between both ends of the high temperature filter 12 is measured using the differential pressure gauge 13. This measured value is sent as a signal to the controller 16.

The signal sent from the differential pressure gauge 13 is processed by the controller 16. At this time, the amount of iron in the iron cladding is calculated and determined from the differential pressure using the method described above.

Next, in the high temperature anion filter 14,
Anions are converted to OH ^- ions. after that,
The high temperature conductivity meter 15 measures the conductivity and sends the signal to the controller 16 .

A metal hydroxide or the like is used as the high temperature anion filter. With this filter,
Even if anions such as HCO ₃ ^- and HSiO ₃ ^- are present,
Since it is converted into OH ^- ions, it is easy to convert the cation concentration from the conductivity.

The signal sent from the high temperature conductivity meter 15 is processed by the controller 16. At this time, the amount of nonferrous metal ions in the corrosion product is determined by arithmetic processing using the relationship shown in FIG. Using the previously determined iron content, the ratio (weight ratio) of the iron content to the nonferrous metal ion content is determined. Water quality control is performed so that the value of this ratio falls within the range of 2 to 8.

When the ratio of the amount of iron to the amount of nonferrous metal ions calculated by the controller 16 is smaller than 2, the valve 19 is opened by a signal from the controller 16, and the nonferrous metal ions in the water supply are removed by the high temperature cation filter 21.
When the weight ratio falls within the range of 2 to 8, the valve 19 is closed by a signal from the controller 16. The above operation also reduces the absolute value of nonferrous metal ions, which are the source of radioactive corrosion products. High temperature cation filter 21
uses metal oxides.

When the ratio of the amount of iron to the amount of non-ferrous metal ions calculated by the controller 16 is greater than 8, the valve 17 is opened by a signal from the controller 16, and the iron cladding in the water supply is removed by the high temperature cladding filter 20.
When the weight ratio falls within the range of 2 to 8, a signal from the controller 16 closes the valve 17. High temperature clad filter 20 is the same as high temperature filter 12.

In this way, the surface dose rate of the plant piping is reduced, and maintenance and inspection as described above is facilitated.
In addition, it becomes easier to handle radioactive solid waste generated by repairing structural materials, etc.

Example 2 will be explained with reference to FIG.

In Example 1, when the ratio of the iron content in the iron cladding to the nonferrous metal ion content is out of the range of 2 to 8, the water quality of the water supply is improved by removing the iron cladding or nonferrous metal ions in the water supply. However, the same effect can be achieved by installing a bypass line in the condensate demineralizer 4 and opening and closing the valve 17 in response to a signal from the controller 16. However, the amount of iron cladding and nonferrous metal ions brought into the reactor is larger than in Example 1, and the effect is correspondingly smaller.

In addition, in this example, the ionic corrosion products were
Although the measurement was performed using a high-temperature conductivity meter, the concentration of individual ions may also be measured using a high-temperature ion sensor.

〔Effect of the invention〕

According to the present invention, it is possible to optimally control the ratio (weight ratio) of the amount of iron in the iron cladding to the amount of nonferrous metal ions in the reactor feed water, and to firmly fix radioactive ions to the ferrite layer on the surface of the fuel rod. Since the absolute amount of non-ferrous metal ions, which are the source of corrosion products, can be reliably reduced, the surface dose rate of the primary system piping can be reduced most effectively.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the relationship between the differential pressure of the filter and the iron cladding concentration, FIG. 3 is a diagram showing the relationship between the electrical conductivity and the nonferrous metal ion concentration, and FIG. FIG. 4 is a diagram showing the relationship between the surface dose rate of piping and the ratio of Fe concentration to Ni ion concentration in the water supply, and FIG. 5 is a diagram showing another embodiment of the present invention. [Explanation of symbols] 1... Turbine, 2... Condenser, 3... Condensate pump, 4... Condensate desalinator, 5...
... Feed water pump, 6... Low pressure feed water heater, 7... Boost pump, 8... High pressure feed water heater, 9... Reactor pressure vessel, 10... Valve, 11... Valve, 12... High temperature filter, 13...Differential pressure gauge, 14...High temperature anion filter, 15...High temperature conductivity meter, 16
...Controller, 17...Valve, 18...Valve, 19...
Valve, 20... High temperature clad filter, 21...
High temperature cation filter.

Claims (1)

[Scope of Claims] 1. In a nuclear power plant equipped with a nuclear reactor, a turbine condenser, a low-pressure feedwater heater, a high-pressure feedwater heater, and water supply piping connecting these, in the reactor feed water flowing through the water supply piping. A means for measuring the amount of iron cladding, a means for measuring the amount of nonferrous metal ions in the reactor feed water, and a ratio of the amount of iron in the ferrous cladding to the amount of nonferrous metal ions is 2 to 2, based on the measurement results by each of the means. 8 (weight ratio), a cladding filter that reduces the amount of iron cladding and an ion filter that reduces the amount of nonferrous metal ions are provided in the water supply piping system, respectively. Nuclear plant. 2. The nuclear power plant according to claim 1, wherein the means for measuring the amount of iron cladding is a filtration device equipped with a differential pressure gauge and having a function of measuring the amount of iron cladding by measuring the differential pressure. . 3. Claim 1, wherein the means for measuring the amount of nonferrous metal ions is a conductivity meter.
A nuclear power plant as described in paragraph or paragraph 2. 4. A bypass line of the water supply piping is provided between the high-pressure feed water heater and the nuclear reactor, and means for measuring the amount of iron cladding on the bypass line, means for measuring the amount of nonferrous metal ions, and means for measuring the amount of iron cladding are provided on the bypass line. Claim 1, characterized in that a cladding filter that reduces the amount of nonferrous metal ions and an ion filter that reduces the amount of nonferrous metal ions are provided.
A nuclear power plant as described in paragraph or paragraph 2. 5. A bypass line of the water supply piping is provided between the high pressure feed water heater and the nuclear reactor, and means for measuring the amount of iron cladding on the bypass line, means for measuring the amount of nonferrous metal ions, and means for measuring the amount of iron cladding are provided on the bypass line. Claim 3, characterized in that a clad filter that reduces the amount of nonferrous metal ions and an ion filter that reduces the amount of nonferrous metal ions are provided.
Nuclear power plant as described in Section. 6. A bypass line of the water supply piping is provided between the high pressure feed water heater and the nuclear reactor, and means for measuring the amount of iron cladding and means for measuring the amount of nonferrous metal ions are provided on the bypass line, and the means for measuring the amount of nonferrous metal ions are provided on the bypass line. Claim 1 or 2, characterized in that a cladding filter for reducing the amount of iron cladding and an ion filter for reducing the amount of nonferrous metal ions are provided between the water container and the high-pressure feed water heater. Nuclear power plant as described in Section. 7. A bypass line of the water supply piping is provided between the high pressure feed water heater and the nuclear reactor, and means for measuring the amount of iron cladding and means for measuring the amount of nonferrous metal ions are provided on the bypass line, and A nuclear power plant according to claim 3, characterized in that a cladding filter for reducing the amount of iron cladding and an ion filter for reducing the amount of non-ferrous metal ions are provided between the water container and the high-pressure feed water heater. plant.