JP2015110818A - Surface treatment method and surface treatment system of heat transfer pipe of steam generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and a system capable of suppressing Ni elution from inside a heat transfer pipe during PWR operation, and suppressing decline of AOA potential or rise of exposure dosage.SOLUTION: A surface treatment method of a heat transfer pipe of a steam generator comprising a Ni-based alloy includes steps for: conducting treatment liquid into the heat transfer pipe; adjusting pH of the treatment liquid to be 5.5 or higher and 6.5 or lower; and adjusting a surface potential of the heat transfer pipe to be 0 V.vs.SHE or higher. In the surface treatment method, a surface film comprising an oxide of Fe and/or a complex oxide of Cr and Fe is formed on the surface of the heat transfer pipe.

Description

  The present invention relates to a surface treatment method and a surface treatment system for a heat transfer tube of a steam generator.

  Heat transfer tubes used in steam generators (SG) have changed from stainless steel (SUS) to Ni-based alloys such as Alloy 600, Alloy 690TT, Alloy 800, etc., with stress corrosion cracking (SCC) resistance as a priority issue. It was. Although these Ni-based alloys are excellent in SCC resistance, it has been known that they are likely to cause a decrease in AOA (Axial offset anomaly) potential and an increase in exposure dose during high-load operation.

  Among these, the cause of the decrease in the AOA potential is considered as follows. For example, in the primary system of a pressurized water reactor (PWR), the heat transfer tube is one of the members having the largest contact area with the liquid. Since a Ni-based alloy is used for this heat transfer tube, the proportion of Ni ions that dissolve into the primary cooling water is greater than that of Fe ions. Therefore, it is considered that excessive Ni ions precipitate as NiO in the subcooled boiling portion during high-load operation, and the B (boron) compound taken into NiO causes a decrease in output.

  As for the cause of the increase in exposure dose, surplus Ni ions eluted in the primary cooling water are reduced and deposited on the fuel surface as metallic Ni (including Co), and the primary system is oxidized when the reactor plant is shut down. It is considered that this metallic Ni elutes in the primary cooling water due to the atmosphere, and adheres to the equipment surface around the entire primary system.

  Therefore, as a method of reducing the elution of Ni from the heat transfer tube, the inner surface of the heat transfer tube is subjected to a blasting process using beads having a small particle diameter, and then the high temperature water is circulated on the inner surface of the heat transfer tube. A method for forming a film has been proposed (see, for example, Patent Document 1). As described above, methods for suppressing a decrease in AOA potential and an increase in exposure dose have been studied for heat transfer tubes made of a Ni-based alloy.

Japanese Patent Laying-Open No. 2005-213538

  An object of the present invention is to provide a method and system for suppressing Ni elution from the inside of a heat transfer tube during PWR operation and suppressing a decrease in AOA potential and an increase in exposure dose.

  One aspect of the heat treatment tube surface treatment method of the present invention is a surface treatment method for a heat transfer tube of a steam generator made of a Ni-based alloy, the step of flowing a treatment solution into the heat transfer tube, and the treatment solution Adjusting the pH of the tube to 5.5 or more and 6.5 or less, and adjusting the surface potential of the heat transfer tube to 0V. vs. And a step of adjusting to more than SHE, and forming a surface film made of an oxide of Fe and / or a composite oxide of Cr and Fe on the surface of the heat transfer tube.

  One aspect of the heat transfer tube surface treatment system of the present invention is a surface treatment system for a steam generator heat transfer tube made of a Ni-based alloy, the treatment liquid line circulating the treatment liquid in the heat transfer tube, and the treatment A weak acid adjusting tank which is arranged on the liquid line and adjusts the pH of the treatment liquid to 5.5 or more and 6.5 or less, and the surface potential of the heat transfer tube is set to 0V. vs. A potential adjusting mechanism for adjusting to SHE or higher is provided, and a surface film made of an oxide of Fe and / or a composite oxide of Cr and Fe is formed on the surface of the heat transfer tube.

  According to the present invention, Ni elution during PWR operation can be suppressed, and a decrease in AOA potential and an increase in exposure dose can be suppressed.

The schematic block diagram which shows the surface treatment system of the heat exchanger tube of the steam generator of embodiment. The potential-pH diagram of Ni at 25 ° C. The electric potential-pH figure of Cr in 25 degreeC. The electric potential-pH figure of Fe in 25 degreeC. Ni potential-pH diagram at 300 ° C. The electric potential-pH figure of Cr in 300 degreeC. The potential-pH diagram of Fe at 300 ° C. The graph which shows the relationship between the temperature of boric acid aqueous solution, and pH.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram schematically illustrating a main part of a surface treatment system 100 for a heat transfer tube of a steam generator according to an embodiment. A surface treatment system 100 shown in FIG. 1 performs a surface treatment in a heat transfer tube 2 made of a Ni-based alloy and installed in a steam generator 1. The heat transfer pipe 2 is connected to a primary cooling water supply pipe 3 for introducing the primary cooling water therein, and a primary cooling water outlet pipe 4 for deriving the internal primary cooling water, and the primary cooling water supply pipe 3, The heat transfer pipe 2 and the primary cooling water outlet pipe 4 constitute a primary cooling water line 5. The primary cooling water line 5 is provided with a pump 6 for circulating the primary cooling water.

  A steam chamber 7 is formed above the inside of the steam generator 1. A secondary cooling water supply pipe 8 that introduces secondary cooling water and a steam discharge pipe 9 that discharges steam are connected to the steam chamber 7. During the operation of the steam generator 1, the primary cooling water generated in a nuclear reactor (not shown) is introduced into the heat transfer pipe 2, and the secondary cooling water supplied to the steam chamber 7 while flowing through the heat transfer pipe 2. And heat-exchange the secondary cooling water to generate steam. Further, the primary cooling water cooled by heat exchange is returned from the heat transfer pipe 2 to the nuclear reactor through the primary cooling water outlet pipe 4.

  A weak acid adjusting tank 11 is connected to the primary cooling water supply pipe 3 and the primary cooling water outlet pipe 4 via a surface treatment line 10. A treatment liquid pump 12 for transferring the treatment liquid L is interposed in the surface treatment line 10 and is configured to circulate the treatment liquid L between the heat transfer tube 2 and the weak acid adjustment tank 11.

  The weak acid adjustment tank 11 is connected to an oxygen gas cylinder 13 for supplying oxygen gas and nitrogen gas to the weak acid adjustment tank 11, a nitrogen gas cylinder 14 and a sampling pipe 15 for sampling the processing liquid L in the weak acid adjustment tank 11, respectively. Yes. On the sampling tube 15, a dissolved oxygen meter 16 for measuring the dissolved oxygen concentration and a pH meter 17 for measuring the pH of the processing liquid L are installed. The sampling pipe 15 is provided with a pump 18 for transferring the processing liquid L, and the processing liquid L is returned to the weak acid adjustment tank 11 after the dissolved oxygen concentration and pH are measured on the sampling pipe 15.

  In addition, the surface treatment system 100 includes a potential measuring device 19 that measures the surface potential of the heat transfer tube 2. The heat transfer tube 2 and the potential measuring device 19 are connected by a heat transfer tube sample electrode connection line 20, and a reference electrode 21 is installed in the vicinity of the opening of the primary cooling water outlet tube 4 in the heat transfer tube 2. Valves 22 to 25 for switching the flow path to the primary cooling water line 5 or the surface treatment line 10 are interposed in the primary cooling water line 5 and the surface treatment line 10.

  Here, the corrosion potential of Ni, Fe, and Cr will be described. FIG. 2 is a potential-pH diagram of Ni at a temperature of 25 ° C. From FIG. 2, it can be seen that Ni ions have a characteristic that the pH is approximately 8.5 or less and the corrosion potential is easily stabilized in the vicinity of 0 V. vs. SHE and is easily dissolved in water.

  FIG. 3 is a potential-pH diagram of Cr at a temperature of 25 ° C. It can be seen that Cr forms an oxide in the range of about pH 5.5 to 12 and corrosion potential in the range of about -1.1 V. vs. SHE to 0.5 V. vs. SHE, and is easily passivated. FIG. 4 is a potential-pH diagram of Fe at a temperature of 25 ° C. From FIG. 4, it can be seen that Fe forms an oxide in the range of about pH 2 or higher and a corrosion potential in the range of about -0.3 V. vs. SHE or higher and 0.8 V. vs. SHE or lower, and is easily passivated. Therefore, Ni ions are dissolved by bringing a liquid having a pH of 5.5 or more and 8 or less and a corrosion potential of ± 0 V.vs.SHE or more into contact with the surface of the alloy containing Ni, Cr and Fe. Fe and Cr can be led to a state where they are likely to be oxides.

  5, 6, and 7 are potential-pH diagrams of Ni, Cr, and Fe, respectively, at a temperature of 300 ° C. At a temperature of 300 ° C., the pH at which Ni is easily dissolved is about 5.5, which is more acidic than at a temperature of 25 ° C., but at pH 5.5, Fe and Cr are in a region where oxides are easily formed. There is no change in the tendency of Ni to dissolve more easily than Fe and Cr.

  Next, the surface treatment method of this embodiment will be described. First, before starting the nuclear reactor plant, the valves 22 to 25 are opened and closed in a state where the heat transfer tube 2 is assembled in the steam generator 1, that is, in an assembled state of the steam generator 1, and the heat transfer tube 2 and the surface treatment line 10 are opened. Constitutes a circulation flow path. From the weak acid adjustment tank 11, an acidic aqueous solution is supplied as the treatment liquid L to the primary cooling water supply pipe 3, and the treatment liquid L is circulated through the surface treatment line 10.

  The treatment liquid L forms a surface coating on the inner surface of the heat transfer tube 2 in the process of flowing through the heat transfer tube 2. The heat transfer tube 2 is made of a Ni-based alloy containing Ni, Cr, and Fe. Therefore, as described above, from the relationship between the corrosion potential and the pH, for example, the pH of the treatment liquid L is adjusted to 5.5 or more, and the surface potential of the heat transfer tube 2 is set to 0V. vs. By adjusting to SHE or higher, Ni on the inner surface of the heat transfer tube 2 is eluted as Ni ions, and an oxide of Fe and / or a composite oxide of Cr and Fe is formed, thereby enriching Fe or Fe and Cr. Surface coating can be formed. Further, since the pH at which Ni easily dissolves decreases as the temperature rises (changes to the acidic side), the treatment liquid L is adjusted to pH 6.5 or less.

  For adjusting the pH of the treatment liquid L, an aqueous solution of boric acid or oxalic acid is preferably used. Boric acid is preferred because it is a chemical used in neutron absorbers. Moreover, since oxalic acid decomposes at a high temperature, it has little influence on the nuclear reactor plant and is preferable.

  The pH of the treatment liquid L is adjusted by adding a boric acid aqueous solution (or oxalic acid aqueous solution) to the treatment liquid L in the weak acid adjustment tank 11 based on the measured value of the pH meter 17. The surface potential of the heat transfer tube 2 is adjusted by supplying oxygen gas and nitrogen gas from the oxygen gas cylinder 13 and the nitrogen gas cylinder 14 to the weak acid adjustment tank 11 based on the measured value of the potential measuring device 19. By increasing the dissolved oxygen concentration in the treatment liquid L, the corrosion potential of the treatment liquid L can be raised, and by reducing the dissolved oxygen concentration, the surface potential of the heat transfer tube 2 can be lowered. In addition, by supplying nitrogen gas to the processing liquid L, the dissolved oxygen concentration in the processing liquid L can be reduced and the surface potential of the heat transfer tube 2 can be reduced.

  From the results of FIGS. 5 to 7, the temperature of the boric acid aqueous solution (or oxalic acid aqueous solution) is not particularly limited, and may be room temperature (about 25 ° C.) to 300 ° C. Further, the circulation flow rate and the circulation time of the treatment liquid L are appropriately changed according to the acidic aqueous solution used as the treatment liquid L and the material constituting the heat transfer tube 2.

  The concentration of the boric acid aqueous solution (or oxalic acid aqueous solution) is preferably in the range of 0.005 to 5.0 mol / L. Here, FIG. 8 is a graph showing the results of measuring the relationship between the temperature and pH of the boric acid aqueous solution. From FIG. 8, the boric acid aqueous solution at 25 ° C. had a pH of about 4.7 at a concentration of 1.0 mol / L, a pH of about 5.1 at 0.11 mol / L, and a pH of about 5.6 at 0.0093 mol / L.

  The treatment liquid L contains the eluted Ni ions by flowing through the heat transfer tube 2, and NiO or metal Ni may be deposited. Therefore, it is preferable to provide an ion exchange device 26 that adsorbs and removes Ni ions in the treatment liquid L between the primary cooling water outlet pipe 4 and the weak acid adjustment tank 11 in the surface treatment line 10. It is preferable to include a filter 27 for filtering and removing solids such as Ni. Thereby, the usage efficiency of the process liquid L can be improved and the amount of waste liquids can be reduced.

  Moreover, after performing the surface treatment of the heat exchanger tube 2 by the surface treatment method of the present embodiment, the valves 22 to 25 are opened and closed to configure the primary cooling water line 5 to start the nuclear reactor plant. At this time, in order to wash the treatment liquid L on the inner surface of the heat transfer tube 2, it is preferable to pass pure water through the heat transfer tube 2 before the reactor plant is started. However, since boric acid is contained in the primary cooling water as described above, when the boric acid aqueous solution is used to adjust the pH of the treatment liquid L, there is little influence on the nuclear reactor plant even if cleaning is not performed.

  Note that the temperature adjustment of the treatment liquid L in the weak acid adjustment tank 11, the control of the supply amount of the boric acid aqueous solution, and the control of the supply amount of oxygen gas and nitrogen gas are the measuring instrument and control device normally installed in the nuclear reactor plant. Can be performed.

  As described above, according to the surface treatment method and the surface treatment system of the present embodiment, a surface film made of an oxide of Fe and / or a composite oxide of Cr and Fe is formed on the surface of a heat transfer tube formed of a Ni-based alloy. Thus, it is possible to suppress Ni elution during PWR operation, and to suppress a decrease in AOA potential and an increase in exposure dose.

  As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Steam generator, 2 ... Heat transfer pipe, 3 ... Primary cooling water supply pipe, 4 ... Primary cooling water outlet pipe, 5 ... Primary cooling water line, 6, 18 ... Pump, 7 ... Steam chamber, 8 ... Secondary cooling Water supply pipe, 9 ... steam discharge pipe, 10 ... surface treatment line, 11 ... weak acid adjustment tank, 12 ... treatment liquid pump, 13 ... oxygen gas cylinder, 14 ... nitrogen gas cylinder, 15 ... sampling pipe, 16 ... dissolved oxygen meter, 17 ... pH meter, 19 ... Potential measuring device, 20 ... Heat transfer tube sample electrode connection line, 21 ... Reference electrode, 22, 23, 24, 25 ... Valve, 26 ... Ion exchange device, 27 ... Filter, 100 ... Surface treatment system, L: Treatment liquid.

Claims (5)

A surface treatment method for a heat transfer tube of a steam generator made of a Ni-based alloy,
Flowing the treatment liquid into the heat transfer tube;
Adjusting the pH of the treatment liquid to 5.5 or more and 6.5 or less;
The surface potential of the heat transfer tube is set to 0V. vs. A step of adjusting to more than SHE,
A surface treatment method for a heat transfer tube, comprising forming a surface film made of an oxide of Fe and / or a composite oxide of Cr and Fe on the surface of the heat transfer tube.   The surface treatment method according to claim 1, wherein the pH of the treatment liquid is adjusted with an aqueous boric acid solution.   The surface treatment method according to claim 1, wherein the pH of the treatment liquid is adjusted with an oxalic acid aqueous solution.   The surface treatment method according to claim 1, wherein the surface potential is adjusted by dissolving oxygen in the treatment liquid and adjusting a dissolved oxygen concentration. A surface treatment system for a heat transfer tube of a steam generator made of a Ni-based alloy,
A treatment liquid line for circulating the treatment liquid in the heat transfer tube;
A weak acid adjusting tank which is arranged on the processing liquid line and adjusts the pH of the processing liquid to 5.5 or more and 6.5 or less;
The surface potential of the heat transfer tube is set to 0V. vs. Equipped with a potential adjustment mechanism that adjusts more than SHE,
A surface treatment system for a heat transfer tube, wherein a surface film made of Fe oxide and / or a composite oxide of Cr and Fe is formed on the surface of the heat transfer tube.

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