JPH06148386A - Reducing method of corrosion product of nuclear power plant - Google Patents

Abstract

PURPOSE:To prevent a corrosion product from sticking on a fuel cladding tube and to reduce a radioactive corrosion product by using the fuel cladding tube of which the zeta potential of the corrosion product is changed or regulated. CONSTITUTION:A zeta potential regulating device 21 is provided on the outlet side of a recirculation pump 15 of a BWR plant. Zeta potential varies according to differences in pH and the form of a crystal and the zeta potential of an iron oxide is so conditioned that it shows a plus when the pH is on the acid side and a minus when the pH is on the alkali side and is inverted (becomes zero) in a neutral area. When the regulating device 21 is provided and thereby the zeta potential of a clad is regulated, a repulsive force acts on the surface of a fuel cladding tube and the clad. The form of the clad changes according to temperature and the concentration of dissolved oxygen and the particle size is made minute by an ultrasonic vibration bath. Thereby the zeta potential is regulated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing corrosion products in a nuclear power plant which reduces the amount of radioactive corrosion products formed on the surface of a fuel cladding tube.

[0002]

2. Description of the Related Art In a boiling water reactor (BWR),
Particulate metal oxides (generally called clad) and ionic impurities brought in from the water supply system are fixed on the surface of the fuel cladding tube by the boiling concentration phenomenon. Further, in a pressurized water reactor (PWR), since the boiling phenomenon does not exist on the surface of the fuel cladding tube, the adhesion amount of the clad is smaller than that of the BWR, but the adhesion phenomenon is observed.

At the surface of fuel cladding, they are activated by the irradiation of neutrons and become radioactive corrosion products. The whole amount of generated radioactivity is not fixed on the surface of the fuel cladding tube as it is, but part of it is exfoliated in the form of particles or eluted in the form of ions, and the radioactivity concentration in the primary coolant Will be increased.

Further, as a process of generating radioactivity, there is a process of peeling / eluting from the in-core structural material and similarly being fixed on the surface of the fuel cladding tube to be activated, and the core structural material being directly activated and radiated. A process of elution as a function is also considered.

Radioactivity generated through various processes is
Using the primary coolant as a medium, it reaches the entire primary system of the plant and pollutes each system. Furthermore, the liquid contact part of the primary coolant is almost a metallic material, and the contamination is considered to be a phenomenon that radioactivity is taken into this metallic material.

[0006] The incorporation of particulate radioactivity is mainly a process of physical adsorption on the surface of a metal material or adhesion by gravity settling, while ionic radioactivity is the surface of a metal material when it corrodes. It is co-deposited as an oxide in the formed corrosion film or is incorporated by an isotope exchange reaction.

For example, it is considered that ⁶⁰ Co ²⁺ is incorporated into the film by the following two equations with the corrosion of steel materials. _{^{60 Co 2+ + 2Fe + 4H 2}} O + 6e - = CoFe 2 O 4 + 4H 2 60 Co 2+ + CoFe 2 O 4 = Co 2+ + 60 CoFe ± 2 O 4

In this way, the metal material in contact with the primary coolant takes up radioactivity, the amount of which increases over time due to the growth of the corrosion film and the rate at which the amount of radioactivity taken in due to radioactive decay decreases. It will continue to increase until it is balanced.

[0009]

When radioactivity is accumulated on the surface of pipes and equipment, the air dose rate increases and the radiation exposure dose of workers who work in the vicinity increases, which in turn leads to a periodic inspection period of the plant. The total exposure dose (total manrem) in Japan.

As a result, the increase in the number of workers leads to an increase in the cost of the periodic inspection and an extension of the period of the periodic inspection. On the other hand, it becomes a social problem and also hinders the construction of a new plant. Against this background, vigorous research has been conducted on methods for suppressing the accumulation of radioactivity and methods for removing once accumulated radioactivity (decontamination).

By comprehensively studying the transfer behavior of these radioactivity, in order to suppress the increase of the air dose rate due to the adhesion of the radioactive corrosion products to the surface of the primary system piping and the surface of the equipment, (1) Does not generate radioactive corrosion products, (2)
Keeping radioactivity at the site of generation and not transferring it to the reactor water, (3)
Does not increase the concentration by removing the radioactivity transferred to the reactor water, (4)
It can be seen that there are possible measures such as preventing the radioactivity from adhering to the pipe surface and equipment, and (5) removing the adhering radioactivity.

Of these, the present invention relates to (1) a technique for preventing the generation of radioactive corrosion products, which suppresses the adhesion of the clad to the surface of the fuel cladding tube to reduce the generation of radioactive corrosion products to reduce the space dose. It is to provide a method for suppressing corrosion products of a nuclear power plant, which contributes to the reduction of radiation exposure of workers by suppressing the increase in the rate.

The present invention has been made to solve the above-mentioned problems. In a nuclear power plant, in a process in which a corrosion product adhered to the surface of a fuel cladding tube is converted into a radioactive corrosion product by neutron irradiation. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for preventing corrosion products from adhering to a fuel cladding tube and, as a result, reducing the amount of radioactive corrosion products generated on the surface of the fuel cladding tube.

[0014]

SUMMARY OF THE INVENTION The present invention relates to a corrosion product suppressing method for a nuclear power plant, which suppresses electrostatically depositing corrosion products on the surface of the fuel cladding tube, wherein the corrosion products of the fuel cladding tube are suppressed. The zeta potential of the fuel clad tube and the corrosion product is changed by changing the zeta potential of the fuel cell, using a fuel clad tube whose zeta potential is adjusted in advance, or by adjusting the water quality during the operation of the nuclear power plant. And

[0015]

Function: The zeta potential of the clad is adjusted so that the clad receives a repulsive force on the surface of the fuel cladding tube. That is, the same potential of the cladding tube is adjusted to the potential direction of the clad to electrostatically repel both.

That is, the main generation sites of the radioactive corrosion products are the fuel cladding surface and the core structure, as described above. Of these, a clad containing iron oxide as a main component adheres to the surface of the fuel cladding tube and is activated and becomes a radioactive corrosion product. Therefore, preventing the clad from adhering to the cladding is the most effective method for suppressing the generation of radioactive corrosion products.

The adhesion and deposition mechanism of the cladding on the boiling heat transfer surface is generally considered as follows.
That is, on a clean surface with no deposits initially, high-temperature water containing the clad is carried to the heat transfer surface by boiling and is concentrated near the surface to be attached by electrostatic interaction or the like.

Further, once the clad layer is formed, the clad is deposited by the filter effect of the clad layer. From this, it is estimated that the adhesion of the clad can be suppressed if the approach of the clad to the heat transfer surface can be prevented electrostatically, for example.

The clad observed in a nuclear power plant is classified according to its crystal morphology. Hematite (αFe
₂ O ₃ ), magnetite (Fe ₃ O ₄ ), goethite (γFeOOH), lepidocrocite (αFeOOH)
It is also reported that there is an amorphous component which does not exhibit a crystalline form.

These clads have a unique electrokinetic potential (zeta potential) in water. The zeta potential is a portion of the potential difference at the interface between a solid and a liquid, which effectively acts on the electrokinetic phenomenon, and represents the potential difference between the electrically fixed phase on the solid phase surface and the liquid.

FIG. 1 shows an example of measurement of zeta potential of iron oxide as a reagent. In the figure, the results are summarized with pH as a parameter. As is clear from FIG. 1, the zeta potential changes depending on the pH and also differs depending on the crystal form.

In general, the zeta potential of iron oxide is positive on the acidic side and negative on the alkaline side. In addition, the potential is inverted in the neutral region (the potential becomes zero: P
ZC) There are pH conditions.

By the way, ultrapure water is used in the primary system of BWR, and it is operated in the neutral region as the pH.
Therefore, for example, when the pH is defined as 7, it can be seen that the zeta potential is completely different due to the difference in crystal form.

On the other hand, zirconium alloys such as zircaloy 2 and zircaloy 4 are currently used for fuel cladding in nuclear power plants. The surface of these zirconium alloys is covered with a zirconium oxide (ZrO ₂ ) layer by oxidation.

This oxide layer is applied before the use of the cladding tube by an autoclave treatment called a blackening treatment, which is carried out for product inspection of the cladding tube. Further, in recent fuel cladding tubes, blackening treatment may not be carried out and the surface of the bare metal may be used, but the zirconium oxide layer is immediately formed upon contact with high temperature and high pressure water.

FIG. 2 shows an example of measuring the zeta potential of this zirconium oxide. In the initial process of the phenomenon that the clad adheres to the surface of the fuel cladding tube, it has been described that the adhesion starts due to the electrostatic interaction between the two, but the adhesion is largely affected by the zeta potential, for example.

That is, if the zeta potential of zirconium oxide on the surface of the fuel cladding tube and the zeta potential of the clad have exactly the same pH dependence, a repulsive force acts on both to repel each other. On the other hand, if the pH dependence is completely opposite, it will be met under any condition. Therefore, if the zeta potentials of the both can be maintained in a state in which repulsive force acts on each other, it is possible to suppress the clad adhesion to the surface of the fuel cladding tube.

Since the zeta potential of the clad differs depending on the crystal form as described above, it is possible to change the zeta potential by suppressing the crystal form of the clad. Further, FIG. 3 shows that even if hematite has the same crystal form, the zeta potential differs depending on the size of the crystal grain size. That is, it is considered possible to arbitrarily control the zeta potential of the clad by such means.

Further, the zeta potential of zirconium oxide on the surface of the fuel cladding tube can be controlled in the same manner as the clad because it changes depending on the degree of unevenness of the surface. Therefore,
Appropriate control of both zeta potentials can suppress the adhesion of the clad to the surface of the fuel cladding tube.

[0030]

EXAMPLE An example of a method for suppressing corrosion products in a nuclear power plant according to the present invention will be described with reference to FIGS. 4 and 5. FIG. 4 schematically shows a first embodiment in which the present invention is applied to a conventional BWR. In this embodiment, the zeta potential of the clad is adjusted to prevent adhesion to the surface of the fuel cladding tube.

Reference numeral 1 in FIG. 4 indicates a nuclear reactor in which steam generated in the core 3 turns the turbine 5. The steam that has worked in the turbine 5 returns to water in the condenser 7. The water is sent to the condensate purification system 11 by the condensate pump 9, and after removing impurities, it is preheated by the feed water heater 13 and returned to the reactor 1. Also,
The reactor water is forcedly circulated by the recirculation pump 15 and the jet pump 17. Further, a part of the reactor water flowing through the recirculation system is purified by the reactor water purification system 19.

In such a BWR plant, a zeta potential adjusting device 21 is installed on the outlet side of the recirculation pump 15. With this device, the zeta potential of the clad is adjusted so that the surface of the fuel cladding tube and the clad are repulsive to each other.

As a method for adjusting the zeta potential, for example, a change in the morphology of the clad or a change in the grain size can be used. The morphology of the clad can be changed by the temperature and the dissolved oxygen concentration. Further, the particle size can be reduced by an ultrasonic vibration bath, while it can be grown by a physical action such as vibration or stirring, or by an increase in aging time or dissolved oxygen concentration.

Of course, the zeta potential adjusting device 21 can be attached to the inlet side of the recirculation pump 15 or after the final stage of the feed water heater. In addition, in a plant without a recirculation system, it may be possible to install the clad in the furnace independently at the site where the clad circulates.

FIG. 5 shows a second embodiment in which the present invention is applied to a conventional BWR. In this embodiment, the zeta potential of the clad is constantly monitored and the zeta potential of the fuel cladding surface is adjusted. In FIG. 5, the same parts as those in FIG. 4 are designated by the same reference numerals, and the description of the overlapping parts will be omitted.

The zeta potential of the clad sampled through a normal sampling line is measured, and the zeta potential on the surface of the fuel cladding tube is adjusted so that repulsive forces act on each other.
It is adjusted by 21. The zeta potential on the surface of the fuel can be adjusted by, for example, insulating the fuel in bundle units and electrically polarizing it.

When the clad concentration and zeta potential during the operation of the plant are predicted, the zeta potential on the surface of the fuel cladding tube is adjusted in advance by a method such as surface processing or surface treatment, and repulsive forces are always applied during use. It is also possible to offer it while working.

Further, even in an operating plant, it is possible to control the water quality so that a repulsive force acts on the clad and the surface of the fuel cladding tube by a method such as pH control.

The technique of electrostatically controlling the zeta potential on the surface of the clad or the fuel cladding tube can be applied to improve the performance of the water treatment equipment for collecting the clad if the reverse principle is used. . That is, for example, when the clad is to be collected by a mixed bed type condensate demineralizer, the zeta potentials of the clad and the ion exchange resin may be adjusted so that attractive forces act on each other. become.

[0040]

According to the present invention, since the repulsive force acts on the clad and the surface of the fuel clad tube, it is possible to suppress the adhesion of the clad to the surface of the fuel clad tube. As a result, the amount of radioactive corrosion products generated can be significantly reduced.
By reducing the amount of radioactive corrosion products generated on the surface of the fuel cladding tube, the amount of radioactivity transferred to the reactor water is small and the amount of radioactivity adhering to the piping surface and equipment surface can be suppressed. Reduction is achieved.

Further, since the amount of radioactive corrosion products existing on the surface of the fuel cladding tube is small, the radioactivity is not scattered during the exchange and transportation of the fuel assembly, and the fuel assembly can be handled remarkably. It will be easy.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing an example of measurement of zeta potential of iron oxide as a reagent.

FIG. 2 is a characteristic diagram showing an example of measuring zeta potential of zirconium oxide.

FIG. 3 is a characteristic diagram showing a measurement example of zeta potential of hematite having different particle sizes.

FIG. 4 is a system diagram showing a first embodiment of a system in which the present invention is applied to a conventional BWR.

FIG. 5 is a system diagram showing a second embodiment of a system in which the present invention is applied to a conventional BWR.

[Explanation of symbols]

1 ... Reactor, 3 ... Core, 5 ... Turbine, 7 ... Condenser, 9
… Condensate pump, 11… Condensate purification system, 13… Water heater, 15…
Recirculation pump, 17 ... Jet pump, 19 ... Reactor water purification system,
21 ... Zeta potential regulator.

Claims (1)

[Claims] 1. A method for suppressing corrosion products of a nuclear power plant, which suppresses electrostatic adhesion of corrosion products to the surface of a fuel cladding tube, wherein the zeta potential of the corrosion product of the fuel cladding tube is changed. Corrosion production in a nuclear power plant characterized by using a fuel cladding tube whose zeta potential is adjusted in advance, or changing the zeta potential of the fuel cladding tube and the corrosion product by adjusting the water quality during operation of the nuclear power plant. Object suppression method.