JP2009229388A - Nuclear power plant, fuel assembly and method for reducing radiation exposure in nuclear power plant - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reducing radiation exposure in a nuclear power plant which can further reduce radiation exposure even if the burnup of fuel assemblies is increased further. <P>SOLUTION: A BWR plant 1 includes a core 4 loaded with fuel assemblies 5 and feedwater pipes 9 which are connected with a condenser 8 and a reactor pressure vessel (RPV)3 and where a hollow filter 10 is placed. The placement of the hollow filter 10 restrains the concentration of iron oxide in the feedwater supplied into the RPV 3 to 1×10<SP>-9</SP>mol/kg or less. A ferrite coating is formed on the outside face of a cladding tube used for each fuel rod included in the fuel assemblies 5. Even if cobalt oxide in reactor water adheres to the ferrite coating, it is exfoliated from the outside face of the cladding tube through the agency of the electrostatic repulsive force generated between the cobalt oxide and triiron tetraoxide. This restrains nonradioactive cobalt from being activated on the outside face of the cladding tube and reduces the concentration of radioactive cobalt in the reactor water. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a radiation exposure reducing method for a nuclear power plant, a nuclear power plant, and a fuel assembly, and more particularly to a radiation exposure reducing method for a nuclear power plant suitable for application to a boiling water nuclear power plant, a nuclear power plant, and a fuel assembly. .

  In a boiling water nuclear power plant (BWR plant), further reducing the radiation exposure of workers during periodic inspection is an important issue from the viewpoint of plant health. The radiation source of the radiation exposure of the workers at the time of the regular inspection is mainly cobalt 60 which has adhered and accumulated on the inner surfaces of the recirculation system piping and the reactor water purification system piping. That is, non-radioactive cobalt is eluted by corrosion from the metal member of the BWR plant that is in contact with the reactor water (cooling water) or the feed water. This non-radioactive cobalt precipitates and adheres as cobalt oxide to the dryout surface existing on the fuel rod surface of the fuel assembly installed in the core of the nuclear reactor due to the enrichment effect accompanying nucleate boiling. Non-radioactive cobalt adhering to the fuel rod surface is irradiated with neutrons generated by fission of nuclear fuel existing in the fuel rod and activated to become cobalt 60. Cobalt 60 is transferred to the reactor water by peeling of cobalt oxide containing cobalt 60 from the fuel rod surface or by elution of the cobalt oxide. The cobalt 60 adheres to and accumulates on the inner surface of piping (for example, recirculation piping and reactor water purification piping) through which reactor water flows. The accumulation rate of cobalt 60 is proportional to the cobalt 60 concentration in the reactor water and the oxide film growth rate on the inner surface of the pipe. Therefore, if the concentration of cobalt 60 in the reactor water is reduced, the accumulation of cobalt 60 on the inner surface of the pipe can be suppressed, and the radiation exposure of workers during the periodic inspection can be reduced. The reactor water is cooling water that flows in the reactor pressure vessel and in pipes connected to the reactor pressure vessel other than the water supply pipe and the main steam pipe.

Japanese Patent Application Laid-Open No. 10-197672 describes a method of fixing cobalt to the surface of a fuel rod in order to reduce the concentration of cobalt 60 in the reactor water. In this method, a layer of ferric trioxide (α-Fe ₂ O ₃ ) is previously formed on the surface of the fuel rod, and cobalt oxide (CoO) adhering to the surface of the fuel rod is converted into cobalt ferrite ( The shape is changed to CoFe ₂ O ₄ ). Cobalt ferrite suppresses dissolution of cobalt adhering to the fuel rod surface. That is, since the cobalt 60 is fixed to the fuel rod surface, the concentration of the cobalt 60 in the reactor water is reduced.

  Japanese Patent Application Laid-Open No. 5-264786 describes a technique for suppressing the adhesion and accumulation of cobalt on the surface of a fuel rod as another method for reducing the concentration of cobalt 60 in the reactor water. The deposition of cobalt on the fuel rod surface increases in proportion to the iron oxide concentration in the reactor water. JP-A-5-264786 focuses on this phenomenon and suppresses the iron concentration of the feed water to 0.05 ppb or less, thereby suppressing cobalt from adhering to the fuel rod surface in the nuclear reactor. Furthermore, carbonic acid, nitrogen or dinitrogen monoxide is injected into the reactor water to control the pH of the reactor water to be weakly acidic and promote the dissolution of cobalt oxide adhering to the fuel rod surface. Such a method described in Japanese Patent Application Laid-Open No. 5-264786 reduces the residence time of cobalt on the fuel rod surface and suppresses activation of non-radioactive cobalt into cobalt 60 by neutron irradiation. Therefore, the concentration of cobalt 60 in the reactor water is reduced.

  Japanese Patent Application Laid-Open No. 6-148386 describes a method for suppressing corrosive organisms in a nuclear power plant. This corrosive organism control method suppresses adhesion of iron oxide to the outer surface of the cladding tube by controlling the surface potential of the surface of the fuel rod, that is, the outer surface of the cladding tube of the fuel rod and the iron oxide in the reactor water. ing. For this reason, it can suppress that cobalt accompanies iron oxide and adheres to a cladding tube outer surface. Also in the corrosive organism suppression method, activation of cobalt can be suppressed, and the concentration of cobalt 60 in the reactor water can be reduced.

  Japanese Patent Laid-Open No. 63-52092 describes that nickel ferrite and cobalt ferrite are formed on the outer surface of the cladding tube of the fuel rod of the fuel assembly loaded in the reactor core during operation of the nuclear power plant. Before loading the core, an iron oxide layer (cladding layer) is formed on the outer surface of the cladding tube, and fuel rods and fuel assemblies are manufactured using the cladding tube. A fuel assembly using a cladding tube having an iron oxide layer formed on the outer surface is loaded into the core, and then the operation of the nuclear power plant is started. During operation of the nuclear power plant, nickel and cobalt contained in the reactor water come into contact with the iron oxide layer on the outer surface of the cladding tube and change its chemical form into ferrite oxide. For this reason, stable nickel ferrite and cobalt ferrite are formed on the outer surface of the cladding tube, and the elution of radioactive cobalt from the cladding tube into the reactor water can be suppressed. Therefore, the concentration of the radionuclide in the reactor water can be reduced.

  JP-A-2006-38483 discloses that a ferrite film is formed on the inner surface of a nuclear plant pipe (for example, a recirculation system pipe of a BWR plant) in order to suppress adhesion of radionuclides.

Japanese Patent Laid-Open No. 10-197672 Japanese Patent Laid-Open No. 5-264786 JP-A-6-148386 JP-A 63-52092 JP 2006-38483 A

  In the method described in JP-A-10-197672, cobalt is fixed as cobalt ferrite on the outer surface of the cladding tube, so that the concentration of cobalt 60 ions in the reactor water can be reduced. However, in this method, cobalt ferrite on the surface of the fuel rod is peeled off from the surface of the fuel rod due to the shearing force of the cooling water flow and transferred to the reactor water. For this reason, the oxide concentration containing cobalt 60 in the reactor water increases. In addition, when the fuel assembly stays in the core longer due to the higher burnup of the fuel assembly, the amount of non-radioactive cobalt adhering to the fuel rod surface is increased, and the concentration of cobalt 60 ions in the reactor water is increased. To increase.

  In JP-A-5-264786, carbonic acid, nitrogen or dinitrogen monoxide is injected into the reactor water to control the pH of the reactor water to be weakly acidic. However, since carbonic acid or the like may increase the sensitivity of stress corrosion cracking of the structural member, it is not preferable to inject it into the reactor water.

  The method for inhibiting corrosive organisms described in JP-A-6-148386 suppresses the adhesion of iron oxide to the outer surface of the cladding tube by making the surface potential of the iron oxide and the outer surface of the cladding tube the same sign. Can do. However, even if iron oxide does not adhere to the outer surface of the cladding tube, cobalt adheres to the outer surface of the cladding tube. Furthermore, since the surface potential of cobalt oxide has an opposite sign to the surface potential of iron oxide and the outer surface of the cladding tube in the reactor water, it cannot be suppressed that the cobalt oxide adheres to the outer surface of the cladding tube.

  Further, the radiation exposure reducing method described in Japanese Patent Application Laid-Open No. 63-52092 is applied to an oxide layer formed on the outer surface of a cladding tube, which is formed before being loaded on a reactor core, after the operation of a nuclear power plant is started. Nickel and cobalt ferrite are formed on the outer surface of the cladding tube by contacting nickel and cobalt contained in water. Nickel and cobalt taken into the outer surface of the cladding tube due to the formation of these ferrites are activated by neutron irradiation to become radioactive cobalt. Nickel ferrite and cobalt ferrite are stable and the elution of radioactive cobalt is suppressed, but since the elution is not completely prevented, a trace amount of radioactive cobalt is eluted in the reactor water. When a fuel assembly having a further increased burnup is loaded on the core, the period of stay of the fuel assembly in the core becomes longer. When nickel ferrite and cobalt ferrite are formed on the outer surface of each cladding tube of the fuel assembly whose burnup is further increased by the method described in JP-A-63-52092, the length of stay in the core is increased. The amount of radioactive cobalt eluted from the outer surface of the cladding tube into the reactor water also increases. Also, the amount of radioactive cobalt produced on the outer surface of the cladding tube increases.

  An object of the present invention is to provide a method for reducing radiation exposure of a nuclear power plant, a nuclear power plant, and a fuel assembly that can further reduce radiation exposure even when the burnup of the fuel assembly is further increased.

  The feature of the present invention that achieves the above-described object is that the reactor is operated in a state in which a fuel assembly having a plurality of fuel rods using a cladding tube having a ferrite film formed on the outer surface is loaded in the reactor core. There is to do.

  During the operation of the nuclear reactor, the cobalt oxide adhering to the outer surface of the cladding tube, that is, the surface of the ferrite film is peeled off by the action of electrostatic repulsive force generated with the compound constituting the ferrite film. For this reason, even in a fuel assembly having a further increased burnup, the time during which the cobalt oxide adheres to the outer surface of the cladding tube becomes extremely short, so that non-radioactive cobalt contained in the cobalt oxide is not contained in the cladding tube. It is suppressed from being activated while adhering to the outer surface. The amount of radioactive cobalt eluted from the outer surface of the cladding tube into the reactor water is reduced, and the concentration of radioactive cobalt in the reactor water is also reduced. As a result, the deposition and accumulation of radioactive cobalt on the inner surface of a pipe or the like that is connected to the reactor and through which the reactor water flows is suppressed, and the surface dose rate of the pipe or the like is reduced. Therefore, even when the burnup of the fuel assembly is further increased, it is possible to further reduce the radiation exposure of workers who carry out periodic inspections after the operation of the nuclear power plant is stopped.

  The above object is also achieved by operating the nuclear reactor in a state where a fuel assembly having a plurality of fuel rods using a cladding tube having a hydrophilic oxide film formed on the outer surface is loaded in the core of the nuclear reactor. Can be achieved.

  Due to the boiling of the reactor water, the outer surface of the cladding tube, that is, the dryout surface generated on the surface of the hydrophilic oxide film is quickly submerged by the action of the hydrophilic oxide film. For this reason, the cobalt oxide adhering to the dry-out surface becomes a readily soluble cobalt hydroxide and is dissolved in the reactor water. Even when a hydrophilic oxide film is formed on the outer surface of the cladding tube, the time during which the cobalt oxide adheres to the outer surface of the cladding tube becomes extremely short. Therefore, in the case where the burnup of the fuel assembly is further increased by forming a hydrophilic oxide film on the outer surface of the cladding tube as in the case where the ferrite film is formed on the outer surface of the cladding tube, It is possible to further reduce the radiation exposure of workers who carry out periodic inspections after the operation is stopped.

  According to the present invention, radiation exposure can be further reduced even when the burnup of the fuel assembly is further increased.

  The inventors have studied a method that can further reduce the radiation exposure of workers during the periodic inspection when the fuel assembly is further burned up. The contents of this study will be described below. In the above examination, the radiation exposure reduction method described in JP-A-63-52092 was also considered.

The inventors examined the surface potential of the oxide particles. The oxide particles are positively charged by the adsorption of hydrogen ions and negatively charged by the adsorption of hydroxide ions. The adsorption of hydrogen ions or hydroxide ions depends on the oxide composition, and further changes depending on the pH of the reactor water. FIG. 3 shows the pH dependence of the surface potential of ferric trioxide (α-Fe ₂ O ₃ ), triiron tetroxide (Fe ₃ O ₄ ), and cobalt oxide (CoO). The horizontal axis in FIG. 3 indicates the pH of the reactor water. In the reactor water, ferric trioxide is negatively charged, and triiron tetroxide and cobalt oxide (CoO) are positively charged. Therefore, ferric trioxide and cobalt oxide (CoO) are electrically adsorbed, and triiron tetroxide and cobalt oxide (CoO) are electrically repelled. When a ferric trioxide layer is formed on the outer surface of the cladding tube in contact with the reactor water, cobalt oxide (CoO) is likely to adhere to the outer layer, that is, the outer surface of the cladding tube, and triiron tetraoxide is formed on the outer surface of the cladding tube. When the layer is formed, cobalt oxide (CoO) becomes difficult to adhere to the outer surface. Also, ferric trioxide and ferric tetroxide are electrostatically adsorbed.

  By utilizing the electrostatic repulsive force of the above-mentioned triiron tetroxide and cobalt oxide (CoO), the adhesion of cobalt oxide (CoO) to the outer surface of the cladding tube can be suppressed, and the radioactivity in the reactor water The concentration of cobalt can be reduced. The present invention has been made based on such characteristics.

  Due to the nuclear fission of the nuclear fuel material present in the fuel rods of the fuel assembly loaded in the reactor core, the reactor water rising in the fuel assembly is heated and boils. The cobalt oxide present in the reactor water adheres to the outer surface of the cladding tube of the fuel rod by using the boiling as a driving force. The process of adhesion of cobalt oxide to the outer surface of the cladding tube at this time will be described with reference to FIG. Bubbles are generated on the outer surface of the cladding tube by boiling of the reactor water flowing outside the cladding tube of the fuel rod ((A) in FIG. 4). The thin film at the interface between the generated bubbles and the outer surface of the cladding tube evaporates ((B) in FIG. 4). By evaporation of the thin film, cobalt ions existing in the thin film exceed the solubility and are deposited and adhered to the outer surface of the cladding tube as cobalt oxide ((C) of FIG. 4).

In the radiation exposure reduction method described in JP-A-63-52092, ferric trioxide was previously deposited on the outer surface of the cladding tube, and was deposited by boiling the reactor water as shown in FIG. Cobalt oxide (CoO) is reacted with ferric trioxide to produce cobalt ferrite (CoFe ₂ O ₄ ) (see FIG. 5). Since cobalt ferrite (CoFe ₂ O ₄ ) has a lower solubility than cobalt oxide (CoO), it is difficult to redissolve in the reactor water from the outer surface of the cladding tube. That is, the concentration of cobalt 60 in the reactor water is reduced by increasing the degree of adhesion of cobalt to the outer surface of the cladding tube and suppressing the remelting of cobalt into the reactor water. However, a certain amount or more of ferric trioxide is required to react cobalt oxide (CoO) with ferric trioxide. When the reaction does not occur, as is apparent from the characteristics of FIG. 8 showing the relationship between the degree of adhesion of cobalt to the cladding tube and the ⁶⁰ Co ion concentration in the reactor water, the adhesion of cobalt oxide to the cladding tube. Since the degree is moderate, the activated cobalt is eluted in the reactor water, and the ⁶⁰ Co ion concentration in the reactor water is increased. Further, since the solubility of cobalt ferrite in the reactor water is not zero, some of the cobalt activated in the cobalt ferrite is also eluted.

  By forming a ferrite film (triiron tetroxide film) on the outer surface of a zirconium alloy cladding tube in advance, cobalt oxide (CoO) adhering to the surface of the ferrite film due to boiling of the reactor water is in contact with the triiron tetroxide. It peels with the electrostatic repulsive force which arises in (refer FIG. 6). For this reason, the period during which cobalt oxide (CoO) adheres to the outer surface of the cladding tube, specifically, the surface of the ferrite film, is remarkably shortened. The remarkable shortening of the period suppresses the activation of non-radioactive cobalt contained in cobalt oxide (CoO), that is, the production of cobalt 60. By reducing the degree of adhesion of cobalt to the outer surface of the cladding tube, activation of cobalt is suppressed, and the concentration of radioactive cobalt (for example, cobalt 60) in the reactor water is reduced. Adhesion and accumulation of radioactive cobalt on the inner surface of a pipe or the like that is connected to the reactor and through which reactor water flows are suppressed, and the surface dose rate of the pipe or the like is reduced. Therefore, even when a fuel assembly having a further increased burnup is used, it is possible to further reduce the radiation exposure of a worker who performs a periodic inspection after the nuclear power plant is shut down.

  As described above, the formation of the ferrite film on the outer surface of the cladding tube can reduce the concentration of radioactive cobalt in the reactor water, but by reducing the amount of iron oxide brought into the reactor from the feed water, the radioactive cobalt in the reactor water is reduced. The concentration of can be further reduced.

  The iron oxide brought into the reactor from the feed water exists mainly as ferric trioxide in the reactor water. When a ferritic coating (triiron tetroxide coating) is formed on the outer surface of the cladding tube, ferric trioxide and ferrous tetroxide are electrostatically adsorbed (see Fig. 3), so ferric trioxide contained in reactor water Adheres to the outer surface of the cladding tube, that is, the surface of the ferrite film (see FIG. 7). When ferric trioxide adheres to the outer surface of the cladding tube, the electrostatic repulsion caused by ferric tetroxide is weakened, so cobalt oxide (CoO) contained in the reactor water is deposited on the surface of the adhering ferric trioxide. Adhere to. Ferric trioxide adhering to the outer surface of the cladding tube reacts with cobalt oxide (CoO) to produce cobalt ferrite (see FIG. 5). Cobalt contained in this cobalt ferrite is activated by neutron irradiation. Although the concentration of radioactive cobalt in the reactor water is smaller than when no ferrite film is formed on the outer surface of the cladding tube, the activated cobalt is eluted into the reactor water, so that the concentration in the reactor water increases.

The amount of ferric trioxide adhering to the ferrite film formed on the cladding tube by reducing the concentration of ferric trioxide in the reactor water, that is, reducing the iron oxide concentration of the feed water supplied to the reactor Decrease. As a result, the amount of cobalt oxide (CoO) adhering to the outer surface of the cladding tube is reduced, and the amount of radioactive cobalt in the reactor water is further reduced. In particular, by reducing the iron oxide concentration in the feed water to 1 × 10 ⁻⁹ mol / kg or less, the amount of ferric trioxide brought into the reactor is significantly reduced, and ferrite formed on the outer surface of the cladding tube The amount of ferric trioxide adhering to the surface of the film is drastically reduced. For this reason, the quantity of the radioactive cobalt produced | generated by the outer surface of a cladding tube becomes very small, and the density | concentration of the radioactive cobalt in a reactor water is further reduced.

The phenomenon shown in FIGS. 5 and 6 will be described in detail with reference to FIG. The degree of adhesion shown in FIG. 8 means the ease of transfer of cobalt adhering to the outer surface of the cladding tube into the reactor water. When the degree of fixation is large, the cobalt adhering to the outer surface of the cladding tube is not easily transferred to the reactor water due to elution. When the degree of fixation is small, the cobalt adhering to the outer surface of the cladding tube quickly moves into the reactor water. By increasing or decreasing the degree of cobalt fixation, the ⁶⁰ Co ion concentration in the reactor water can be reduced. That is, as in the conventional example described in JP-A-63-52092 (FIG. 5), activation of cobalt adhering to the outer surface of the cladding tube is promoted when increasing the degree of fixation of cobalt. However, since the degree of radiocobalt migration into the reactor water is small, the ⁶⁰ Co ion concentration in the reactor water is reduced. When a ferrite film is formed on the outer surface of the cladding tube to reduce the degree of adhesion of cobalt (FIG. 6), activation of cobalt adhering to the outer surface of the cladding tube is suppressed, so that the ⁶⁰ Co ion concentration in the reactor water is reduced.

  Japanese Patent Application Laid-Open No. 2006-38483 describes a method of forming a ferrite film on the surface of a structural member made of stainless steel (or carbon steel) such as piping of a nuclear power plant in contact with the reactor water for the purpose of suppressing adhesion of radionuclides. is doing. This ferrite film uses a diffusion suppressing action to prevent cobalt 60 ions in the reactor water from adhering to and accumulating on the surface of the structural member. Cobalt ions in the reactor water are taken in when an oxide film is formed on the surface of the structural member. That is, there are cases where iron ions eluted due to corrosion of the structural member are taken in when re-deposited and when chromium oxide is formed on the surface of the structural member. As shown in FIG. 9, the ferrite film is formed as a barrier layer between the structural member and the reactor water, suppresses the diffusion of oxygen from the reactor water to the structural member, and further, iron ions enter the reactor water from the structural member. Suppresses diffusion. With this function, the ferrite film suppresses corrosion of the structural member. The ferrite film also suppresses the diffusion of cobalt 60 ions in the reactor water into the structural member. With the above action, the ferrite film described in Japanese Patent Application Laid-Open No. 2006-38483 suppresses the accumulation and accumulation of cobalt 60 ions contained in the reactor water on the surface of the structural member.

  The formation of the ferrite film on the outer surface of the cladding tube of the fuel rod uses the electrostatic action of this ferrite film to suppress the adhesion of cobalt oxide to the outer surface of the cladding tube and The object is to suppress radioactive cobalt from being activated on the outer surface of the cladding tube. That is, the purpose of forming the ferrite film on the outer surface of the cladding rod of the fuel rod is different from the purpose of forming the ferrite film described in Japanese Patent Laid-Open No. 2006-38483. The cladding tube is made of a zirconium alloy. As shown in FIG. 10, the oxygen contained in the reactor water diffuses in the zirconium oxide film formed on the surface of the cladding tube (zirconium alloy) and reacts with zirconium at the interface with the cladding tube. Therefore, cobalt ions are not taken up by reprecipitation in the cladding tube.

As described above, by forming a ferrite film on the outer surface of the cladding tube of the fuel rod, the action of electrostatic repulsion can be utilized and adhesion of cobalt oxide to the outer surface of the cladding tube can be suppressed. The inventors also measured the surface potential of each metal oxide in high-temperature water for metal oxides other than the metal oxides shown in FIG. 3, and further, based on the ionic form in high-temperature water, The pH at which the surface potential of the metal oxide becomes zero (hereinafter referred to as the equipotential point) was determined. The measurement results of the surface potential of each metal oxide and the obtained equipotential points are shown together in FIG. Taking cobalt as an example, cobalt takes the chemical form of Coi ²⁺ , Co (OH) ⁺ , Co (OH) ₂ , Co (OH) ₃ ⁻ in high temperature water. The concentration of each cobalt ion for these chemical forms varies depending on the pH of the hot water. At the equipotential point of cobalt oxide, the total charge of cobalt ions of all chemical forms is considered to be zero. Therefore, the pH satisfying the equation (1) was determined and used as the equipotential point of the cobalt oxide.

2 [Co ²⁺ ] + [Co (OH) ⁺ ] − [Co (OH) ₃ ⁻ ] = 0 (1)
In the formula (1), the value in [] is the concentration of the ionic species indicated in [].

When the water in contact with the metal oxide is more acidic than the pH at the equipotential point of the metal oxide, the surface potential of the metal oxide becomes positive. When the water in contact with the metal oxide is more alkaline than the pH at the equipotential point of the metal oxide, the surface potential of the metal oxide becomes negative. From the results shown in FIG. 11, the metal oxides having the same sign as cobalt oxide in the vicinity of pH 5.6 of the reactor water of the BWR plant are triiron tetroxide, nickel ferrite and zinc ferrite. Therefore, if a ferrite film of one of these ferrites is formed on the outer surface of the cladding tube in advance, peeling of the cobalt oxide can be promoted by electrostatic repulsion with the film. In addition, you may form a membrane | film | coat on a cladding tube outer surface in the state which mixed these ferrites. Furthermore, intermediate compounds (Ni _X Fe _{3 -X} O ₄ , Ni _{1 -X} Zn _X Fe ₂ O ₄ , Zn _X Fe _{3 -X} O ₄ or Ni _Y Zn _Z Fe _{3 -YZ} O ₄ (where X, For Y, Z, and Y + Z, a coating of 0 to 1)) may be formed on the outer surface of the cladding tube.

  However, since the compound contained in the coating formed on the outer surface of the cladding tube is activated, it is desirable that the solubility of the compound is small. The solubility of each compound, that is, each metal oxide is shown in FIG. As is clear from FIG. 12, the solubility of nickel ferrite is smaller than that of other metal oxides. That is, in order to reduce the concentration of the radionuclide in the reactor water, it is desirable to form a nickel ferrite film on the outer surface of the cladding tube. When a zinc ferrite coating is formed on the outer surface of the cladding tube, the amount of radioactive zinc 65 generated by neutron irradiation can be reduced by using zinc in which zinc 64 is depleted. Formation of a triiron tetroxide film is also desirable.

The surface potential of ferric trioxide present in the reactor water has a different sign in the vicinity of the pH of the reactor water, not only for triiron tetroxide but also for nickel ferrite and zinc ferrite. For this reason, ferric trioxide is easily adsorbed to each of nickel ferrite and zinc ferrite. When ferric trioxide particles adhere to these ferrites, the electrostatic repulsion caused by nickel ferrite and zinc ferrite, respectively, is weakened in the same manner as the above-described triiron tetraoxide, and the adhering ferric trioxide This is undesirable because cobalt oxide adheres to the particles. Therefore, when the nickel ferrite and zinc ferrite films are formed on the outer surface of the cladding tube, it is desirable to suppress the concentration of iron oxide contained in the feed water to 1 × 10 ⁻⁹ mol / kg or less. In order to suppress the iron oxide concentration of the feed water to 1 × 10 ⁻⁹ mol / kg or less, a hollow filter or a filter desalting device, which is an iron oxide removing device, may be used in the feed water pipe.

  Injecting hydrogen into the reactor water in a nuclear reactor after starting a nuclear power plant loaded with a fuel assembly having a plurality of fuel rods manufactured using a cladding tube with a ferrite coating on the outer surface. Is desirable. By the action of the injected hydrogen, the concentration of oxygen and hydrogen peroxide in the reactor water is reduced, and the corrosion of the in-reactor structure installed in the reactor can be suppressed. Therefore, the amount of ferric trioxide generated on the surface of the reactor internal structure and transferred to the reactor water can be reduced, and the amount of radionuclide (for example, cobalt 60) produced on the fuel rod surface is further reduced.

Instead of the ferrite film, a hydrophilic oxide film may be formed in advance on the outer surface of the cladding tube in contact with the reactor water. The hydrophilic oxide film is formed on the outer surface of the cladding tube before the fuel assembly is loaded on the core. By forming the hydrophilic oxide film on the outer surface of the cladding tube, the outer surface of the cladding tube becomes hydrophilic, and the portion of the outer surface of the cladding tube that has been dried out by nucleate boiling can be quickly submerged. Cobalt oxide contained in the reactor water adheres to the dry-out surface of the outer surface of the cladding tube due to the concentration effect by nucleate boiling. However, since this dry-out surface is quickly submerged by the action of the hydrophilic oxide film, the deposited cobalt oxide is precipitated as cobalt hydroxide (Co (OH) ₂ ) and hydrolyzed to cobalt oxide. (CoO). Since cobalt hydroxide is easier to dissolve than cobalt oxide, it is desirable to contact water when cobalt is in a hydroxide state in order to dissolve cobalt quickly.

Titanium oxide is suitable as a substance for forming a hydrophilic oxide film on the outer surface of the cladding tube. The surface of titanium oxide becomes hydrophilic when irradiated with light. However, some oxides of titanium adsorb cobalt to form a compound. For example, titanium oxide (TiO ₂ ) is used as an adsorbent for cobalt ions. For this reason, it is desirable to use an oxide of titanium that does not adsorb cobalt to form a compound. Examples of such titanium oxide include transition metal salts of titanic acid (for example, iron titanate, zinc titanate, and zirconium titanate).

When ferric trioxide present in the reactor water adheres to the surface of the hydrophilic oxide film, the effect of making the outer surface of the cladding tube hydrophilic is reduced by the attached ferric trioxide. In order to improve the effect of making the outer surface of the cladding tube hydrophilic, it is desirable to reduce the amount of ferric trioxide attached to the surface of the hydrophilic oxide film on the outer surface of the cladding tube. This is to reduce the amount of ferric trioxide brought into the reactor by water supply. As in the case of forming a ferrite film on the outer surface of the cladding tube, it is desirable to suppress the iron oxide concentration in the feed water to 1 × 10 ⁻⁹ mol / kg or less.

  A method for reducing radiation exposure in a boiling water nuclear power plant (BWR plant) according to embodiment 1, which is a preferred embodiment of the present invention, will be described with reference to FIGS.

  The BWR plant 1 of this embodiment includes a nuclear reactor 2, a turbine 7, a condenser 8, and a water supply pipe 9. The nuclear reactor 2 has a nuclear reactor pressure vessel (hereinafter referred to as RPV) 3 and a core 4 disposed in the RPV 3. A plurality of fuel assemblies 5 with higher burnup are loaded on the core 4. The fuel assembly 5 has a plurality of fuel rods in which a plurality of fuel pellets made of nuclear fuel material are filled in a cladding tube made of a zirconium alloy. A ferrite film is formed on the outer surface of the cladding tube in contact with the reactor water. A plurality of jet pumps (not shown) are disposed in an annular downcomer formed between the RPV 3 and the core 4. The main steam pipe 6 connected to the RPV 3 is connected to the turbine 7. The water supply pipe 9 communicates the condenser 8 and the RPV 3. A hollow filter (condensate filter) 10, a condensate demineralizer 11, a feed water pump 12, and a feed water heater 13 are installed in the feed water pipe 9 in this order from the upstream. A filtration desalting apparatus may be used in place of the hollow filter 10. The filtration desalination apparatus is configured by pre-coating a powder ion exchange resin on a support member through which water can pass. A bleed steam pipe 14 connected to the main steam pipe 6 is connected to the feed water heater 13. A hydrogen injection device 19 is connected to the water supply pipe 9 between the condensate demineralizer 11 and the water supply pump 12.

  The recirculation system provided in the BWR plant 1 has a recirculation system pipe 15 and a recirculation pump 16 provided in the recirculation system pipe 15. One end of the recirculation system pipe 15 is connected to a nozzle (not shown) provided in the RPV 3 and communicated with a downcomer. The other end of the recirculation pipe 15 is connected to a riser pipe (not shown) that is disposed in the downcomer of the RPV 3 and connected to the nozzle of the jet pump. The reactor water purification system has a reactor water purification system pipe 17 and a reactor water purification device 18 installed in the reactor water purification system pipe 17. The reactor water purification system pipe 17 is connected to the recirculation system pipe 15 and the water supply pipe 9. A sampling pipe 20 is connected to the water supply pipe 9 between the connection point of the hydrogen injection device 19 to the water supply pipe 9 and the condensate demineralizer 11. A sampling pipe 21 is connected to the reactor water purification system pipe 17.

  When the BWR plant 1 is in operation, the reactor water sucked into the recirculation system pipe 15 from the downcomer in the RPV 3 by driving the recirculation pump 16 is boosted by the recirculation pump 16 and jetted through the riser pipe. It is ejected from the nozzle of the pump. The reactor water present in the downcomer around the nozzle is sucked into the jet pump by the jet flow and discharged from the jet pump. Reactor water discharged from the jet pump is guided into the reactor core 4 and rises between the fuel rods in the fuel assembly 5. The reactor water is heated by heat generated by fission of nuclear fuel material existing in each fuel rod, and a part thereof becomes steam. Moisture is removed from the steam by an air / water separator (not shown) and a steam dryer (not shown) in the RPV 3, the steam is guided to the turbine 7 through the main steam pipe 6, and the turbine 7 is rotated. A generator (not shown) connected to the turbine 7 rotates to generate electric power. The steam discharged from the turbine 7 is condensed in the condenser 8. The water generated by this condensation is boosted as feed water by the feed water pump 12 and supplied to the downcomer in the RPV 3 through the feed water pipe 9.

The feed water passes through the hollow core filter 10, the condensate demineralizer 11 and the feed water heater 13 while flowing in the feed water pipe 9. The hollow filter 10 is generated in the condenser 8 and removes corrosion products (for example, ferric trioxide which is an iron oxide) contained in the water supply. Since the hollow core filter has high iron removal performance, the concentration of iron oxide contained in the feed water that has passed through the hollow core filter 10 is suppressed to 1 × 10 ⁻⁹ mol / kg or less. For this reason, the density | concentration of the iron oxide brought into RPV3 with water supply falls. The condensate demineralizer 11 prevents the seawater components (sodium ions and chloride ions) from entering the RPV 3 when the seawater used for condensation of steam leaks through the heat transfer pipe in the condenser 8. To remove the seawater component. The extraction steam pipe 14 extracts a part of the steam flowing in the main steam pipe 6. The feed water heater 13 heats the feed water flowing through the feed water pipe 9 using the steam extracted by the extraction steam pipe 14. Heated water supply is supplied into the RPV 3.

  The reactor water in the RPV 3 is guided into the reactor water purification system piping 17 through the recirculation system piping 15. Impurities (such as oxides containing radionuclides) contained in the reactor water are removed by the reactor water purification device 17 and returned to the RPV 3 through the water supply pipe 9.

  During operation of the BWR plant, hydrogen is injected from the hydrogen injection device 19 into the feed water flowing through the feed water pipe 9. The injected hydrogen is guided into the RPV 3 together with the water supply. This hydrogen reacts with oxygen and hydrogen peroxide present in the reactor water in the RPV 3 to produce water. For this reason, each amount of oxygen and hydrogen peroxide contained in the reactor water is reduced. As a result, the production of iron oxide (for example, ferric trioxide) in the RPV 3, the recirculation system pipe 15, and the reactor water purification system pipe 17 is suppressed.

  A fuel assembly 5 having a plurality of fuel rods using a cladding tube having a ferrite film formed on the outer surface is manufactured before being loaded into the core 4. The production of the fuel assembly 5 will be described below. In manufacturing the fuel assembly 5, a ferrite film is formed on the outer surface of the cladding tube. A detailed configuration of the film forming apparatus 25 used for forming the ferrite film will be described with reference to FIG. The film forming apparatus 25 includes a cladding tube storage container 26, a circulation pipe 27, an iron (II) ion implantation apparatus 28, a pH adjusting agent injection apparatus 33, an oxidant injection apparatus 38, a circulation pump 43, a heater 44 and a desalter 47. It has. The valve 48, the circulation pump 43, the valve 49, the heater 44, the valves 50 and 51, the oxidant injection device 38, the pH adjuster injection device 33, the iron (II) ion injection device 28, and the valve 52 circulate in this order from the upstream. The pipe 27 is provided. Both ends of the circulation pipe 27 are connected to the cladding tube storage container 26. The valve 54, the cooler 45, and the valve 55 are installed in a pipe 53 that bypasses the valve 49, the heater 44, and the valve 50 and is connected to the circulation pipe 27. A pipe 56 that bypasses the valve 51 is connected to the circulation pipe 27. A valve 57, a filter 46, a desalter 47 and a valve 58 are installed in the pipe 56. A drain pipe 59 provided with a valve 60 is connected to the circulation pipe 27. An inert gas introduction pipe 61 and a vent pipe 62 are connected to the cladding tube storage container 26.

  The iron (II) ion implantation apparatus 28 includes a chemical solution tank 29, an injection pump 30, and an injection pipe 31. The chemical tank 29 is connected to the circulation pipe 27 by an injection pipe 31 in which an injection pump 30 and a valve 32 are installed. The chemical solution tank 29 is filled with a medicine containing divalent iron (II) ions prepared by dissolving iron with formic acid. This drug contains formic acid. In addition, as a chemical | medical agent which dissolves iron, the organic acid or carbonic acid which becomes a counter anion of iron (II) ion can be used not only formic acid. The pH adjuster injection device 33 includes a chemical tank 34, an injection pump 35, and an injection pipe 36. The chemical liquid tank 34 is connected to the circulation pipe 27 by an injection pipe 36 provided with an injection pump 35 and a valve 37. The chemical tank 34 is filled with hydrazine which is a pH adjusting agent. The oxidant injection device 38 includes a chemical tank 39, an injection pump 40, and an injection pipe 41. The chemical tank 39 is connected to the circulation pipe 27 by an injection pipe 41 in which an injection pump 40 and a valve 42 are installed. The chemical tank 39 is filled with hydrogen peroxide as an oxidizing agent. A pH meter 63 is installed in the circulation pipe 27 downstream of the connection point between the injection pipe 31 and the circulation pipe 27.

  A method for forming a ferrite film on a cladding tube using the film forming apparatus 25 will be described in detail. First, the cladding tube 22 is stored in the cladding tube storage container 26. The cover of the cladding tube storage container 26 is opened, and a plurality of cladding tubes 22 made of zirconium alloy (for example, Zircaloy) sealed at both ends with rubber or the like are placed in the cladding tube storage container 26. The cladding tube 22 is not filled with nuclear fuel material. After a predetermined number of cladding tubes 22 are arranged in the cladding tube storage container 26, a lid is attached to the cladding tube storage container 26, and the cladding tube storage container 26 is sealed. Open valves 48-52, 54, 55, 57, 58. Valves 32, 37 and 42 are closed. The valve 60 is opened to supply water from the drain pipe 59 into the cladding tube storage container 26. Water is filled in the cladding tube storage container 26, and the cladding tube 22 is immersed in water. The circulation pipe 27 and the pipes 53 and 56 are also filled with water. When supplying this water, the air in the piping such as the cladding tube storage container 26 and the circulation piping 27 is discharged from the vent pipe 62. When the water supply is finished, the valve 60 is closed. Nitrogen (or argon) is supplied from the inert gas introduction pipe 61 into the cladding tube storage container 26, and oxygen contained in the water in the cladding tube storage container 26 is removed.

  Water present in the cladding tube storage container 26 and the circulation piping 27 is heated. The valves 54, 55, 57, and 58 are closed to drive the circulation pump 43. Water existing in the cladding tube storage container 26 and the circulation pipe 27 is circulated through the circulation pipe 27. The heater 44 is activated to heat the circulating water, and the temperature of the water is raised to 90 ° C.

  A drug containing divalent iron (II) ions, a pH adjusting agent, and an oxidizing agent are injected into the circulation pipe 27. These chemicals injected into the circulation pipe 27 are mixed with the circulating water to generate a film-forming aqueous solution (film-forming liquid). The film-forming aqueous solution is guided into the cladding tube storage container 26 and contacts the outer surface of each cladding tube 22 stored in the cladding tube storage container 26. The film-forming aqueous solution circulates in the closed loop formed by the cladding tube storage container 26 and the circulation pipe 27 by the operation of the circulation pump 43.

  The injection of each drug will be specifically described. The valve 32 is opened to drive the injection pump 30, and the chemical solution containing iron (II) ions and formic acid is returned from the chemical solution tank 29 to the water flowing in the circulation pipe 27 (or the coating returning from the cladding tube storage container 26. Injecting aqueous solution). By opening the valve 37 and driving the injection pump 35, a pH adjusting agent (for example, hydrazine) is injected from the chemical tank 34 into the water (or film-forming aqueous solution) flowing through the circulation pipe 27. The pH meter 63 measures the pH of the film-forming aqueous solution that flows in the circulation pipe 27. Based on this measured pH value, the rotation speed of the infusion pump 35 (or the opening degree of the valve 37) is adjusted, and the pH of the film-forming aqueous solution is adjusted to pH 5.5 to 9.0, for example, 7.0. As the pH adjuster, it is desirable to use an organic alkaline solution such as hydrazine and ethanolamine. The valve 42 is opened and the injection pump 40 is driven to inject hydrogen peroxide, which is an oxidant, into water (or a film-forming aqueous solution) flowing from the chemical tank 39 through the circulation pipe 27. As the oxidizing agent, in addition to hydrogen peroxide, a chemical in which ozone or oxygen is dissolved may be used.

  The film-forming aqueous solution produced by the injection of each of the above drugs has a temperature of 90 ° C. and a pH of 7.0. It is necessary to form a dense ferrite film on the outer surface of the cladding tube 22 so that cobalt ions in the reactor water do not directly adhere to the outer surface of the cladding tube 22 during operation of the BWR plant. At the time of film formation, the temperature of the film-forming aqueous solution needs to be 60 ° C. or higher in order to promote a chemical reaction that forms a dense ferrite film on the outer surface of the cladding tube 22. When the temperature of the film-forming aqueous solution exceeds 200 ° C., a dense ferrite film cannot be formed on the outer surface of the cladding tube 22. For this reason, in order to form a dense ferrite film, the temperature of the film-forming aqueous solution is set to 200 ° C. or lower. When the temperature of the film-forming aqueous solution exceeds 100 ° C., in order to suppress boiling of the film-forming aqueous solution, it must be pressurized and the film-forming apparatus must have a pressure-resistant structure. The film forming apparatus becomes larger. Therefore, the temperature of the aqueous solution for film formation in the film formation treatment is preferably 100 ° C. or less, which does not require the film forming apparatus 25 to have a pressure resistant structure.

  By supplying the film forming aqueous solution into the cladding tube storage container 26, iron (II) ions contained in the film forming aqueous solution are adsorbed on the outer surface of the cladding tube 22. The adsorbed iron (II) ions react with hydrogen peroxide contained in the film-forming aqueous solution, and a ferrite film, that is, a ferrite film containing magnetite as a main component (hereinafter referred to as magnetite film) is applied to the cladding tube 22. Formed on the outer surface. A magnetite film is formed on the outer surface of the cladding tube 22 by the reactions shown in the equations (2) and (3).

  After the magnetite film having a predetermined thickness is formed on the outer surface of the cladding tube 22, the heating by the heater 44 is stopped, the driving of the injection pumps 30, 35, 40 is stopped, and the valves 32, 37, 42 are closed. Whether or not a magnetite film having a predetermined thickness has been formed on the outer surface of the cladding tube 22 is determined, for example, based on the elapsed time after the injection of the medicine into the circulation pipe 27 is started. After the heating by the heater 44 is stopped, the valves 54 and 55 are opened and the valves 49 and 50 are closed. The film-forming aqueous solution flowing in the closed loop flows through the pipe 53 and is cooled by the cooler 45. When the temperature of the film-forming aqueous solution decreases to about 20 ° C., the cooling of the film-forming aqueous solution by the cooler 45 is stopped. Then, the valves 57 and 58 are opened and the valve 51 is closed. The film-forming aqueous solution whose temperature has decreased flows in the pipe 56 and is supplied to the filter 46 and the desalter 47. The filter 46 removes solids such as particles contained in the film-forming aqueous solution. The ion component contained in the film-forming aqueous solution is removed by the desalter 47.

  After the solid content and the ion component in the film-forming aqueous solution are removed, the circulation pump 43 is stopped. The valves 49, 50, 51 are opened, and further the valve 60 is opened, and the water in the cladding tube storage container 26, the circulation pipe 27 and the pipes 53, 56 is discharged through the drain pipe 59. In addition, when it is necessary to leave water in the cladding tube storage container 26, the valve 48 located below is closed when draining.

  With the above operation, the magnetite film can be formed on the outer surface of the cladding tube 22. By storing the plurality of cladding tubes 22 in the cladding tube storage container 26, a magnetite film can be formed on the outer surface of the plurality of cladding tubes 22 at a time. The cladding tube 22 on which the magnetite film is formed is taken out from the cladding tube storage container 26. Thereafter, another plurality of cladding tubes 22 are accommodated in the cladding tube storage container 26 and the above-described operation is repeated, whereby a magnetite film can be formed on the outer surfaces of those cladding tubes 22.

  An ultrasonic transmission device is installed in the cladding tube storage container 26, and ultrasonic cleaning of the cladding tube 22 in the cladding tube storage container 26 is performed when a film-forming aqueous solution is supplied to the filter 46 and the desalter 47. . By this ultrasonic cleaning, ferrite adhered loosely to the outer surface of the cladding tube 22 can be removed. The removed ferrite is removed by the filter 46 or the like.

  After drainage from the system of the film forming apparatus 25 is completed, the cover of the cladding tube storage container 26 is opened, and a plurality of cladding tubes 22 having a magnetite film formed on the outer surface are taken out from the cladding tube storage container 26. After these cladding tubes 22 are dried, the fuel assembly 5 is manufactured. A lower end plug is welded to one end of the cladding tube 22, and the cladding tube 22 is filled from the other end where a plurality of fuel pellets are opened. Furthermore, necessary components such as a coil spring are inserted into the cladding tube 22. The upper end plug is welded to the other end of the cladding tube 22, and the cladding tube 22 is sealed with a plurality of pellets stored therein. The fuel rod is completed through the above steps.

  The lower ends of the plurality of fuel rods are held by the lower tie plate, and the upper ends of the fuel rods are held by the upper tie plate. These fuel rods are held at a plurality of positions in the axial direction by a plurality of fuel spacers so that a mutual interval becomes a predetermined interval. A channel box that surrounds a plurality of fuel rods bundled with fuel spacers is attached to the upper tie plate. A fuel assembly 5 having a plurality of fuel rods with a magnetite film formed on the outer surface is completed. A water rod is disposed at the center of the cross section of the fuel assembly 5. The fuel assembly 5 having a higher burnup can be realized by storing fuel pellets made of nuclear fuel material with a higher enrichment in the fuel rods.

  The plurality of fuel assemblies 5 manufactured as described above are loaded into the core 4 of the RPV 3 in the period of the periodic inspection after the operation of the BWR plant is stopped. That is, the lid (not shown) of the RPV 3 is removed, and the steam dryer, the steam separator, etc. installed in the RPV 3 are taken out into the RPV 3. Among the plurality of fuel assemblies 5 loaded in the core 4, a plurality of spent fuel assemblies 5 having reached the end of their life are taken out and transferred to a fuel storage pool (not shown) outside the RPV 3. A plurality of new fuel assemblies 5 having a plurality of fuel rods having magnetite films formed on the outer surface are loaded into the core 4. After such fuel exchange is completed and the periodic inspection is completed, the taken out device is installed in the RPV 3 and a lid is attached to the RPV 3. Then, the operation of the BWR plant is started.

During operation of the BWR plant, the feed water passes through the hollow filter 10, so that the iron oxide concentration of the feed water supplied into the RPV 3 through the feed water pipe 9 is suppressed to 1 × 10 ⁻⁹ mol / kg or less. For this reason, the density | concentration of the iron oxide (for example, ferric trioxide) contained in a reactor water falls remarkably. The reactor water rises along the outer surface of the cladding tube 22 of each fuel rod provided in each fuel assembly 5 in the core 4. Since the magnetite film is formed on the outer surface of each cladding tube 22, the cobalt oxide adhering to the surface of the magnetite film on the outer surface of the cladding tube 22 due to boiling of the reactor water is in contact with the triiron tetroxide contained in the magnetite film. It peels with the electrostatic repulsive force which arises between (refer FIG. 6). For this reason, the period when the cobalt oxide adheres to the surface of the magnetite film, that is, the outer surface of the cladding tube 22, is remarkably shortened. As a result, even when the fuel assembly 5 whose burnup is further increased is loaded on the core 4, activation of non-radioactive cobalt contained in the cobalt oxide is suppressed, and radioactive cobalt in the reactor water (for example, cobalt 60) is suppressed. The concentration of is reduced. Adhesion and accumulation of radioactive cobalt on the inner surface of a pipe or the like that is connected to the reactor and through which reactor water flows are suppressed, and the surface dose rate of the pipe or the like is reduced. Therefore, even when the burnup of the fuel assembly is further increased, it is possible to further reduce the radiation exposure of workers who carry out periodic inspections after the operation of the nuclear power plant is stopped.

In this embodiment, the oxide concentration of the feed water supplied to the RPV 3 is suppressed to 1 × 10 ⁻⁹ mol / kg or less by the installation of the hollow filter 10, so that the concentration of radioactive cobalt in the reactor water is further reduced. The For this reason, the radiation exposure of the worker can be further reduced.

  When a nickel ferrite film is formed on the outer surface of the cladding tube 22, the chemical tank 29 is filled with a solution containing divalent iron (II) ions and divalent nickel (II) ions. By supplying a 90 ° C. film-forming aqueous solution containing iron (II) ions, nickel (II) ions, hydrazine and hydrogen peroxide into the cladding tube storage container 26, the outer surface of the cladding tube 22 in the cladding tube storage container 26 A nickel ferrite film is formed on the surface.

  It is also possible to form a magnetite film on the outer surface of the fuel rod constituted by sealing the nuclear fuel material in the cladding tube 22, that is, on the outer surface of the cladding tube 22, using the film forming device 25. A magnetite film can be formed on the outer surface of the cladding tube 22 by supplying the coating film-forming aqueous solution into the cladding tube storage container 26 storing the fuel rods.

  Another example of forming a ferrite film on the outer surface of the cladding tube 22 will be described with reference to FIG. The film forming apparatus 25 shown in FIG. 2 forms a magnetite film on the outer surface of the cladding tube 22 in batch mode, whereas the film forming apparatus 25A shown in FIG. 13 continuously forms a magnetite film on the outer surface of the cladding tube 22. It is a device to do. The film forming apparatus 25A includes a chamber 70 having openings 71 and 72 through which the cladding tube 22 passes, transport apparatuses 73 and 74, an iron (II) ion implanter 87, a pH adjuster injector 88, and an oxidant injector 89. It has. The iron (II) ion implantation apparatus 87 includes a spray 75, an injection pump 76, and a chemical solution tank 77 filled with a medicine containing divalent iron (II) ions and formic acid. The pH adjuster injection device 88 has a spray 78, an injection pump 79, and a chemical tank 80 filled with hydrazine as a pH adjuster. The oxidant injector 89 includes a spray 81, an injection pump 82, and a chemical tank 83 filled with hydrogen peroxide as an oxidant. The sprays 75, 78 and 81 are arranged in the chamber 70.

  The cladding tube 22 is inserted into the chamber 70 through the opening 71 by the transport device 73 from one end. A chemical containing iron (II) ions and formic acid in the chemical tank 77 is sprayed from the spray 75 toward the outer surface of the cladding tube 22. Hydrazine in the chemical tank 80 is sprayed from the spray 78 toward the outer surface of the cladding tube 22. Hydrogen peroxide in the chemical tank 83 is sprayed from the spray 81 toward the outer surface of the cladding tube 22. Although not shown in the drawing, each medicine sprayed from each spray is heated to 90 ° C. by each electric heater installed in the chemical liquid tanks 77, 80, 83 and the sprays 75, 78, 81, etc. The chamber 70 is also heated to 90 ° C. by an electric heater. An inert gas (for example, nitrogen) is supplied into the chamber 70 through the inert gas introduction pipe 84. For this reason, the inside of the chamber 70 is an inert gas atmosphere. A drain pipe 85 connected to the bottom of the chamber 70 discharges each medicine sprayed in the chamber 70 to the outside.

  As described above, the sprayed drug containing iron (II) ions and formic acid, hydrazine and hydrogen peroxide flow down along the outer surface of the cladding tube 22 inserted into the chamber 70. For this reason, iron (II) ions are adsorbed on the outer surface of the cladding tube 22, and a magnetite film is formed on the outer surface of the cladding tube 22 by the reactions of the equations (2) and (3). The cladding tube 22 is moved in the chamber 70 toward the opening 72 by the transport device 73. When one end of the cladding tube 22 passes through the opening 72, the cladding tube 22 is moved by the transport device 74. The moving speed of the cladding tube 22 is set so that a magnetite film having a predetermined thickness is formed on the outer surface of the cladding tube 22. The cladding tube 22 having a magnetite film formed on the outer surface is taken out through the opening 72. While removing the cladding tube 22 through the opening 72, another cladding tube 22 is inserted into the chamber 70 from the opening 71 by the transport device 73. For this reason, a magnetite film can be continuously formed on the outer surface of the cladding tube 22. By inserting a plurality of cladding tubes 22 into the chamber 70 at a time, a magnetite film can be formed on more cladding tubes 22.

  A radiation exposure reducing method for a BWR plant according to embodiment 2, which is another embodiment of the present invention, will be described below. The BWR plant used in the radiation exposure reduction method of the present embodiment has a plurality of clad tubes 22 in which a ferrite film is formed on the outer surface as the fuel assembly 5 loaded in the core 4 of the BWR plant used in the first embodiment. A fuel assembly having a plurality of fuel rods using a cladding tube 22 having a hydrophilic oxide film (for example, iron titanate film) formed on the outer surface is used instead of the fuel assembly having the fuel rods. In the BWR plant used in this embodiment, a plurality of fuel assemblies 5 each having a plurality of fuel rods using a cladding tube 22 having a hydrophilic oxide film formed on the outer surface thereof are loaded on the core 4. The other structure of this BWR plant is the same structure as the BWR plant 1 used in Example 1. Instead of the cladding tube 22 having the iron titanate film formed thereon, a cladding tube 22 having a zinc titanate film or zirconium titanate film formed on the outer surface may be used.

The operation of the BWR plant in which the fuel assembly 5 having a plurality of fuel rods using the cladding tube 22 having the hydrophilic oxide film formed on the outer surface is loaded on the core 4 is started. During the operation of this BWR plant, the feed water passes through the hollow filter 10 as in the first embodiment, so the iron oxide concentration of the feed water supplied into the RPV 3 is 1 × 10 ⁻⁹ mol / kg or less. It is suppressed. For this reason, the density | concentration of the iron oxide (for example, ferric trioxide) contained in a reactor water falls remarkably. The reactor water rises along the outer surface of the cladding tube 22 of each fuel rod provided in each fuel assembly 5 in the core 4. Since the hydrophilic oxide film is formed on the outer surface of each cladding tube 22, the surface of the hydrophilic oxide film, that is, the dryout surface generated on the outer surface of the cladding tube 22 due to boiling of the reactor water is hydrophilic. It is submerged quickly by the action of the oxide film. For this reason, the cobalt oxide adhering to the dry-out surface is easily dissolved into cobalt hydroxide (Co (OH) ₂ ) and dissolved in the reactor water. Cobalt oxide can be prevented from adhering to the outer surface of the cladding tube 22 and being activated for a long time. As a result, the concentration of radioactive cobalt (for example, cobalt 60) in the reactor water can be reduced even when the fuel assembly 5 with further increased burnup is loaded on the core 4. Adhesion and accumulation of radioactive cobalt on the inner surface of a pipe or the like that is connected to the reactor and through which reactor water flows are suppressed, and the surface dose rate of the pipe or the like is reduced. Therefore, even when the burnup of the fuel assembly is further increased, it is possible to further reduce the radiation exposure of workers who carry out periodic inspections after the operation of the nuclear power plant is stopped.

Also in this embodiment, since the oxide concentration of the feed water supplied to the RPV 3 is suppressed to 1 × 10 ⁻⁹ mol / kg or less by the installation of the hollow filter 10, the concentration of radioactive cobalt in the reactor water is further reduced. The

  The cladding tube 22 having a hydrophilic oxide film formed on the outer surface is manufactured using a film forming apparatus 25B shown in FIG. By applying a chemical vapor deposition method using a metal alkoxide as a raw material, a titanium oxide film can be formed on the outer surface of the cladding tube 22. The film forming apparatus 25B includes a chemical vapor deposition apparatus 90, a raw material supply apparatus 93, and an exhaust apparatus 95. The chemical vapor deposition apparatus 90 includes a chamber 91 and a heating device 92 provided in the chamber 91. The raw material supply device 93 is connected to the chamber 91 by a raw material supply pipe 94. The exhaust device 95 is connected to the chamber 91 by an exhaust pipe 96.

  The formation of a hydrophilic oxide film on the outer surface of the cladding tube 22 using the film forming apparatus 25B will be described. A plurality of cladding tubes 22 whose both ends are to be formed are accommodated in a chamber 91. The exhaust device 95 is driven to evacuate the chamber 91. The inside of the chamber 91 is heated by the heater 92. A raw material necessary for forming a titanium oxide film on the outer surface of the cladding tube 22 is supplied from the raw material supply device 93 into the chamber 91. This raw material is a metal alkoxide. For example, when forming an iron titanate film, titanium alkoxide and iron alkoxide are used, and when forming a zinc titanate film, titanium alkoxide and zinc alkoxide are used. When a zirconium titanate film is formed, titanium alkoxide and zirconium alkoxide are used.

  For example, titanium alkoxide and iron alkoxide are supplied from a raw material supply device 93 through a raw material supply pipe 94 into a chamber 91 which is heated in a vacuum. The supplied titanium alkoxide and iron alkoxide evaporate in the chamber 91, and iron titanate adheres to the outer surface of the cladding tube 22. In this way, an iron titanate film is formed on the outer surface of the cladding tube 22. Heating by the heater 92 is stopped and the chamber 91 is cooled to room temperature (for example, 20 ° C.). The operation of the exhaust device 95 is stopped and the pressure in the chamber 91 is set to atmospheric pressure. Thereafter, the cladding tube 22 having a hydrophilic oxide film (for example, iron titanate film) formed on the outer surface is taken out from the chamber 91. A fuel assembly is manufactured using the cladding tube 22.

  A plurality of fuel assemblies 5 each having a plurality of fuel rods using a manufactured cladding tube 22 formed with a hydrophilic oxide film that does not form a compound by adsorbing cobalt on the outer surface thereof after the operation of the BWR plant is stopped. In FIG. 2, the core is loaded in the RPV 3. After the periodic inspection is completed, the BWR plant in which the fuel assemblies 5 with higher burnup are loaded on the core 4 is started. During the operation of the BWR plant, as described above, the dry-out surface generated on the outer surface of the cladding tube 22 is immediately submerged by the action of the hydrophilic oxide film, so that the concentration of radioactive cobalt in the reactor water is reduced. The

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram of the boiling water nuclear plant which applies the radiation exposure reduction method of the boiling water nuclear plant of Example 1 which is one suitable Example of this invention. It is a block diagram of the membrane | film | coat formation apparatus which forms a ferrite membrane | film | coat on the outer surface of the cladding tube used for the fuel rod of the fuel assembly loaded in the core of the boiling water nuclear power plant shown in FIG. It is a characteristic view which shows the pH dependence of the surface potential of each metal oxide. It is explanatory drawing which shows the process in which a cobalt oxide adheres to the outer surface of a cladding tube by boiling of reactor water. It is explanatory drawing which shows the process in which a cobalt ferrite adheres to the outer surface of a cladding tube in a prior art example. It is explanatory drawing which shows the behavior of the cobalt oxide in the ferrite membrane | film | coat surface which is an outer surface of the cladding tube of the fuel assembly shown in FIG. FIG. 2 is an explanatory diagram showing the state of adhesion of cobalt oxide to the surface of a ferrite film that is the outer surface of the cladding tube of the fuel assembly shown in FIG. 1 when there is a large amount of iron oxide supplied from feed water. It is a characteristic view which shows the relationship between the adhesion degree of cobalt to a cladding tube, and the density | concentration of the cobalt 60 ion in a reactor water. It is explanatory drawing which shows the function of the ferrite membrane | film | coat formed in the surface of the structural member of a BWR plant. It is explanatory drawing which shows the behavior of the oxygen ion and electron in the outer surface of a cladding tube made from a zirconium alloy. It is explanatory drawing which shows pH of the reactor water during the operation period of the equipotential point of a metal oxide and a BWR plant. It is explanatory drawing which shows the solubility of the metal oxide in a reactor water. It is a block diagram of the other example of the film formation apparatus which forms a ferrite film in the outer surface of a cladding tube. The outer surface of the cladding tube used for the fuel rod of the fuel assembly loaded in the core of the boiling water nuclear plant to which the radiation exposure reducing method for the boiling water nuclear plant of the second embodiment which is another embodiment of the present invention is applied. It is a block diagram of the membrane | film | coat formation apparatus which forms a ferrite membrane | film | coat.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Boiling water type nuclear power plant, 2 ... Reactor, 3 ... Reactor pressure vessel, 4 ... Reactor core, 5 ... Fuel assembly, 6 ... Main steam piping, 7 ... Turbine, 8 ... Condenser, 9 ... Feed water piping DESCRIPTION OF SYMBOLS 10 ... Hollow filter, 12 ... Feed water pump, 15 ... Recirculation system piping, 17 ... Reactor water purification system piping, 19 ... Hydrogen injection apparatus, 22 ... Cladding pipe, 25, 25A, 25B ... Film formation apparatus, 26 ... Clad tube storage container, 27 ... circulation piping, 28,87 ... iron (II) ion implantation device, 33,88 ... pH adjuster injection device, 38,89 ... oxidizer injection device, 43 ... circulation pump, 44,92 ... Heater, 45 ... Cooler, 46 ... Filter, 47 ... Desalter, 70 ... Chamber, 75, 78, 81 ... Spray, 73, 74 ... Transport device, 90 ... Chemical vapor deposition device, 93 ... Raw material supply device, 95 ... exhaust device.

Claims (19)

  Radiation exposure of a nuclear plant characterized in that the nuclear reactor is operated with a fuel assembly having a plurality of fuel rods using a cladding tube having a ferrite coating formed on the outer surface, loaded in the core of the nuclear reactor. Reduction method.   Radiation exposure of a nuclear plant characterized in that a fuel assembly having a plurality of fuel rods using a cladding tube having a ferrite film formed on the outer surface is loaded into a reactor core and then the reactor is operated. Reduction method.   The method for reducing radiation exposure of a nuclear power plant according to claim 1 or 2, wherein iron oxide contained in feed water supplied to the nuclear reactor is removed. The method for reducing radiation exposure of a nuclear power plant according to claim 3, wherein the concentration of the iron oxide in the feed water is set to 1 × 10 ^-9 mol / kg or less by removing the iron oxide.   5. The nuclear plant radiation exposure according to claim 1, wherein the compound forming the ferrite film is at least one compound selected from triiron tetroxide, nickel ferrite, and zinc ferrite. Reduction method.   A nuclear power plant in which the nuclear reactor is operated in a state where a fuel assembly having a plurality of fuel rods using a cladding tube having a hydrophilic oxide film formed on the outer surface is loaded in the core of the nuclear reactor. Of reducing radiation exposure.   The method for reducing radiation exposure of a nuclear power plant according to claim 6, wherein iron oxide contained in feed water supplied to the nuclear reactor is removed. The method for reducing radiation exposure of a nuclear power plant according to claim 7, wherein the concentration of the iron oxide in the feed water is set to 1 × 10 ⁻⁹ mol / kg or less by removing the iron oxide.   The method for reducing radiation exposure of a nuclear power plant according to any one of claims 6 to 8, wherein the hydrophilic oxide film is a titanium oxide film that does not form a compound with cobalt.   The method for reducing radiation exposure of a nuclear power plant according to claim 9, wherein the compound forming the titanium oxide film is a transition metal salt of titanic acid.   The method for reducing radiation exposure of a nuclear power plant according to claim 10, wherein the transition metal salt of titanic acid is at least one compound selected from iron titanate, zinc titanate and zirconium titanate.   The method for reducing radiation exposure of a nuclear power plant according to any one of claims 1, 2, and 6, wherein hydrogen is introduced into the nuclear reactor during operation of the nuclear reactor.   A nuclear power plant comprising: a core loaded with a plurality of fuel assemblies having a plurality of fuel rods using a cladding tube having a ferrite film formed on an outer surface; and a pump device for supplying reactor water to the core. .   Characterized in that it comprises a core loaded with a plurality of fuel assemblies having a plurality of fuel rods using a cladding tube having a hydrophilic oxide film formed on the outer surface, and a pump device for supplying reactor water to the core. Nuclear plant.   The nuclear power plant according to claim 13 or 14, wherein an iron oxide removing device is installed in a water supply pipe for introducing water into the nuclear reactor containing the core.   The nuclear power plant according to claim 15, wherein the iron oxide removing device is one of a hollow filter device and a filtration desalting device.   The nuclear power plant according to claim 13 or 14, wherein a hydrogen injection device for injecting hydrogen into a nuclear reactor containing the core is provided.   A plurality of fuel rods filled with nuclear fuel material in a cladding tube having a ferrite film formed on the outer surface, a plurality of fuel spacers for bundling these fuel rods, a lower holding member for holding the lower end of each fuel rod, A fuel assembly comprising an upper holding member for holding an upper end portion of the fuel rod.   A plurality of fuel rods filled with nuclear fuel material in a cladding tube having a hydrophilic oxide film formed on the outer surface; a plurality of fuel spacers for bundling these fuel rods; and a lower holding member for holding the lower end of each fuel rod; And an upper holding member for holding an upper end portion of each fuel rod.

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