JP5591454B2 - Reactor water radioactivity reduction method and nuclear power plant - Google Patents

Description

  The present invention relates to a technique for reducing the radioactivity of a nuclear power plant. In particular, iron is injected into the reactor water of a nuclear power plant, and radioactive ions generated in the core are reacted with the iron to cause ferrite on the fuel rod surface. The present invention relates to a reactor water radioactivity reducing method and a nuclear power plant that remove radionuclides from reactor water.

  In general, nuclear power plants use metallic materials such as stainless steel and nickel-base alloys as materials for equipment and piping. Some of these metallic materials are activated by neutron irradiation to generate radionuclides. For example, in the activation of nickel, radioactive cobalt 58 is generated by the neutron absorption reaction shown in Formula (1).

⁵⁸ Ni (n, p) ⁵⁸ Co (1)
Then, from the cobalt 58, radioactive cobalt 60 is generated by the neutron absorption reaction shown in the formula (2).

⁵⁹ Co (n, γ) ⁶⁰ Co (2)
Some of these radionuclides dissolve in the reactor water and circulate in the piping and adhere to the inner wall surface of the piping. This increase in the radionuclide adhering to the inner wall surface of the pipe may lead to an increase in the dose of workers.

Conventionally, as a technique for reducing reactor water radioactivity in nuclear power plants, the ratio of iron to nickel or cobalt is controlled so as to fall within a certain range, and cobalt 58 or cobalt 60 is reacted with iron on the fuel rod surface to produce ferrite (NiFe ₂ O _4, CoFe ₂ O ₄₎ which is removed from the reactor water by reduction is known (see Patent documents 1 and 2). And iron ion (refer patent document 1) and iron oxide (refer patent document 2) are selected as a form of the iron inject | poured into a reactor water.
JP 2000-9889 A Japanese Patent Laid-Open No. 7-20277

  Iron ions are highly reactive with nickel and cobalt, and a relatively large proportion of iron ions injected into the reactor water does not reach the fuel rod surface. As a result, there is a concern that excess iron that is out of quantitative balance with nickel or cobalt forming ferrite is injected. On the other hand, iron oxides have very low reactivity with nickel and cobalt compared to iron ions. For this reason, the ratio which is not provided for ferrite formation on the fuel rod surface among the iron oxides injected into the reactor water increases. As a result, as in the case of iron ions, there is a concern that excess iron that is out of quantitative balance with nickel or cobalt forming ferrite is injected. Iron dissolved in the reactor water may be mixed into equipment in the reactor water circulation path, such as a steam generator or a filter, and shorten the life of the reactor.

  The present invention was made in view of the above circumstances, and the reactor water radioactivity can be satisfactorily ferritized in the fuel rod surface without using iron ions and iron oxides, and the reactor water radioactivity concentration can be reduced. An object is to provide a reduction method and a nuclear power plant.

In order to achieve the above-described object, in the method for reducing reactor water radioactivity according to the present invention, iron is injected into the reactor water of a nuclear power plant, and the radioactive ions generated in the reactor core are reacted with the iron so as to react with the surface of the fuel rod. In the reactor water radioactivity reduction method for removing radionuclides from the reactor water by making it ferritic, the iron is injected into the reactor water in the form of organic acid iron, and iron fumarate is used as the organic acid iron. It is characterized by using .

Further, in the nuclear power plant according to the present invention, when iron is injected into the reactor water, the radioactive ions generated in the reactor core react with the iron to ferritize on the fuel rod surface, and the radionuclide is generated from the reactor water. In the nuclear power plant to be removed, the form of iron injected into the reactor water is organic acid iron, and the organic acid iron is iron fumarate .

  According to the present invention, the reactor water-soluble nickel can be satisfactorily ferritized on the fuel rod surface without using iron ions and iron oxides, and the reactor water radioactivity concentration can be reduced.

  Embodiments of a reactor water radioactivity reducing method and a nuclear power plant according to the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a system diagram of a principal part of a nuclear power plant 100 according to the present embodiment. This nuclear power plant 100 is an example to which the method for reducing reactor water radioactivity according to the present invention is applied.

  As shown in FIG. 1, the nuclear power plant 100 includes a reactor pressure vessel 101, a water supply system (102, 103), a reactor recirculation system (104, 105), a reactor water purification system (106, 107, 108), a residual heat removal system (109, 110, 111), a high-pressure feed water heater 112, and an iron injection device 120.

  The water supply system supplies reactor water to the reactor pressure vessel 101 that houses the core C where the fuel rods are arranged, and is composed of a water supply system pipe 102 and a water supply system pump 103. The reactor recirculation system controls the reactor power by forcibly circulating the reactor water and controlling the reactor water supply to the core C, and includes a recirculation system pipe 104 and a recirculation pump 105. The reactor water purification system removes impurities in the reactor water by passing the reactor water through the ion exchange resin, and is composed of a filter 106, a reactor water purification system pipe 107, and a reactor water purification system pump 108. The The residual heat removal system removes the decay heat of the fuel after the reactor is shut down, and includes a heat exchanger 109, a residual heat removal system pipe 110, and a residual heat removal system pump 111. The heat exchanger 109 is configured to lower the temperature of the reactor water led to the reactor water purification system, and to increase the temperature of the coolant introduced from the reactor water purification system to the water supply system.

  FIG. 2 is a diagram showing the iron injection device 120 of the nuclear power plant 100.

  The iron injection device 120 of the nuclear power plant 100 removes the radionuclide from the reactor water by injecting iron into the reactor water and reacting with the iron injected with radioactive ions on the fuel rod surface to make it ferritic. .

  As shown in FIG. 1, the iron injection device 120 includes a position P1 upstream of the feed water system pump 103 in the feed water system pipe 102, a position P2 downstream of the filter 106 in the reactor water purification system pipe 107, and a recirculation system pipe 104. It is provided at a position P3 downstream of the recirculation pump 105 and a position P4 upstream of the heat exchanger 109 in the residual heat removal system piping 110. From these positions P1 to P4, iron fumarate and iron oxalate belonging to organic acid iron are injected. In addition, the position which inject | pours organic acid iron does not need to be the positions P1-P4, and considers the radioactivity reduction effect of reactor water, and sets it suitably.

  The iron injection device 120 includes a storage tank 121, a stirring mechanism 122, a dispersion promoting mechanism 123, an injection pump 124, and a valve 125.

  The storage tank 121 stores a solution containing organic acid iron to be injected into the reactor water. The stirring mechanism 122 stirs the solution inside the storage tank 121 and uniformly disperses the organic acid iron in the solution. The dispersion promoting mechanism 123 stirs the solution when the organic acid iron stored in the storage tank 121 is poured into the reactor water, and uniformly disperses the organic acid iron in the solution. By providing the stirring mechanism 122 and the dispersion promoting mechanism 123, the organic acid iron can be uniformly injected into the reactor water even when the organic acid iron is insoluble in the medium. The injection pump 124 is for injecting organic acid iron into the high internal pressure piping at the positions P1 to P4, and the valve 125 is used for switching between the operation of injecting and stopping the organic acid iron.

The injection amount of the organic acid iron is adjusted so that the iron concentration in the reactor water is twice the nickel concentration. Note that “concentration” refers to mass percent concentration. The injection amount of the organic acid iron is set in consideration of the quantitative balance with the nickel-based ferrite formed on the fuel rod surface by the injection of the organic acid iron so as not to cause excessive injection of iron. . That is, the amount considering that the iron content of nickel-based ferrite (NiFe ₂ O ₄ ) is _{twice the} iron content of iron fumarate (C ₄ H ₂ FeO ₄ ) and iron oxalate (FeC ₂ O ₄ ) It is. It should be noted that the amount of organic acid iron injected need not be adjusted so that the iron concentration in the reactor water is twice the nickel concentration. Considering the water quality in the current nuclear power plant, if the organic acid iron is injected so that the iron concentration in the reactor water falls within the range of 0.05 ppb to 0.5 ppb, the damage to the plant caused by the injected iron can be ignored. In addition, it is important to adjust the injection amount of the organic acid iron so that the conductivity of the reactor water does not exceed the limit value.

  The operation of injecting the organic acid iron is preferably performed while the nuclear power plant 100 is in operation and the reactor water is circulating. By injecting the organic acid iron during the circulation of the reactor water, it is possible to suppress the organic acid iron from adhering in the piping of the reactor water circulation path, and the organic acid iron can be smoothly transported to the core C where the fuel rods are located.

  Next, the verification test of the reactor water radioactivity reduction method applied to the nuclear power plant 100 and the result will be described.

This demonstration test is a reactivity confirmation test between iron compounds and nickel, and the test conditions and test procedures are as follows.
1) Four iron compounds containing 2.5 mg of iron, iron oxide hydroxide (αFeOOH), iron oxide hydroxide (γFeOOH), iron fumarate (C ₄ H ₂ FeO ₄ ) and iron oxalate (FeC ₂ O ₄ ) And put into a Teflon test tube with a capacity of 20 mL.
2) Add 15 mL of pure water to each test tube, and add an aqueous nickel sulfate solution containing 1.25 mg of nickel.
3) Apply ultrasonic treatment to each test tube to disperse the contents in the test tube uniformly.
4) Then, place the test tube in an autoclave containing pure water.
5) Close the autoclave lid and heat at 285 ° C for about 17 hours.
6) After completion of the test, the contents of each test tube are filtered through a filter having a pore diameter of 0.1 μm, and the form and composition ratio of the reaction product are determined using a fluorescent X-ray analyzer and an X-ray diffractometer.

  FIG. 3 is a diagram showing the results of a reactivity confirmation test between an iron compound and nickel.

As a result of the test, as shown in FIG. 3, in iron oxide hydroxide (αFeOOH, γFeOOH), about 40% of the contained iron did not react with nickel and changed to hematite (Fe ₂ O ₃ ). On the other hand, most of the iron contained in iron fumarate and iron oxalate was changed to nickel ferrite (NiFe2O4). That is, it is expected that iron fumarate and iron oxalate, which are organic acid irons injected into the reactor water, will change to nickel at a high rate and adhere to the fuel cladding surface. Since nickel and cobalt have similar chemical properties, it is expected that organic acid iron reacts with cobalt at a high rate to change to ferrite and adhere to the surface of the fuel cladding.

  According to the nuclear power plant 100, the reactor water-soluble nickel can be satisfactorily ferritized on the fuel rod surface without using iron ions and iron oxides, and the reactor water radioactivity concentration can be reduced.

  As described above, the method for reducing reactor water radioactivity and the nuclear power plant according to the present invention have been described based on one embodiment. However, the specific configuration is not limited to this embodiment, and departs from the gist of the invention. Unless otherwise, design changes and additions are permitted.

  For example, in the present embodiment, an example in which one of iron fumarate and iron oxalate is used as the organic acid iron is shown, but both may be mixed and used. In addition, the type of organic acid iron is not particularly limited. Even if it is other organic acid iron, if it reacts with nickel and cobalt and forms a ferrite, the effect of the present invention can be acquired.

  Further, the present invention can be applied to either a boiling water nuclear power plant or a pressurized water nuclear power plant. FIG. 1 shows a configuration of a boiling water nuclear power plant as an example, and a pressurized water nuclear power plant does not require a reactor recirculation system (104, 105).

The principal part system diagram of the nuclear power plant of this embodiment. The figure which shows the iron injection apparatus of a nuclear power plant. The figure which shows the result of the reactivity confirmation test with an iron compound and nickel.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... Nuclear power plant, 101 ... Reactor pressure vessel, 102 ... Feed water system piping, 103 ... Feed water system pump, 104 ... Recirculation system piping, 105 ... Recirculation system pump, 106 ... Filter, 107 ... Reactor water purification system Pipe, 108 ... Reactor water purification system pump, 109 ... Heat exchanger, 110 ... Residual heat removal system pipe, 111 ... Residual heat removal system pump, C ... Reactor core, 120 ... Iron injection device, 121 ... Storage tank, 122 ... Stirring mechanism, 123 ... dispersion promotion mechanism, 124 ... injection pump, 125 ... valve, P1 to P4 ... injection position of organic acid iron.

Claims (11)

Reactor water activity reduction that removes radionuclides from the reactor water by injecting iron into the reactor water of a nuclear power plant and reacting the radioactive ions generated in the core with the iron to make it ferritic on the fuel rod surface In the method
Injecting the iron into the reactor water in the form of organic acid iron,
The organic iron, reactor water radioactivity reduction how to comprising using the fumarate iron.   2. The method for reducing reactor water radioactivity according to claim 1, wherein the organic acid iron is injected into the reactor water such that the iron concentration in the reactor water is in the range of 0.05 ppb to 0.5 ppb. .   The method of claim 1, wherein the organic acid iron is injected into the reactor water such that the iron concentration in the reactor water is twice the nickel concentration in the reactor water. .   The said organic acid iron is inject | poured into the said reactor water from at least 1 piping among water supply system piping, reactor water purification system piping, recirculation system piping, and residual heat removal system piping. Reactor water radioactivity reduction method.   2. The method for reducing reactor water radioactivity according to claim 1, wherein the organic acid iron is injected in a state in which the reactor water is circulated.   The method for reducing reactor water radioactivity according to claim 1, wherein the organic acid iron is injected into the reactor water so that the conductivity of the reactor water does not exceed a limit value.   The method for reducing reactor water radioactivity according to claim 1, wherein the solution containing the organic acid iron is stored in a storage tank, the solution is stirred inside the storage tank, and the stirred solution is injected into the reactor water. In the nuclear power plant in which the radioactive ions generated in the reactor core are reacted with the iron and ferritized on the surface of the fuel rods to remove the radionuclides from the reactor water by injecting iron into the reactor water.
The form of the iron injected into the reactor water is organic acid iron,
The nuclear power plant, wherein the organic acid iron is iron fumarate.   The nuclear power plant according to claim 8, wherein the iron concentration in the reactor water is 0.05 ppb to 0.5 ppb.   The nuclear power plant according to claim 8, wherein the iron concentration in the reactor water is twice the nickel concentration in the reactor water.   9. The nuclear power plant according to claim 8, wherein the organic acid iron injection portion is provided in at least one of a water supply system pipe, a reactor water purification system pipe, a recirculation system pipe, and a residual heat removal system pipe. .

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