JP2874943B2 - Reactor shutdown method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for shutting down a nuclear reactor, and in particular, to reducing a dose rate on a pipe surface of a reactor water recirculation system and a reactor water purification system. On the Reactor Shutdown Act.

[Conventional technology]

An oxide film containing a radioactive substance is formed on the water-contacting surface of piping, equipment, and the like in contact with the primary cooling water of a nuclear power plant, causing an increase in the pipe surface dose rate. In particular,
When the dose rate on the pipe surface of the reactor water recirculation system and the reactor water purification system pipe increases, the radiation dose (exposure radiation dose) received by workers during the periodic shutdown, which is performed once a year after the reactor is shut down, And its reduction is a heavy issue.

This pipe surface dose rate is greatly affected by the reactor shutdown operation just before the periodic inspection, in addition to the primary cooling water radioactivity concentration during steady-state operation. This is because sudden changes in reactor pressure and temperature associated with the reactor shutdown operation promote the separation of the cladding containing a large amount of radioactive material and the dissolution of radioactive ions attached to the fuel cladding tube surface. .

In particular, a rapid decrease in the furnace pressure promotes the generation of bubbles (bubbles) and the separation of the adhered cladding on the fuel cladding tube surface due to the increase in the bubble volume. The radioactivity generated by the detachment of the adhered cladding on the fuel cladding tube surface adheres to the water-contacting surfaces of the reactor water recirculation system and the reactor water purification system piping, resulting in an increase in the pipe surface dose rate. The adhered cladding on the fuel cladding tube surface is formed by the adhesion and deposition of metal ions and cladding (mainly oxides and hydroxides of iron) contained in primary cooling water due to corrosion of piping materials and the like. It is easier to peel off than the formed oxide film.

According to the Abstracts of the Atomic Energy Society of Japan, Fall Meeting of 1988, J18, Vol. II, p. 142 (1988), the fuel cladding tube was controlled by controlling the rate of void rise near the fuel rods when the reactor was shut down. It is said that the separation of the adhered cladding on the surface can be suppressed. In the same document, a method of keeping the reactor water temperature drop rate as low as 15 ° C / hr or less, and a furnace pressure of 50 kg / c in addition to the above method
Two types of reactor shutdown methods, a method of holding at m ² for 3 hours, are shown. In the latter method, the amount of ⁶⁰ Co radioactivity generated due to the separation of the adhered cladding on the fuel cladding surface is reduced from 5 Ci to 2 Ci
It is shown that it was able to reduce to.

However, the radioactivity still rises due to the operation of shutting down the reactor, which leaves a problem that the dose rate on the pipe surface is raised more than in the steady output operation. In order to make the pipe surface dose rate at the time of periodic inspection equal to or less than that at the time of steady output operation, radioactive substances are removed from the oxide film containing radioactive substances already formed on the pipe surface by the reactor shutdown operation. There is a need to.

[Problems to be solved by the invention]

The above prior art has a certain effect in suppressing the separation of the adhered cladding on the fuel cladding tube surface, but from an oxide film containing a radioactive substance already formed on the piping surface without a reactor shutdown operation, Removal of radioactive material is not considered.

The present invention suppresses the separation of the adhered cladding on the surface of the fuel cladding tube due to the reactor shutdown operation, and furthermore, the radioactive material formed on the water contact surface of the reactor water recirculation system and the reactor water purification system piping. Is intended to remove
Furthermore, it is another object of the present invention to reduce the radiation dose received by workers during periodic inspections by reducing the dose rate on the pipe surface.

[Means for solving the problem]

To achieve the above object, according to the present invention, in a reactor shutdown operation, before the primary cooling water is cooled, at least one of a complexing agent or a reducing agent is injected into the primary cooling water to dissolve the dissolved radioactive material. The reactor shutdown method characterized by removing the reactor water purifier, and in the reactor shutdown operation, while maintaining the pressure in the reactor higher than the saturated steam pressure at the reactor water temperature, A reactor shutdown method characterized by injecting at least one of hydrogen gas, a complexing agent, or a reducing agent into primary cooling water and removing dissolved radioactive materials by a reactor water purification device. In operation, at least one primary cooling water is introduced into the reducing atmosphere until the power is reduced and the reactor water temperature reaches a temperature below 80 ° C.
A reactor shutdown method characterized by maintaining the reactor for more than an hour and removing dissolved radioactive materials with a reactor water purification device.In the reactor shutdown operation, the output is reduced and the reactor water temperature is reduced to 80 ° C or less. The atomic pressure is characterized by increasing the partial pressure of hydrogen gas in the reactor to at least 0.1 kg / cm ² for at least one hour until the temperature is reached, and removing dissolved radioactive materials by a reactor water purification device. This is the furnace shutdown method.

As described above, the present invention relates to a reactor shutdown operation,
By keeping the pressure inside the reactor higher than the saturated vapor pressure at the reactor water temperature and suppressing the boiling of the reactor water, the separation of the clad adhering to the surface of the fuel cladding tube is suppressed, and the hydrogen gas or Injecting at least one of a complexing agent or a reducing agent into the primary cooling water to promote the dissolution of the oxide film containing radioactive substances formed on the water contact surface of the reactor water recirculation system and the reactor water purification system piping. This is a reactor shutdown method in which dissolved radioactive materials are removed by a reactor water purification device to reduce the pipe surface dose rate.

Further, a preferred aspect of the present invention is that, in the reactor shutdown operation, the pressure in the reactor is maintained higher than the saturated vapor pressure at the reactor water temperature for at least 3 hours, and the primary cooling water temperature is set at 200 C. and maintained for 3 hours or more, hydrogen gas was injected into the primary cooling water, and at 200 ° C or less, the reactor water cooling rate was set to 30 ° C / hr or less and at least one of the complexing agent or reducing agent was added to the primary cooling water. Into the reactor water recirculation system and the reactor water purification system to promote the dissolution of the oxide film containing radioactive material formed on the water contact surface of the piping and equipment, and dissolve the dissolved radioactive material in the reactor water purification system This is a reactor shutdown method that removes and reduces the pipe surface dose rate.

In the present invention, as the complexing agent, specifically, for example, ethinezaminetetraacetic acid (abbreviated as EDTA), acetylacetone, dithizone, 2,2′-dipyridyl, ethylenediamine, 8-hydroxyquinoline, 1,10-phenanthroline , N, N'-di (2-aminoethyl) ethylenediamine, (abbreviated as Trien), 2,2 ', 2 "-triaminotriethylamine (abbreviated as Tren), cyanide, oxalate, oxy Acid salt, aminic acid salt, picolinate salt and the like, and specific examples of the reducing agent include chromium divalent ion, vanadium divalent ion, copper monovalent ion, iron divalent ion, titanium Trivalent ions,
It is selected from niobium trivalent ions, molybdenum trivalent ions, and molybdenum tetravalent ions.

Furthermore, in the present invention, by applying a voltage between the pipe or the device through which the primary cooling water flows and the primary cooling water and energizing the cooling water, the cooling water can be brought into a reducing atmosphere, and the dissolution of the oxide film is promoted. be able to.

[Action]

In order to suppress the separation of the clad adhering to the fuel cladding tube surface, it is most effective to suppress the generation of bubbles. When the reactor is shut down, bubbles tend to be generated due to a decrease in reactor pressure, and the generated bubbles expand in volume due to a decrease in pressure due to a decrease in temperature, and the rising speed in the reactor water greatly increases. The increase in the rising speed of the bubble disturbs the flow of the reactor water near the fuel cladding surface, increases the shearing force with the fuel cladding surface, and promotes the separation of the adhered cladding on the surface.

The present invention has an effect of suppressing the generation of bubbles by maintaining the reactor pressure higher than the saturated vapor pressure at the reactor water temperature while reducing the reactor pressure and the reactor water temperature by the reactor shutdown operation. . Ideally, the time during which the reactor pressure is maintained higher than the saturated vapor pressure is preferably from the start of the reactor shutdown operation to a reactor water temperature of 100 ° C. or less at which the saturated vapor pressure becomes equal to or less than the atmospheric pressure. Of course, even if the holding time is short, the effect is commensurate with that. However, when the furnace pressure is reduced to the saturated vapor pressure, there is a negative factor that accelerates the generation of bubbles. Must be maintained above the saturated vapor pressure. In addition, in order to reduce the amount of bubbles generated per unit time (bubble generation speed), it is desirable to reduce the rate of decrease in furnace pressure as much as possible.

Further, the present invention selectively dissolves a radioactive oxide film formed on a pipe surface of a reactor water recirculation system and a reactor water purification system in accordance with a reactor shutdown operation, and dissolves dissolved radioactive ions into a reactor water purification system. The removal by the device has the effect of reducing the pipe surface dose rate. In order to selectively promote the dissolution of the oxide film on the reactor water recirculation and reactor water purification system piping without promoting as much as possible the melting of the adhered cladding on the fuel cladding tube surface, which is the source of radioactivity, An environment for primary cooling water capable of activating a difference in solubility (dissolution rate) based on a difference in crystal form (chemical form) between the adhered cladding and the oxide film is formed.

Generally, the oxide film formed on the piping and equipment of the reactor water recirculation system and the reactor water purification system is made of magnetite (Fe
₃ O ₄ ) and hematite (α-Fe ₂ O ₃ ). Radioactive substances (for example, ⁶⁰ Co and ⁵⁸ Co) are taken into crystals of these magnetites and hematites, but since their amounts are extremely small, their dissolution characteristics are determined by the dissolution of magnetites and hematites. The dissolution of these oxides in pure water is represented by the following reaction formula.

Fe ₃ O ₄ + 8H ⁺ + 2e ⁻ → 3Fe ²⁺ + 4H ₂ O (1) Fe ₂ O ₃ + 6H ⁺ + 2e ⁻ → 2Fe ²⁺ + 3H ₂ O (2) That is, the dissolution of magnetite and hematite is caused by proton (H ⁺⁾ and electrons (e ^- proceeds as the reducing reaction that receives provision of). Therefore, the reactor shutdown operation in which the primary cooling water is set to the reducing atmosphere can promote the dissolution of the oxide film on the pipe surface and reduce the pipe surface dose rate. In order to accelerate the reduction reactions of formulas (1) and (2) when the reactor is shut down, a reducing agent such as the above-mentioned chromium divalent ion (Cr ²⁺ ), vanadium divalent ion (V ²⁺ ), or EDTA And a complexing agent such as picolinic acid can be injected into the primary cooling water. In particular, reducing agents have a great effect, but Cr ²⁺ and V ²⁺ have pH <7.
In this case, since the oxidation-reduction potential of V ³⁺ / V ²⁺ and Cr ³⁺ / Cr ²⁺ is lower than that of water, water as a solvent is reduced instead of the intended oxide film. To prevent this, a complexing agent is also added to chelate, eg Fe ²⁺ / EDTA, Cr ²⁺ / pipyridine, V ²⁺
/ Picoline salt is more effective.

On the other hand, 90% or more of the clad adhering to the surface of the fuel cladding tube is nickel ferrite (NiFe ₂ O ₄ ) in a boiling water reactor employing a double condensing purification device in recent years. The dissolution of this nickel ferrite is represented by the following equation.

NiFe ₂ O ₄ + 8H ⁺ + 2e ⁻ → Ni ²⁺ + 2Fe ²⁺ + 4H ₂ O (3) That is, the dissolution of nickel ferrite is a reduction reaction similarly to the formulas (1) and (2). Thus, the addition of a reducing or complexing agent or complexing agent accelerates its dissolution. However, the dissolution rate of nickel ferride is much lower than that of magnetite and hematite. Generally, these dissolution rates are as follows:

Fe ₃ O ₄ > α-Fe ₂ O ₃ ≫NiFe ₂ O ₄ (4) For example, in a solution of EDTA 0.1 mol / l, 60 ° C. and pH = 5,
Assuming that the dissolution rate of nickel ferrite is 1, hematite is 75 and magnetite is 100. Therefore, almost no dissolution of the clad attached to the fuel cladding tube surface,
Dissolution of the oxide film on the surfaces of the piping and equipment of the reactor water recirculation system and the reactor water purification system is greatly promoted, and the dose rate on the piping surface can be reduced.

In addition, the primary cooling water is changed to a reducing atmosphere (1) and (2).
As another method of promoting the reaction of the formula, as an anode of piping or equipment, an anode is placed in primary cooling water, voltage is applied and electricity is applied, and the oxide film on the pipe surface etc. is cathode-polarized and dissolved. There is a way. Fe ²⁺ dissolved
In order to chelate Fe ²⁺ / EDTA or the like, it is more effective to inject a complexing agent into primary cooling water and to energize.

In addition, there is a method of injecting hydrogen gas in primary cooling water as a method for promoting dissolution of an oxide film on a reactor water recirculation system and a reactor water purification system and an oxide film on the surface of equipment. As will be described later in detail as an example, a reducing atmosphere is formed by injecting hydrogen gas into water, so that dissolution of an oxide film, which is a reduction reaction, can be promoted. As shown in FIG. 10, the effect of accelerating the dissolution of hydrogen gas injection is greater at higher temperatures.
At 0 ° C, it becomes about 4 times.

〔Example〕

 Hereinafter, examples of the present invention will be described in detail.

Example 1 In this example, hydrogen gas, a complexing agent or a reducing agent was injected into primary cooling water in accordance with a reactor shutdown operation, and piping and equipment surfaces of a reactor water recirculation system and a reactor water purification system were injected. This is an embodiment for promoting the dissolution of the oxide film.

FIG. 1 is a schematic diagram showing a primary cooling water circulation system of a boiling water nuclear power plant having an injection device for hydrogen gas, a complexing agent or a reducing agent. After the steam generated in the reactor 1 is rotated by the turbine 2, the steam is condensed in the condenser 3 to be condensed. The condensate is sent to a condensate filter 5 by a condensate pump 4 and further passes through a condensate desalter 6 to remove iron clad and impurity metal ions contained therein. Further, the water is pumped by a water supply pump 7 and heated by passing through water supply heaters 8 arranged in several stages, whereby water is supplied to the reactor 1. Part of the reactor water in the reactor circulates through the reactor water recirculation system and circulates between the reactor 1 and the recirculation pump 9, and part of the water is guided to the reactor water purification system. After being purified by the reactor water purification device 10, it is circulated to the reactor 1 by the CUW pump 11.

In the primary cooling water circulation system as described above, hydrogen gas, a complexing agent or a reducing agent injection device 12 is supplied to the reactor water recirculation system.
Is installed, and hydrogen gas, complexing agent or reducing agent is injected into the primary cooling water when the reactor is stopped.

The installation position of the hydrogen gas, complexing agent or reducing agent injection device may be located downstream of the recirculation pump of the reactor water recirculation system as shown in FIG. 2, or FIG. 3, FIG. 4 and FIG. As shown in the figure, upstream of the reactor water purification system of the reactor water purification system, CUW
It may be upstream or downstream of the pump. Also, as shown in FIG. 6, it may be directly injected into the nuclear reactor. Further, as shown in FIG. 7 and FIG. 8, it may be upstream and downstream of the feed water heater. In addition, as long as the primary cooling water circulation system flows into the nuclear reactor, the hydrogen gas, complexing agent, or reducing agent injection device may be located anywhere.

Along with the reactor shutdown operation, the hydrogen gas, complexing agent, or reducing agent was injected from the hydrogen gas, complexing agent, or reducing agent injection device, resulting in a reactor water recirculation system and a reactor water purification system. Radioactive ions generated by the dissolution of the oxide film on the pipes and equipment surfaces are removed from the primary cooling water by the reactor water purification device 10.

Example 2 FIG. 9 shows a cathode polarization method for piping and equipment as an example of a method for changing the primary cooling water to a reducing atmosphere when the reactor is stopped. A DC power supply 14 is connected to a pipe 13 of a reactor water recirculation system or a reactor water purification system from which an oxide film is to be removed so that the pipe 13 serves as a cathode, and an anode 15 is provided in the primary cooling water. The anode is made of an insoluble anode material such as a platinum wire or carbon fiber coated with a net-like rubber or synthetic resin that is insulative and allows liquid to permeate.

At the same time as the reactor shutdown operation, a DC power is supplied from the DC power supply 14 so that the potential becomes lower than the natural potential (corrosion potential) of the base metal in the pipe 13. Then, the base material of the pipe 13 does not dissolve, but only the oxide film on the inner surface dissolves. Radioactive ions generated by dissolution of the oxide film are removed from the primary cooling water by the reactor water purification device 10.

Example 3 An example in which an experimental study was performed based on the present invention will be described.

Fig. 10 shows the dissolution rate (unit time, unit Fe ₃ O ₄ surface area of Fe ₃ O ₄ ) when a magnetite (Fe ₃ O ₄ ) plate was put in pure water in an autoclave and hydrogen gas was injected and not.
(Dissolution amount of ions). The amount of hydrogen gas injected was such that the hydrogen gas partial pressure was 5 kg / cm ² . As is clear from the figure, the dissolution rate of magnetite is considerably higher than in the case where hydrogen gas was injected (marked in the figure) and the case where hydrogen gas was not injected (marked in the figure). In particular, at a reactor water temperature of 200 ° C. or higher and a steady output operation, the dissolution rate at the time of injecting hydrogen gas is about 4 to 10 times larger.

FIG. 11 is magnetite in pure water in the autoclave (Fe ₃ O ₄₎ Put the plates, further 0.002m complexing agents EDTA · 2NH ₄
This is the result of measuring the dissolution rate by adding the solution to a concentration of ol / l. At temperatures below 220 ° C, the pure water in Fig. 10 (△
Compared with (marked), the dissolution rate is at least 100 times greater due to the addition of EDTA. However, at a temperature of 220 ° C. or higher, the dissolution rate sharply decreases. This is because EDTA decomposes at 220 ° C. or higher.

〔The invention's effect〕

According to the present invention, at the time of reactor shutdown operation, the pressure of the reactor is maintained higher than the saturated vapor pressure at the reactor water temperature and the boiling of the reactor water is suppressed, whereby the reactor adheres to the fuel cladding tube surface. In addition to suppressing the separation of the clad, at least one of hydrogen gas or a complexing agent or a reducing agent was injected into the primary cooling water, and formed on the water contact surface of the reactor water recirculation system and the reactor water purification system piping. By promoting the dissolution of the oxide film containing radioactive substances and removing the dissolved radioactive substances by the reactor water purification device, the pipe surface dose rate at the time of periodic inspection can be reduced. Therefore, it is possible to significantly reduce the radiation exposure of workers engaged in periodic inspections of the nuclear power plant.

[Brief description of the drawings]

FIG. 1 is a system diagram of primary cooling water of a boiling water nuclear power plant used in an example of the present invention, and FIGS. 2 to 8 are diagrams showing addition of hydrogen gas, a complexing agent or a reducing agent used in an example of the present invention. FIG. 9 is a partial system diagram of primary cooling water of a boiling water nuclear power plant showing a position, FIG. 9 is a partial sectional view of piping of a reactor water recirculation system and a reactor water purification system used in an example of the present invention, and FIG. FIG. 11 is a graph showing the dissolution rate of magnetite at hydrogen gas pressure according to the present invention, and FIG. 11 is a graph showing the dissolution rate of magnetite when EDTA is added according to the present invention. 1: Reactor, 2: Turbine, 3: Condenser, 4: Condensate pump, 5: Condensate filter, 6: Condensate desalinator, 7: Feedwater pump, 8: Feedwater heater, 9: Recirculation Pump, 10: Furnace water purification device, 11: CUW pump, 12: Injection device for hydrogen gas, complexing agent or reducing agent, 13:
Reactor water recirculation system or reactor water purification system piping, 14: DC power supply, 15: anode

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takayuki Matsumoto 4026 Kuji-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi Research Laboratory, Hitachi, Ltd. (72) Katsumi Osumi 3-1-1 Sachimachi, Hitachi City, Ibaraki Co., Ltd. Hitachi Works Hitachi Plant (72) Inventor Naoshi Usui 3-2-1 Sachimachi, Hitachi City, Ibaraki Prefecture Within Hitachi Engineering Co., Ltd. (56) References JP-A-1-269090 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) G21C 19/30 G21F 9/28

Claims (8)

(57) [Claims] In the reactor shutdown operation, at least one of a complexing agent or a reducing agent is injected into the primary cooling water before the primary cooling water is cooled, and the dissolved radioactive material is passed through the reactor water purification device. Reactor shutdown method characterized by removal. 2. In the reactor shutdown operation, the pressure in the reactor is maintained higher than the saturated vapor pressure at the reactor water temperature, and at least one of hydrogen gas, a complexing agent or a reducing agent is added to the primary cooling water. A method for shutting down a nuclear reactor, characterized by removing radioactive materials injected into a reactor and dissolving them with a reactor water purification device. 3. The reactor according to claim 1, wherein the primary cooling water is maintained in a reducing atmosphere for at least one hour before the power is reduced and the reactor water temperature is reduced to 80 ° C. or less during the reactor shutdown operation. Reactor shutdown method comprising removing water by a reactor water purification device. 4. In the reactor shutdown operation, the hydrogen gas partial pressure in the reactor is reduced for at least one hour until the power is reduced and the reactor water temperature becomes 80 ° C. or less. kg/
Reactor shutdown method, characterized in that the radioactive material is removed by a reactor water purification device to at least ² cm2. 5. The reactor shutdown operation, wherein the pressure inside the reactor is maintained at least higher than the saturated vapor pressure at the reactor water temperature for at least 3 hours and the primary cooling water temperature is maintained at 200 ° C. or higher.
Hold for more than an hour, and inject hydrogen gas into the primary cooling water, and at 200 ° C or lower, set the reactor water cooling rate to 30 ° C / hr or less and inject at least one of the complexing agent or reducing agent into the primary cooling water, A reactor shutdown method comprising removing dissolved radioactive materials with a reactor water purification device. 6. The method according to claim 1, wherein the complexing agent is ethylenediaminetetraacetic acid, acetylacetone, dithizone, 2,2'-dipyridyl, ethylenediamine, 8-hydroxyquinoline, 1,10-phenanthroline, N, N'-
Di (2-aminoethyl) ethylenediamine, 2,2 ', 2 "
A reactor shutdown method, characterized in that it is at least one of triaminotriethylamine, cyanide, oxalate, oxyacid salt, amate, picolinate. 7. The method according to claim 1, wherein the reducing agent is divalent chromium ion, divalent vanadium ion, monovalent copper ion, divalent iron ion, trivalent titanium ion, trivalent niobium ion, A method for shutting down a nuclear reactor, comprising at least one of molybdenum trivalent ions and molybdenum tetravalent ions. 8. The reactor shutdown method according to claim 1, wherein a voltage is applied between a pipe or a device through which the primary cooling water flows and the primary cooling water.