JPH03267797A - Nuclear reactor stopping method - Google Patents

Abstract

PURPOSE:To reduce the radiation exposure of periodic inspection operators greatly by injecting at least one of hydrogen gas, a complexing agent, and a reducing agent into primary cooling water before cooling, and dissolving and removing radioactive materials. CONSTITUTION:Steam generated by a nuclear reactor 1 is condensed 3 into water after a turbine 2 is rotated, sent to a condensate filter 5 by a condensate pump 4, and passed through a condensate demineralizer 6 to remove an iron clad and impurity gold ions from it. Then the condensate is fed to the nuclear reactor 1 by a feed water pump 7 through a feed water heater 8, part of it flows in a nuclear reactor water recirculation system, and further part of it is purified by a reactor water purifying device 10 and circulated to the nuclear reactor 1. At this time, an injecting device 12 for the hydrogen gas, complexing agent, or reducing agent is installed in the nuclear reactor water recirculation system and when the nuclear reactor 1 is at a stop, the hydrogen gas, complexing agent, or reducing agent is injected into the primary cooling water. Consequently, radioactive ions generated by the oxidized film dissolution of the nuclear reactor water recirculation system and purification system are removed by the device from the primary cooling water.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for shutting down a nuclear reactor, and in particular, a method for reducing the pipe surface dose rate of reactor water recirculation system and reactor water purification system piping. Regarding the nuclear reactor shutdown law.

[Conventional technology]

An oxide film containing radioactive substances is formed on the water-contact surfaces of piping, equipment, etc. that come into contact with the primary cooling water of nuclear power plants, causing high dose rates on piping surfaces. In particular, when the surface dose rate of the reactor water recirculation system and reactor water purification system piping increases, the amount of radiation that workers receive (exposure radiation dose) during periodic inspections that are performed approximately once a year after shutting down the reactor.
, and its reduction is an important issue.

In addition to the radioactivity concentration of the primary cooling water during steady-state operation, the pipe surface dose rate is greatly influenced by the reactor shutdown operation immediately before periodic inspections. This is because the rapid changes in reactor pressure and temperature associated with reactor shutdown operations promote the peeling off of the cladding, which contains large amounts of radioactive materials, adhering to the surface of the fuel cladding tubes, and the dissolution of radioactive ions. .

In particular, a rapid decrease in furnace pressure promotes the generation of bubbles and the increase in bubble volume, which promotes the peeling off of the cladding adhered to the surface of the fuel cladding tube. Radioactivity generated by the peeling of the adhering cladding on the surface of the fuel cladding tube adheres to the water contact surfaces of the reactor water recirculation system and reactor water purification system piping, resulting in an increase in the piping surface dose rate. The deposited crud on the surface of the fuel cladding tube is caused by the adhesion of crud (mainly iron oxides and hydroxides) and metal ions contained in the primary cooling water due to corrosion of piping materials, etc.
It is a precipitated substance that peels off more easily than the oxide film formed on the pipe surface.

Atomic Energy Society of Japan "Autumn Conference of 1986" Abstracts, J18. According to Vol. 1, page 142 (1988), it is possible to suppress the peeling of the cladding adhered to the surface of the fuel cladding by performing operations to suppress the rate of rise of voids near the fuel rods when the reactor is shut down. In the same document, the reactor water temperature drop rate is 15℃.
Two methods for shutting down the nuclear reactor are shown: a method in which the reactor pressure is maintained low and constant at 50 kg/cm' or less for 3 hours, and a method in which the reactor pressure is maintained at 50 kg/cm' for 3 hours in addition to the above method.

It has been shown that the latter method was able to reduce the amount of 60Co radioactivity generated due to peeling off of the cladding adhered to the surface of the fuel cladding tube from 5Ci to 2Ci.

However, there still remains the problem that radioactivity increases due to reactor shutdown operations, which causes the pipe surface dose rate to be higher than during steady power operation. In order to make the pipe surface dose rate equal to or lower than that during regular power operation during periodic inspections, radioactive materials must be removed from the oxide film containing radioactive materials that has already formed on the pipe surface during reactor shutdown operations. There is a need to.

[Problem to be solved by the invention]

The above-mentioned conventional technology is effective to some extent in suppressing the peeling of adhering cladding on the surface of fuel cladding tubes, but radioactive No consideration is given to removing substances.

The present invention suppresses the peeling of adhering cladding on the surface of fuel cladding tubes during reactor shutdown operations, and prevents radioactive substances formed on the water-contact surfaces of reactor water recirculation system and reactor water purification system piping. The aim is to reduce the radiation dose received by workers during periodic inspections by reducing the radiation dose rate on the surface of pipes compared to conventional methods.

[Means to solve the problem]

In order to achieve the above object, the present invention provides hydrogen gas,
A nuclear reactor shutdown method characterized by injecting at least one of a complexing agent or a reducing agent into primary cooling water and removing dissolved radioactive substances with a reactor water purification device, and in the reactor shutdown operation, Maintaining the pressure within the reactor higher than the saturated vapor pressure at the reactor water temperature, and injecting at least one of hydrogen gas, a complexing agent, or a reducing agent into the primary cooling water;
The reactor shutdown method is characterized by removing dissolved radioactive materials with a reactor water purification system, and in the reactor shutdown operation, the power is reduced until the reactor water temperature drops to 80℃ or less. In the meantime, the reactor shutdown method is characterized by maintaining the primary cooling water in a reducing atmosphere for at least one hour or more, and removing dissolved radioactive materials with a reactor water purification system. , the hydrogen gas partial pressure inside the reactor is reduced to 0.1 for at least 1 hour before the power is reduced and the reactor water temperature reaches 80°C or below.
kg/ctn2 or higher, and the dissolved radioactive materials are removed by a reactor water purification system.

As described above, the present invention maintains the pressure inside the reactor higher than the saturated steam pressure at the reactor water temperature during reactor shutdown operations, and suppresses boiling of the reactor water, thereby increasing the surface of the fuel cladding tube. In addition to suppressing the peeling of adhered crud, at least one of hydrogen gas, a complexing agent, or a reducing agent is injected into the primary cooling water, and connections between the reactor water recirculation system and reactor water purification system piping are This is a nuclear reactor shutdown method that promotes the dissolution of the oxide film containing radioactive substances formed on the water surface, removes the dissolved radioactive substances with a reactor water purification system, and reduces the pipe surface dose rate.

Furthermore, a preferred aspect of the present invention is to maintain the pressure inside the reactor higher than the saturated steam pressure at the reactor water temperature for at least 3 hours during the reactor shutdown operation, and to maintain the primary cooling water temperature at 200°C. ℃ or higher for 3 hours or more, hydrogen gas is injected into the primary cooling water, and when the temperature is below 200℃, the reactor water cooling rate is set to 30℃/hr or less, and at least one of the complexing agent or reducing agent is injected into the primary cooling water. Injected into cooling water to promote the dissolution of oxide films containing radioactive materials formed on water-contact surfaces of reactor water recirculation system and reactor water purification system piping and equipment.
This is a reactor shutdown method that removes dissolved radioactive materials using a reactor water purification system and reduces the pipe surface dose rate.

In the present invention, the complexing agent is specifically, for example,
Ethylenediamine four vinegar! (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=-) riaminotriethylamine (Tre
n11! The reducing agent is selected from cyanide, oxalate, oxyacid, amino acid salt, picolinate, etc., and the reducing agent is specifically, for example, chromium trivalent ion, vanadium trivalent ion, etc. valent ions, monovalent copper ions,
divalent iron ion, trivalent titanium ion, trivalent niobium ion,
Selected from trivalent molybdenum ions and tetravalent molybdenum ions.

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

[For production]

In order to suppress the peeling of the cladding attached to the surface of the fuel cladding tube, it is most effective to suppress the generation of bubbles. When a nuclear reactor is shut down, bubbles are likely to be generated due to the drop in reactor pressure, and the bubbles that are generated expand in volume due to the drop in pressure associated with the drop in temperature, significantly increasing the rate of rise in reactor water. An increase in the rising speed of the bubbles disturbs the flow of reactor water near the surface of the fuel cladding tube, increases the shearing force with the surface of the fuel cladding tube, and promotes separation of the cladding adhered to the surface.

The present invention has the effect of suppressing the generation of bubbles by maintaining the reactor pressure higher than the saturated steam pressure at the reactor water temperature while reducing the reactor pressure and reactor water temperature by reactor shutdown operations. .

Ideally, the time period for which the reactor pressure is maintained higher than the saturated steam pressure is from the start of the reactor shutdown operation until the reactor water temperature is 100° C. or lower, at which the saturated steam pressure becomes lower than the atmospheric pressure. of course,
Even if the holding time is short, the effect is commensurate with that, but since there is a negative factor that accelerates the generation of bubbles when reducing the furnace pressure to the saturated vapor pressure, it is necessary to saturate the furnace pressure for at least 3 hours. Must be maintained above vapor pressure. Furthermore, in order to keep the amount of bubbles generated per unit time (bubble generation rate) low, it is desirable to reduce the rate of decrease in furnace pressure as low as possible.

Furthermore, the present invention selectively dissolves the radioactive oxide film formed on the surface of the reactor water recirculation system and reactor water purification system pipes during reactor shutdown operations, and uses the dissolved radioactive ions to purify the reactor water. By removing it with a device, it has the effect of reducing the pipe surface dose rate. In order to selectively promote the dissolution of the oxide film on the surface of the reactor water recirculation system and reactor water purification system piping, without promoting the dissolution of the adhered cladding on the surface of the fuel cladding tube, which is the source of radioactivity, as much as possible. , the solubility (
Forms a primary cooling water passageway that can take advantage of differences in dissolution rate).

Generally, the oxide film formed on the piping and equipment of the reactor water recirculation system and the reactor water purification system is magnetite (Fe,
04) and hematite (αFe2D3).

Radioactive substances (for example, "CO+" Co) are incorporated into these magnetite and hematite crystals, but since the amount is extremely small, the dissolution characteristics are determined by the dissolution of the magnetite and hematite. The dissolution of these oxides in pure water is expressed by the following reaction formula.

FeJ4+88"+ 2e-→3Fe"+ 4820
- (1) Fez03+68"+2e-→2Fe"
+3820 − (2) That is, the dissolution of magnetite and hematite involves protons (H”) and electrons (e-
) proceeds as a reduction reaction. Therefore, by performing a reactor shutdown operation in which the primary cooling water is brought into a reducing atmosphere, it is possible to promote dissolution of the oxide film on the pipe surface and reduce the pipe surface dose rate. In order to promote the reduction reactions of formulas [1) and (2) during reactor shutdown, the above divalent chromium ions (
This can be done by injecting a reducing agent such as Cr''), divalent vanadium ion (■2+), or a complexing agent such as EDTA or picolinic acid into the primary cooling water.Reducing agents are particularly effective, but Cr2+ and v2+ reduce the oxidation-reduction potential of pl (V"/V" when < 7, Cr3+/Cr2+ is less than water, so they reduce the solvent water instead of the desired oxide film. This can be prevented. In addition, a complexing agent is also added to form a chelate, e.g.
.

It is more effective to use Cr''/bipyridyl and v2+/picolinate.

On the other hand, in recent boiling water reactors that employ double condensate purification devices, more than 90% of the cladding attached to the surface of the fuel cladding is nickel ferrite (N+FezL).
It is. The dissolution of this nickel ferrite is expressed by the following equation.

N+FeJ<+88" + 2e -Ni" + 2Fe" + 4) +20
- (3) That is, the dissolution of nickel ferrite is also a reduction reaction similar to (1) II and (2) equations. Therefore, the addition of a reducing agent or complexing agent accelerates its dissolution. However, the dissolution rate of nickel ferrite is much lower than that of magnetite and hematite. Generally, these dissolution rates are as shown in the following equation.

FeJ,>a-Fe*Gs> N1Pex04
-... (4) For example, E D T A O, 1m
ol/1. In a solution at 60°C and pH = 5, if the dissolution rate of nickel ferrite is 1, hematite is 75
, for magnetite it is 100. Therefore, it greatly promotes the dissolution of the oxide film on the surface of piping and equipment in the reactor water recirculation system and the reactor water purification system, while hardly dissolving the crud attached to the surface of the fuel cladding tube, thereby reducing the dose rate on the piping surface. It becomes possible to reduce the amount.

Another method of creating a reducing atmosphere in the primary cooling water and promoting the reactions of formulas 1) and [2] is to use piping or equipment as a cathode, place an anode in the primary cooling water, and apply a voltage to energize it. Therefore, there is a method of dissolving the oxide film on the surface of the piping by cathodically polarizing it. Dissolved Fe'
In order to chelate + and the like with Fe 2 + /EDTA, it is more effective to inject a complexing agent into the primary cooling water and energize it.

In addition, methods for promoting the dissolution of oxide films on the piping and equipment surfaces of the reactor water recirculation system and the reactor water purification system include:
There is a method of injecting hydrogen gas into the primary cooling water. As will be described in detail as an example below, by pressurizing hydrogen gas into water, a reducing atmosphere can be formed and the dissolution of the oxide film, which is a reduction reaction, can be promoted. As shown in Figure 10,
The dissolution promoting effect of hydrogen gas injection is greater at high temperatures;
At ℃, it is about 4 times as large.

〔Example〕

Examples of the present invention will be described in detail below.

Example 1 In this example, hydrogen gas,
This is an example in which a complexing agent or a reducing agent is injected into the primary cooling water to promote dissolution of oxide films on the surfaces of piping and equipment in the reactor water recirculation system and the reactor water purification system.

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 nuclear reactor 1 performs the work of rotating the turbine 2, it is blown into the condenser 3 and condensed to become condensed water. This condensate is sent to a condensate filter 5 by a condensate pump 4, and further passes through a condensate demineralizer 6 to remove iron cladding and impurity metal ions contained therein. Furthermore, the water supply pump 7
Water is pumped and heated by passing through feed water heaters 8 arranged in several stages, and is supplied to the nuclear reactor 1. A part of the reactor water in the reactor flows through the reactor water recirculation system, is circulated to and from the reactor 1 by the recirculation pump 9, and further part of it is guided to the reactor water purification system. After being purified by the reactor water purification system 10, it is circulated to the reactor 1 by the CUW pump 11.

In the primary cooling water circulation system as described above, an injection device 12 for injecting hydrogen gas, a complexing agent, or a reducing agent into the reactor water recirculation system is provided.
and inject hydrogen gas, complexing agent, or reducing agent into the primary cooling water when the reactor is shut down.

The installation position of the hydrogen gas, complexing agent, or reducing agent injection device may be downstream of the recirculation pump of the reactor water recirculation system as shown in Figure 2, or as shown in Figures 3, 4, and 5. As shown in the figure, upstream of the reactor water purification system, CUW
It may be upstream or downstream of the pump. Alternatively, as shown in FIG. 6, it may be directly injected into the nuclear reactor. Furthermore, Figures 7 and 8
It may be upstream and downstream of the feedwater heater as shown.

If there is another primary cooling water circulation system that flows into the reactor,
The hydrogen gas, complexing agent or reducing agent injection device may be located anywhere.

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

Example 2 As an example of a method for bringing primary cooling water into a reducing atmosphere during reactor shutdown, a cathode polarization method for piping and equipment is shown in FIG. 9. Connect the DC power supply 14 to the pipe 13 of the reactor water recirculation system or the reactor water purification system from which you want to remove the oxide film so that it becomes the cathode, and connect the γ node 1 to the primary cooling water.
Install 5. The anode is made of an insoluble anode material such as platinum wire or carbon fiber coated with a mesh rubber or synthetic resin that is insulating and allows liquid to pass through.

Along with the reactor shutdown operation, the DC power supply 14 is turned on so that the potential is lower than the natural potential (corrosion potential) of the base metal of the piping 13.
DC power is applied from the In this case, the base material of the pipe 13 is not dissolved, but only the oxide film on the inner surface thereof is dissolved. Radioactive ions generated by the dissolution of the oxide film are transferred to the reactor water purification system 1.
removed from the primary cooling water at 0.

Example 3 An example in which experimental studies were conducted based on the present invention will be described.

Figure 10 shows magnetite in pure water inside an autoclave)
(Fe304) plate was inserted, and the dissolution rate (dissolution amount of Re ions per unit time, unit Fe, O, and surface area) was measured with and without pressurizing hydrogen gas. The amount of hydrogen gas injected is hydrogen gas partial pressure of 5 kg/cm2.
I made it so that

As is clear from the figure, the dissolution rate of magnetite is considerably higher when hydrogen gas is pressurized (○ mark in the figure) than when it is not injected (△ mark in the figure). In particular, at reactor water temperatures of 200° C. or higher during steady power operation, the dissolution rate during injection of hydrogen gas is about 4 to 10 times higher.

Figure 11 shows magnetite in pure water in an autoclave)
(Fe30−) plate was added, and the complexing agent EDTA-2 was added.
These are the results of measuring the dissolution rate by adding NH to a concentration of 0.002 mol/12.

At temperatures below 220℃, use pure water as shown in Figure 10 (marked △ in the figure).
By adding EDTA, the dissolution rate is more than 100 times higher than that of EDTA. However, at temperatures above 220°C, the dissolution rate decreases rapidly. This is EDTA 2
This is because it decomposes at temperatures above 20°C.

〔Effect of the invention〕

According to the present invention, during a reactor shutdown operation, the pressure inside the reactor is maintained higher than the saturated steam pressure at the reactor water temperature, and boiling of the reactor water is suppressed, so that no fuel adheres to the surface of the fuel cladding tube. At least one of hydrogen gas, a complexing agent, or a reducing agent is injected into the primary cooling water to suppress the peeling of the cladding formed on the grafted surfaces of the reactor water recirculation system and reactor water purification system piping. By promoting the dissolution of the oxide film containing radioactive substances and removing the dissolved radioactive substances using a reactor water purification system, it is possible to reduce the pipe surface dose rate during periodic inspections.

Therefore, it is possible to significantly reduce the amount of radiation exposure of personnel engaged in periodic inspection work at nuclear power plants.

[Brief explanation of drawings]

Figure 1 is a system diagram of the primary cooling water of a boiling water nuclear power plant used in an example of the present invention, and Figures 2 to 8 are additions of hydrogen gas, complexing agent, or reducing agent used in an example of the present invention. FIG. 9 is a partial system diagram of the primary cooling water of a boiling water nuclear power plant showing the location; FIG. 9 is a partial sectional view of the piping of the reactor water recirculation system and reactor water purification system used in an example of the present invention; FIG. 10 11 is a graph showing the dissolution rate of magnetite during hydrogen gas injection according to the present invention, and FIG. 11 is an ED concerning the present invention.
It is a graph showing the dissolution rate of magnetite in the addition of TA. 1: Reactor, 2: Turbine, 3: Condenser, 4: Condensate pump, 5: Condensate filter, 6: Condensate demineralizer, 7 Two feed water pumps, 8: Feed water heater 9: Recirculation pump , 10: Reactor water purification system, 11: CUW pump, 12: Hydrogen gas, complexing agent or reducing agent injection device, 13: Reactor water recirculation system or reactor water purification system piping, 14: DC power supply, 15 near node

Claims (1)

[Claims] 1. In a reactor shutdown operation, before the primary cooling water is cooled, at least one of hydrogen gas, a complexing agent, or a reducing agent is injected into the primary cooling water to remove dissolved radioactive substances. A nuclear reactor shutdown method characterized by removal using a reactor water purification system. 2. During reactor shutdown operations, the pressure within the reactor is maintained higher than the saturated steam pressure at the reactor water temperature, and at least one of hydrogen gas, a complexing agent, or a reducing agent is injected into the primary cooling water. , a nuclear reactor shutdown method characterized by removing dissolved radioactive materials with a reactor water purification system. 3. During reactor shutdown operations, the primary cooling water is maintained in a reducing atmosphere for at least one hour until the power is reduced and the reactor water temperature drops to 80°C or less, and dissolved radioactive materials are removed. A nuclear reactor shutdown method characterized by removal using a reactor water purification system. 4. During reactor shutdown operations, reduce the partial pressure of hydrogen gas in the reactor to 0.1 kg/kg for at least 1 hour until the power is reduced and the reactor water temperature reaches 80°C or below.
A method of shutting down a nuclear reactor, which is characterized by reducing the amount of radioactive material to cm^2 or more and removing dissolved radioactive materials using a reactor water purification system. 5. During reactor shutdown operations, maintain the pressure inside the reactor higher than the saturated steam pressure at the reactor water temperature for at least 3 hours, and maintain the primary cooling water temperature at 200°C or higher for 3 hours or more. , hydrogen gas was injected into the primary cooling water, the reactor water cooling rate was kept below 30°C/hr at temperatures below 200°C, and at least one of the complexing agent or reducing agent was injected into the primary cooling water and dissolved. A nuclear reactor shutdown method characterized by removing radioactive materials using a reactor water purification system. 6. Claim 1, 2 or 5, 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'.
A nuclear reactor shutdown method characterized by at least one of 2″-triaminotriethylamine, cyanide, oxalate, oxyacid, amino acid salt, and picolinate. 7. Claims 1 and 2. In description 3 or 5, the reducing agent is
It is characterized by being at least one of divalent chromium ions, divalent vanadium ions, monovalent copper ions, divalent iron ions, trivalent titanium ions, trivalent niobium ions, trivalent molybdenum ions, and tetravalent molybdenum ions. Nuclear reactor shutdown law. 8. A nuclear reactor shutdown method according to any one of claims 1 to 5, characterized in that a voltage is applied between a pipe or device through which primary cooling water flows and the primary cooling water.