JPH08297196A - Boiling water nuclear power plant - Google Patents

Abstract

PURPOSE: To mitigate the corrosive environment of a primary cooling system by providing a device reducing the dissolved oxygen concentration in the circulation path of the primary cooling water, and suppressing the accumulation of an oxidizing constituent. CONSTITUTION: The primary cooling water separated from steam is again returned to a core through a mixing plenum 4, a downcomer 5, a jet pump, and a lower plenum 7. Part of the cooling water contributes to the drive of the jet pump from the downcomer 5 through a recirculation pump 6 and a recirculation pipe. Part of the primary cooling water flowing in the recirculation pipe is removed with impurities by a reactor purifying system 8 then is merged with the fed water. A dissolved oxygen concentration reducing device 23 installed on the recirculation pipe removes the gas constituent from the liquid together with the steam generated by heating and boiling. The extracted gas containing oxygen is processed as off-gas. The waste heat in a power plant such as the waste heat generated when steam is condensed by a heat pump can be effectively utilized for feeding heat to the dissolved oxygen concentration reducing device 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear power plant in which a corrosive environment in a boiling water reactor primary cooling system is mitigated.

[0002]

2. Description of the Related Art Intergranular stress corrosion cracking (hereinafter referred to as IGSCC) of a structural material of a nuclear reactor is said to occur when all three factors of material composition, stress and water quality are in unfavorable states. Conventionally, reactor structural materials, especially SUS304 steel, have been subjected to heat treatment for reducing carbon content and residual stress relaxation, and have been operated on a sufficiently safe side from the viewpoint of IGSCC. Thus, the measures taken so far are IGSCC
Among the above three factors, the material and the stress are two factors. In recent years, in a boiling water reactor, the concentration of an oxidizing component, which is one of the third factors, is reduced. Hydrogen injection has been attempted as seen in Japanese Patent Publication No. 57-3086.

FIG. 2 shows a main system of a conventional boiling water reactor primary system. In the figure, 1 is a reactor core, 2 is an upper plenum, 3 is a steam separator, 4 is a mixing plenum,
5 is a downcomer, 6 is a recirculation pump, 7 is a lower plenum, 8 is a furnace cleaning system, 9 is a water heater, 10 is a condensate demineralizer, 11A is a high pressure turbine, 11B is a low pressure turbine, 1
2 is a generator operated by these turbines, 13 is an off-gas treatment device, 14 is a rare gas hold-up, 15
Is a condenser, 16 is a reducing agent injection device, 17 is a water supply pipe, 1
Reference numeral 8 is a main steam pipe, 19 is a recirculation pipe, 20 is a jet pump, 21 is a water supply pump, and 22 is a condensate pump.

The heat generated by the nuclear fission reaction in the nuclear reactor core 1 is removed by converting the primary cooling water into steam. The steam is separated from the primary cooling water in the upper plenum 2 and the steam separator 3, and the steam is carried to the high-pressure turbine 11A and the low-pressure turbine 11B, and the pressure thereof drives the generator 12 to generate electricity. The steam is returned to water in the condenser 15, and the condensate demineralizer 10, the water supply pump 21, the water supply heater 9, the water supply pipe 1
It is supplied to the primary cooling water via 7. On the other hand, the primary cooling water separated from the steam returns to the core again via the mixing plenum 4, the downcomer 5, the jet pump 20, and the lower plenum 7. A part of the primary cooling water from the downcomer 5 contributes to the driving of the jet pump through the recirculation pump 6 and the recirculation pipe 19. Part of the primary cooling water flowing through the recirculation pipe 19 is removed of impurities by the furnace cleaning system 8 and then joined to the feed water.

In an example in which hydrogen is injected into the water supply system after the condenser of the boiling water reactor primary cooling system, a reducing agent injection device 16 is arranged upstream of the water supply pump 21, and the injected hydrogen is irradiated with water in the core. The purpose was to reduce the concentration of the oxidizing component in each part of the primary cooling system including the recirculation system 6 by combining with the oxidizing component generated as a result of decomposition.

However, the downside of hydrogen injection is an increase in turbine system dose rate. This is because the dose rate of the turbine system increases 5 to 10 times at a certain hydrogen injection amount, and nitrogen-16 generated by the nuclear reaction between oxygen-16 and neutrons in the core is the same as the injected hydrogen. This is due to the fact that the reaction changes to highly volatile ammonia. Since the turbine building has less shielding than the reactor building, an increase in the turbine system dose rate causes an increase in the dose rate around the turbine building.

[0007] In overseas plants with a large power plant site, the increase in dose rate is not a problem in many cases, but since the site is limited in domestic nuclear power plants, the hydrogen injection amount due to the increase in boundary dose rate. It is conceivable that there may be restrictions. Mitigation of the corrosive environment in reactor water by the conventional technique is possible when sufficient injection can be performed, but there is a possibility that sufficient injection cannot be performed because the turbine system dose rate increases as described above.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a boiling water reactor and a nuclear power plant in which the corrosive environment in the primary cooling system is mitigated.

[0009]

The present invention has a corrosive environment in a primary cooling system by suppressing the accumulation of oxidizing components by having a device for reducing the dissolved oxygen concentration in the circulation path of the primary cooling water. Constitutes a relaxed nuclear power plant.

[0010]

When the radiation passes through the medium, ionization and excitation are successively performed on the tracks. The secondary electrons emitted along with the primary ionization by radiation still have sufficient energy, and thus cause further ionization.
If the energy of the secondary electrons is large, another track branched from the track of the primary particle is created, and further ionization occurs. If the energy of secondary electrons is small enough (100 eV
(Below) Secondary ionization and excitation occur near the position of primary ionization (about 10Å), so that a cluster of ion pairs and excited species is created. This community is called a spur in radiation chemistry. The distribution of spurs generated along the track depends on the type of radiation and energy.

The decomposition reaction of water by radiation can be expressed as shown in Chemical formula 1, but the final product and concentration differ depending on the distribution of spurs and impurities. In general, when the linear energy loss (LET) of radiation is low, the number of products (G value) generated by energy absorption of 100 eV is as follows.

[0012]

Embedded image

G (e _aq ) = G (OH) = 2.6 ± 0.3
G (H) = 0.60, G (H ₂ ) = 0.45,
G (H ₂ O ₂ ) = 0.75 In the radiation with high LET, the spurs are densely packed, so that recombination of radicals easily occurs and the G value of the radical decreases, but the G value of the molecular product is To increase. The final products are oxygen, hydrogen, and hydrogen peroxide as major products. Since boiling is occurring in the core of a boiling water reactor, some gas components such as oxygen and hydrogen are transferred into steam. Hydrogen peroxide undergoes thermal decomposition and surface decomposition as it circulates.
Decomposes in a manner that produces oxygen. Other components remaining in the reactor water also decompose or react with each other as they circulate, maintaining different steady-state concentrations at each site.

As a comprehensive view, among the components produced by radiolysis of water, reducing components (hydrogen, etc.) are more likely to migrate into vapor than oxidizing components (oxygen, hydrogen peroxide, etc.). Therefore, reactor water is generally in an oxidizing environment. Hydrogen injection is intended to relax the oxidizing atmosphere of the reactor water by adding hydrogen that reduces an oxidizing component to the reactor water under irradiation of radiation.

According to the present invention, an oxidizing atmosphere of the primary cooling water is relaxed by providing a device for reducing the dissolved oxygen concentration in the circulation path of the primary cooling water. In primary cooling water, there are various oxidizing components in addition to oxygen, but their concentration rapidly shifts to an equilibrium state under intense radiation. Therefore, by reducing the oxygen concentration, which is one of the main oxidizing components, the accumulation of all the oxidizing components in the primary cooling water can be suppressed, and the entire oxidizing atmosphere of the primary cooling water can be relaxed.

The present invention works effectively even when the concentration of the oxidizing component in the primary cooling water is reduced by injecting a reducing agent such as hydrogen. In the present invention, since the oxidizing component is reduced before the core in which the reducing agent acts, the oxidizing component remaining by the reducing agent is reduced, and primary cooling water in which the corrosive environment is mitigated can be provided. . In particular, when the hydrogen injection amount is small, the reducing effect of the oxidizing component by the reducing agent is not sufficient. Therefore, the use of the present invention makes it possible to effectively reduce the oxidizing component.

From the above, it has been clarified that a nuclear plant in which the corrosive environment in the primary cooling system is mitigated can be constructed by installing an apparatus for reducing the dissolved oxygen concentration in the boiling water reactor primary cooling system.

[0018]

The present invention will be described below with reference to examples. FIG.
Is an example of a boiling water nuclear power plant to which the present invention is applied. The heat generated by the nuclear fission reaction in the reactor core 1 is removed by converting the primary cooling water into steam. The steam is separated from the primary cooling water in the upper plenum 2 and the steam separator 3, and the steam is high pressure turbine 11A and low pressure turbine 1
It is carried to 1B, and the pressure drives the generator 12 to generate electricity. The steam is returned to the water in the condenser 15, and the condensate demineralizer 1
0, the water supply pump 21, the water supply heater 9, and the water supply pipe 17 to supply the primary cooling water. On the other hand, the primary cooling water separated from the steam returns to the core again via the mixing plenum 4, the downcomer 5, the jet pump 20, and the lower plenum 7. A part of the primary cooling water from the downcomer 5 contributes to the driving of the jet pump through the recirculation pump 6 and the recirculation pipe 19. Part of the primary cooling water flowing through the recirculation pipe 19 is
After the impurities are removed by the furnace cleaning system 8, they join the water supply. The dissolved oxygen concentration reduction device 23 installed in the recirculation pipe 19 removes the gas component from the liquid together with the vapor accompanying the boiling due to the heating boiling. The extracted gas containing oxygen is treated as off-gas. For supplying heat to the dissolved oxygen concentration reducing device 23, waste heat in the plant such as waste heat at the time of steam condensate can be effectively used by using a heat pump.

The recommended gas component removal rate varies depending on the plant, its operating conditions, and water quality conditions. This is because the accumulation status of oxidizing components varies depending on plant parameters.

The prediction results of this embodiment in an 800 MWe class boiling water reactor will be described below. A dissolved oxygen concentration reduction device 23 is installed in the recirculation pipe 19 to
When the dissolved oxygen concentration in the total reactor water flowing through the shroud is reduced to 30%, the dissolved oxygen concentration at the shroud inner surface 25 is 76.1 p.
The concentration of hydrogen peroxide is reduced from pb to 48.9 ppb to about 65% before installation, and the hydrogen peroxide concentration is reduced from 288 ppb to 230 ppb to about 80% before installation. Furthermore, when hydrogen is injected to the extent that (amount of hydrogen injected into feed water) / (core flow rate) reaches 3.5 ppb, the dissolved oxygen concentration is reduced from 41.4 ppb to 13.7 ppb, approximately 30 before installation.
%, The hydrogen peroxide concentration is reduced from 213 ppb to 140 ppb to about 65% before installation.

FIG. 3 shows an embodiment in which the hydrogen peroxide decomposition promoting unit 24 is installed in the mixing plenum 4 in addition to the embodiment shown in FIG. By decomposing hydrogen peroxide, which cannot be removed by the dissolved oxygen concentration reducing device 23, into oxygen by a surface reaction, the oxidizing component can be removed more effectively.
The hydrogen peroxide decomposition accelerating unit 24 can be configured without affecting the nuclear reactor structural material by the multilayer mesh filter structure using the same material as the structural material. Also, platinum catalysts, etc.
It can also be constituted by a catalyst.

Similar to the embodiment shown in FIG. 1, 800 MWe
The prediction result of this example in a class boiling water reactor will be described. In the hydrogen peroxide decomposition promoting unit 24 installed in the mixing plenum 4, 70% of the hydrogen peroxide in the cooling water flowing through the mixing plenum 4 was decomposed, a corresponding amount of oxygen was generated, and the degassing installed in the recirculation pipe 19 was performed. Gas equipment 23
When the dissolved oxygen concentration of the entire reactor water flowing through the recirculation pipe 19 is reduced to 30%, the dissolved oxygen concentration at the inner surface 25 of the shroud is changed from 76 ppb to 3 ppb, which is about 3% before installation.
In addition, the hydrogen peroxide concentration is drastically reduced from 288 ppb to 93 ppb, about 35% before installation. Furthermore, when hydrogen was injected to the extent that (amount of hydrogen injected into feed water) / (core flow rate) was 3.5 ppb, the dissolved oxygen concentration was reduced from 41 ppb to 0.6 ppb, approximately 2% before installation, and the hydrogen peroxide concentration was reduced. Is reduced from 213 ppb to 65 ppb, about 30% before installation. Even if only the hydrogen peroxide decomposition accelerating part 24 is installed, there is an effect of reducing oxidative components, but in that case the dissolved oxygen concentration on the inner surface 25 of the shroud is 9.5 ppb, which is about 3 times that of 3 ppb when both equipments are installed. The hydrogen oxide concentration is 127 ppb, which is about 1.3 times the 94 ppb when both facilities are installed.

FIG. 4 shows an embodiment in which the dissolved oxygen concentration reducing device 23 in the embodiment shown in FIG. 1 is changed from the heating boiling type to the reduced pressure boiling type. Since the primary cooling water is boiled by reducing the pressure, it is not necessary to supply heat to the degassing device 23, and the device can be simplified. The effect is similar to that of the embodiment shown in FIG.

FIG. 5 shows that in the embodiment shown in FIG. 1, a dissolved oxygen concentration reducing device 23 is installed downstream of the furnace cleaning system 8,
This is an example in the case of using a film made of a substance that selectively permeates oxygen. When using a substance that is difficult to use at high temperatures, such as an oxygen permeable membrane, install it in the part where the primary cooling water temperature is low in the furnace purification system. However, the furnace cleaning system 8 has a small flow rate, and is inferior in effect to the embodiment shown in FIG.

FIG. 6 shows an embodiment in which the dissolved oxygen concentration reducing device 23 is constructed by using a substance which combines with oxygen to produce a solid oxide. Dissolved oxygen concentration reduction device 23
Indicates a case where the recirculation pipe 19 is installed. Fine iron powder is stored inside the dissolved oxygen concentration reduction device 23 and is in contact with the primary cooling water. Oxygen in the primary cooling water flowing through the recirculation pipe reacts with iron, and the dissolved oxygen concentration reduction device 2
Stay inside 3. Fine iron powder changes into iron oxide with use, so it must be replaced or regenerated at an appropriate time. The effect is inferior to the embodiment shown in FIG. 1 because the oxygen removal efficiency is expected to be slightly low.

[0026]

According to the present invention, by installing a device for reducing the dissolved oxygen concentration in the circulation path of the primary cooling water, it is possible to construct a nuclear power plant in which the corrosive environment is mitigated in the primary cooling system.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram of a boiling water reactor primary cooling system.

FIG. 3 is a block diagram of an embodiment in which a hydrogen peroxide decomposition promoting unit 24 is installed in the mixing plenum 4 in addition to the embodiment shown in FIG.

FIG. 4 is a block diagram of an embodiment when the dissolved oxygen concentration reducing device 23 in the embodiment shown in FIG. 1 is changed from the heating boiling type of FIG. 1 to a reduced pressure boiling type.

FIG. 5 is a block diagram of an embodiment in which the dissolved oxygen concentration reducing device 23 is configured by an oxygen selective permeable membrane and is installed in a low temperature part of the furnace cleaning system 8.

FIG. 6 is a recirculation pipe 1 in which the dissolved oxygen concentration reduction device 23 is configured by using a substance that combines with oxygen to generate a solid oxide.
9 is a block diagram of an embodiment when installed in FIG.

[Explanation of symbols]

1 ... Reactor core, 2 ... Upper plenum, 3 ... Steam separator,
4 ... mixing plenum, 5 ... downcomer, 6 ... recirculation pump, 7 ... lower plenum, 8 ... furnace cleaning system, 10 ... condensate demineralizer, 11A ... high pressure turbine, 11B ... low pressure turbine, 15 ... condenser, 16 ... Reducing agent injection device, 18 ... Main steam piping, 21 ... Water supply pump, 23 ... Dissolved oxygen concentration reducing device, 25 ... Shroud inner surface.

Claims (7)

[Claims] 1. A boiling water nuclear power plant comprising a device for reducing the dissolved oxygen concentration of primary cooling water in a boiling water nuclear reactor. 2. The boiling water nuclear power plant according to claim 1, further comprising a hydrogen peroxide decomposition accelerating portion in the flow path of the primary cooling water, and a device for reducing a dissolved oxygen concentration downstream thereof. 3. The hydrogen injection into the primary cooling water.
Alternatively, the method for mitigating a corrosive environment according to claim 2. 4. A boiling water nuclear power plant in which the device for reducing the dissolved oxygen concentration according to claim 1 or 2 boils primary cooling water in the device. 5. The boiling water nuclear power plant according to claim 4, wherein the boiling method of the primary cooling water is heating, decompression, or a combination thereof. 6. A boiling water nuclear power plant using a substance that selectively permeates oxygen in the apparatus for reducing the dissolved oxygen concentration according to claim 1 or 2. 7. A boiling water nuclear power plant that uses a substance that combines with oxygen to form a solid oxide in the apparatus for reducing the dissolved oxygen concentration according to claim 1 or 2.