JP2654050B2 - Nuclear power plant - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention operates by controlling the reactor water conductivity of a nuclear power plant, especially a primary reactor system, and maintains the soundness of materials more firmly. The present invention relates to a nuclear power plant.

(Prior Art) The coolant of a nuclear power plant is in a state of high temperature and high pressure during operation, which is a severe environmental condition for structural materials, and the corrosion behavior of structural materials is an important problem. Particularly in a boiling water reactor (hereinafter referred to as BWR), stress corrosion cracking of austenitic stainless steel pipe welds etc. ｛below SCC (S
tress Corossion Cracking).

The SCC phenomenon described above has three factors: material, stress,
It is supposed to occur when the environment overlaps. As a material factor, there is a condition of a SUS304 stainless steel welded portion. That is, since chromium carbide precipitates in the welded portion due to the thermal effect during welding, a chromium-deficient layer is formed, which causes a reduction in proof stress. Also, regarding the stress, residual thermal stress on the member due to heat during welding is becoming a problem, and the removal of residual stress by improving the welding method is being attempted. Further, factors on the environmental side include the presence of impurities such as chlorine ions and dissolved oxygen in addition to the corrosive environment of high-temperature water.

At nuclear power plants, the water quality of the reactor coolant is strictly controlled, and the primary water system of BWR plants is kept as neutral as possible. On the other hand, dissolved oxygen has the following problems. In other words, oxygen is constantly generated in the core by radiolysis of water.
The existence of dissolved oxygen of about 0 to 300 ppb is inevitable.

Therefore, as an environmental factor for SCC, the presence of dissolved oxygen is more important than the presence of impurities. However, at the reactor water temperature during reactor operation (285 ° C), to generate SCC in SCC-sensitive materials,
About 200 ppb of dissolved oxygen is sufficient.

In recent years, hydrogen injection technology into reactor water has been developed as a countermeasure for the above-mentioned SCC, and some BWs where it is difficult to take countermeasures from the material side
It has been put to practical use in the R plant. This technology reduces the concentration of dissolved oxygen in the reactor water by injecting hydrogen into the reactor water to make the reactor water excessive in reducing hydrogen and by radiochemically recombining oxygen and hydrogen in the presence of radiation. It is to let.

An example of hydrogen injection into an existing plant (Dressde No. 1 reactor in the United States) will be described with reference to FIGS. FIG. 3 shows a system diagram of the BWR plant. In this figure, a reactor core 1 is accommodated in a reactor pressure vessel 2, and steam generated in the reactor core 1 is worked by a turbine 3, then guided to a condenser 4, where it is cooled and condensed and condensed. This condensate is condensate pump 5, condensate purification system 6, high pressure condensate pump 7, feed water heater 8,
The temperature is increased and pressurized through the water supply pump 9, and the reactor pressure vessel 2
Is injected into. On the other hand, part or all of the reactor water in the reactor pressure vessel 2 is recirculated by the reactor recirculation pump 10. This recirculation forces the core flow to be increased and more heat is removed from the core 1.

Thus, hydrogen injection in the BWR plant is performed at an injection point 11 between the condensate purification system 6 and the high-pressure condensate pump 7.

FIG. 4 is a diagram showing a dissolved oxygen reduction effect in a hydrogen injection test of the plant. In this figure, the vertical axis is the dissolved oxygen concentration (ppb) in the reactor water collected in the recirculation system, and the horizontal axis is the dissolved hydrogen concentration (ppb) in the feed water. From this figure, as the hydrogen concentration in the feed water increases, It can be seen that the dissolved oxygen concentration is reduced. The symbol in the figure indicates the reactor power.
The value at 81% is shown, and the symbol at そ れ ぞ れ indicates the value at 95%.

In the above hydrogen injection test, an index of the effect of reducing the dissolved oxygen concentration by hydrogen injection is determined from the dissolved oxygen concentration in the reactor water (taken from the recirculation system). It is necessary to set the index value to 20 ppb or less. However, this value is limited to the above-described test, differs from plant to plant, and a generally accepted reference value has not been determined.

On the other hand, some plants measure the corrosion potential of the material (SUS304 stainless steel) in addition to the dissolved oxygen concentration. In this measurement, the corrosion potential of a sample electrode exposed to the environment is measured, and thereby the effect of hydrogen injection is grasped. It is said that the occurrence of SCC is suppressed when the corrosion potential reaches a certain value. This corrosion potential is another important indicator of the hydrogen injection effect,
As described above, it is also possible to control the reactor water quality only by using the dissolved oxygen concentration instead of using it together.

(Problems to be Solved by the Invention) In a conventional hydrogen injection method, a dissolved oxygen concentration in reactor water and / or a material corrosion potential is adopted as an index indicating an effect of hydrogen injection. That is, if the corrosive environment is suppressed until those values fall below a certain value,
It is assumed that the occurrence of SCC is suppressed.

However, another major water quality factor affecting SCC limits is conductivity. It is recognized that the conductivity of the reactor water is dominated by impurity ions in the reactor water. Depending on the type of the impurity ions, it is generally recognized that the higher the conductivity, the more easily SCC is generated. The value of the conductivity involved in the SCC generation varies from plant to plant, and a general value that can be applied to each plant cannot be obtained. Therefore, even in an environment with the same value of dissolved oxygen concentration and material corrosion potential, some plants may or may not exhibit corrosion susceptibility. Conversely, the values of the dissolved oxygen concentration and the material corrosion potential, which indicate the SCC generation limit, differ for each plant.

On the other hand, under hydrogen injection conditions, the reactor water changes from an oxidizing environment to a reducing environment, so that the chemical form of the radioactive N-16 compound generated by the (n, p) reaction of O-16 changes to volatile. As a result, there is a side effect of increasing the radiation dose rate of the main steam system, which causes problems such as an increase in the exposure of operating employees and an increase in the skyshine dose rate at the plant site boundary. . FIG. 5 is a diagram showing the relationship between the hydrogen injection amount and the main steam-based radiation dose rate. The vertical axis represents the main steam-based radiation dose rate (relative value), and the horizontal axis represents the hydrogen injection amount (arbitrary unit).

The present invention has been made based on the above-described circumstances, and mainly relates to control of electric conductivity of reactor water, dissolved oxygen concentration, suppression of SCC generation by controlling material corrosion potential, and a nuclear power plant that has solved the above-mentioned problems. It is intended to provide.

[Structure of the Invention] (Means for Solving the Problems) A nuclear power plant of the present invention includes a reactor pressure vessel, a reactor core housed in the reactor pressure vessel, and a main steam system for guiding steam generated in the reactor core. A recirculation system that recirculates coolant in the reactor pressure vessel, a turbine that works on steam in the main steam system, a condenser that condenses steam discharged from the turbine, A water supply system that feeds water into the reactor pressure vessel and includes a high-pressure condensate pump, a condensate purification system, a low-pressure condensate pump, a feedwater heater, and a feedwater pump;
A hydrogen injection system for injecting water between the condensate purification system and the low-pressure condensate pump of the water supply system, wherein the reactor water of the reactor recirculation system is purified and returned to the reactor pressure vessel. Measure the conductivity and dissolved oxygen concentration of the reactor water in the reactor coolant purification system and the recirculation system before flowing into the reactor coolant purification system, and measure the hydrogen based on the measurement results and the conductivity / dissolved oxygen correlation curve. A sample analysis rack for determining the injection amount and maintaining the conductivity of the reactor water at 0.1 μs / cm or less is provided.

(Action) The SCC of SUS304 sensitized material, which is the primary material of the reactor, was examined for its generation limit in relation to the two environmental factors of dissolved oxygen and conductivity. When the dissolved oxygen concentration exceeded a certain level, the BWR It was found that the austenitic stainless steel exhibited sensitivity to SCC under the operating conditions of, and its generation limit was lower as the dissolved oxygen concentration was higher. Furthermore, even if the dissolved oxygen concentration is somewhat high when the conductivity is low, no SCC sensitivity is observed, especially when the conductivity is 0.1 μs / cm or less, the dissolved oxygen concentration is 100 ppb.
It was also found that no SCC sensitivity was observed.

Generally, the dissolved oxygen concentration of BWR plants in Japan is 100-200p
From the above results, if the conductivity of the reactor water is kept at 0.1 μs / cm or less, no SCC is generated. In the unlikely event that the SCC is generated even though the conductivity is 0.1 μs / cm or less, a small amount of hydrogen should be injected to improve the water quality to a range where SCC does not occur. Can be.

(Embodiment) FIG. 1 is a system diagram of one embodiment of the present invention, in which the same parts as those in FIG. However, in this figure, the turbines are shown as a high-pressure turbine 3a and a low-pressure turbine 3b. A reactor coolant purification system 13 branches from the reactor recirculation system 12, and the purified reactor water is returned to the reactor pressure vessel 2. Reactor water is sampled from the reactor coolant purification system 13 by a sampling line 14, and the sampled reactor water is subjected to measurement of conductivity, dissolved oxygen concentration, and the like in a sample analysis rack 15. In the figure, reference numeral 16 denotes a hydrogen injection facility.

The sample analysis rack 15 measures the dissolved oxygen concentration and the reactor water conductivity.The reactor coolant purification system is controlled based on the measurement result of the reactor water conductivity, and the reactor water conductivity is 0.1 μs.
Water quality is controlled so that it does not exceed / cm.

Fig. 2 shows SC of SUS304 sensitized material which is the primary material of nuclear reactor
For C, its emission limit is defined as the dissolved oxygen and conductivity.
The experimental data obtained by examining the correlation between two environmental factors are shown. The ■ in the conductivity / dissolved oxygen correlation curve indicates the occurrence of SCC, and the □ indicates no SCC. From this figure, it can be seen that when the dissolved oxygen concentration exceeds a certain level, the austenitic stainless steel exhibits sensitivity to SCC under the operating conditions of the BWR, and that the generation limit decreases as the dissolved oxygen concentration increases. Furthermore, it can be seen that SCC sensitivity is not observed when the conductivity is low even if the dissolved oxygen concentration is somewhat high, and that SCC sensitivity is not observed even when the dissolved oxygen concentration is 100 ppb or more when the conductivity is 0.1 μs / cm or less.

Generally, the dissolved oxygen concentration of BWR plants in Japan is 100-200p
In FIG. 2, if the conductivity of the reactor water is kept at 0.1 μs / cm or less, no SCC is generated. In the unlikely event that the SCC is generated even though the conductivity is 0.1 μs / cm or less, a small amount of hydrogen should be injected to improve the water quality to a range where SCC does not occur. Can be.

[Effects of the Invention] As is clear from the above, in the nuclear power plant of the present invention, hydrogen injection for converting dissolved oxygen into water is not required at all just by controlling the conductivity of reactor water, or if injection is performed, May be extremely small, and the rate of increase of the radiation dose rate by N-16 in the main steam system can be suppressed. Also, as a result, the exposure of employees can be reduced,
An increase in the skyshine dose rate at the plant site boundary can be suppressed. This is also shown in the diagram of FIG.

[Brief description of the drawings]

FIG. 1 is a system diagram of one embodiment of the present invention, FIG. 2 is a diagram showing the correlation between the conductivity and the dissolved oxygen concentration which affect the SCC generation limit of sensitized stainless steel, and FIG. 3 is a diagram of a conventional nuclear power plant. FIG. 4 is a system diagram, FIG. 4 is a diagram showing a dissolved oxygen reduction effect in a hydrogen injection test of the plant, and FIG. 5 is a diagram showing a relationship between the hydrogen injection amount and the main steam radiation dose rate. 1 ... core, 2 ... reactor pressure vessel, 3 ... turbine,
3a High-pressure turbine, 3b Low-pressure turbine, 4 Condenser, 5 High-pressure condensate pump, 6 Condensate purification system, 7
Low-pressure condensate pump, 8: Feed water heater, 9: Feed water pump, 10: Recirculation pump, 11: Injection point, 12: Reactor recirculation system, 13: Reactor coolant purification system, 14 ... Sampling line, 15 ... Sample analysis rack, 16 ... Hydrogen injection equipment

Claims (1)

(57) [Claims] 1. A reactor pressure vessel, a reactor core housed in the reactor pressure vessel, a main steam system for guiding steam generated in the reactor core, and a recirculation system for recirculating a coolant in the reactor pressure vessel. A circulation system, a turbine that works on the steam of the main steam system,
A condenser for condensing steam discharged from the turbine, and a condenser for sending the condensate to the reactor pressure vessel, wherein the high-pressure condensate pump, the condensate purification system, the low-pressure condensate pump, the feed water heater, A water supply system including a water supply pump, and a hydrogen injection facility for injecting the condensate between the condensate purification system and the low-pressure condensate pump of the water supply system, wherein the reactor water of the reactor recirculation system is purified. The reactor coolant purification system to be returned into the reactor pressure vessel, and the conductivity and dissolved oxygen concentration of the reactor water in the recirculation system before flowing into the reactor coolant purification system were measured. A nuclear power plant comprising: a sample analysis rack for determining a hydrogen injection amount based on a rate / dissolved oxygen correlation curve and maintaining a reactor water conductivity of 0.1 μs / cm or less.