JPH01197698A - Nuclear power plant - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Purpose of the Invention (Field of Industrial Application) The present invention operates a nuclear power plant, particularly in a nuclear reactor, by controlling the reactor water conductivity in the subsystem, thereby further solidifying the material integrity. This relates to a nuclear power plant that was maintained.

(Prior Art) The coolant of a nuclear power plant is in a high temperature and high pressure state during its operation, which is a harsh environmental condition for structural materials, and the corrosion behavior of structural materials has become an important issue. In particular, in boiling water reactors (hereinafter referred to as BWR), stress corrosion cracking (hereinafter referred to as 5CC) of austenitic stainless steel piping welds, etc.
(Stress Corrosion Crack
There is an example in which a phenomenon called (abbreviation for ing) occurred.

The above SCC phenomenon is caused by three factors: materials.

It is said that this occurs at Uζ where stress and environment are superimposed. As for the material factor, there is a condition that the rust welded part is 5US304 stainless steel. That is, since chromium carbide is precipitated in the welded part due to the thermal effect during welding, a chromium-deficient layer is generated, which causes a decrease in yield strength. In addition, with regard to stress, residual thermal stress on members due to heat during welding has become a problem, and efforts are being made to eliminate residual stress by improving welding methods. Furthermore, environmental factors include the presence of impurities such as chlorine ions, dissolved oxygen, etc. in addition to the corrosive environment of high-temperature water.

In nuclear power plants, the water quality of the reactor coolant is strictly controlled, and the primary water system of the BWR plant is kept as neutral and pure as possible. On the other hand, there are the following problems regarding dissolved oxygen. In other words, oxygen is constantly produced in the reactor core by radiolysis of water, and 2
The presence of dissolved oxygen on the order of 00 to 300 PPb is unavoidable.

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

In recent years, hydrogen injection technology into reactor water has been developed as a countermeasure against SCC mentioned above, and some B
It is put into practical use at WR plants.

This technology reduces the dissolved oxygen concentration in the reactor water by injecting hydrogen into the reactor water to make the reactor water excessively hydrogen-reducing, and then recombining oxygen and hydrogen radiochemically in the presence of radiation. It is something that makes you

An example of hydrogen injection in an existing plant (Dresgen No. 1 reactor in the United States) will be explained with reference to FIGS. 3 and 4. FIG. 3 shows a system diagram of the BWR plant.

In this figure, a reactor core 1 is housed in a reactor pressure vessel 2, and steam generated in the reactor core 1 performs work in a turbine 3, and then is led to a condenser 4, where it is cooled and condensed to condense. This condensate is heated and pressurized through a condensate pump 5, a condensate purification system 6, a high-pressure condensate pump 7, a feed water heater 8, and a feed water pump 9, and then injected into the reactor pressure vessel 2. On the other hand, part or all of the reactor water within the reactor pressure vessel 2 is recirculated by the reactor recirculation pump 10. This recirculation forces the core flow rate to increase and more heat is removed from the core 1.

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 the effect of reducing dissolved oxygen in the hydrogen injection test of the plant. In this figure, the vertical axis is the dissolved oxygen concentration (ppb) in the reactor water sampled in the recirculation system, and the horizontal axis is the feedwater dissolved hydrogen concentration (ppb). You can see how the dissolved oxygen concentration is reduced. Note that in the figure, the 0 mark indicates the value when the reactor output is 81%, and the 1 mark indicates the value when the reactor output is 95%.

In the above hydrogen injection test, the dissolved oxygen concentration in the reactor water (collected from the recirculation system) was used as an indicator of the effect of hydrogen injection on reducing the dissolved oxygen concentration. It is said that it is necessary to keep the index value below 20 ppb. However, this value is limited to the above-mentioned test and varies depending on the plant, and no generally accepted standard value has been established.

On the other hand, in some plants, in addition to dissolved oxygen concentration, material (
The corrosion potential of SUS304 stainless steel has been measured. This measurement measures the corrosion potential of a sample electrode exposed to the environment, and thereby attempts to understand the effect of hydrogen injection. It is believed that when this corrosion potential reaches a certain value, the occurrence of SCC is suppressed. This corrosion potential is another important indicator of the hydrogen injection effect, and it is also possible to manage the quality of reactor water by using it alone, rather than in combination with the dissolved oxygen concentration as described above.

(Problem to be solved by the invention) In the conventional hydrogen injection method.

Dissolved oxygen concentration in reactor water and/or material corrosion potential are used as indicators of the effectiveness of hydrogen injection.

In other words, if the corrosive environment is suppressed until these values fall below a certain value, the occurrence of SCC will be suppressed. '- However, another major water quality factor that affects the SCC generation limit is electrical conductivity.

Impurity ions in the reactor water control the electrical conductivity of the reactor water, and it is generally recognized that the higher the electrical conductivity, the more likely SCC will occur, although it depends on the type of impurity ions. The value of electrical conductivity involved in the generation of SCC varies depending on the plant, and it is not possible to determine a general value that is applicable to each individual plant.

Therefore, even in environments with the same dissolved oxygen concentration and material corrosion potential, some plants may or may not exhibit corrosion susceptibility. Conversely, the values of dissolved oxygen concentration and material corrosion potential, which indicate the SCC generation limit, differ from plant to plant.

On the other hand, under hydrogen injection conditions, the reactor water changes from an oxidizing environment to a reducing environment, so the chemical form of the radioactive N-16 compound generated in the 0-16 (n,p) reaction changes to volatile. As a result, the side effect of increasing the radiation dose rate in the main steam system will occur, leading to problems such as increased radiation exposure for employees during operation and increased skyshine dose rate at the plant site boundary. 6 Figure 5 is a diagram showing the relationship between hydrogen injection amount and main steam system radiation dose rate, where the vertical axis is the main steam system radiation dose rate (relative value) and the horizontal axis is the hydrogen injection amount (arbitrary unit). be.

The present invention has been made based on the above circumstances, and is a nuclear power plant that solves each of the above problems by suppressing the occurrence of SCC mainly by controlling the conductivity of reactor water and secondly by controlling the dissolved oxygen concentration and material corrosion potential. 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 that guides steam generated in the reactor core. a recirculation system that recirculates the coolant in the reactor pressure vessel; a turbine that performs work on the steam of the main steam system; a condenser that condenses the steam discharged from the turbine; A water supply system that sends water to 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, and the condensate purification system of this water supply system. and a reactor coolant purification system that purifies reactor water in the reactor recirculation system and returns it to the reactor pressure vessel, the hydrogen injection equipment being injected between the low pressure condensate pump and the reactor pressure vessel.

The conductivity and dissolved oxygen concentration of the reactor water in the recirculation system before flowing into the reactor coolant purification system are measured, and the reactor coolant purification system is controlled based on the measurement results, and the conductivity of the reactor water is controlled. 0.
It is characterized by being provided with a sample analysis rack that maintains the sample density at 1 μ5 acs or less.

(Function) SCC of 5US304 sensitizing material, which is a nuclear reactor-related material
We investigated the correlation between two environmental factors, dissolved oxygen and electrical conductivity, to determine the limit for its occurrence, and found that when the dissolved oxygen concentration exceeds a certain level, austenitic stainless steel becomes susceptible to SCC under BWR operating conditions. It was found that the higher the dissolved oxygen concentration, the lower the limit for its occurrence. Furthermore, it was also found that when the conductivity decreases, SCC susceptibility is not observed even if the dissolved oxygen concentration is high to some extent, and in particular, when the conductivity is 0.1 μ8/a1 or less, SCC susceptibility is not observed even when the dissolved oxygen concentration is 100 pPt) or higher. Ta.

Generally, the dissolved oxygen concentration in BWR plants in Japan is 100~
200 ppb, and based on the above results, the conductivity of the reactor water was reduced to 0.
．． If it is kept below 1 μs/l, no SCC will occur. In addition, in the event that SCC occurs even though the conductivity is below 0°1 μs/as, by injecting a small amount of hydrogen, the water quality will be improved to the extent that SCC will not occur. be able to.

(Embodiment) FIG. 1, in which the same parts as in FIG. 3 are denoted by the same reference numerals, is a system diagram of one embodiment of the present invention. 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 is installed branching off from the reactor recirculation system 12, and purified reactor water is returned to the reactor pressure vessel 2. Further, reactor water is sampled from the reactor coolant purification system 13 through a sampling line 14, and the sampled reactor water is placed in a sample analysis rack 15.
In Figure 0, where measurements of electrical conductivity, dissolved oxygen concentration, etc. are carried out, 16 indicates hydrogen injection equipment.

Note that the sample analysis rack 15 measures the dissolved oxygen concentration and reactor water conductivity, and the reactor coolant purification system is controlled based on the measurement results of the reactor water conductivity, so that the reactor water conductivity is 0.1.
Water quality is controlled so as not to exceed μ8/Ql.

Figure 2 shows experimental data for investigating the SCC generation limit of 5US304 sensitizing material, which is a reactor-grade material, by examining the correlation between two environmental factors: dissolved oxygen and electrical conductivity. c-'i's mouth is SCC
None are shown respectively.

From this figure, when the dissolved oxygen concentration exceeds a certain level, B
It can be seen that under WR operating conditions, austenitic stainless steel becomes susceptible to SCC, and that the higher the dissolved oxygen concentration, the lower the limit for its occurrence. Furthermore, when the conductivity becomes low, SCC susceptibility is not observed even if the dissolved oxygen concentration is high to some extent, especially when the conductivity is 0.1
If the dissolved oxygen concentration is less than μS/mouth and the dissolved oxygen concentration exceeds 100 ppb, S
It can also be seen that CC sensitivity is not observed.

Generally, the dissolved oxygen concentration in BWR plants in Japan is 100~
200 ppb, and from Figure 2 the conductivity of the reactor water is 0.1.
If it is kept below μs/cm, no SCC will occur. In addition, in the event that SCC occurs even though the conductivity is below 0.1 μs/cm, by injecting a small amount of hydrogen, the water quality will be improved to the extent that SCC will not occur. be able to.

[Effects of the Invention] As is clear from the above, in the nuclear power plant of the present invention, by simply managing the conductivity of reactor water, there is no need for hydrogen injection at all to convert dissolved oxygen into water, or if injection is required, The amount of N-16 in the main steam system can be suppressed, and the rate of increase in the radioactive weight ratio due to N-16 in the main steam system can be suppressed. In addition, as a result, it is possible to reduce the amount of radiation exposure of employees,
It is possible to suppress the increase in skyshine dose rate at the plant site boundary. This is also shown in the diagram of FIG.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of one embodiment of the present invention, Fig. 2 is a diagram showing the correlation between electrical conductivity and dissolved oxygen concentration affecting the SCC generation limit of sensitized stainless steel, and Fig. 3 is a diagram of a conventional nuclear power plant. A system diagram, FIG. 4 is a diagram showing the dissolved oxygen reduction effect in the hydrogen injection test of the plant, and FIG. 5 is a diagram showing the relationship between the amount of hydrogen injection and the main steam system radiation dose rate. 1...Reactor core 2...Reactor pressure vessel
3...) - Bin 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...
...Water heater 9...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)

[Claims] A reactor pressure vessel, a reactor core housed in the reactor pressure vessel, a main steam system that guides steam generated in the reactor core, a recirculation system that recirculates coolant in the reactor pressure vessel, and a main steam system that guides steam generated in the reactor core. A turbine that performs work on the steam in the steam system, a condenser that condenses the steam discharged from the turbine, a high-pressure condensate pump, and a condensate purifier that sends this condensate to the reactor pressure vessel. A water supply system including a low-pressure condensate pump, a feedwater heater, and a water supply pump, and hydrogen injection equipment for injecting hydrogen between the condensate purification system and the low-pressure condensate pump of the water supply system, A reactor coolant purification system that purifies the reactor water in the reactor recirculation system and returns it to the reactor pressure vessel, and the conductivity and dissolved reactor water in the recirculation system before it flows into the reactor coolant purification system. The oxygen concentration is measured, and the reactor coolant purification system is controlled based on the measurement results, and the conductivity of the reactor water is reduced to 0.1.
A nuclear power plant, characterized in that it is equipped with a sample analysis rack that maintains the temperature below μs/cm.