JPH01272997A - Feed water heater - Google Patents

Abstract

PURPOSE:To extremely suppress the radiation dose in an atomic power plant by using a ferritic stainless steel imparted with an oxide film to form the tubes of a feed water heater disposed in the body. CONSTITUTION:Both ends of the many heater tubes 27 are fixed to both tube plates 25, 26 and are provided to open in inlet side and outlet side water chambers 28, 29. A feed water inlet 30 is provided to an upper stream side water chamber end plate 23 and a feed water outlet 31 to a down stream side water chamber end plate 24, respectively. Further, a heating medium inlet 32 and heating medium outlet 33 for admitting and discharging the heating medium are provided to a body cylinder 22. The feed water introduced from the inlet 30 into the water chamber 28 is heated by a heat exchange with the steam as the heating medium introduced from the inlet 32 into the cylinder 22 during the passage through the inside of the tubes 27 and is discharged through the water chamber 29 from the outlet 31. Since the tubes 27 are constituted of the ferritic stainless steel, the corrosion products therefrom are extremely suppressed and the quantity of the radioactive nuclide to be formed in the reactor core is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION (Industrial Field of Application) The present invention relates to a feed water heater, and more particularly to a feed water heater with an improved heater tube.

(Prior Art) A feedwater heater for a boiling water nuclear power plant heats condensate from a condenser as feedwater and guides it to a reactor pressure vessel. Heating of the feed water is accomplished by heat exchange while the feed water flows through the heater tubes of the feed water heater.

Such a heater tube has a large contact area with the water supply of approximately 20,000 m in order to increase heat exchange efficiency. Therefore, from the viewpoint of corrosion prevention, the heater tube is made of austenitic stainless steel, which is difficult to corrode. Although this austenitic stainless steel has a low corrosion rate and a small amount of corrosion as shown in FIG. 4, it has a high content of 1. When the amount of N1 is large, the amount of Co present as an impurity also increases.

Generally, the amount of CO eluted from stainless steel into a liquid is proportional to the product of the corrosion rate of stainless steel and the N content. Therefore, a high N1 content in the stainless steel constituting the heater tube means an increase in the amount of CO leached into the feed water. If the heater tube is constructed from austenitic stainless steel, this results in an increased Co concentration in the feed water.

Furthermore, since the heater tube has a large surface area in contact with the liquid, as in the double series, more than 90% of the Co amount in the water supply is eluted from the heater tube.

Co dissolved in the feed water is led to the reactor core in the reactor pressure vessel along with Ni, Fe, etc. in the feed water and is irradiated with neutrons, where it changes into radionuclides such as Co-60, Kn-54, and Co-58. . Among these, Co-60 has a significantly higher radiation dose than the others. Therefore, if a large amount of GO is present in the water supply, the amount of Co-60 emitted will also increase, increasing the radiation dose in the plant, and there is a risk that Plan 1 - operators and work tools will be exposed to radiation.

Therefore, from the viewpoint of reducing the exposure of plant operators and the like, ferritic stainless steel that does not contain N1 has been considered for use in feed water heater tubes.

(Problem to be Solved by the Invention) However, as shown in FIG. 4, the amount of corrosion in ferritic stainless steel is extremely large compared to that in Austety 1-1 stainless steel, and the above-mentioned plan to reduce corrosion products or Unfavorable from the perspective of reducing radiation exposure for operators.

The present invention has been made in consideration of the above circumstances, and an object thereof is to provide a feed water heater that can significantly suppress the radiation dose in a nuclear power plant.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention is installed in the water supply system of nuclear power generation plan 1~, and a large number of heating tubes are arranged inside the main body, and inside the heater tube. In the feed water heater that heats the feed water by exchanging heat while the feed water passes through the feed water heater, the heater tube is made of stainless steel having an oxide film formed on its surface. .

(Function) According to the feedwater heater of the present invention, corrosion products from the heater tube are significantly suppressed, so corrosion products leached into the feedwater are also significantly reduced, and radioactivity in the core is accordingly reduced. The generation rate of nuclides can also be reduced.

(Example) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of the present invention.

In the figure, the main body 2I of the feed water heater of the present invention is composed of a cylindrical main body shell 22, and an upstream ice chamber head plate 23 and a downstream ice chamber head plate 24 attached to both ends of the main body body 22. 23.
Tube plates 25 and 26 are disposed at the boundaries between 24 and the main body shell 22, respectively. An inlet water chamber 28 is formed surrounded by the tube plate 25 and the upstream ice chamber mirror plate 23, and is surrounded by the tube plate 26 and the F
A first side water chamber 29 is formed at the outset surrounded by the flow side water chamber mirror plate z4.

Both ends of the large number of heater tubes 27 are connected to both tube plates 25.
．． 26, and the water chamber 28 on the inlet side and 14 side
It is opened at 0.29° and provided. Also, a water supply port 30 is formed in the −L flow side ice chamber head plate 23, and a water supply outlet 31 is formed in the downstream side ice chamber head plate 24.Furthermore, the main body +5!
22 is a heating medium population 3 into which heating refrigerant flows and is discharged.
2 and a heating medium outlet 33 are formed. therefore,
The water supplied from the water supply person 1030 and guided into the water measurement chamber 28 is heated by steam as a heating medium guided from the heater medium 32 into the main body shell 22 while passing through the heater tube 27. The water is exchanged and heated, and flows out from the water supply outlet 31 via the ice water chamber 9 on the outlet side. Further, the steam as a heating medium guided into the main body shell 22 is cooled by heat exchange and flows out from the heating medium amount 1'' 33.

The heater tube 27 is made of ferritic stainless steel.

A dense and strong oxide film that forms on the tube surface in a specific temperature range acts as a barrier and can effectively suppress the generation of corrosion products in the water supply during plant operation. For this reason, at the time of tube manufacture, the tube surface is actively annealed at a temperature of 700 to 1100°C to form a dense oxide film that is highly effective in suppressing the generation of corrosion products. be.

Such an oxide film can be formed during the material manufacturing process of ferritic stainless steel, but in that case, it is formed by machining or pickling, which is conventionally performed as a subsequent processing step. It is important not to remove the oxidized film.

Further, in the present invention, in order to obtain an oxide film having a good effect of suppressing the generation of corrosion products as described above, it is preferable that the annealing temperature of the tube is 700 to 1100"C, more preferably 750"C. ℃~1050℃.

If the heat treatment temperature is less than 700°C, the sensitization resistance will be poor,
On the other hand, heat treatment at a temperature exceeding 1050° C. is undesirable because functional properties, particularly yield strength and tensile strength, are inferior.

The annealing step 1- is completed after water cooling or rapid cooling in the above temperature range.

The corrosion rate of ferritic stainless steel with an oxide film added is 0.1ny/a#/20days, and that of the one without an oxide film is 0.6+ng/aiT/2
It is 0 days.

(See Figure 4) Therefore, by applying the film, the elution amount is
/6.

Here, FIG. 4 is a graph showing the change over time in the amount of corrosion in a corrosion test of Ferrite 1 stainless steel to which an oxide film was applied, Ferrite 1 to type stainless steel to which no oxide film was applied, and austenitic stainless steel. The dissolved oxygen concentration in the test water was approximately 50 ppb, and the test temperature was 280
It is °C.

FIG. 2 is a system diagram in which the feed water heater according to the present invention is installed in a water supply system of the heater drain forward pump-up type to the boiling water type nuclear power generation plan 1.

In the figure, steam generated within a reactor pressure vessel 1 is led to a high-pressure steam turbine 3 via a main steam line 2 to drive a turbine rotor. The steam that has done work in the high-pressure steam turbine 3 is guided to the low-pressure steam turbine 5 via a moisture separator and reheater 4, and drives a turbine rotor. The moisture separator reheater 4 guides steam from the reactor pressure vessel 1, removes moisture from the steam that has done work in the high-pressure steam turbine 3, and reheats the steam.

The steam that has been guided to the low-pressure steam turbine 5 and has done work is cooled and condensed in the condenser 6 to become condensed water. This condensate is led to the condensate purification system 7 where it is filtered and desalinated, and then the water supply system 8
water is sent to the water supply. A low-pressure feedwater heater 9 and a high-pressure feedwater heater 11 are sequentially installed in the water supply system 8 from the upstream side. After the feed water is heated in stages by these feed water heaters 9 and 11, it is led to the reactor pressure vessel 1.

The heating medium that heats the feed water by exchanging heat with the feed water in the high-pressure feed water heater 11 is the steam that functions as a heating medium in the moisture separation reheater 4 and flows out. Further, as the heating medium of the low pressure feed water heater 9, a part of the steam heated in the moisture separation reheater 4 and guided to the low pressure steam turbine 5 is used. The heating medium flowing out from these high pressure and low pressure feed water heaters 11 and 9 is transferred to a high pressure drain recovery line 12 and a low pressure 1, respectively.
It is sent to the water supply lines upstream of the high-pressure feed water heater 11 and the low-pressure feed water heater 9 via the carbon recovery line 10, respectively, and becomes water supply.

The heating medium, which has become the feed water, is heated together with other feed water in the high pressure feed water heater 11 and the low pressure feed water heater 9, and is guided to the reactor pressure vessel 1. A water supply system in which the heating medium of the low-pressure and high-pressure water heaters 9 and 11 is guided directly to the water supply line without being purified is referred to as a forward pump system.

That is, in this water supply system, since the heating medium is directly sent to the water supply without a purification device as described above, it is particularly desirable to reduce corrosion products.

By the way, as described above, in the feed water heater of the present invention, the heating tube 27 is made of ferritic stainless steel coated with an oxide film. In this way, the Ferrai 1 series stainless steel with an oxide film has a 1/6th reduction in the number of corrosion product generating groups in the feed water compared to the Ferrai 1 series stainless steel that does not have an oxide film. The amount of radionuclides produced in the reactor core within the pressure vessel 1 is significantly reduced.

As a result, the amount of radioactive radicals in nuclear power plants is reduced, making it possible to reduce the amount of radiation exposure for workers.

Moreover, since the amount of corrosion products in the water supply and steam is reduced, the water supply system can be of a heater 1-rainbow 1-pump-up system.

Therefore, the heating medium of the low-pressure and high-pressure feedwater heaters 9, 11 is directly transferred to the feedwater heaters 9, 11 without passing through the condensate purification system 7.
11 for heating.

As a result, the heating medium is guided to the condenser 6, cooled and solidified, and then introduced to the condensate purification system 7 in a one-way water supply system (described later).
(See Figure 3) has a thermal economy advantage.

FIG. 3 is a system diagram when the feed water heater of the present invention is installed in a cascade water supply system. Components that are the same as those in FIG. 2 that have already been described are designated by the same reference numerals, and their description will be omitted.

In this cascade type water supply system, the heating medium from the high-pressure feedwater heater 11 is guided to the low-pressure feedwater heater 9 and used as a heating medium again, and the heating medium from the low-pressure feedwater heater 9 is transferred to the condenser 6.
The water supply system differs from the water supply system shown in FIG. 2 in that it is constructed so that the condensate water is introduced into water and purified by the condensate purification system 7. If we ignore the second advantage of ``thermoeconomics'' in this skating water supply system, corrosion products in the heating medium can be reliably removed in the condensate purification system 7, so the amount of radionuclides produced in the reactor core can be reduced. The radiation dose of the plant can be further reduced.

〔Effect of the invention〕

As explained above, according to the feed water heater of the present invention, the large number of feed water heater tubes disposed inside the main body are made of ferritic stainless steel coated with an oxide layer. 4 radionuclides in the reactor core by reducing corrosion products leaching from the reactor tubes into the feed water.
As a result, the radiation dose in a nuclear power plant can be significantly suppressed.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of an embodiment of the present invention, Fig. 2 is a system diagram of a boiling water nuclear power plant in which the feed water heater of the present invention is installed in a water supply system of the heater drain forward pump amplifier type, and Fig. 3 Figure 4 is a system diagram of a boiling water nuclear power generation plan l- in which the feed water heater of the present invention is installed in the water supply system of the Kasugou 1~ method, and Figure 4 shows ferritic stainless steel with an oxide film and ferrite without an oxide film. 1 is a graph showing changes over time in the amount of corrosion in corrosion tests of stainless steels and Austety 1 to stainless steels. 1 Reactor pressure vessel 3・High pressure steam turbine 4・
Moisture separation reheater 5 ・Low pressure steam turbine 6...
Condenser 7-・Condensate purification system 8 Water supply system
9...Low pressure feed water heater 10...High pressure drain recovery line 11...High pressure feed water heater 12...Low pressure drain recovery line 21...Heater main body 22...Heater main body shell 23.24...Ice chamber mirror plate 25.26
・-Tube plate 27・Heater tube 28.2
9... Ice house 30 - Water supply inlet 31 - Water supply outlet 32... Heating medium inlet 33 Heating medium outlet (8733) Agent Patent attorney Yoshiaki Inomata (and 1 other person) Figure 4

Claims (1)

[Claims] 1) In a feed water heater installed in the water supply system of a nuclear power plant, in which a large number of heater tubes are arranged inside the main body, and the feed water is heated by heat exchange while passing through the heater tubes, the heating This water heater is characterized in that the tube is made of ferritic stainless steel with an oxide film formed on its surface.