JPH04126902A - Feedwater heater - Google Patents

Abstract

PURPOSE:To substantially reduce the amount of radiation at a nuclear power plant and contribute to lessening of the exposure of workers at such a plant to radiation by providing a heater tube formed of a ferritic stainless steel and treated by electrolytic polishing. CONSTITUTION:A heater tube 23 is formed of a ferritic stainless steel SUS 434 and treated by electrolytic polishing. The corrosion rate of electropolished ferritic stainless steel SUS 434 is 0.05mg/cm<2>/20 days, the electrolytic polishing decreasing the amount of elution to about 1/10. This effect is considered to be due to smoothing of the surface and hence diminution of the surface area by the electropolishing treatment. With corrosion products generated in feedwater reduced to about 1/10 in quantity, the quantity of the radionuclide produced in the core of a reactor pressure vessel 1 also decreases considerably. At a nuclear power plant, a decrease in the amount of radiation results and it becomes possible to reduce the amount of the radiation to which the workers are exposed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a feedwater heater, and more particularly to a feedwater heater with an improved heater tube.

(Prior Art) A feed water heater for a boiling water nuclear power plant heats condensate from a condenser as feed water 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.

In order to increase heat exchange efficiency, such heater tubes are designed to have a large contact area with the water supply of approximately 20,000 nm. Therefore, from the viewpoint of corrosion prevention, conventional heater tubes are difficult to corrode. Made of austenitic stainless steel.

(Problems to be Solved by the Invention) Although this austenitic stainless steel has a low corrosion rate and a small amount of corrosion as shown in curve 1 in Figure 4,
The Ni content is high. Note that FIG. 4 shows the relationship between the amount of corrosion of stainless steel and the test time.

When the amount of Ni in stainless steel increases, 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 Ni content. Therefore, if the stainless steel that makes up the heater tube contains a lot of Ni, the co
This means an increase in the amount of elution into the water supply. Therefore, when the heater tube is constructed from austenitic stainless steel, the Co concentration in the feed water increases. In addition, since the wetted area of the heater tube is large as in the double series, more than 90% of the cobalt in the water supply will be eluted from the heater tube.

The Co eluted into the feed water is guided to the reactor core in the reactor pressure vessel together with Ni Fe, etc. in the feed water, where it is irradiated with neutrons and transformed into radionuclides such as 60Co, ``Mn,'' Co, etc. Of these, Co has a significantly higher radiation dose than other nuclides. Therefore, if a large amount of Co exists in the water supply, the amount of 611 Co generated will increase, and the radiation dose of the blunt will increase. Brandt operators may be exposed to radiation.

Therefore, from the viewpoint of reducing radiation exposure for workers, consideration is being given to applying ferritic stainless steel that does not contain Ni to the feed water heater tube.

For these reasons, it is desirable to apply Ni-free ferritic stainless steel to the feed water heater tube. However, as shown by curve 3 in FIG. 4, the amount of corrosion in ferritic stainless steel is extremely large compared to that in austenitic stainless steel (curve 1).

Therefore, there is a problem in that simply changing the steel type cannot be expected to have a significant effect on reducing corrosion products or reducing worker radiation exposure.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a feed water heater that can significantly suppress the radiation dose in a nuclear power plant and contribute to reducing the radiation exposure of blunt workers. It is in.

[Structure of the Invention] (Means for Solving the Problems) The present invention is installed in a water supply system of a nuclear power plant, and a large number of heater tubes are arranged inside the main body to constitute a feed water heater, and the water supply heater A feed water heater in which the feed water of the system is heated by heat exchange while passing through the heater tube, wherein the heater tube is made of ferritic stainless steel subjected to electrolytic polishing treatment. do.

(Function) The heater tube is made of ferritic stainless steel, and its inner and outer surfaces are smoothed by electrolytic polishing. As a result, the amount of constituent elements such as Ni and Co eluted from the heater tube into the feed water is reduced, and the amount of eluted elements is also suppressed, reducing the amount of wind carried into the reactor. by this,
The amount of radionuclides produced in the reactor core can be reduced.

(Example) An example of the feed water heater according to the present invention will be described based on the drawings. FIG. 1 shows an embodiment of the feed water heater according to the present invention.
FIG. 2 is a system diagram showing a first example of a boiling water nuclear power plant in which the feed water heater in FIG. 1 is integrated into a heater drain forward pump-up type water supply system.

In FIG. 2, steam generated within the reactor pressure vessel 1 is led to a high pressure steam turbine 5 through a main steam line 3 to drive a turbine rotor. The steam that has done work in the high pressure steam turbine 5 passes through the moisture separator reheater 7 and then passes through the low pressure steam turbine 9.
is guided to drive the turbine rotor. The moisture separation reheater 7 guides steam from the reactor pressure vessel, removes moisture from the steam that has done work in the high-pressure steam turbine 5, and reheats the steam.

The steam that has been guided to the low pressure steam turbine 9 and has done work is cooled and condensed in the condenser II to become condensed water. This condensate is led to the condensate purification system 13 where it is filtered and desalinated, and then the water supply system 15
water is sent to the water supply. A low-pressure feed water heater 17 and a high-pressure feed water heater 19 are sequentially installed in the water supply system 15 from the upstream side. The feedwater is heated in stages by these feedwater heaters 17, 19 and then led to the reactor pressure vessel.

The heating medium that heats the feed water by exchanging heat with the feed water in the high-pressure feed water heater 19 functions as a heating medium in the moisture separation reheater 7, and the steam that flows out is used. Further, as the heating medium of the low-pressure feed water heater 17, a part of the steam heated in the moisture separator reheater 7 and guided to the low-pressure steam turbine 9 is used. The heating medium flowing out from these high-pressure and low-pressure feedwater heaters 19 and 17 passes through the high-pressure drain recovery line 20 and the low-pressure drain recovery line 18 to the water supply lines upstream of the high-pressure feedwater heater 19 and the low-pressure feedwater heater 17, respectively. Water is supplied to each of them. The heating medium that has become the feed water is heated together with other feed water by the high pressure feed water heater 19 and the low pressure feed water heater 17, and is led to the reactor pressure vessel 1. A water supply system in which the heating medium of the low-pressure and high-pressure water heaters 17 and 19 is directly guided to the water supply line without being purified is called a heater drain forward pump-up water supply system.

As shown in FIG.
It is configured by arranging. In other words, the main body 21
It is composed of a cylindrical body shell 25, and a second-stream ice chamber mirror plate 27 and a downstream ice chamber mirror plate 29 attached to both ends of the main body shell 25. Tube plates 31 and 33 are disposed at the boundaries between the second-stream side and downstream side ice chamber mirror plates 27 and 29 and the main body shell 25, respectively. Surrounded by the tube plate 31 and the upstream ice chamber mirror plate 27, a water chamber 35 on the person t1 side is formed, and an outlet side water chamber 37 is formed surrounded by the tube plate 33 and the downstream ice chamber mirror plate 29.

The ends of the plurality of heater tubes 23 are fixed to both tube sheets 31.33 and open into the inlet and outlet ice chambers 35.37. In addition, the upstream side ice chamber mirror plate 27 has a water supply container 1.
39 and a water supply mill [141] are formed on the downstream side ice chamber mirror plate 29, respectively. Further, the main body cylinder 25 has a heating medium 43 and a heating medium 1 which flows into and discharges a heating medium.
:11 D 45 is formed. Therefore, the water supplied from the water supply 39 into the inlet water chamber 35 is transferred from the heating medium 43 to the main body body 2 while passing through the heater tube 23.
5 is heated by heat exchange with steam as a heating medium, and flows out from the water supply outlet 41 via the outlet side ice chamber 37. Further, the steam as a heating medium guided into the main body shell 25 is cooled by heat exchange and flows out from the heating medium outlet 45.

The heater tube 23 is made of ferritic stainless steel 5US
It consists of 434. In this embodiment, this tube is subjected to electrolytic polishing.

Ferritic stainless steel 5U with electrolytic polishing treatment
Corrosion rate of S434 is O, Q5mg/cnf/20d
ays te, untreated it is 0.06mg/cn
f/20 days.

(Curve 2 in Figure 4) Therefore, by electropolishing, the elution amount is reduced to about 1/IO.

This is thought to be because the electrolytic polishing process smoothed the surface and reduced the surface area.

Here, curve 2 in Figure 4 shows the change over time in the amount of corrosion in a corrosion test for ferritic stainless steel subjected to electrolytic polishing, curve 3 for untreated ferritic stainless steel, and curve 1 for austenitic stainless steel. are shown respectively. The dissolved oxygen concentration in the test water is approximately 5 opppb, and the test temperature is 280°C.

However, with the heater tube 23 made of ferritic stainless steel subjected to electrolytic polishing treatment, the amount of corrosion products generated in the water supply is reduced to about 1710 compared to the case of using untreated ferritic stainless steel. , the generation of radionuclides generated in the reactor core within the reactor pressure vessel 1 is also significantly reduced. As a result, the radiation dose in the nuclear power plant decreases, making it possible to reduce the radiation exposure of workers.

Furthermore, since less corrosion products enter the water supply and steam, the water supply system can be of the heater drain forward pump-up type. Therefore, the heating medium of the low-pressure and high-pressure feedwater heaters 17, 19 can be led directly to the feedwater heater 1719 and heated without passing through the condensate purification system 13. As a result, it is more advantageous in terms of thermoeconomics than a cascade type water supply system (FIG. 3), which will be described later, in which the heating medium is guided to the condenser 11, cooled and solidified, and then introduced to the condensate purification system 13.

A second example in which the feed water heater having the above configuration is incorporated into a Norskate water supply system will be described. In this second example, the heater tube 23 is made of ferritic stainless steel provided with an oxide coating as described above.
7.19 was installed in the cascade water supply system shown in Figure 3. In this cascade type water supply system, the heating medium from the high-pressure feedwater heater 19 is guided to the low-pressure feedwater heater 17 and used as a heating medium again, and the heating medium from the low-pressure feedwater heater 17 is guided to the condenser 11, where it is condensed. It is configured to be purified by a purification system 13. In the case of this second example, if the second advantage of "thermal economy" is ignored, corrosion products in the heating medium can be reliably removed by the condensate purification system 13. Therefore, the amount of radionuclides produced in the reactor core can be further reduced, and the radiation dose in the plant can be further reduced.

[Effects of the Invention] According to the present invention, since a large number of heater tubes disposed inside the main body are made of electrolytically polished ferritic stainless steel, no elution from the heater tubes into the water supply occurs. It is possible to reduce the amount of radionuclides produced in the reactor core by reducing the amount of corrosion products generated. As a result, radiation doses in nuclear power plants can be significantly reduced.

[Brief explanation of drawings]

Fig. 1 is a vertical sectional view showing the feed water heater according to the present invention, Fig. 2 is a longitudinal sectional view showing the feed water heater according to the present invention;
The figure is a system diagram showing the first example of a boiling water nuclear power generation plant in which the feedwater heater in Figure 1 is incorporated into the water supply system of the heater drain forward pump-up method. ] - A system diagram showing a second example of a boiling water nuclear power generation plant incorporated in a single-door water supply system, and Figure 4 shows the ferritic stainless steel subjected to the electrolytic polishing treatment according to the present invention and the conventional example. FIG. 2 is a characteristic diagram showing changes over time in the amount of corrosion in a corrosion test of untreated ferritic stainless steel and austenitic stainless steel. ”...Reactor pressure vessel 5...High pressure steam turbine 9
...Low pressure steam turbine 15...Water supply system 17...
Low-pressure feed water heater 19... High-pressure feed water heater 21...
・Main body 23... Heater tube (87
33) Agent: Yoshiaki Inomata, patent attorney (and 1 other person)

Claims (1)

[Claims] It is installed in the water supply system of a nuclear power plant, and a large number of heater tubes are arranged inside the main body to constitute a feed water heater, and the feed water of the water supply system undergoes heat exchange while passing through the heater tubes. 1. A feed water heater for heating, wherein the heater tube is made of electrolytically polished ferritic stainless steel.