JPS62108195A - Feedwater heater for nuclear reactor - 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 [Field of Application of the Invention] The present invention relates to a feedwater heater provided for heating condensate in, for example, a boiling water nuclear power plant.

[Background of the invention]

In experimental reactors of boiling water nuclear power plants (hereinafter referred to as BWR plants), copper alloys were used for feedwater heater heating tubes, but copper eluted from the heating tubes was deposited on the fuel rods and turbines inside the reactor. It has been found that various inconveniences occur due to the corrosion, and so-called corrosion-resistant 304 stainless steel is used in commercial furnaces (Power, June,
S-12 (1979)).

However, as experience in operating commercial reactors increases,
Co-58 and Go-
The accumulation of 60 radionuclides and the increase in the radiation dose surrounding them, as well as the mechanism of accumulation of Co-58 and Co-60, have been somewhat understood (Procaeding of W.
ater Che+5istry n, paper
44, BN 5 (1980)). The outline is as follows. Some of the nickel, cobalt, or iron in the form of ions or solid particles (commonly called cladding) eluted from the reactor and its upstream constituent materials adheres to the surfaces of the fuel rods in the reactor, where they are irradiated with neutrons. Go-58, Co-'60゜Mn-5
4, becomes radioactive nuclides such as Fe-59.

Some of these radionuclides are eluted or separated from the surface of the fuel rods, and some of them re-adhere to the inner surfaces of piping and equipment in the reactor system, increasing the radiation dose around them. Based on past operational experience in actual plants, the larger the amount of iron brought into the reactor, the greater the amount of Co-6 in the reactor water.
It is known that the concentration is high, and therefore the radiation dose of the two piping equipment in the reactor-subsystem will be high. This is because cobalt and nickel in the reactor water adheres to the surface of the iron cladding and then to the surface of the fuel rods. This is thought to be due to the intense elution and peeling from the rod surface. Furthermore, the greater the amount of nickel and cobalt brought into the reactor, the more radioactivity increases.

Therefore, it is considered effective to reduce the amount of iron cladding, nickel, and cobalt emitted in order to reduce the radiation dose of the nine components of the reactor-subsystem piping of a boiling water nuclear power plant. Therefore, we strengthened the condensate desalination equipment and adopted corrosion-resistant steel for the condenser, condensate piping, bleed pipe, drain piping, etc. to suppress the amount of iron crud released into the water supply, and to reduce the amount of iron crud released into the water supply. Plant A was constructed using type 304 stainless steel with a cobalt content of 0.05% (previously it contained about 0.25% cobalt), and the elution of cobalt was also suppressed. As shown, in Plant A, iron cladding clearly decreased and the cobalt concentration in the reactor water was lower or almost the same as in other plants.
It was observed that Co-58 in the reactor water was high and the surface dose of secondary system piping was also high.

Table 1 Such a phenomenon is unexpected and contradicts the conventional common knowledge in related technology or academic concepts.

[Purpose of the invention]

The present invention aims to provide a nuclear reactor water supply system that provides a plant with low radioactivity, and is characterized by a feed water heater using a heating tube made of ferritic stainless steel.

[Summary of the invention]

The feed water heater for BWR according to the present invention is characterized in that its heating tube is made of a material selected from ferritic stainless steel.

As a result of systematic and fundamental research, the present inventors found that
Because there is extremely little iron cladding, the form of nickel that adheres to the fuel rod surface is different from the conventional ferrite type oxide (NiFe).
It was found that this is because nickel oxide (N i O) rather than nickel oxide (N i O) is the main form. That is,
Neutral water of nickel oxide for ferrite type oxide, 2
The solubility ratio at 88°C is approximately 1,000, and the nickel oxide attached to the fuel rod surface is iron ferrite (NiFe).
This is because activated Co-58 is released into the primary cooling water more easily and in large quantities than in the case of zOa).

Therefore, in order to form iron ferrite, countermeasures such as partial bypass of the condensate purification system and iron ion implantation can be considered, but this is not preferable since radionuclides such as M n -54 and Fe -59 will increase. No, the fundamental solution lies in suppressing the release of nickel into the cooling water.

[Embodiments of the invention]

FIG. 1 is a diagram showing a system diagram of a boiling water nuclear power plant to which a water supply-heater according to an embodiment of the present invention is applied. Steam generated in the reactor pressure vessel 1 is sent to the turbine 3 through the main steam pipe 2. The steam that has done work in the turbine 3 is condensed and cooled in a condenser 4 to become condensate, which is pressurized by a condensate pump 5, passes through a water supply pipe 6, and is purified in a condensate desalination device 7.
It is sent to the low pressure feed water heater 8. The feedwater heated by the low-pressure feedwater heater 8 is pressurized by the feedwater pump 10 and sent to the high-pressure feedwater heater 9, where it is further heated and returned to the reactor pressure vessel 1 via the water supply piping 6. In a commercial power generation plant, two to three low-pressure feedwater heaters 8 and high-pressure feedwater heaters 9 are each arranged in series, although this varies depending on the power generation capacity. A group of feed water heaters are installed in parallel.

FIG. 2 is a sectional view of a feed water heater according to an embodiment of the present invention. The details differ depending on the plant or whether it is a low-pressure feedwater heater or a high-pressure feedwater heater, but it can basically be explained using this diagram. Water is supplied from the water supply inlet pipe 20 to the water chamber 2
1a, passes through the heating pipe 22, and is sent out from the ice chamber 21b through the water supply outlet pipe 23. A large number of heating tubes are provided to form a tube group, which opens into the water chamber 21 through a tube plate 24 and is supported to a body 26 by a tube support plate 25. On the other hand, heated steam extracted from the turbine flows into the vessel from the steam inlet pipe 27. Further, the condensed water that has been heated by the heater located on the higher pressure side of the heater also flows into the vessel from the drain inlet pipe 28. These heating fluids come into contact with the surface of each heating tube 22 in the tube group, exchange heat with the feed water flowing through the tubes, condense, become drain, accumulate at the bottom of the vessel, and pass through the drain cooler 29. and is discharged from the drain outlet 30. The non-condensable gas that has flown into the heating fluid is guided in its flow direction through the heater by the tube support plate 25 along with the flow of the heating fluid, and is discharged outside the device from the non-condensable gas outlet 31 at the end of the body. is discharged.

Fuselage 26. The tube sheet 24, water chamber 21 and tube support plate 25 are typically carbon steel dust. On the other hand, heating Ir! 22 is made of ferritic stainless steel.

Steel needs strength and workability to be used as a heating tube.

Weldability etc. must also be considered. In the ferritic stainless steel used in the present invention, unavoidable impurities contained in a normal steel manufacturing process can be tolerated.

In addition, the reason why the temperature may be limited to 140°C or higher in the present invention is that, as shown in Figure 3, the release rate of austenitic stainless steel (SUS304), which is conventionally used as a heating tube, in neutral water is This is because it becomes noticeable at temperatures above 140℃, and below 140℃, conventional
This is because the object of the present invention can be achieved even if 304 is used.

An example in which the feed water heater of the present invention is applied will be explained with reference to FIG. From the tank 34 that adjusts water quality.

Water whose dissolved oxygen and conductive dust have been adjusted in the same way as in the actual machine is sent to a preheater 36 by a pump 35, and then sent to a feed water heater 37.
The pressure vessel 38 is heated with water.

The water heated and evaporated by the electric heater 39 is condensed in the cooler 40 and further passed through the ion exchange resin tower 41 to the adjustment tank 34.
It is now back to . The grafting part of this device is the preheater 3.
6 and the feed water heater 37 are all made of titanium and no nickel is released from the device.

The preheater 36 is made of 5US304 and the feed water heater is made of ferritic stainless steel. The water is heated from room temperature to 140°C in the preheater 36, and further heated to 140°C in the feed water heater 37.
It is heated to 40°C and boiled at 280°C in a pressure vessel 38 with an electric heater 39. 10 with this device
Although the system was operated for 00 hours, only 1 pb or less of nickel was found in the water in the pressure vessel 38.

〔Effect of the invention〕

According to the present invention, there is no elution of nickel from stainless steel, which has the effect of reducing radioactivity.

[Brief explanation of drawings]

Fig. 1 is a system diagram of a boiling water nuclear power plant according to an embodiment of the present invention, Fig. 2 is a sectional view of the feed water heater, Fig. 3 is a characteristic diagram of the steel discharge rate of 5US304 steel, and Fig. 4 FIG. 3 is an explanatory diagram of another embodiment of the present invention. 1... Reactor pressure vessel, 8... Low pressure feed water heater, 9
...High pressure feed water heater 32 Fuel rod 18-Sequential distribution
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Claims (1)

[Claims] 1. A nuclear power plant feed water heater for heating condensate and supplying it to a nuclear reactor, characterized in that a heating tube for heating condensate is made of a material selected from ferritic stainless steel. Feed water heater.