JPH0424434B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a method for suppressing cobalt elution from a cobalt-containing metal used in high-temperature water, and in particular to a method for suppressing cobalt elution from a feedwater heater heater tube of a primary cooling piping of a nuclear power plant. This invention relates to a method suitable for reducing.

[Prior art]

Pipes, pumps, valves, etc. used in the primary cooling water system of nuclear power plants are made of stainless steel, Stellite, Inconel, Incoloy, Hastelloy, Colmonoy, etc. When these metals are used for a long period of time, they are damaged by corrosion and their constituent metal elements are eluted into the primary cooling water and brought into the nuclear reactor. Most of these eluted metal elements become oxides and adhere to the fuel rods, and upon irradiation with neutrons, they become radioactive nuclides such as ⁶⁰ Co, ⁵⁸ Co, ⁵¹ Cr, and ⁵⁴ Mn. The radioactive nuclides attached to the fuel rods are re-eluted into the primary cooling water and suspended as ions or insoluble solid components (hereinafter referred to as crat).
Some of the floating radionuclides are removed in the demineralizer of the reactor water purification system, but the radionuclides that are not removed are absorbed into structural materials, mainly made of stainless steel, while circulating in the primary cooling water system. Adheres to surfaces. For this reason, the dose rate on the surface of the structural material increases, causing the problem of radiation exposure of workers during maintenance and inspection. Therefore, as a method to prevent an increase in the surface dose rate of structural materials, oxygen is injected into the water supply system in order to reduce the amount of iron cracks brought into the reactor, which causes radionuclides to adhere to the fuel rods. Many plants are implementing measures to prevent corrosion damage to water supply piping.

On the other hand, various analyzes have revealed that the radiation exposure of nuclear plant workers is mainly due to ⁶⁰ Co (for example, G. Romeo, Proceedings of The 7th
International Congress on Metallic Congress
P1456.1978). This is because the decay energy intensity of the gamma rays of ⁶⁰ Co is high at 1.17 MeV and 1.33 MeV, and the half-life is long at 5.26 years, which increases the surface dose rate over a long period of time once it adheres to structural materials.
Therefore, in order to reduce the surface dose rate of structural materials, it depends on how to suppress the adhesion of ⁶⁰ Co to the structural materials.

The production of ⁶⁰ Co in a nuclear power plant is due to the (n, γ) reaction of the stable isotope ⁵⁹ Co in a nuclear reactor. Therefore, by suppressing the inflow of ⁵⁹ Co into the reactor, it is possible to reduce the generation of ⁶⁰ CO and, in turn, reduce the radiation exposure of workers (for example, S. Uchida et al.,
Water Chemistry, BNNES, 1980, Paper
44, 1980). Therefore, methods of strengthening the reactor purification system and preparing piping to prevent ⁶⁰ Co from adhering to the recirculation piping are being considered. However, even with this method, water supply systems and other
Corrosion of the structural materials of the primary cooling water system cannot be completely prevented. For this reason, it is not possible to eliminate radioactive substances in the primary cooling water, and therefore it is not possible to prevent an increase in the surface dose rate due to adhesion of radioactive substances to structural materials.

In particular, the primary source of cobalt in the feed water system is the stainless steel used in the feed water heater heater tube. In other words, stainless steel usually contains about 0.2% cobalt as an impurity, and where this cobalt is actually used, it comes into contact with high-temperature water (for example, in feed water heaters for boiling water reactors). water above 35℃ and below 225℃)
It is eluted as the stainless steel inside corrodes. Therefore, it is being considered to use low-cobalt stainless steel, which has a cobalt content lowered to about 1/4 of the usual amount, for the heater tube of the feed water heater. However, the use of low cobalt stainless steel
This poses material cost problems and has not been widely adopted.

[Purpose of the invention]

An object of the present invention is to provide a method for suppressing cobalt elution that can reduce elution of cobalt from a metal containing cobalt and chromium.

[Summary of the invention]

The present invention was made based on the discovery that the rate of cobalt elution from stainless steel is extremely dependent on temperature and is closely related to the properties of the oxide film formed on the surface of stainless steel. A dense oxide film rich in the chromium element contained in the metal is formed on the surface of the metal, and the vertical axis represents the cobalt elution rate from the metal into the water in contact with it, and the horizontal axis represents the time the metal is immersed. Cobalt elution rate at water immersion temperature
It is constructed so that cobalt elution from cobalt-containing metals can be reduced by contacting it with high-temperature water at a temperature higher than the immersion temperature at which the cobalt elution rate peaks in the immersion temperature relationship curve. be.

[Embodiments of the invention]

Conventionally, there have been few reports on the measurement of the elution rate of cobalt from stainless steel, partly because the elution rate is extremely low, and no detailed studies have been made on the temperature dependence of the elution rate. However, the inventors determined that ^{10 -3 mdm} (mg/d
This makes it possible to measure elution rates on the order of ^m2・month). As a result of basic experiments regarding the cobalt elution rate from this stainless steel, it was found that the cobalt elution rate shows a peak temperature, that is, a maximum value, at around the immersion temperature of 240°C. The results are shown in FIG.

Figure 1 shows the temperature dependence of the cobalt elution rate from stainless steel under reactor water supply conditions (dissolved oxygen concentration of about 20 ppb). As a result of surface observation using secondary electron images of these immersed samples,
On the surface of the sample whose immersion temperature was below 240℃,
No oxide film was present in areas where grain boundaries or cracks were present. On the other hand, in samples immersed at temperatures higher than 240°C, all grain boundaries and cracks were covered with an oxide film. In the former case, there is no oxide film at grain boundaries or cracks, so cobalt mainly elutes from there.
On the other hand, in the latter case, it is thought that cobalt elution is reduced because it is covered with an oxide film. Furthermore, elemental analysis of the oxide film using an ion microanalyzer revealed a chromium-rich layer. It has also been found that the elution rate of cobalt decreases in proportion to t ^-1/2 , where t is the immersion time, under any immersion temperature conditions. Furthermore, it has become clear that the cobalt elution rate of stainless steel that has been immersed at high temperatures changes in accordance with the elution rate change curve at high temperatures when it is immersed again in water at a lower temperature. Ta. Therefore, elution of cobalt can be reduced by pre-immersing stainless steel at a temperature higher than the operating temperature, that is, higher than the peak temperature (FIG. 1).

Feedwater heaters in nuclear plants are typically 35
The condenser outlet water, which is around ℃, is heated in several stages,
It is heated to a maximum of 225℃. Therefore, the heater tube of the feed water heater should be adjusted to the peak temperature (240°C) in advance.
By contacting the product with high-temperature water at a temperature higher than 200 °C for 2 to 300 hours to form an oxide film that protects against cobalt elution, this oxide film acts as a barrier and can suppress cobalt elution during use.

By the way, as a result of oxide film property analysis, the oxide film is Fe ₃ O ₄ with a spinel structure (M〓O・M〓 ₂ O ₃ ).
It was found that when cobalt and Cr partially substituted, a dense oxide film was formed, which was effective in suppressing the elution of cobalt. Therefore, whether or not elution of cobalt can be suppressed depends on whether a dense oxide film can be formed that covers grain boundaries and cracks.

Examples of the present invention will be described below. However, the present invention is not limited to these examples in any way.

Example Dissolved oxygen concentration flowing through stainless steel containing 0.22% cobalt at a rate of 1 m/sec
It was immersed in 20 ppb water at various temperatures of 200°C, 240°C, and 270°C, and the change in cobalt elution rate over time was measured. The test results are shown in Figure 2.

In FIG. 2, curve 10 is for an immersion temperature of 240°C, curve 12 is for an immersion temperature of 200°C, and curve 14 is for an immersion temperature of 270°C.
As is clear from FIG. 2, it can be seen that the cobalt elution rate decreases in proportion to (time) ^-1/2 .
As a result of secondary electron images of samples immersed for 600 hours, grain boundaries were clearly observed in samples immersed at 200°C and 240°C, whereas grain boundaries were clearly observed in samples immersed at 270°C. It was observed that the field was completely covered by an oxide film. In addition, as a result of examining the distribution of Cr and Fe in the depth direction using an ion microanalyzer, the sample immersed at a temperature of 270°C showed that the Cr concentration in the oxide film was lower than that in the base material, as shown by the solid line 16 in Figure 3. It became clear that the price was increasing. In addition, the broken line 18 shown in FIG.
is the concentration distribution of Fe.

Example Immersion temperature 200℃, 240℃, 270℃ as in the example
The sample that had been immersed for 600 hours was immersed again in high-temperature water at 240°C, where the elution rate of cobalt is maximum, and changes in the elution rate were measured. FIG. 4 shows the test results.

In Figure 4, curve 20 is the result of 600 hours of treatment at a dipping temperature of 240°C, and curve 22
Curve 24 is for a sample treated for 600 hours at an immersion temperature of 200°C, and curve 24 is for a sample treated for 600 hours at an immersion temperature of 270°C.

The following can be seen from Figure 4. In addition, in FIG. 4, each elution rate curve corresponding to the immersion time up to point A is the curve 10, 12, 14 shown in FIG.
is the same as After point A (after immersion time 600h)
shows dissolution rate curves 20, 22, and 24 when all test pieces were immersed in water at 240°C. Even if a sample that has been immersed for 600 hours at 270°C to form an oxide film is immersed in water at a lower temperature of 240°C, its elution rate curve 24 will remain the same as the elution rate curve at 270°C. It can be seen that the cobalt elution rate is suppressed by the formation of the oxide film. On the other hand, a sample immersed for 600 hours at 200°C becomes
It was found that the elution rate curve 22 did not change along the elution rate curve at 200°C, but increased significantly to a rate close to the elution rate curve 20 at 240°C.

As mentioned above, by treating metals containing cobalt and chromium by immersing them in high-temperature water higher than the peak temperature (240°C), it is possible to suppress the elution of cobalt and, in turn, reduce the surface dose rate of the primary piping system. This can reduce the radiation exposure of nuclear plant workers. Furthermore, this embodiment also reduces the occurrence of crud due to the reduction in the amount of corrosion, and can be applied regardless of the type of nuclear power plant. cobalt when it is in contact with high-temperature water (water at temperatures above 35°C and below 225°C in the example of feed water heaters for boiling water reactors) in places where stainless steel is actually used. It is effective in suppressing elution and can also be applied to water supply systems of general steam generators.

Further, when the above embodiment is applied to a nuclear power plant, the following method can be used.

(1) Before installing the feedwater heater in a nuclear power plant, check the dissolved oxygen concentration on the inner and outer surfaces of the heater tube.
Aqueous water adjusted to 10-50 ppb (PH at 20℃
6.5 to 7.2) is heated to 270℃ to 280℃ while injecting water to form a dense chromium-rich oxide film on the inner and outer surfaces of the heater tube before installing it in the plant.

(2) For feedwater heaters in existing nuclear power plants, high-temperature water of about 270°C is brought into contact with the heater tube for a certain period of time, either by introducing high-temperature water from the outside or by using high-temperature water from the turbine extraction system. An oxide film can be formed.

In the above embodiment, the feed water heater heater tube of a nuclear power plant was explained.
For example, it can be applied to the water supply system of a pressurized water reactor secondary system or the water supply system of a general steam generator. Furthermore, although stainless steel has been described in the above embodiments, other metals such as stellite may also be used.

〔Effect of the invention〕

As explained above, according to the present invention, the metal containing cobalt and chromium is heated at the peak temperature (240
The elution of cobalt can be reduced by immersing the metal in high-temperature water (higher than 30°F (°C)) to form a dense chromium-rich oxide film on the metal surface.

[Brief explanation of drawings]

Figure 1 shows the relationship between the immersion rate of stainless steel and the cobalt elution rate, Figure 2 shows the relationship between the immersion temperature and time of stainless steel, and the cobalt elution rate, and Figure 3 shows the relationship between the immersion rate of stainless steel and the cobalt elution rate. A diagram showing the concentration distribution of chromium and iron in the formed stainless steel. Figure 4 is a diagram showing the relationship between the immersion time and the cobalt elution rate when stainless steel that has been immersed for 600 hours is immersed again at 240 ° C. 10...Immersion temperature 240℃, 12...Immersion temperature 200
℃, 14...Immersion temperature 270℃, 16...Cr concentration curve, 18...Fe concentration curve, 20...Immersion temperature 240℃, 600 hours treatment, 22...Immersion temperature 200℃,
600 hours treatment, 24... Immersion temperature 270°C, 600 hours treatment.

Claims (1)

[Claims] 1. In a method for suppressing cobalt elution from a cobalt-containing metal, which reduces the elution of cobalt element from a metal containing cobalt element and chromium element into water with which the surface of the metal comes into contact during use, the surface of the cobalt-containing metal is The dense chromium-rich oxide film contained in the chromium element is expressed as follows: the rate of elution of cobalt from the metal into the water it comes into contact with is taken as the vertical axis, and the immersion temperature of the water in which the metal is immersed is taken as the horizontal axis. A method for suppressing cobalt elution from a cobalt-containing metal, the method comprising forming a cobalt-containing metal by contacting it with high-temperature water at a temperature higher than the immersion temperature at which the cobalt elution rate peaks in a cobalt elution rate-immersion temperature relationship curve.