JP2000199751A - Determining method for corrosion resistance of zirconium alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a determining method, in which the generation of the acceleration of a corrosion speed can be determined more precisely by a method wherein an AC voltage is applied to a zirconium alloy oxide film, an AC current which is output is measured and the corrosion resistance of a zirconium alloy is determined on the basis of the frequency dependence of an impedance. SOLUTION: In this method, an AC voltage ΔE is applied to a zirconium alloy oxide film, an AC current ΔI which is output is measured, and an impedance Z=ΔE/ΔI is found. Then, the frequency dependence of the obtained impedance Z is investigated. For example, in a frequency region of 10-3 to 104 Hz in a Bode diagram on logarithm vs. a phase angle θ at a frequency (ω/2π), a frequency region at 2 decades or more and in the range of a phase angle absolute value of 20 to 70 deg. exists continuously. In addition, inside the frequency region, a region in which a data point at [the logarithm of a logarithm vs. an impedance Z at the frequency (ω/2π)] can be approximated by a straight line (±0.15) at an inclination of -1/2 exists continuously for 1 decade or more. At this time, it is determined that the acceleration of a corrosion speed is generated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for judging the corrosion resistance of a zirconium alloy used in a nuclear reactor, and more particularly to an evaluation method effective for judging the acceleration of uniform corrosion.

[0002]

2. Description of the Related Art At present, zirconium alloys are widely used as fuel cladding tubes and core structural materials in boiling water reactors, pressurized water reactors and the like.

The most commonly used zirconium alloy so far is zircaloy-2 (Sn: 1.2 to 1.7).
Wt%, Fe: 0.07 to 0.20 wt%, Cr: 0.0
5 to 0.16% by weight, Ni: 0.03 to 0.08% by weight,
The balance: Zr) and Zircaloy-4 (Sn: 1.2)
To 1.7% by weight, Fe: 0.18 to 0.24% by weight, C
r: 0.07 to 0.13% by weight, balance: Zr)
In addition, Zr-2.5% Nb, Zr-1% Nb alloy and the like are also applied to the nuclear reactor.

[0004] The above-mentioned alloys are alloys developed mainly in consideration of neutron economy and corrosion resistance. However, in a boiling water reactor, lens-shaped local corrosion called nodular corrosion occurs on the surface of the material during the operation of the reactor,
That was a problem.

[0005] Nodular corrosion grows with neutron irradiation, and may become exfoliated when the corrosion layer becomes thicker. Accordingly, the occurrence of nodular corrosion not only causes the structural material to lose its thickness, but also increases the radiation concentration in the coolant due to the exfoliation of the corroded layer, and may increase the radiation dose during the periodic inspection.

An α + β phase for preventing nodular corrosion,
Alternatively, a heat treatment method of heating to a β-phase temperature range for a short time and quenching (Japanese Patent Publication Nos. 61-45699 and 63-5822).
No. 3) and a change in alloy composition (Sekisui 60-434)
No. 50, No. 62-228442).

[0007] With the above-mentioned improvement, the occurrence of nodular corrosion is suppressed, and the form of corrosion is changing to uniform corrosion in which an oxide film as a corrosion product grows uniformly.

[0008] The above-mentioned materials having improved nodular corrosion resistance sufficiently fulfill their functions under the operating conditions of actual furnaces. However, at present, high burnup of fuel is planned to improve the economic efficiency of nuclear power plants.

When the residence time of the reactor fuel assembly in the reactor is prolonged, the amount of corrosion increases and the environment becomes more severe for the material. By way of example, there is concern that the corrosion rate may be low for an initial period of time, but increase rapidly with high burnup.

As described above, when the corrosion rate is accelerated,
Since the amount of corrosion increases at an accelerated rate compared to before the acceleration of the corrosion rate, not only does the thickness of the structural material decrease as in the case of nodular corrosion, but also the radiation concentration in the coolant increases due to the peeling of the corrosion layer. In addition, there is a possibility that the radiation dose at the time of the periodic inspection will increase.

[0011] In addition, the acceleration of the corrosion rate increases the amount of hydrogen generated by the oxidation reaction, and it is considered that the hydrogen permeability increases due to the destruction of the fine and stable oxide film, so that the zirconium alloy substrate absorbs the hydrogen. Hydrogen absorption increases,
Structural materials may be embrittled.

[0012] From the above, it is important to know whether or not the acceleration of the corrosion rate has occurred, and a corrosion resistance evaluation method for determining the acceleration of the corrosion rate is desired.

Until now, the judgment of the occurrence of the acceleration of the corrosion rate has been made as follows.
Judgment was made based on changes in the amount of corrosion with respect to the amount of oxidation and time. That is, the weight increase due to corrosion or the thickness of the oxide film was measured and compared between samples, or the measured corrosion amount was determined based on the increase in the amount of change with time.

[0014]

However, since the uniform corrosion of the zirconium alloy proceeds through the movement of oxygen ions and electrons in the formed oxide film, the property of the oxide film serving as a barrier to such movement is considered. It is considered that the change in the corrosion rate causes an increase in the corrosion rate.

As described above, since the acceleration of the corrosion rate largely depends on the properties of the oxide film, it may occur even when the amount of corrosion is relatively small or when the amount of change in the amount of corrosion is small.

When used in an actual nuclear reactor, it is difficult to monitor the amount of corrosion at any time. In practice, measurement is performed at every irradiation cycle of about one year. Therefore, if the acceleration of the corrosion rate occurs immediately before the measurement, there is almost no change in the amount of corrosion, and it is impossible to determine the acceleration of the corrosion rate by a method based on the amount of corrosion so far.

As described above, for the zirconium alloy,
There is a demand for an evaluation method for more accurately determining whether or not the corrosion rate has accelerated.

An object of the present invention is to provide a method for judging the corrosion resistance of a zirconium alloy, which more accurately judges whether or not the corrosion rate has accelerated.

[0019]

The gist of the present invention to achieve the above object is as follows.

[1] An AC current ΔI output by applying an AC voltage ΔE to the oxide film formed on the surface of the zirconium alloy is measured, and the corrosion resistance is determined by the frequency dependence of the impedance Z based on Z = ΔE / ΔI. And a method for determining the corrosion resistance of a zirconium alloy.

[2] The frequency dependence of the impedance Z is expressed in a frequency range of 10 to ³ Hz to 10 ⁴ Hz in a Bode diagram (logarithm of frequency (ω / 2π) vs. phase angle θ).
Continuously more than 2 decades, the absolute value of the phase angle is 20 to
There is a frequency range of 70 degrees, and the Bode diagram (logarithm of frequency (ω / 2π) v
s A region where the data point in the logarithm of the impedance Z can be approximated by a straight line having a slope of -1/2 (± 0.15) exists continuously for one or more decades, or a Bode diagram (frequency (ω / 2π) The data points in the frequency range of 10 to ³ Hz to 10 ⁴ Hz in the logarithm of logarithm vs. the logarithm of impedance Z are plotted on a straight line (± 0.
15), a region that can be approximated by 15) exists continuously for one or more decades, and a data point of a Nyquist diagram (real part Z ′ of impedance Z vs. imaginary part Z ″ of impedance Z) has a slope of 1 in that frequency region. The method for judging the corrosion resistance of a zirconium alloy according to the above, wherein it is judged that the corrosion rate has accelerated when the straight line of (± 0.2) can be approximated.

That is, according to the present invention, an AC voltage ΔE is applied to a zirconium alloy oxide film, the output AC current ΔI is measured, and the frequency dependence of the impedance Z evaluated from Z = ΔE / ΔI is examined (hereinafter referred to as “Z = ΔE / ΔI”). , AC impedance measurement method) to determine whether or not the corrosion rate is accelerated.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for judging the occurrence of acceleration of the corrosion rate by using the AC impedance measurement method will be described below.

It is known that the corrosion rate is accelerated in a corrosion test using an autoclave as in the case where a zirconium alloy is used in an actual nuclear reactor. An example is shown in FIG.

FIG. 4 shows that Zircaloy-2 was heated at 400.degree.
The change with time of the increase in corrosion (weight increase: mg / dm ² ) when corroded in steam at 5 atm for about 8000 hours is shown.

FIG. 4 shows that the slope of the approximate straight line changes between 800 hours and 1000 hours, indicating that the corrosion rate is accelerated.

Among the test pieces after the corrosion test, a test piece in which the corrosion rate was not accelerated (corrosion test time: about 300 hours later) and a test piece in which the corrosion rate was accelerated (corrosion test) (Test time: after about 1300 hours), the impedance was measured, and the difference in impedance before and after the acceleration of the corrosion rate was examined.

In an AC impedance measurement in which an AC voltage ΔE is applied, an output AC current ΔI is measured, and a frequency dependence of an impedance Z evaluated by Z = ΔE / ΔI is measured, an AC voltage ΔE and an AC current ΔI Are represented by complex numbers, respectively.

[0029]

ΔE = | ΔE | exp (jωt) ΔI = | ΔI | exp [j (ωt−θ)] The impedance Z is

[0030]

Z = ΔE / ΔI = (| ΔE | / | ΔI |) exp (jθ) = Z (cosθ + jsinθ) = Zcosθ + jZsinθ = Z ′ + Z ″

Usually, the analysis of the impedance measurement results
The Bode diagram 1 (frequency (ω / 2
π) vs. logarithm of impedance Z), Bode plot 2 (logarithm of frequency (ω / 2π) vs. phase angle θ), and Nyquist plot (real part Z ′ v of impedance Z)
s The imaginary part Z '') of the impedance Z is used.

FIG. 1 shows the results of impedance measurement of two types of test pieces, one in which the corrosion rate was not accelerated and the other in which the corrosion rate was accelerated.

FIG. 1 is a Bode diagram 1, FIG. 2 is a Bode diagram 2, and FIG. 3 is a Nyquist diagram. In each figure, the closed circles indicate the results before the corrosion rate was accelerated, and the open triangles indicate the results after the corrosion rate was accelerated.

From FIGS. 1 to 3, the frequency dependence of the impedance was clearly different before and after the acceleration of the corrosion rate.
Therefore, for the zirconium alloy oxide film, the AC voltage ΔE
Is applied, the output alternating current ΔI is measured, and Z = Δ
It was found that by examining the frequency dependence of the impedance Z evaluated from E / ΔI (hereinafter referred to as an AC impedance measurement method), it was possible to determine whether or not the corrosion rate had accelerated.

In order to further narrow down the judging conditions, those having different corrosion test conditions (for example, 300 ° C. water corrosion test, 40 ° C.
Impedance measurement was performed on various zirconium alloys having different oxide film thicknesses (up to 60 μm) and compositions having different oxide film thicknesses (0 ° C. or 530 ° C. corrosion test: actual furnace irradiation).

The results are summarized. In the impedance response after the acceleration of the corrosion rate, at least one of the following two characteristics that were not commonly observed before the acceleration of the corrosion rate was observed. .

In the Bode diagram (logarithm of frequency (ω / 2π) vs. phase angle), the absolute value of the phase angle is 20 to 70 degrees at 2 decade or more continuously in the frequency range of 10 to ³ Hz to 10 ⁴ Hz. And a Bode diagram (frequency (ω
/ 2π) vs. the logarithm of impedance Z), there is a continuous range of ldecade or more where data points can be approximated by a straight line with a slope of -1/2 (± 0.15).

In the Bode diagram (logarithm of frequency (ω / 2π) vs. logarithm of impedance Z), 10 to ³ Hz
In the frequency domain of 〜１０10 ⁴ Hz, the area where the data point can be approximated by a straight line with a slope of −−１ (± 0.15) is continuously
and the data points of the Nyquist diagram (real part Z ′ of impedance Z vs. imaginary part Z ″ of impedance Z) have a slope of 1/2 (±
0.2) can be approximated by a straight line.

Accordingly, the results of measuring the frequency dependence of the impedance Z are shown in the Bode diagram (frequency (ω / 2
In logarithmic vs phase angle θ) of π), 10 ~ ³ Hz~10
^In a frequency range of ⁴ Hz, there is a frequency range in which the absolute value of the phase angle is in the range of 20 to 70 degrees at least 2 decades continuously, and a Bode diagram (frequency (ω / 2π ) Vs. the logarithm of impedance Z), the data point is a straight line with a slope of -1/2 (± 0.
An area that can be approximated by 15) exists continuously for one or more decades.

Bode diagram (logarithm of frequency (ω / 2π) v
In logarithm) of s impedance Z, 10 ~ ³ Hz~1
0 ⁴ Hz of continuous area which can be approximated by a straight line of -1/2 data points the slope in the frequency domain (± 0.15) ldeca
data points in the Nyquist diagram (real part Z 'of impedance Z vs. imaginary part Z''of impedance Z) which can be approximated by a straight line having a slope 1 (Soil 0.2) .

When at least one of the above two is recognized, it can be determined that the corrosion rate of the zirconium alloy has accelerated.

Next, the AC impedance measuring method according to the present invention is a method generally used for analyzing an electrode reaction such as a corrosion reaction, and the measuring apparatus will be described.

A test piece made of a zirconium alloy having an oxide film formed on its surface is used for the working electrode, and a specific solution is used for the electrolytic solution.

A measuring cell is filled with an electrolytic solution, and a working electrode,
A measuring section for immersing the reference electrode and the counter electrode, applying an AC voltage ΔE between the working electrode and the counter electrode, and measuring the response current ΔI, controlling the measurement, and impedance Z expressed by Z = ΔE / ΔI A measurement device having a control recording unit for calculating and recording the value is used.

In this embodiment, 0.05 mol of an electrolyte is used.
Using a 1 / l sodium sulfate solution, degassing was performed by passing a gas such as Ar. Degassing continued during the measurement. In addition to the sodium sulfate solution, an acid solution such as 0.1 N sulfuric acid or hydrochloric acid, or an alkaline solution such as NaOH was measured. However, the measurement was essentially the same as that using a 0.05 mol / l sodium sulfate solution. There was no difference between them, and it was confirmed that it was effective for judging the occurrence of accelerated uniform corrosion.

A test piece was prepared as follows and attached to a measurement cell. The fuel cladding tube was cut into a length of 20 mm and a half length, and a portion of the oxide film other than the measurement portion was removed by mechanical polishing for conduction with the measurement portion, and a lead wire was attached. This test piece was attached by pressing against a measurement cell with a window so that only the surface of the oxide film to be measured was in contact with the electrolyte.

After attaching the test piece, the measuring cell
Left for hours. This takes into account the time required for the electrolyte to enter pores and cracks in the oxide film and reach a steady state.

After standing for 12 hours, the potential of spontaneous immersion was measured for 30 minutes, and thereafter, an AC voltage was applied with reference to this potential to measure the AC impedance.

The applied AC voltage at this time was a very small voltage of 20 to 100 mV in order to avoid an unsteady state due to DC polarization. All measurements were performed at room temperature.

As the samples measured, ten kinds of alloys having different compositions as shown in Table 1 were used. These were irradiated for 4 cycles in an actual nuclear reactor, and showed an increase in weight at this time.

[0051]

[Table 1]

The presence or absence of the acceleration of the corrosion rate was distinguished from the difference in the frequency dependence of the impedance. This result was shown as breakaway. Those in which the corrosion rate was accelerated were indicated by white circles, and those in which they did not occur were indicated by x.

5 to 7 show the measurement results of alloys 8 and 10 having the closest corrosion increase (weight increase) among them.

FIG. 5 is a Bode diagram 1, FIG. 6 is a Bode diagram 2, and FIG. 7 is a Nyquist diagram. 5 to 7, solid circles indicate alloy 10 and open triangles indicate alloy 8.

When the two types of measurement results are distinguished by the presence or absence of the corrosion rate from the difference in the frequency dependence of the impedance, the alloy 8 accelerates the corrosion rate and the alloy 10
It was found that no corrosion rate acceleration occurred.

As a result, there was no significant difference in weight increase, and it was difficult to determine whether or not the corrosion rate was accelerated.
By examining the frequency dependence of the impedance of the alloy 10 and the alloy 10, the presence / absence of breakaway could be easily examined.

Further, as shown in FIG. 3, the presence or absence of acceleration of the corrosion rate could be determined without monitoring the temporal change of the corrosion amount.

[0058]

According to the method for determining corrosion resistance of the present invention, it is possible to more accurately determine whether or not breakaway of uniform corrosion has occurred in a zirconium alloy.

Further, since the usage limit of the zirconium alloy material can be more accurately grasped, it is economical and safe.

[Brief description of the drawings]

FIG. 1 is a graph showing the frequency dependence of impedance before and after the occurrence of accelerated corrosion rate in a corrosion test of zircaloy-2 in steam at 400 ° C. and 105 atm.

FIG. 2 is a graph showing the frequency dependence of the phase angle before and after the occurrence of accelerated corrosion rate in a corrosion test of zircaloy-2 in steam at 400 ° C. and 105 atm.

FIG. 3 is a Nyquist diagram before and after the occurrence of an accelerated corrosion rate in a corrosion test of zircaloy-2 in steam at 400 ° C. and 105 atm.

FIG. 4 is a graph showing the time-dependent change in weight increase in a corrosion test of zircaloy-2 in steam at 400 ° C. and 100 atm.

FIG. 5 is a graph showing the frequency dependence of the impedance of two alloys irradiated for four cycles in one example of the results of the example of the present invention.

FIG. 6 is a graph showing the frequency dependence of the phase angle of two alloys irradiated for four cycles in an example of the results of the example of the present invention.

FIG. 7 is a Nyquist diagram of two alloys irradiated for 4 cycles in an example of the results of the example of the present invention.

Continuing from the front page (72) Inventor Shuichi Nanagawa 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Power & Electricity Development Division, Hitachi, Ltd. (72) Inventor Yoshinori Eito 7, Omika-cho, Hitachi City, Ibaraki Prefecture No. 2 F-term (Reference) in Power & Electricity Development Division, Hitachi, Ltd. 2G060 AA10 AD04 AE28 AE40 AF03 AF06 EA08 2G075 AA03 BA17 CA38 DA02 DA14 EA01 FA01 FB10 FB16 FC12 FD02 GA21 GA34

Claims (2)

[Claims] 1. An AC current Δ output by applying an AC voltage ΔE to an oxide film formed on a surface of a zirconium alloy.
I, and the impedance Z based on Z = ΔE / ΔI
A method for determining the corrosion resistance of a zirconium alloy, comprising determining the corrosion resistance based on the frequency dependence of the corrosion resistance. 2. The frequency dependence of the impedance Z is expressed in a frequency range of 10 to ³ Hz to 10 ⁴ Hz in a Bode diagram (logarithm of frequency (ω / 2π) vs. phase angle θ).
Continuously more than 2 decades, the absolute value of the phase angle is 20 to
There is a frequency range of 70 degrees, and the Bode diagram (logarithm of frequency (ω / 2π) v
s A region where the data point in the logarithm of the impedance Z can be approximated by a straight line having a slope of -1/2 (± 0.15) exists continuously for one or more decades, or a Bode diagram (frequency (ω / 2π) The data points in the frequency range of 10 to ³ Hz to 10 ⁴ Hz in the logarithm of logarithm vs. the logarithm of impedance Z are plotted on a straight line (± 0.
15), a region that can be approximated by 15) exists continuously for one or more decades, and the data point of the Nyquist diagram (real part Z ′ of impedance Z vs. imaginary part Z ″ of impedance Z) has a slope of 1 The method for determining the corrosion resistance of a zirconium alloy according to claim 1, wherein it is determined that acceleration of the corrosion rate has occurred when it can be approximated by a straight line of (± 0.2).