JPH11344597A - Decontamination method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a decontamination method having high crud dissolving power, capable of reducing corrosivity to a metallic material and being excellent in workability by dissolving/removing metallic oxide sticking in a nuclear reac tor cooling water system by using a reducing acid solution by setting a specific concentration oxalic acid solution containing hydrazine to a specific pH value. SOLUTION: A reducing acid solution (a reducing decontamination agent) is adjusted to pH 2 to 3 by adding hydrazine to a low concentration oxalic acid aqueous solution having the oxalic acid concentration of 0.05 to 0.3 wt.%. Crud dissolving power lacks when the oxalic acid concentration is less than 0.05 wt.%, and corrosivity increases when the concentration exceeds 0.3 wt.%. The crud dissolving power lacks when pH of the reducing decontamination agent is larger than 3, and the corrosivity increases when the pH is smaller than 2. A temperature when dissolving iron oxide by using such a reducing decontamination agent is suitably about 80 deg.C or more. Thus, since the oxalic acid concentration is low and hydrazine is contained as well, while the corrosivity is relieved, reduction in dissolving power can be compensated by the dissolution promoting action by increasing the pH.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a decontamination method for dissolving and removing metal oxides adhering in a reactor cooling water system.

[0002]

2. Description of the Related Art The surfaces of metallic materials such as equipment and piping of a primary cooling water system of a nuclear reactor are magnetite (Fe ₃ O ₄ ), hematite (Fe ₂ O ₃ ), nickel ferrite (NiFe ₂ O ₄ ).
Chromium ferrite containing some trivalent chromium (FeC)
r ₂ O ₄ ). And in these crystal lattices,
⁶⁰ Co, ⁵⁸ Co, ⁵⁴ radionuclides are accumulated incorporated such Mn, causing exposure amount increases in nuclear power plants workers. For this reason, so-called decontamination for removing the radioactive cladding is required. In particular, the development of a chemical decontamination technique for dissolving and removing the cladding using a chemical decontamination agent has been advanced, and several techniques have been put into practical use.

In these chemical decontamination techniques, it is of course important that the above-mentioned radioactive cladding can be efficiently dissolved and removed, but it is also extremely important that the base metal of the constituent material is not damaged by corrosion. Become. More recently, reduction of radioactive waste generated by decontamination has been emphasized, and development of excellent decontamination technology has been desired from the comprehensive viewpoint of decontamination effect, material integrity, and waste reduction.

In general, a reducing solution containing an organic acid such as citric acid or oxalic acid or a chelating agent such as ethylenediaminetetraacetic acid (EDTA) is effective for dissolving an iron-based metal oxide. In order to dissolve oxides containing chromium, a method of oxidizing and dissolving trivalent chromium to hexavalent chromium with a solution containing an oxidizing agent such as permanganate is effective. In practice, a method of repeatedly performing oxidation and dissolution is adopted.

[0005] In particular, as a reducing solubilizer, many decontaminants containing oxalic acid, which have the best dissolving power of iron oxides, have been proposed (JP-B-45-37360, JP-B-1-19473). Gazette, Japanese Patent Publication No. 1-53440, and JP-A-57-9885).

As the oxidative decontamination agent, a permanganate to which an alkali agent is added (Japanese Patent Publication No. 45-37360)
Japanese Patent Application Laid-Open No. HEI 5-53440) and an acid obtained by adding an acid such as nitric acid to a permanganate (Japanese Patent Publication No. 1-53440) has a strong oxidizing power and is effective for dissolving chromium.

On the other hand, from the viewpoint of reducing the amount of decontamination waste,
A method of capturing dissolved metal ions with an ion-exchange resin and reducing the volume to a small amount of solid waste is exclusively adopted.

[0008]

However, although oxalic acid has a very good dissolving power for iron-based oxides, oxalic acid is a metal material, particularly a sensitized stainless material (such as a weld heat affected zone).
There is a disadvantage that it is highly corrosive to water. For this reason, there is a need for measures to reduce the corrosiveness of oxalic acid, such as increasing the pH of the solution by adding an alkaline agent, or limiting the concentration of the decontaminant, the decontamination temperature and the decontamination time. , Solution pH
The rise in the temperature has the problem that the dissolving power of metal oxides is reduced at the same time as the corrosiveness is alleviated.The relaxation of the decontamination agent concentration, decontamination temperature and decontamination time also This is a negative factor.

Further, Japanese Patent Application Laid-Open No. 57-9885 discloses that
Hydrazine is added to an oxalic acid solution of about 5% as a means of not lowering the dissolving power of the oxide even when the pH is increased,
A method of adjusting the pH to 3 to 4.5 has been proposed,
A hydrazine oxalate solution having a pH of 3 or more has low solubility in water, so there is a problem in the on-site workability of preparing a high-concentration solution. In addition, since the use concentration of the drug is high, the cost of the drug increases and the amount of waste is increased. There is a disadvantage that it increases.

Furthermore, Japanese Patent Publication No. 1-19473 discloses that
Hydrazine is added to a mixed solution of malonic acid and oxalic acid of 1 to 10% in an amount of 500 to 5000 mg / L, and another alkali agent is added thereto to adjust the pH to 2.5 to 6 (preferably 3 to 10).
A composition adjusted to 4.5) has been proposed, but this composition also has a high use concentration of the drug, so that the cost of the drug increases and
There is a disadvantage that the amount of waste increases.

On the other hand, in the case of an oxidative decontamination agent, in a case where an alkali (eg, KOH) is added to a permanganate, ions derived from the alkali, eg, K ⁺ ions, impose a load on an ion exchange resin used for waste treatment. The disadvantage of increasing the amount of waste and increasing the amount of waste.
O ₃ ) is added to ions derived from an acid, for example, NO ₃
^{In addition to the} ions increasing the load on the ion exchange resin, acidic permanganate solutions have the disadvantage of being highly corrosive to various materials.

The present invention solves the above-mentioned conventional problems, has a high clad dissolving power, has low corrosiveness to metal materials, has good on-site workability, and has a small amount of decontamination waste. It is an object of the present invention to provide a decontamination method which is excellent in economical efficiency.

[0013]

The decontamination method of the present invention comprises a step of dissolving and removing metal oxides adhering in a reactor cooling water system with an oxalic acid solution. Is a reducing acid solution having an oxalic acid concentration of 0.05 to 0.3% by weight, containing hydrazine and having a pH of 2 to 3.

In the reducing acid solution according to the present invention, since the concentration of oxalic acid is low and hydrazine is contained, the corrosiveness on the material is remarkably reduced as compared with the solution containing only oxalic acid, but the pH is increased. It is possible to compensate for the decrease in the dissolving power of the metal oxide by the dissolution promoting action of hydrazine.

In the present invention, the metal oxide is dissolved with a permanganate solution before or after the step of dissolving and removing the metal oxide with a reducing acid solution (hereinafter sometimes referred to as “reduction decontamination”). It is preferable to perform an oxidative dissolution step of oxidizing and dissolving chromium in the oxide to hexavalent chromium (hereinafter, may be referred to as “oxidative decontamination”). In particular, the reduction dissolution step and the oxidation dissolution step are alternately performed at least. By repeating each step twice, an excellent decontamination effect can be obtained.

[0016]

Embodiments of the present invention will be described below in detail.

The reducing acid solution (hereinafter sometimes referred to as "reducing decontaminant") used in the present invention is prepared by, for example, adding hydrazine to a low-concentration aqueous solution of oxalic acid having an oxalic acid concentration of 0.05 to 0.3% by weight. To adjust the pH to 2-3.

If the oxalic acid concentration in the reductive decontamination agent is less than 0.05% by weight, the clad dissolving power is insufficient, and if it exceeds 0.3% by weight, the corrosiveness may be increased.

When the pH of the reducing and decontaminating agent is higher than 3, the dissolving power of the clad becomes insufficient, and when the pH is lower than 2, the corrosiveness increases.

The temperature at which the iron-based oxide is dissolved using such a reducing decontamination agent is preferably 80 ° C. or more, and in consideration of the decontamination effect and the workability of the heating operation, the temperature is 85 to 95 ° C.
Is optimal. Usually, one reduction decontamination step is performed at such a temperature for about 2 to 8 hours.

As the oxidative decontamination agent used in the oxidative decontamination step performed before or after the reduction decontamination step, it is preferable to use a neutral solution of permanganate. That is, conventionally, the ability to oxidize and dissolve trivalent chromium in chromium oxide to hexavalent chromium has been considered to be stronger when the liquid property is made acidic or alkaline, but the chromium content is relatively high. In the case of a small amount of oxide, for example, a clad in which chromium is concentrated in an oxide near the base material such as a BWR (boiling water reactor), a neutral aqueous solution of permanganate is sufficiently effective. Demonstrate. Then, by using a neutral aqueous solution of permanganate, the amount of waste is reduced by reducing the ion exchange load derived from excess acids and alkali components in the subsequent treatment of the decontamination waste liquid with the ion exchange resin. Can be very advantageous.

As the permanganate of the oxidative decontamination agent, neutral salts of alkali metals such as sodium, potassium and lithium salts of permanganate can be used. Also,
The use concentration of permanganate is preferably 0.01 to 0.1% by weight, particularly preferably 0.03 to 0.06% by weight.

The temperature of the oxidative decontamination treatment using such an oxidative decontamination agent is preferably 80 ° C. or higher, and is most preferably 85 to 95 ° C. in consideration of the decontamination effect and the workability of the heating operation. Usually, one oxidative decontamination step is performed at such a temperature for about 2 to 4 hours.

The procedure for applying the decontamination method of the present invention to actual decontamination of a reactor cooling water system is as follows.

When the target is a BWR plant, reductive decontamination is first performed, and then the decontaminant components contained in the reductive decontaminant and dissolved metal ions and radioactive ions are captured and purified by an ion exchange resin. After performing oxidative decontamination, a permanganate is decomposed by adding a reducing decontaminating agent containing hydrazine and oxalic acid, and then a second reduction decontamination is performed. If the removal of radioactive ions is not sufficient at this stage, the oxidative decontamination and reduction decontamination may be repeated. on the other hand,
In the case of a PWR (Pressurized Water Reactor) plant, since chromium oxide is distributed over all layers of the cladding, oxidative decontamination is performed first, and then performed in the order of reduction decontamination, oxidative decontamination, and reduction decontamination. Just do it.

In the treatment of the decontamination waste liquid, the oxalic acid and hydrazine components of the reducing decontamination agent can be decomposed by an existing method such as an oxidizing agent or UV treatment in order to reduce the load on the ion exchange resin. It is.

[0027]

The present invention will be described more specifically below with reference to examples and comparative examples.

Examples 1 to 3, Comparative Example 1 0.2 wt% oxalic acid aqueous solution (pH = 1.8) (Comparative Example 1), and hydrazine was added to adjust the pH to 2.0 (Example 1). Sensitized SUS30 prepared by subjecting 500 mL of the solution adjusted to 2.2 (Example 2) and 2.5 (Example 3) to heat treatment at 650 ° C. for 3 hours as a corrosion test piece.
The four materials and carbon steel (STPT42) were immersed two by two, and kept at 95 ° C. with stirring for 8 hours to conduct a corrosion test.

As a result, as shown in FIG. 1A, the corrosion of the sensitized SUS304 material was accelerated when the pH was not adjusted, whereas the corrosion was accelerated when the pH was adjusted to 2 to 2.5 with hydrazine. Was found to hardly progress. Further, as shown in FIG. 1 (b), in the case of carbon steel, it was clear that the corrosion rate decreased with increasing pH.

[0030] In Examples 4-6, the test loop of the hydrogen injection simulated environment in Comparative Example 2 BWR (using ⁵⁸ Co as a tracer), to produce a clad simulated film on SUS304 specimen surface, divided by the following decontamination conditions A dyeing test was performed.

[Decontamination conditions] First reduction decontamination: 0.2 wt% oxalic acid aqueous solution (p
H = 1.8) (Comparative Example 2), and hydrazine was added thereto to adjust the pH to 2.0 (Example 4), 2.5 (Example 5),
500 mL of each solution adjusted to 3.0 (Example 6)
Each of the test pieces with the simulated film was immersed in the
After the decontamination treatment for a time, it was washed with water. Oxidative decontamination: The test piece was transferred to a 0.05% by weight aqueous solution of potassium permanganate, decontaminated at 95 ° C. for 1 hour, and washed with water. The second reduction decontamination was performed under the same conditions as the first reduction decontamination.

After the test, the radiation dose equivalent ratio of each test piece was measured, and the ratio to the value before the test (decontamination coefficient = DF) was calculated.

As a result, as shown in FIG.
There is a peak of DF (decontamination effect) in the vicinity, and it was confirmed that by neutralizing a part of oxalic acid with hydrazine, the decontamination performance can be maintained or enhanced while mitigating the corrosive environment of the material.

Example 7 A decontamination test according to the method of the present invention was performed using pipe materials cut out from the recirculation system of an actual BWR plant as test pieces.

In the test, 300 mL of a 0.18% by weight oxalic acid aqueous solution adjusted to pH = 2.5 with hydrazine was placed in a 500 mL beaker, and the test piece was immersed and subjected to reduction decontamination treatment at 95 ° C. for 4 hours. (First reduction decontamination). afterwards,
The test piece was taken out and the radiation dose equivalent rate was measured to determine the decontamination effect (DF). Subsequently, the test piece was transferred to a 0.05% by weight aqueous solution of potassium permanganate, and subjected to oxidative decontamination treatment (first oxidative decontamination) at 95 ° C. for 2 hours. This operation was repeated to obtain the DF after each step.

As a result, as shown in Table 1, it was confirmed that a high DF of 17 was already obtained by the first reduction decontamination, and a higher DF was obtained by repeating oxidative decontamination and reduction decontamination. Was done.

From these results, according to the method of the present invention, the expected decontamination effect (D
Depending on F), the objective may be achieved only by one step of reductive decontamination, and it can be seen that a better decontamination effect can be obtained by combining with oxidative decontamination as required.

[0038]

[Table 1]

[0039]

As described above in detail, according to the decontamination method of the present invention, the metal oxide adhering in the reactor cooling water system can be efficiently used without causing corrosion damage to the base metal of the constituent material. It can be decontaminated by reduction.

In particular, according to the decontamination method of the present invention, a high decontamination effect can be obtained by alternately repeating reduction decontamination and oxidative decontamination.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of Examples 1 to 3 and Comparative Example 1.

FIG. 2 is a graph showing the results of Examples 4 to 6 and Comparative Example 2.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroo Yoshikawa 2-2-22 Kitahama, Chuo-ku, Osaka-shi, Osaka Inside Kurita Engineering Co., Ltd. (72) Inventor Makoto Nagase 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd.

Claims (3)

[Claims] 1. A decontamination method comprising a step of dissolving and removing metal oxides adhering in a reactor cooling water system with an oxalic acid solution, wherein the oxalic acid solution has an oxalic acid concentration of 0.05 to 0.1. A decontamination method comprising 3% by weight, a hydrazine-containing reducing acid solution having a pH of 2 to 3. 2. The method according to claim 1, wherein before or after a reduction dissolution step of dissolving and removing the metal oxide with a reducing acid solution,
A decontamination method comprising an oxidative dissolution step of oxidizing and dissolving chromium in the metal oxide into hexavalent chromium with a permanganate solution. 3. The decontamination method according to claim 2, wherein the reduction-dissolution step and the oxidation-dissolution step are alternately repeated at least twice for each step.