JPS6112239B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for treating nuclear power plant waste liquid, and more particularly to a method for preventing corrosion and scaling of a nuclear power plant waste liquid treatment device made of stainless steel. Radioactive liquid waste discharged at a nuclear power plant cannot be discharged outside the plant, so it is treated in a waste liquid treatment facility provided within the plant. Radioactive waste liquids treated in this facility include recycled waste liquid of ion exchange resins, floor drains, and equipment drains. These waste liquids contain sodium sulfate generated from the sodium hydroxide and sulfuric acid used to regenerate the ion exchange resin, iron oxides (Fe ₂ O ₃ , Fe ₃ O ₄ ) and Suppressing corrosion of iron hydroxide (Fe(OH) ₃ ) cladding, sodium chloride, calcium chloride, and magnesium chloride caused by contamination with seawater used as cooling water, calcium and magnesium compounds in floor drain water, or auxiliary cooling system equipment. Contains sodium nitrite and sodium chromate, which are used as agents. Therefore, in order to ensure safety, it is common practice to collect these waste liquids in a waste liquid concentrator, evaporate them with heated steam, concentrate and reduce the volume, solidify them, and then place them in drums for burial treatment. However, when concentrating the waste liquid, various problems arise as the concentration of the liquid increases. One problem is that as concentration progresses, the concentration of chlorine ions and sulfate ions, which are corrosion factors, increases, causing pitting corrosion, crevice corrosion, and stress corrosion cracking in the stainless steel that makes up the concentrator. This is a problem where the amount of energy decreases. Two of these are scales (CaSO ₄ , CaCO ₃ and Mg(OH) ₂ , etc.), which are hardly water-soluble precipitates, and iron oxides and The problem is that hydroxide gets mixed in and sticks. When scale adheres, the thermal efficiency of the concentrator decreases, and gaps are formed between the scale and the tube wall of the concentrator, and the concentration of chlorine ions further promotes crevice corrosion. To address this problem, chromate-based anticorrosive agents have traditionally been added to wastewater to prevent corrosion, but chromate actually accelerates corrosion of stainless steel when chlorine and sulfate ions are present in large quantities. do. In addition, polymerized phosphate-based corrosion inhibitors react with calcium and magnesium in the waste liquid to produce calcium and magnesium orthophosphates that are sparingly soluble in water.
This not only adheres to the can wall and forms scale, reducing the thermal efficiency of the concentrator, but also has no anti-corrosion effect on the stainless steel materials that constitute the concentrator. Recently, phosphonic acids and aminophosphonic acids have been developed as organophosphate corrosion inhibitors that have properties similar to polyphosphates and can be used as corrosion inhibitors for iron-based materials in highly concentrated circulating cooling water systems. However, these anticorrosive agents have almost no anticorrosive effect on stainless steel, although they have an anticorrosive effect on iron. Furthermore, as a scale adhesion prevention agent, polymaleic acid has been developed for removing calcium sulfate and magnesium hydroxide scales that precipitate in the high-temperature brine section of seawater desalination equipment. However, although polymaleic acid has a scale-preventing effect, it does not have a corrosion-preventing effect on stainless steel for waste liquid concentrators. Therefore, conventional corrosion inhibitors or scale inhibitors do not have a sufficient anticorrosive effect on stainless steel materials used in nuclear power plant waste liquid treatment equipment. For this reason, it has not been possible to completely prevent pitting corrosion, crevice corrosion and stress corrosion cracking of stainless steel. An object of the present invention is to provide a method for treating nuclear plant waste liquid that can prevent corrosion of a nuclear power plant waste liquid treatment apparatus and precipitation and fixation of scale within the apparatus. The present inventors have developed a nuclear power plant waste liquid treatment device,
In particular, as a result of searching for chemical substances that have an anticorrosion effect on stainless steel materials used in concentrators, we discovered that orthophosphate is extremely effective. In particular, when orthophosphate is used in combination with one or more components selected from organic compounds having a carboxyl group, 8-hydroxyquinoline, phosphonic acid, or water-soluble salts thereof, it has a corrosive effect on stainless steel materials and also causes fixed scale. The present invention has been achieved based on the discovery that it has a preventive action. In the present invention, the orthophosphate added to the nuclear power plant wastewater is M/PO ₄ (M: allurimetal).
These are orthophosphates with a molar ratio of 1 to 3, that is, tertiary phosphate, secondary phosphate, and primary phosphate. Among these orthophosphates, dibasic sodium phosphate is particularly effective in preventing corrosion of stainless steel materials. The anticorrosion effect of orthophosphate on stainless steel materials is shown through test examples. The anode polarization curve of a commercially available SUS316L steel material in a simulated waste liquid concentrate containing the components shown in Table 1 was measured.

[Table] 105℃ with chloride ion concentration of 0 to 10000ppm,
As shown in Figure 1, the anodic polarization curve of SUS316L in a simulated concentrated solution of pH 7.0 shows that as the Cl ^- concentration increases, the anodic current increases on the base potential side; It can be seen that the corrosion resistance decreases as the Cl ^- concentration increases. Second
The figure shows the anode polarization curve of SUS316L when 0 to 10,000 ppm of PO ₄ ^3- (Na ₂ HPO ₄ ) is added to the waste liquid simulated concentrate shown in Table 1 containing 1,000 ppm of Cl ^- .
As is clear from the figure, the potential at which the anode current increases rapidly shifts to the noble potential side as the amount of PO ₄ ^3- added increases.
It can be seen that the corrosion resistance of SUS316L improves as the amount of PO ₄ ^3- added increases. Next, the pitting potential, which is often used to compare the degree of corrosion, was calculated from the anode polarization curve. The pitting corrosion potential was a potential corresponding to an anode current density of 200 μA/cm ² used by the Stainless Steel Local Corrosion Subcommittee of the Corrosion Prevention Association. Therefore, when the pitting corrosion potential is determined from the anode polarization curves shown in FIGS. 1 and 2 and plotted against the PO ₄ ^3- concentration, the result is as shown in FIG. 3. Third
From the figure, the pitting potential without Cl ^- , i.e.
To create a potential that does not cause corrosion on SUS316L,
When containing 100 ppm of Cl ^- , it is necessary to add 100 ppm or more of PO ₄ ^3- . In other cases PO ₄ ^3-
It can be seen that it is necessary to add 1000 ppm or more. As described above, it can be seen that the corrosion resistance of SUS316L in a waste liquid simulated concentrate containing Cl ^- is significantly improved by adding PO ₄ ^3- . Incidentally, if you calculate the anodic polarization curve using a polymerized phosphate such as tripolyphosphate in the same manner as above,
As shown in Fig. 4, the anodic polarization behavior of SUS316L remains almost the same even if the amount of tripolyphosphate added increases. Therefore, it can be seen that polymerized phosphates have no anticorrosion effect on stainless steel materials. In particular, in the present invention, orthophosphate and a component selected from organic compounds having a carboxyl group, 8-hydroxyquinoline, and phosphonic acid or water-soluble salts thereof are added together to the nuclear power plant wastewater. Examples of organic compounds having a carboxyl group include carboxylic acids such as succinic acid, malonic acid, maleic acid, and sikolic acid; oxycarboxylic acids such as gluconic acid, citric acid, tartaric acid, and malic acid; ethylendiaminetetraacetic acid (EDTA); Mention may be made of aminopolycarboxylic acids such as nitrotriacetic acid (NTA). Further, water-soluble salts thereof such as sodium salts, potassium salts, ammonium salts, etc. can also be used. Examples of the phosphonic acid include nitrilotrimethylphosphonic acid and aminotrimethylphosphonic acid, and water-soluble salts thereof, such as sodium salts, potassium salts, and ammonium salts, can also be used. When orthophosphate is added alone to nuclear plant wastewater, it reacts with calcium and magnesium in the wastewater to produce calcium phosphate and magnesium phosphate, which prevents corrosion of stainless steel materials, but completely prevents the precipitation of fixed scale. cannot be prevented. However, when orthophosphate is used in combination with an organic compound having a carboxyl group, 8-hydroxyquinoline, phosphonic acid, or a water-soluble salt thereof, the drawbacks of orthophosphate, which is originally a precipitated film-forming anticorrosive agent, are overcome. At the same time, it is possible to prevent the precipitation of fixed scale, and the anticorrosion effect of orthophosphate is not impaired. Figures 5, 6, 7 and 8 are
Citric acid, maleic acid, succinic acid and EDTA-2Na were added alone to the simulated concentrated waste liquid shown in the table.
Or the anodic polarization curve when added in combination with sodium phosphate is shown. According to these figures, citric acid, maleic acid, succinic acid and
50ppm of EDTA-2Na alone or PO ₄ ^3-
When combined with 100 ppm, the anode current increases rapidly on the base potential side. However, citric acid,
Maleic acid, succinic acid and EDTA-2Na
When 50ppm or 1000ppm is combined with 1000ppm of PO ₄ ^3- , the increase in anode current shifts to the noble potential side. Furthermore, when plotting the relationship between citric acid, maleic acid, succinic acid, and EDTA concentrations and pitting corrosion potential, the results are as shown in Figures 9, 10, 11, and 12, and the pitting potential when PO ₄ ^3- is not added is: 0.4V
(vs, SCE), but when 1000 ppm of PO ₄ ^3- is added, the pitting potential becomes significantly nobler to 0.8V (vs, SCE), indicating that corrosion resistance is improved. Since the above pitting corrosion potential is the same as when the waste liquid does not contain Cl ^- , by adding an organic compound with a carboxyl group together with orthophosphate, SUS316L can prevent corrosion due to Cl ^- attack. It can be seen that the corrosion resistance is improved without any damage. Figures 13 and 14 show the results when oxine (8-hydroxyquinoline) and phosphonic acid (aminotrimethylphosphonic acid) were added alone or in combination with sodium phosphate to the simulated concentrated waste liquid shown in Table 1. The anodic polarization curve is shown. According to these figures, oxine and phosphonic acid exhibit almost the same behavior as organic compounds having a carboxyl group, indicating the anticorrosive effect of their combined addition with orthophosphate. In addition, when the relationship between the concentrations of oxine and phosphonic acid and the pitting corrosion potential is plotted, Figures 15 and 16 show
As shown in the figure, the pitting potential is about 0.4V when PO ₄ ^3- is not added, but when 1000 ppm of PO ₄ ^3- is added, the pitting potential becomes significantly nobler to 0.8V, improving corrosion resistance. I understand that. In the present invention, the amount of orthophosphate added to the nuclear power plant wastewater is determined by the amount of Cl ^- contained in the wastewater.
It depends on the concentration, but around 50ppm is sufficient.
There is no harm in adding too much. Furthermore, the amount of carboxyl group-containing organic compounds, oxins, and phosphonic acids added to nuclear plant wastewater varies depending on the amount of calcium, magnesium, iron compounds, etc. contained in the wastewater, but around 1000 ppm is sufficient. Yes, and there is no harm in adding too much. Example 1 Ca ²⁺ , which is a scale forming component shown in Table 2,
Contains Mg ²⁺ , Fe ₃ O ₄ , Fe ₂ O ₃ , Fe(OH) ₃ and Cl ^-
pH7.0, 15% Na ₂ SO ₄ SUS316 in waste liquid simulated concentrate
We investigated the scale adhesion and corrosion occurrence after a manufactured sheath heater was inserted and left under boiling conditions for 100 hours. The results are shown in Table 3.

【table】

【table】

[Table] As shown in Table 3, in the case of the conventional method, CaSO ₄ ,
Fixed scale consisting of a mixture of CaCO ₃ , Mg(OH) ₂ , Fe ₃ O ₄ , and Fe ₂ O ₃ was thickly adhered, and there was particularly a large amount of fixed scale at the gas-liquid boiling interface. Pitting corrosion also occurred. On the other hand, with the method according to the present invention, no corrosion was observed and the amount of deposited scale was small. Especially maleic acid, succinic acid,
In the case of EDTA-2Na, the amount of fixed scale precipitated was small. Example 2 The amount of scale deposited and the state of corrosion were investigated in the same manner as in Example 1, except that oxine and phosphonic acid were used instead of the organic compound having a carboxyl group in Example 1. The results are shown in Table 4.

[Table] Table 4 shows that the combination of oxine or phosphonic acid with orthophosphate has the effect of preventing corrosion and fixed scale. Example 3 50 ppm of maleic acid and PO ₄ ^3- (Na ₂ HPO ₄ ) were added to a SUS316L concentrator that condenses and reduces the volume of ion exchange resin regeneration waste liquid and floor drain waste liquid from A nuclear power plant.
We added 1000ppm and conducted actual tests for about one year. As a result, almost no scale adhesion was observed, and there was no occurrence of corrosion and the product was sound. In comparison, pitting corrosion, crevice corrosion, and stress corrosion cracking in welds occurred in the concentrator used in the conventional method, which did not contain any additives, after about six months. As described above, according to the present invention, it is possible to prevent corrosion and scaling of a waste liquid treatment device of a nuclear power plant. In particular, in nuclear power plants, scale is also radioactive, so being able to prevent scale adhesion has a great effect on reducing the radiation exposure of workers.

[Brief explanation of the drawing]

Figure 1 shows changes in the amount of Cl ^- added to the waste liquid.
Figure 2 shows the anode polarization curve of SUS316L. Figure 2 shows the anode polarization curve of SUS316L depending on the amount of PO ₄ ³ added in the waste liquid. Figure 3 shows the anode polarization curve of SUS316L.
Diagram showing the relationship between pitting potential and PO ₄ ^3- concentration, 4th
The figure shows the anodic polarization curve of SUS316L depending on the amount _of tripolyphosphate added in ^the waste liquid.
A diagram showing the anode polarization curve of SUS316L, Figure 6 shows SUS316L in maleic acid and PO ₄ ^3- added waste liquid.
Figure 7 shows the anodic polarization curve of SUS316L in succinic acid and PO ₄ ^-added waste liquid, Figure 8 shows the anodic polarization curve of SUS316L in succinic acid and PO 4
Figure 9 shows the anodic polarization curve of SUS316L in PO ₄ ^3- added waste liquid, Figure 9 shows the relationship between pitting potential of SUS316L and citric acid concentration, Figure 10 shows the
Figure 11 is a diagram showing the relationship between the pitting potential of SUS316L and maleic acid concentration. Figure 12 is a diagram showing the relationship between the pitting potential and succinic acid concentration of SUS316L.
Figure 13 shows the relationship between the pitting corrosion level of SUS316L and _EDTA - ^2Na concentration.
Figure 4 shows the relationship between phosphonic acid and PO ₄ ^3- added waste liquid.
Diagram showing the anode polarization curve of SUS316L, No. 15
The figure shows the relationship between the pitting corrosion potential of SUS316L and the oxine concentration, and FIG. 16 shows the relationship between the pitting corrosion potential of SUS316L and the phosphonic acid concentration.

Claims (1)

[Claims] 1 For a nuclear power plant supplied in a device in which at least a portion of the inner wall surface is formed of stainless steel, the molar ratio of M/PO ₄ (M: alkali metal) is 1 to 1.
A nuclear power plant characterized in that orthophosphoric acid of No. 3 is added together with one or more components selected from an organic compound having a carboxyl group, 8-hydroxyquinoline, phosphonic acid, or a water-soluble salt thereof. How to treat waste liquid.