RU2169957C2 - Method for decontaminating water-cooled reactor circuits - Google Patents

Abstract

FIELD: nuclear power engineering; low-waste decontamination of power-generating equipment. SUBSTANCE: reactor circuits are treated with solution of nitric acid and nitric-acid aluminum at 80- 100 C for 2-5 h. Upon passing spent solution through ion-exchange filters sodium (potassium) nitrite is introduced in system its concentration being 1-10 mg/kg. Such treatment provides for suppressing corrosion process for long period. Proposed method makes it possible to use low-quality water and to reduce protective concentration of sodium (potassium) nitrite when equipment is given preservation treatment. EFFECT: enhanced decontamination factor and shielding properties of oxide films formed in the process. 4 tbl

Description

 The invention relates to nuclear energy, and in particular to methods of low-waste decontamination of power equipment, and can be used to remove radioactive contaminants from the internal surfaces of the circuits of nuclear power plants, for example, multiple forced circulation circuits of high-power channel reactors (KMPTs RBMK), first water-water circuits power reactors (VVER), prevent secondary radioactive contamination of these circuits, and also be used as supplemented I chemical decontamination on its last stage. The proposed method can also be used to passivate power equipment made of pearlite class steels, for example, the condensate-feed path of nuclear power plants with RBMK, secondary WWER circuits, and contour equipment of thermal power plants.

 In modern nuclear energy, organic acids and complexones are widely used for decontamination purposes. To increase the effectiveness of decontamination, activating additives — fluorides, hydrogen peroxide, hydrazine, etc., are introduced into solutions of acids and complexones [1]. The disadvantages of these methods are high concentrations of chemical reagents, as a result of which after deactivation a large amount of waste is generated, requiring further processing; high corrosiveness of solutions, as a result of which equipment elements made of pearlitic steels are destroyed; multi-stage processing; chemical activation of metal surfaces, as a result of which steel become prone to corrosion damage; increased sorption of radionuclides on activated surfaces.

Also known methods of low-reactive decontamination, described later in the text. Unlike traditional “chemical” decontamination, which produces a large amount of radioactive waste, low-reactive decontamination is less effective in terms of removing radioactive corrosion products in one wash. However, low reagent washing has several advantages over traditional "chemical" decontamination. These include: non-waste (the ability to remove radionuclides from des. Solutions on the ion-exchange filters of the installation itself); less corrosion hazard; takes less time; does not require preliminary preparation and can be carried out at each shutdown of the reactor. A known method of removing radioactive corrosion products [2, 3], used when shutting down the reactor type "CANDU", which sequentially uses the following cycle of operations: temperature cycle - lowering and raising the temperature of the coolant; hydrodynamic cycle - a change in the operating mode of equipment that changes the speed and flow rate of the coolant; redox cycle - alternation of the reduction mode with an excess of H ₂ and the oxidative mode with the addition of O ₂ to a concentration of 50-100 mg / kg. As the authors note, this method shows good results only for surfaces made of Monel alloy. Surfaces made from other materials do not wash well. The first contours of domestic NPPs from the Monel alloy are not manufactured and therefore this method does not find application. In addition, the disadvantages of this method include a complex system of operations for creating cycles, introducing gases into the circuit, the need for preliminary training of personnel. A known method of removing radioactive corrosion products from internal surfaces according to the following mode: reduce the pressure and temperature to atmospheric and 60 ^o C with continuous circulation of the coolant to reduce the concentration of H ₂ in the circuit less than 4 mg / kg, then increase the pressure to several atmospheres and add to the coolant an oxygen-containing solution having an O ₂ content greater than water, and then the pressure is reduced and the coolant is supplied to the treatment plants in order to remove corrosion products, and the cleaned coolant l return to the circuit [4]. This method has the following disadvantages: the concentration of oxygen in water is greater than its solubility at atmospheric pressure, to prevent gas evolution, the pressure in the circuit is increased to several atmospheres, with increasing temperature, the solubility of oxygen in water decreases, and almost all of the oxygen introduced is concentrated in the vapor space of the drum. separators; boiling channel reactors have a developed system of water and steam communications, which leads to uneven distribution of oxygen concentration in the circuit; additional equipment is needed to dispense.

The closest to the proposed invention in technical essence and the achieved results is a method of deactivating the contours of water-cooled reactors by treating with a solution containing 75 - 100 mg / kg of nitric acid for 2 - 10 hours at a solution temperature of 80 - 100 ^o C and subsequent cleaning of the solution on filters special water treatment (CBO) [5]. This method allows to increase the effectiveness of decontamination in comparison with the known methods of low reagent decontamination; reduce secondary sorption of radionuclides; increase the corrosion resistance of steels. In addition, during long shutdowns (during conservation), almost 5 to 10 mg / kg of sodium (potassium) nitrite is required to almost completely prevent corrosion damage to the equipment, which is an order of magnitude - two less than what is required when processing equipment with other decontamination methods.

 The disadvantage of this method is that it is implemented only when processing is carried out on deeply desalted water (χ = 2.0 μS / cm). Otherwise, the secondary sorption of radionuclides on the treated surfaces is increased, the corrosion resistance of passivated steels is reduced, including an order of magnitude - two higher protective concentrations of nitrites in the conditions of operation of the equipment in parking conditions.

 The problem solved by the invention is to expand the functionality of the method to enable processing at high initial electrical conductivity of water, for example, after low-chemical chemical decontamination (if the equipment is heavily contaminated with radionuclides), increasing the efficiency of decontamination and reducing the protective concentration of sodium nitrite (potassium) in parking modes.

The essence of the invention lies in the fact that in the method of processing the contours of water-cooled reactors with a solution containing nitric acid, it is proposed to process the circuit at a temperature of 80-100 ^° C for 2-5 hours with a solution containing aluminum nitrate at a selected ratio of solution components:
Nitric acid - 25-60 mg / kg
Aluminum nitrate - 10-50 mg / kg
It is additionally suggested that after cleaning the spent solution on ion-exchange filters to an electrical conductivity of ≅ 2 μS / cm, sodium (potassium) nitrite should be introduced into the system at a concentration of 1-10 mg / kg. In this case, the corrosion process is almost completely suppressed for a long time (as shown by experiments for a period of 5000 hours or more). It is proposed to prepare a deactivating solution on demineralized water of the reactor cooling circuit.

As a justification of the essence of the claimed combination of features, we cite the following: when nitric acid and aluminum nitrate are added to the solution, the resistance of protective films increases sharply. At the same time, an increase in the decontamination efficiency is also observed. The upper concentration limit of aluminum nitrate should be limited to 50 mg / kg, since there are no significant changes in the corrosion resistance of oxide films, and the decontamination efficiency increases with increasing temperature and processing time, the deactivation coefficient increases. The samples are covered with a protective film only at a processing temperature above 80 ^o C. At a temperature of 120 ^o C precipitation of ferric compounds in the solution is observed after 4 hours of treatment, at a temperature of 100 ^o C after 7 hours of treatment; at 80 ^o C - after 14 hours. At a processing temperature of less than 80 ^o C and a time of less than 2 hours, the decontamination coefficient sharply decreases. Optimum selected: processing time 2-5 hours; processing temperature 80 - 100 ^o C. The addition of aluminum nitrate to the nitric acid solution not only improves the protective properties of oxide films formed on the surface of pearlitic steel, but also allows processing in water of low quality (aluminum ions appear to be the center of crystallization upon surfaces of pearlite steel of a magnetite oxide film, aluminum ions are embedded in the oxide film). The optimal concentration range is nitric acid 25-60 mg / kg. At concentrations less than 25 mg / kg, the corrosion resistance and decontamination coefficient are reduced. At concentrations above 60 mg / kg, corrosion resistance is also reduced. Despite a slight increase in the decontamination coefficient, the concentration of nitric acid should be limited to 60 mg / kg, because at high concentrations it is not practical to remove chemical reagents from the spent solution on standard filters of the installation itself (as a result, a large amount of radioactive waste will be generated).

Examples of specific performance
Example 1. Given to justify the optimal concentration of nitric acid.

Samples of 08Kh18N10T stainless steel and grade 20 pearlitic steel were kept in a polluting solution in demineralized water containing Cs ¹³⁷ , Cr ⁵¹ , Co ₆₀ isotopes with a total activity of 2 • 10 ⁸ Bq / kg (94, 3, 3, respectively) at 270 ^o C for 150 hours. The ratio of the surfaces of the samples to the volume of the solution was 1 cm ² : 5 cm ³ .

Contaminated solutions were treated with solutions of nitric acid at concentrations of 15, 25, 35, 45, 60, 80, 100 and 250 mg / kg and adding 30 mg / kg of aluminum nitrate to each solution. In all cases, the treatment was carried out at a temperature of 95 ^° C. for 2 hours. The samples were washed with demineralized water, the deactivation coefficient was determined, and corrosion tests were performed in demineralized water. After holding the samples for 5 days and a temperature of 20 ± 2 ^o C, the corrosion losses were determined. The experimental results are shown in table 1.

 Example 2. Given to justify the optimal concentration of aluminum nitrate.

Samples from St20 were washed with solutions of nitric acid at a concentration of 45 mg / kg and aluminum nitrate at concentrations of 0; 5; 10; 20; 35; fifty; 75; 100 and 150 mg / kg, respectively. The treatment was carried out at a temperature of 95 ^o C for 3 hours. The samples were put on corrosion tests in demineralized water and the coefficient of deactivation was determined. The experimental results are presented in table 2.

 Example 3. Given to justify the temperature and processing time.

St20 samples contaminated with radionuclides were treated with a solution of nitric acid 45 mg / kg + 30 mg / kg of aluminum nitrate at temperatures of 60, 80, 100 and 120 ^o C (at 120 ^o C in an autoclave) for 1, 2, 3, 5, 7, 10 hours. The deactivation coefficient was determined. The experimental results are shown in table 3.

 From the data shown in table 3, it can be seen that with increasing temperature and processing time, the decontamination coefficient increases.

But, as mentioned above, the samples are covered with a protective film only at a processing temperature above 80 ^o C. At a temperature of 120 ^o C precipitation of ferric iron compounds in the solution is observed after 4 hours of treatment, at a temperature of 100 ^o C after 7 hours of processing, at 80 ^o C - after 14 hours. Therefore, the optimal ones, as in the closest analogue, were selected: processing temperature 80 - 100 ^o C, processing time 2 - 5 hours. It should be noted that with a surface ratio of St20k the solution volume is 1: 250, which complies with the processing conditions of KMPTs NPP with RBMK, precipitation but not observed for 24 hours at a temperature of 100 ^o C.

 Example 4. Given to justify the possibility of processing in water of high electrical conductivity (low quality).

Samples of St20 were processed: a) according to the proposed method — a solution of 45 mg / kg of nitric acid + 30 mg / kg of aluminum nitrate; b) by the method prototype - 75 mg / kg of nitric acid. The treatment was carried out at a temperature of 95 ^o C for 3 hours. The solutions were prepared in demineralized water with the addition of industrial water. After processing, the samples were washed with demineralized water and put on corrosion tests. The experimental results are shown in table 4.

 Example 5. Given to prove a decrease in the protective concentration of nitrite ions in the conditions of conservation of equipment.

 Samples of St20 were processed according to the proposed method and the prototype method (see example 4), thoroughly washed with demineralized water and put on corrosion tests in demineralized water with additives of sodium nitrite of different concentrations (each sample in a separate glass cup). The transition to complete suppression of corrosion of the samples under study was determined visually by the cessation of visible changes occurring on the sample (absence of rust spots) and in solution (the solution remains transparent and colorless). It was established that if corrosion of oxidized samples in nitrite solutions did not begin in the first 3–5 days, the system remains stable for an indefinitely long period. With an insufficient concentration of sodium nitrite, the visible destruction of the oxide film begins after 1 to 3 days and increases in time.

From the above experiments, it was found that the minimum protective concentration of sodium nitrite in demineralized water (with an electrical conductivity of 2 μS / cm) at 20 ^o C was 2 mg / kg for samples processed by the prototype method, and only 0.05 mg / kg for samples processed by the proposed method (to suppress corrosion of non-oxidized samples of St20 requires a concentration of 1000 times greater). Given that for real equipment, the optimal processing mode at individual points in the system may not reach (stagnant zones, branches, various construction materials, insufficient quality of demineralized water, etc.), the protective concentration of sodium nitrite (potassium) should be increased to 1-10 mg / kg But even in this case, if necessary, the water coolant can be cleaned in a matter of hours to normalized values using ion-exchange filters of the power plant itself.

 Example 6. Given to justify the feasibility of additional processing by the proposed method after any chemical decontamination.

Samples contaminated with radionuclides were treated in a solution of 5 g / kg of oxalic acid + 2 g / kg of sodium nitrate at a temperature of 90 ^o C for 5 hours. Then, the solution was diluted five times with water and hydrogen peroxide was introduced at a concentration of 0.2 g / kg and processing continued at 70 ^o C for 3 hours (method "Nitrox" / 5 S. 52 /). Samples were washed with demineralized water and the deactivation coefficient was determined. Then the samples were additionally treated according to the proposed method with a solution of 50 mg / kg of nitric acid + 30 mg / kg of aluminum nitrate at 95 ^o for 3 hours and the coefficient of deactivation was determined. The experimental results were as follows:
a) the CD by the Nitrox method for steel samples 08Kh18N10T was 6.8; for CT20 Cd = 14.

 b) CD by the Nitrox method + processing by the proposed method — for steel 08X18H10T it was 19; for CT20 Cd = 90.

Based on the above material, we can conclude that the proposed method has advantages:
higher deactivation coefficient and higher protective properties of the formed oxide films (see table 2 - the first position corresponds to the prototype method, the second to fifth according to the proposed method). The high efficiency in the treatment of steel with a solution of nitric acid can be explained by the interaction of nitrate ions with iron ions in a weakly acidic medium - the well-known reaction of the "brown ring". Aluminum ions, easily forming mixed compounds, increase the efficiency of interaction of nitrate ions with iron ions, as a result of which the deactivation coefficient increases;
the possibility of processing using solutions in low quality water;
the possibility of reducing the protective concentration of sodium nitrite (potassium) during the conservation of equipment.

Bibliography
1. Decontamination in nuclear power, ed. Ampelogova N.I. et al. M .; Power Publishing. 1982, p. 124 - 125, 127 139.

 2. Atomic Enege of Canada Limited, AECL - 4229, July, 1972.

 3. Materials Research in AECL. 1975. AECL-5227, p. 19-21.

 4. US patent N 4042455 from 09.16.1978.

 5. Sedov V.M., Senin E.V., Nesterenko A.P., Zakharova E.V. "Decontamination of nuclear power plants" - Atomic energy, 1988 v. 65, issue 6, p. 399.

Claims (1)

A method of processing the contours of water-cooled reactors with a solution containing nitric acid, characterized in that the processing of the circuit is carried out at a temperature of 80-100 ° C for 2-5 hours with a solution containing aluminum nitrate at a selected ratio of solution components, mg / kg:
Nitric acid - 25-60
Aluminum nitrate - 10-50

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