RU2059313C1 - Method of decontamination of equipment of water-cooled nuclear reactors - Google Patents

Abstract

FIELD: nuclear power engineering. SUBSTANCE: method consists in alternating treatment of surfaces of equipment by oxidizing solution of potassium permanganate and by reducing solution in which capacity aqueous solution of reagent presenting liquid phase of products of interaction of lignin with nitric acid is used. Alkali up pH 8.2-10.0 is injected into aqueous solution of reagent during last cycle and treatment is continued for the course of 0.5-2.0 h. EFFECT: enhanced cleaning efficiency. 3 tbl

Description

 The invention relates to nuclear technology, namely to the decontamination of radiation-hazardous equipment of water-cooled nuclear reactors having solidly fixed contaminants on the surface.

 The decontamination process is reduced, as a rule, to the removal of solutions of chemical reagents deposits of corrosion products of structural materials (mainly iron oxides) from the surface of the equipment. Due to the presence of chromium and nickel compounds in the oxide film, the equipment is decontaminated in 2 stages (two-method): at the first stage, the equipment is treated with an oxidizing solution based on potassium permanganate (in an alkaline medium or, most effectively, in an acidic environment), at the second the stage is treated with a reducing solution based on various acids. In modern nuclear energy, organic acids and complexones are widely used for these purposes, with oxalic acid and its salts being the most effective deactivating agent.

 In order to increase the effectiveness of decontamination, activating additives such as fluorides, hydrogen peroxide, hydrazine, etc. are introduced into solutions of acids and complexones. Adding known activating additives to acids and complexones, as a rule, improves only one characteristic of the solution and negatively affects its other parameters. For example, the addition of fluorides increases the decontamination efficiency, but leads to a sharp increase in the corrosive effect, the addition of hydrogen peroxide to an oxalic acid solution prevents the formation of secondary oxalate deposits of ferrous iron, which impede purification, but significantly reduces the deactivation efficiency.

 Known adopted for the prototype method of two-way decontamination, which consists in alternately treating the surfaces of the equipment with an oxidizing solution based on potassium permanganate and a reducing solution containing oxalic and citric acids in equal proportions, the process "Citrox".

 The simultaneous presence of oxalate and nitrate ions in the solution has a beneficial effect on the decontamination process. This method is characterized by low deactivation efficiency and the formation of secondary oxalate deposits of ferrous iron and nickel at the recovery stage, which sorb radionuclides and thereby reduce processing efficiency.

 The objective of the invention is to develop a method of decontamination of equipment of water-cooled reactors, which improves the efficiency of decontamination of equipment.

 This is achieved by the fact that the deactivated surfaces of the equipment are subjected to alternate treatment with an oxidizing solution based on potassium permanganate and a reducing solution containing acids, moreover, an aqueous solution containing a reagent representing the liquid phase of the products of the interaction of lignin with nitric acid is used as a reducing solution during the treatment cycle, alkali is added to the reagent solution to a pH of 8.2-10.0 and treatment is continued for 0.5-2 hours. The reagent is obtained by reacting hydrolysis lignin with nitric acid. The reaction products contain two phases, liquid and solid finely divided. The liquid phase is brown and contains about 50 g / l of substances dissolved in water, calculated on the dry residue. The reagent has the following composition for the main products, g-equiv / l: total acidity 1.07; including humic and fulvic acids 0.24; oxalic acid 0.36.

 The content of oxalic acid in the reagent is 16.2 g / l or 0.18 m / l, the use of the specified reagent in the reducing solution leads to an increase in decontamination efficiency by an average of 1.5 times.

 It was also experimentally established that when the pH of the reagent solution is increased to a value of 8.2-10.0, the secondary oxalate deposits of iron and nickel dissolve, which leads to a 2-fold increase in the decontamination efficiency.

 PRI me R 1. Justification of the conditions of use of the reagent when removing secondary oxalate deposits.

The experiments to substantiate the conditions for the use of the reagent were carried out by dissolving the secondary oxalate deposits of ferrous iron (FeC ₂ O ₄ + 2H ₂ O) and nickel (NiC ₂ O ₄ + 2H ₂ O) as follows.

Fat sample (100 mg) was placed in a thermostatic vessels, filled with 25 ml of a solution kept the predetermined time at a temperature of 65 ^{° C,} the solution was filtered and the iron content was determined in samples and nickel solution onto atomnoabsorbtsionnom spectrophotometer. In the experiments, an aqueous reagent solution with a concentration of 5 g / l (in terms of dry residue) was used.

 The experimental results are given in table. 1 and 2.

Dissolution of iron oxalate and nickel in the aqueous reagent solution (concentration 5 g / l, based on the dry residue) in function of pH under the following conditions: sample weight of 100 mg, the volume of solution 25 ml, temperature 65 ^{° C,} time 1 hr (cm. table 1).

 The dissolution of iron and nickel oxalates in an aqueous reagent solution (concentration of 5 g / l) as a function of time (experimental conditions, see table 3) at pH 8.2 (see table 2).

 From the above data it follows that the optimal processing conditions when removing secondary oxalate deposits with the reagent are as follows: pH 8.2-10.0, time 0.5-2 hours

 PRI me R 2. Comparison of the deactivating effect of the prototype method and the proposed method.

 The experimental procedure is as follows.

A series of samples (at least three) cut from equipment 1st PWR circuit (type 1) and MFCC RBMK (Type 2), was placed in the solution in a constant-temperature vessel and kept at ⁹⁰ ° C for 2 hours

The ratio of the volume of the solution to the treated surface was 10 l / m ³ . The deactivating ability was evaluated by the coefficient of deactivation (CD) equal to the ratio of the initial activity of the sample to the final value.

The composition of the solutions during the processing of the stages is as follows;
CITROX (prototype).

Cycle a) 25 g / l nitric acid, 2.5 g / l potassium permanganate,
b) 20 g / l of oxalic acid, 20.0 g / l of citric acid:
c) water washing.

 The proposed method.

Cycle a) 25 g / l nitric acid, 2.5 g / l potassium permanganate,
b) an aqueous solution of the reagent with a concentration of 12.5 g / l (in terms of dry residue),
c) after two stages, alkali is introduced into the solution to a pH of 9.0 and treatment is continued for 1 h in order to remove secondary oxalate deposits.

 The experimental results are given in table. 3.

 From the data table. Figure 3 shows that the use of an aqueous reagent solution as a reducing solution leads to a 1.5-fold increase in the decontamination efficiency, and additionally, treatment in the same solution at pH 9.0 increases the CD by a further 2 times. Thus, the proposed two-way method can improve the efficiency of decontamination of equipment of nuclear reactors. The substances used for the implementation of this method of decontamination are available industrial products, and the new reagent proposed for the purpose of decontamination is obtained from the waste from the hydrolysis production of lignin.

Claims (1)

 METHOD FOR DEACTIVATING EQUIPMENT OF WATER-COOLED NUCLEAR REACTORS, which consists in alternately treating equipment surfaces with an oxidizing solution of potassium permanganate and a reducing solution containing acids, characterized in that an aqueous solution of the reagent is used as the reducing solution, which is the liquid phase of the products of the interaction of lignin with nitric acid and nitric acid In the last cycle of treatment, alkali is added to the aqueous solution of the reagent to a pH of 8.2 to 10.0 and the treatment is continued for 0.5 2 hours

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