RU2340965C1 - Method of chemical decontamination of nuclear power plant equipment - Google Patents

Abstract

FIELD: power production.

SUBSTANCE: invention relates to nuclear power production and mainly, to chemical decontamination methods for radiation hazardous equipment of nuclear reactors and is intended for removing corrosion products on structural materials by means of chemical reagents solutions. Chemical decontamination method for nuclear power plant equipment includes two-bath oxidising and recovering treatment of equipment surfaces by water solutions of chemical reagents and their forced mixing at the given temperature and time. Equipment is decontaminated in the first bath in three stages. Two-stage decontamination is carried in the second bath. At stage 1, equipment is treated with water solution of potassium permanganate and acetic and nitric acids in the first and second bathes. Weight ratio of components is from 1:9:1 to 1:1:9. Solution with total acid concentration from 10.0 to 50.0 g/kg is made in the bath. Concentrated hydrogen dioxide is gradually introduced in the solution made in the first and second bathes. Concentrated solution of ethylenediaminotetraacetic acid with ammonium acetate and hydrazine at their weight ratio 1:0.5:0.1 is introduced to the first bath at the third stage. Solution of ethylenediaminotetraacetic acid with concentration from 10 to 50 g/kg and pH solution from 3.5 to 5.0 is made in the bath.

EFFECT: improvement of decontamination effectiveness and reduction of dose loads of NPP personnel.

3 cl, 5 tbl

Description

The invention relates to nuclear energy, and in particular to methods for chemical deactivation of radiation-hazardous equipment of nuclear reactors, for example, steam generators (GH) of a reactor installation (RU) of nuclear power plants (NPPs) with pressurized water reactors (VVER, PWR).

The process of chemical decontamination (cleaning) is reduced, as a rule, to the removal of chemical products of corrosion of structural materials, including radioactive ones, from the surfaces of the RU circuit equipment with solutions of chemical reagents. It is known [N.I. Ampelogova, Yu.M. Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. Moscow, Energoatomizdat. S.36-39, 113-114, 1982], [V. M. Sedov et al. Chemical technology of coolants of nuclear power plants. Moscow, Energoatomizdat. P.84,249, 1985.], [M. Peak, M. Segal. Chemical decontamination of pressurized water reactors in the UK. Nuclear technology abroad. 1984, No. 12, p.27], that during the operation of RP VVER and PWR NPPs, oxide films complex in chemical and phase composition are formed on the inner surfaces of the primary circuit equipment, which can conditionally be divided into two types:

- protective, strongly bonded to the base metal oxide films (topotactic layer of metal oxide deposits enriched in chromium and having a spinel structure, the composition of which can be represented by the formula - Ni _x Fe ₃ -xy Cr _y O ₄ , where y and x are variables);

- loose deposits (epitactic layer of metal oxide deposits, the composition of which is determined mainly by magnetite and nickel ferrite), accumulating on a dense topotactic layer of operational deposits [V. M. Sedov et al. Chemical technology of coolants of nuclear power plants. Moscow, Energoatomizdat. S.249, 1985].

To remove such films from equipment surfaces, as a rule, a two-stage (two-bath) redox treatment of the equipment with aqueous solutions of chemical reagents is used: first, an alkaline solution of potassium permanganate (KMnO ₄ ), and then an acidic reduction solution. The first bath is designed to oxidize the sparingly soluble Cr ³⁺ oxide, which is part of the sediment structure, to a chromate ion (CrO ₄ ^2- ) soluble in an alkaline medium [N.I. Ampelogova, Yu.M. Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. Moscow, Energoatomizdat. S.131, 1982]. In the second bath, designed to dissolve deposits enriched in magnetite (Fe ₃ O ₄ ) and nickel ferrite (FeNi ₂ O ₄ ), recovery formulations of solutions based on mineral and organic acids or solution compositions containing complexones are usually used [N.I. Ampelogova , Yu.M. Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. Moscow, Energoatomizdat. S.183-186, 1982], [M. Peak, M. Segal. Chemical decontamination of pressurized water reactors in the UK. Nuclear technology abroad. 1984, No. 12, p.30-31].

A two-way method for removing scale from stainless steel is known, which involves boiling parts in a solution of 20% sodium hydroxide (NaOH) with 3% KMnO ₄ and subsequent processing of the parts in sulfuric acid (H ₂ SO ₄ ) or a mixture of hydrofluoric (HF) and nitric (HNO ₃ ) acids at a temperature of 65 ° C [M. Peak, M. Segal. Chemical decontamination of pressurized water reactors in the UK. Nuclear technology abroad. 1984, No. 12, p.28], [United States Steel. Fabrication of USS stainless steel. 2nd ed., 1956, p. 83]. A significant disadvantage of this method is the large corrosive loss of the metal, which is unacceptable for the decontamination of the circuit equipment RU.

Closest to the claimed method is a two-way method for chemical decontamination of pipelines and equipment of nuclear power plants with VVER, including:

- treatment of the internal surfaces with an alkaline solution of potassium permanganate containing 30 g / kg KOH (NaOH) and from 2 to 5 g / kg KMnO ₄ (first bath);

- water washing;

- treatment of internal surfaces with an acid solution of oxalic acid (H ₂ C ₂ O ₄ ) with HNO ₃ containing from 10 to 30 g / kg H ₂ C ₂ O ₄ and 1 g / kg HNO ₃ or 10-30 g / kg H ₂ With ₂ O ₄ and 1-3 g / kg H ₂ O ₂ (second bath).

Decontamination of equipment in each bath is carried out at a temperature of solutions of 90-100 ° C for 1.5-2.0 hours with continuous circulation of solutions. The alternation of solutions (baths) during decontamination is carried out until satisfactory results are obtained [F.Ya. Ovchinnikova. Novovoronezh NPP. Reference and informational materials. Voronezh: Central Black Earth Book Publishing House, p.157, 1979], [Guiding document RD 210.006-90 “Rules for the Technological Design of Nuclear Power Plants (with VVER Reactors)”].

This method is accepted by us as a prototype. The disadvantage of the prototype method is the low efficiency of cleaning equipment surfaces from operational deposits (the average decontamination coefficient (Cd) for 1 cycle, including two baths, is no more than 9) [N.I. Ampelogova, Yu.M. Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p.186, 1982] and the probability of the formation and deposition on the surfaces of decontaminated equipment of secondary insoluble deposits of Fe ²⁺ oxalates [N. I. Ampelogova, Yu.M. Simanovsky, A. A. Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p.130, 1982, 1983].

The objective of the present invention is to provide a method for chemical decontamination of equipment, which allows to increase the efficiency of cleaning the surfaces of equipment and, as a result, reduce dose loads on the personnel of nuclear power plants.

To achieve this technical result, in a method comprising a two-bath redox treatment of the internal surfaces of RU equipment with aqueous solutions of chemical reagents in the mode of their forced mixing at a given temperature and time, it is proposed:

- the process of cleaning (decontamination) of equipment in the first bath is carried out in three stages, and in the second - in two stages;

- in the first and second baths at the first stage of purification, use an aqueous solution of potassium permanganate (KMnO ₄ ) with acetic (CH ₃ COOH) and nitric acids (HNO ₃ ) with a weight ratio of components in the solution from 1: 9: 1 to 1: 1: 9, respectively, and the initial pH of the solution from 1 to 2.5, based on the creation of a solution in the bath with a total acid concentration of 10.0 to 50.0 g / kg;

- at the second stage, concentrated hydrogen peroxide is added portionwise to the solution of the first and second baths; and in the third stage of the first bath, a concentrate of a solution of ethylenediaminetetraacetic acid (EDTA) with ammonium acetate (NH ₄ Ac) and hydrazine (N ₂ H ₄ ) at a weight ratio of 1: 0.5: 0.1, respectively, based on the calculation of creating a solution in the bath with a concentration of EDTA from 10 to 50 g / kg and a pH of the solution from 3.5 to 5.0.

Additionally, the process of decontamination of equipment is proposed to complete:

in the first and second bath in the first processing stage in the absence of free KMnO ₄ in the solution;

in the first bath in the second stage, when the concentration of manganese in the solution is stabilized, and in the third stage, when the specific activity and concentration of Fe, Cr, and Ni are stabilized in the bath solution in the presence of free EDTA in the solution;

in the second bath in the second stage, when the specific activity and concentration of Mn, Fe, Cr and Ni are stabilized in the bath solution in the presence of free H ₂ O ₂ in the solution.

The initial concentration of EDTA in the solution of the first bath is proposed to be determined on the basis of the theoretical metal consumption of EDTA taking into account the chemical and quantitative composition of the deposits.

The essence of the proposed method lies in the fact that the decontamination of radioactive contaminated equipment of nuclear power plants is proposed to be carried out according to the traditional two-bath technology, but with the use of acidic solutions of chemical reagents in both baths and two oxidation stages of processing the equipment in the first and second baths and one reduction (third) stage in first bath. At the first stage of oxidizing treatment of equipment in the first and second baths, it is proposed to use an acetic acid solution of KMnO ₄ with the addition of HNO ₃ in it, and at the second stage (stage of clarification of the solution), hydrogen peroxide is used to destroy the secondary deposits of manganese dioxide and a possible excess of permanganate. It is also suggested that after clarifying the solution of the first and second baths with hydrogen peroxide, continue portioning the dosage in the bath solution of hydrogen peroxide (second stage of oxidative treatment). At the third stage of (recovery) processing of equipment in the first bath, it is proposed to use an acetate solution of EDTA with N ₂ H ₄ .

The proposal to use KMnO ₄ acidic solution at the first stage of cleaning equipment surfaces in the first and second baths is caused by the fact that such a solution, in comparison with an alkaline oxidizing solution, may be more effective in the decontamination of stainless steel and titanium alloys [N. I. Ampelogova, Yu .M.Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p.134, 1982].

It is known that the chemical resistance of dense (protective) oxide films (topotactic layer) firmly adhered to the base metal is due to their enrichment with chromium, the content of which in films reaches 30-45% [N.I. Ampelogova, Yu.M. Simanovsky, A. A.Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p.131, 1982], [V.M. Sedov et al. Chemical technology of coolants of nuclear power plants. M .: Energoatomizdat, p. 248-249, 1985]. In addition, when conducting a decontamination cycle of equipment by any technology on surfaces to be cleaned due to the fact that the initial oxide layer does not have the same composition and thickness [M. Yovchev. Corrosion of thermal power and nuclear power equipment. Moscow, Energoatomizdat. p.161, 1988], the remains of highly enriched chromium oxide films. We experimentally established that after processing samples of 0X18H10T austenitic steel oxide scale samples with redox solutions using the technology of the prototype method, the scale composition of the main chemical elements Fe and Cr changes significantly (see table 1). Dross, at least 2.0 times enriched with chromium and 1.5 times depleted in its iron content. Naturally, the dissolution of highly enriched chromium deposits is possible only in solutions containing an oxidizing agent. This justifies the proposal to use the oxidizing two-stage bath at the final stage of decontamination of equipment using KMnO ₄ at the first stage of acetic acid solution, and hydrogen peroxide at the second stage. This will increase not only the efficiency of dissolution of chromium-rich deposits and, accordingly, the decontamination coefficient (Cd), but will also make it possible to abandon the passivation stage of cleaned equipment surfaces, since acid peroxide solutions are passivators [N.I. Ampelogova, Yu.M. Simanovsky A.A. Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p.130, 1982].

The positive point of using the KMnO ₄ acetic acid solution with the addition of HNO ₃ in the first and second baths of the equipment decontamination is also that the proposed formulation is universal and allows, with a high content of acids (CH ₃ COOH + HNO ₃ ) in the bath solution, to create an optimal pH range from 1.0 for the oxidation of Cr ³⁺ in Cr ⁶⁺ [E.V. Zakharova, V.I. Kazarin, G.N. Meshkova. Improving the methods of decontamination of equipment of the primary circuit of nuclear power plants. Atomic energy, vol. 79, issue 1, pp. 71-74, 1995] up to 2.5 [M. Peak, M. Segal. Chemical decontamination of pressurized water reactors in the UK. Nuclear technology abroad. 1984, No. 12, p.27,], which is impossible in case of using the famous [N.I. Ampelogova, Yu.M. Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p.135, 1982] oxidizing solutions based formulations KMnO ₄ with nitric acid. KMnO ₅ solutions with an HNO ₃ concentration _of more than 10 g / kg have a pH of less than 1.0, and at pH 2.5, the HNO ₃ content in the solution does not exceed 0.5 g / kg, which is clearly insufficient to dissolve deposits containing in its composition Fe, Co and Ni.

It is also positive that the use of oxidizing compounding as the main component of CH ₃ COOH, which has complex-forming properties [N.I. Ampelogova, Yu.M. Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p.126, 1982], will improve the efficiency of extraction from deposits and transfer to a solution of not only chromium, but also other metals (Fe, Ni, Co) included in the composition of deposits, and use in the formulation as an additive HNO ₃ makes it possible to carry out the necessary adjustment of the pH of the solution and prevent the likelihood of the formation and deposition of secondary deposits of metal acetates on the surfaces of the equipment being cleaned.

The reason for using hydrogen peroxide to clarify the solution (the stage of destruction of excess KMnO ₄ and dissolution of secondary deposits of MnO ₂ ) and the continuation of the oxidative stage of processing equipment in the first and second baths due to its periodic dosage in the solution is that H ₂ O ₂ is a salt-free additive with the pH of the solution is close to neutral, which allows to reduce the total salt content of the spent solutions (LRW) formed in the decontamination cycle. A positive point is that the use of H ₂ O ₂ at the final stage of oxidative treatment not only increases the kinetic characteristics of the process of destruction of excess KMnO ₄ in the solution and dissolution of secondary deposits of manganese dioxide (MnO ₂ ), but also allows the oxidation process to continue in deposits of Cr ³⁺ to Cr ^{6+ with} active oxygen formed in solution as a result of chemical reactions of the interaction of H ₂ O ₂ with potassium permanganate and manganese dioxide [R. Ripan, I. Chetyanu. Inorganic chemistry, v.2. Peace. Moscow, 1971, p.430] and partial catalytic decomposition of hydrogen peroxide itself to atomic and molecular oxygen [B.V. Nekrasov. Fundamentals of General Chemistry, v.1. Chemistry, Moscow, p.150, 1985]. It is known, for example [N.I. Ampelogova, Yu.M. Simanovsky, A.A. Trapeznikov. Decontamination in nuclear power. M .: Energoatomizdat, p. 36, 1982] that the introduction of gaseous oxygen into the coolant leads to a significant increase in the content of dissolved chromates, the formation of which probably proceeds according to the reaction:

2Cr ₂ O ₃ + 3O ₂ + 4H ₂ O → 4H ₂ CrO ₄ .

In addition, as a result of chemical-catalytic destruction of H ₂ O ₂ with the formation of gaseous oxygen in the system to be cleaned: solution - films of operational deposits - metal surface, a mode of intensive mixing ("boiling") of the solution is realized, which activates the capture of finely dispersed solution from the cleaned surfaces of the equipment corrosion products [JP Patent No. 62-22119, publication of 05.15.87 No. 6-553, "Inventions of the world"] and carbon-containing insoluble impurities included in the structure of operational pending which, in general, increases the effectiveness of decontamination.

It has been experimentally established that the use of hydrogen peroxide after treating equipment surfaces with acidic KMnO ₄ solutions can completely destroy the excess KMnO ₄ and MnO ₂ precipitate in the solution. The reaction of H ₂ O ₂ with KMnO ₄ and MnO ₂ occurs rapidly with the formation of oxygen both in the solution volume and on the surfaces to be cleaned. The required amount of hydrogen peroxide during the clarification stage in the first and second baths is determined by the stabilization of the concentration of manganese in the solution, and its amount in the solution should correspond to the amount of manganese introduced into the solution in the composition of KMnO ₄ . The criterion for the end of the stage of oxidative treatment of equipment in the second bath (termination of the technological operation of periodically introducing Н ₂ О ₂ into the bath solution) is the stabilization of the concentration of Mn, Fe, Cr, and Ni in the bath solution and specific activity in the presence of free hydrogen peroxide in the solution.

Efficiency of dissolution (estimated from the change in Kd) of metal oxide deposits chromium enriched in acetic acid solution KMnO ₄ + HNO _3, followed by dosing in a solution of concentrated H ₂ O ₂ is illustrated by purification witness samples of heat exchanger tubes (TOT) cut from a steam generator PGV- 440 of power unit No. 3 of the Novovoronezh NPP, the results of which are shown in table 2.

The use in the first bath in the third stage of deactivation of a reducing solution of EDTA with NH ₄ Ac and N ₂ H ₄ with a weight ratio of 1: 0.5: 0.1 and an initial pH of the solution from 3.5 to 5 excludes the possibility of formation in the solution volume and deposition on the surfaces of the equipment to be cleaned secondary deposits, increases the efficiency of the process of dissolution of metal oxides due to:

- the presence in the solution of a reducing agent (N ₂ H ₄ );

- providing optimal for the dissolution of nickel ferrite [M. Peak, M. Segal. Chemical decontamination of pressurized water reactors in the UK. Nuclear technology abroad. 1984, No. 12, p.31] and magnetite [RF patent No. 2203461, BI No. 12, 2003] the pH of the solution from 4 to 5;

- buffering properties of the proposed solution formulation [B.V. Nekrasov. Fundamentals of General Chemistry, vol. 1, Chemistry, Moscow, p. 189, 1965], ensuring the maintenance of the optimum pH range for dissolving nickel ferrite.

A positive aspect of using the proposed reduction formulation based on EDTA with NH ₄ Ac and N ₂ H ₄ is also that the preparation of a solution concentrate with an EDTA content of at least 200 g / kg is not difficult. The concentrate can be prepared without additional heating by simple dissolution of the components in the distillate, following the sequence of dissolution of the components (NH ₄ Ac → EDTA → N ₂ H ₄ ) and their recommended ratio. Tables 3 and 4 show the basic input data for the preparation of 20 m ³ (the volume of a standard tank used at VVER-1000 NPPs) of a concentrated EDTA solution [RF patent, No. 2203461, BI No. 12, pp. 19-20, 2003].

An example implementation of the proposed method

Chemical deactivation of witness samples of heat transfer tubes (TOT) was carried out, cut from the PGV-440 steam generator of unit 3 of the Novovoronezh NPP under static conditions with periodic shaking of solutions in glass flasks equipped with reflux refrigerators. The processing of samples in the first bath, designed for the oxidation of Cr ³⁺ in Cr ⁶⁺ , was carried out for 2.5 hours at a temperature of 90 ± 1 ° C with oxidizing solutions of the composition:

The claimed method: prototype method: KMnO ₄ - 1.0 g / kg KMnO ₄ - 5.0 g / kg CH ₃ COOH - 5.0 g / kg NaOH - 30.0 g / kg HNO ₃ - 5.0 g / kg pH - 12.1 pH 1.3

After the treatment of the test specimen TOT with an oxidizing acetic acid solution of KMnO ₄ with HNO ₃ (the inventive method, the first stage of the first bath), concentrated hydrogen peroxide (second stage of the first bath) was added dropwise into the flask until the solution was completely clarified and the secondary deposits of MnO _{2 were} dissolved (total flask, 0.15 ml H ₂ O ₂₎ witness sample recovered from the flask, washed with water, dried, and measured its radioactivity was determined by Kd. Next, the sample was placed in a clarified hot solution of the first bath, the concentrate of the EDTA reduction solution with NH ₄ Ac and N ₂ H ₄ was introduced into the solution until the concentration of EDTA in the bath solution was 20 g / kg, and the experiment (third stage of the first bath) was continued for another 4 , 1 hours.

After completion of the treatment of the test specimens of TOT with solutions of the first bath, the solutions were drained from the flask, the samples and flasks were washed twice with distillate, the samples were removed from the flasks, dried, their radioactivity was measured, and the CD was determined.

Subsequent processing of test specimens of TOT (second bath) was carried out at a temperature of 90 ± 1 ° C for 9 hours (prototype method) and 4.6 hours (inventive method) with solutions of the composition:

The claimed method: prototype method: KMnO ₄ - 1.0 g / kg H ₂ C ₂ O ₄ - 20.0 g / kg CH ₃ COOH - 5.0 g / kg HNO ₃ - 1.0 g / kg HNO ₃ - 5.0 g / kg pH 2.1 pH 1.3

with periodic extraction of samples from solutions, measurement of their radioactivity and determination of CD. In the experiment (the inventive method), after a 2-hour treatment of the sample with an acidic oxidizing solution of KMnO ₄ , concentrated H ₂ O _{2 was} dosed into the solution.

The results of comparative experiments on the decontamination of TOT witness witnesses of the steam generator PGV-440 of power unit No. 3 of the Novovoronezh NPP, shown in table 5, indicate that the proposed method of decontamination of RP equipment at nuclear power plants, in comparison with the prototype, can significantly increase the efficiency of cleaning equipment surfaces from radioactive operational deposits and reduce dose loads on NPP personnel during routine maintenance and repair of equipment.

The proposed method for the decontamination of pipelines and equipment of VVER-equipped reactors (PWR) can be implemented at nuclear power plants using standard equipment without changing the circuit decisions laid down in standard methods and technologies, taking into account the proposed change in the composition of solutions and their application modes.

Table 1 The elemental composition of the steel oxide grade ОХ18Н10Т before and after the redox treatment cycle using the technology of the prototype method Chemical element The content of elements, in% before processing (ref.) after processing Iron (Fe) 71.54 49.23 Chrome (Cr) 16.80 34.38 Nickel (Ni) 8.87 12.53 Titanium (Ti) 0.63 1.06 Manganese (Mn) 1.77 1,63 Cobalt (Co) 0.21 0.31 Copper (Cu) 0.17 0.16 Zinc (Zn) 0.01 0.02 Vanadium (V) 0.12 0.13 Tungsten (W) 0.02 0.11

table 2 The results of cleaning the surface of a witness sample of a heat exchange tube (TOT) from the side of the primary circuit cut from the PGV-440 steam generator of Unit 3 of the Novovoronezh NPP. The volume of the solution is 100 ml. The surface area to be cleaned is 25 cm ² . Processing temperature - 95 ± 5 ° C. Chemical operations The composition of the solution, g / kg, the sequence of operations Introduced H ₂ O ₂ , ml Processing time, hour And _β , imp · cm ² / min Cd * beg. con. Oxidative treatment of the sample (oxidizing agent - KMnO ₄ ) Oxidizing solution: - 3.0 1170 ** - KMnO ₄ - 5 _CH3COOH - 40 HNO ₃ - 10 pH 1.1 Oxidizing solution clarification Enter H ₂ O ₂ in the spent oxidizing solution 0.36 0.5 - fifty 43,4 Oxidative treatment of the sample (oxidizing agents: H ₂ O ₂ ; atomic oxygen) Periodic input into a clarified oxidative solution of H ₂ O ₂ 0.36 2.0 - 35 62.0 0.36 2.0 - twenty 108,5 0.12 1,5 - fifteen 145.0 0.24 2.0 - 8 271.0 0.24 2.0 - background background Note: * - deactivation coefficients are calculated from the initial value А _{β of the} sample; ** - A _β measurement was not performed on the sample and in the MnO ₂ solution.

Table 3 Basic input data for the preparation of 20 m ^{3 of the} initial concentrate of the washing solution based on EDTA, ammonium acetate and hydrazine according to the proposed method The composition of the initial concentrate, g / l The required amount of chemicals, kg Hydrogen pH Concentrate EDTK - 200 4000 NH ₄ Ac - 100 2000 4.2 N ₂ H ₄ - 20 400 EDTK - 200 4000 NH ₄ Ac - 100 2000 5.4-5.5 N ₂ H ₄ - 20 400 NH ₄ OH to pH 5.4 1.5 m ³ 25% NH ₄ OH

Table 4 The change in the pH of the concentrates of solutions based on EDTA depending on their dilution with distillate. The main component of the concentrate PH initial value Change in pH of the concentrate upon dilution. The concentration of the diluted solution in the main component, g / l one hundred fifty 25 12.5 EDTK 4.2 4.2 4.05 4.05 4.05 EDTK 5.4-5.5 5.3 5.25 5.15 5.15

Table 5 Decontamination coefficients of TOT witness samples cut from the tubing of the PGV-440 steam generator of Unit 3 of the Novovoronezh NPP. Processing temperature - 90 ± 1 ° C. The volume of solution in the flask - 50 cm ^3. V / S ratio = 2.0 cm ³ / cm ² Decontamination Method The composition of the solution, g / kg time hour And _β imp. cm ² / min Cd ** Bath 1 Bath 2 beg. con. prototype KMnO ₄ - 5.0 - 2,5 1595 1545 * 1,03 NaOH - 30.0 pH - 12.0 draining the solution and conducting a water wash - - H ₂ C ₂ O ₄ - 20.0 2.0 1545 745 2.1 4.0 - 690 2,3 HNO ₃ - 1.0 6.0 - 640 2,5 pH 2.1 7.5 - 540 2.9 9.0 - 440 3.6 claimed KMnO ₄ - 1.0 - 2,5 1650 - CH ₃ COOH - 5.0 HNO ₃ - 5.0 pH 1.3 input H ₂ About ₂ 4 drops - 0.1 - 350 4.7 input conc. before - 2,5 - 300 5.5 EDTK - 20.0 3,5 - 250 6.6 NH ₄ A _c - 10.0 4.0 195 8.2 N ₂ H ₄ - 2.0 - - - - pH - 3.5 - - - - draining the solution and conducting a water wash - KMnO ₄ - 1 2.0 - - - CH ₃ COOH - 5 HNO ₃ - 5 pH 1.3 input H ₂ About ₂ 4 drops 0.1 - thirty 55.0 input H ₂ About ₂ 2 drops 1,0 - 10 165.0 input H ₂ About ₂ 1 drops 1,5 - background Background Note: * - A _β measurement was not performed on the sample and in the MnO ₂ solution; ** - CD calculated from the initial value And _{β of the} sample.

Claims (3)

1. The method of chemical decontamination of equipment of nuclear power plants, including two-bath redox treatment of equipment surfaces with aqueous solutions of chemical reagents in the mode of forced mixing at a given temperature and time, characterized in that the process of equipment decontamination in the first bath is carried out in three stages, and the second - in two stages, while in the first and second baths in the first stage, the treatment is carried out with an aqueous solution of potassium permanganate with acetic and nitric acid Tammy in a weight ratio of the components in a solution of 1: 9: 1 to 1: 1: 9, respectively, at the rate established in the bath solution with a total acid concentration of 10.0 to 50.0 g / kg; at the second stage, concentrated hydrogen peroxide is introduced portionwise into the solution of the first and second baths; and in the third stage of the first bath, a concentrate of a solution of ethylenediaminetetraacetic acid with ammonium acetate and hydrazine at a weight ratio of 1: 0.5: 0.1, respectively, based on the calculation of creating in the bath a solution with a concentration of ethylene-diaminetetraacetic acid from 10 to 50 g / kg and a pH of the solution from 3.5 to 5.0. 2. The method according to claim 1, characterized in that the decontamination in the first stage of the first and second baths is carried out at an initial pH of the solution from 1 to 2.5, and in the third stage of the first bath at a pH of the solution from 3.5 to 5.0 . 3. The method according to claim 1, characterized in that the surface treatment of the equipment with solutions is completed: in the first and second baths at the first stage of processing - in the absence of free potassium permanganate in the solution; in the first bath in the second stage, when the concentration of manganese in the solution is stabilized, in the third stage, when the specific activity and concentration of iron, chromium and nickel are stabilized in the bath solution in the presence of free ethylenediaminetetraacetic acid in the solution; and in the second stage of the second bath, when the specific activity and concentration of manganese, iron, chromium and nickel are stabilized in the bath solution in the presence of free hydrogen peroxide in the solution.