Method for decontaminating forced recirculation circuit of nuclear power reactor

FIELD: decontamination engineering.

SUBSTANCE: proposed method includes treatment of circuit coolant with acid solutions and washing. In the process treatment with acid solutions is made by chemical loosening for 2-10 h. Dynamic loosening is effected prior to chemical loosening and then coolant temperature is periodically raised in reactor core to 150-200 °C.

EFFECT: reduced time and enhanced effectiveness of decontamination treatment process.

2 cl, 5 dwg, 1 tbl

 

The invention relates to nuclear power, namely, methods of decontamination, and can be used to remove corrosion products from the internal surfaces of equipment of nuclear power plants, such as contours of the multiple forced circulation (MFCC) boiling water reactors.

Over 75% of the dose rates of staff is due to the gamma radiation of activated corrosion products materials loop. Hence the need to reduce the dose fields from equipment MFCC. When the shutdown unit for repairs decontamination of equipment of the 1st circuit. The process of removing corrosion product deposits on the internal surfaces of the equipment of a nuclear reactor is, as a rule, to the dissolution of their formulations based on organic and mineral acids. The main components of these sediments are the oxides of iron, so part of the solution is oxalic acid, the most effective solvent of these oxides [Nesterenko A.P. and others Kinetics of dissolution of iron oxides and calculation of ionic equilibria in decontamination solutions. Preprint VNIPIET 87-3, M., cniiatominform, 1987]. The efficiency of dissolution of iron oxide deposits increased if the treatment can be done in two stages: a solution of oxalic acid at pH 2.5, then enter the hydrogen peroxide until sod is Rania in a solution of 0.3-0.4 g/l [Sedov V.M., Konstantinov, E., Filippov EM. the Use of hydrogen peroxide to dissolve the oxalate deposition in the decontamination of power plants of solutions. In kN. The study of chemistry, technology and application of radioactive substances. L., LTI them. Lensoviet]. The disadvantages of this method are: significant gas evolution at the stage of entry of hydrogen peroxide, leading to increased release131I in the ventilation; high corrosion loss of carbon steel and non-ferrous metals, which are used in separate parts of the contour of the equipment; chemical activation of metal when handling hydrogen peroxide. The closest analogue is a method of decontamination of the internal surfaces of the contour of the equipment involved in the processing surfaces of the circuit in two stages [Method of decontamination of the internal surfaces of a nuclear reactor. Patent No. 2126182, RU, 10.02.99. Bull. No. 4 - the closest analogue]: at stage I is treated within 30 hours of solution (5-10 g/l)containing acceptors products of water radiolysis, such as nitration at 90-95°; to remove the secondary deposits of ferrous iron oxalate and passivate steel surfaces at stage II is treated with a dilute solution of oxalic acid containing nitrite ion and nitrite ion is injected is a dilute solution of oxalic acid in the form of a salt of nitrous acid with the following concentrations of components: 0.01 to 0.1 g/l of oxalic acid, of 0.015-0.1 g/l nitrite, and decontamination lead for 3-5 hours at a temperature of 85-100°C. the Disadvantages of this method are: the long duration of the procedure of chemical decontamination - up to 35 hours MFCC processing chemicals at selected concentrations; the inefficient removal of corrosion products of zirconium, defining some sections of the MFCC power dose fields.

Objectives of the invention are: to increase the effectiveness of decontamination of primary circuit; to reduce the duration of the procedure of chemical decontamination and corrosion contour of materials during chemical decontamination and subsequent operation.

The invention consists in that in the method of decontamination loop multiple forced circulation of the energy of a nuclear reactor by exposure to the coolant circuit of the acid solution and washing the proposed influence of acid solutions to be implemented in conditions of chemical loosening within 2-10 hours, and before and after chemical tillage implement dynamic loosening by periodically increasing the temperature of the heat carrier in the active zone 150-200°C. in Addition, the proposed operation of the washing be carried out at a linear flow rate of not less than 0.12-0.36 m/s.

The hardware circuit of the multiple push the air circulation made of alloys based on zirconium (of the fuel cladding and technological channels) and different types of steels (pipelines, valves, drum-separators). Contour deposits consist of three types: dense deposits, tightly coupled with the surface, playing the role of protective corrosion films; friable deposits, loosely associated with the surface; sludge (unrelated with the surface of the particles of corrosion products with a diameter of over 10 μm). A feature of the last two is a well-developed surface and sorption them radionuclides from the circuit. Therefore, the ratio of activity to the mass of sediment in all three phases is different. Least reactive dense deposition. Continuous chemical processing circuit solutions of different composition leads to the dissolution not only of mud and loose fat, but thick oxide film and metal corrosion. The absence of a protective film on the metal surface leads to rapid corrosion of the surface for a long period subsequent operation. Decontamination mode chemical loosening unlike conventional chemical decontamination is not aimed at the complete dissolution of the oxide films, which requires a long processing time, and partial dissolution to destroy relations between the sediment particles and transfer them in suspension. Further, such particles and dissolved corrosion products are displayed on the bypass filters clean (BO). For a better understanding and the right combination is the essence of the invention will explore how the processes of decontamination on two types of surface contour: equipment surfaces of the circuit outside the active zone, made mainly from stainless steel and the surfaces of the active contour, i.e. on the rods and technological channels, made of zirconium alloys.

The invention is illustrated figure 1-6, where:

Figa - initial state of the equipment surfaces MFCC RBMK-1000 stainless steel or low-alloy steel after 5000-7000 hours of operation at capacity: 1 - stainless steel or low-alloy steel; 2 - dense oxide layer consisting mainly of magnetite - Fe3O4with the inclusion of particles of zirconium dioxide ZrO2with a particle size of up to 10-5cm; 3 - loose layer of corrosion products of iron and zirconium oxide with a particle size of up to 10-4cm; 4 - large particles of sludge, corrosion products of iron and zirconium oxide with a particle size of about 10-4see

Figure 1, is the initial condition of the surfaces of equipment MFCC RBMK-1000 of zirconium alloys after 5000-7000 hours of operation on power: 5 - zirconium alloy; 6 - thick oxide layer consisting of particles of non-stoichiometric zirconium oxide ZrO2-x(x=0.1 to 0.3) with the inclusion of particles stoichiometric oxide ZrO2and magnetite particle size of up to 10-5cm; 3, 4 - as figa.

Figa - end state of the equipment surfaces MFCC RBMK-1000 stainless steel or low-alloy steel after 5000-7000 hours of operation and detect the activate your product, this surface according to the method of the nearest similar duration 20-30 hours. Designation as figa.

Figb - end state of the equipment surfaces MFCC RBMK-1000 of zirconium alloys after 5000-7000 hours of operation on power and decontamination of the surface according to the method of the nearest similar duration 20-30 hours. Designation as figb.

Figa - end state of the equipment surfaces MFCC RBMK-1000 stainless steel or low-alloy steel after 5000-7000 hours of operation on power and decontamination by the present method. Designation as figa.

Figb - end state of the equipment surfaces MFCC RBMK-1000 of zirconium alloys after 5000-7000 hours of operation on power and decontamination by the present method. Designation as fig.1b.

Figure 4 - histogram of operational output activity of the primary circuit by way of the nearest analogue and by the present method.

5 is a graph correlating the dose of individual sections MFCC RBMK-1000 before and after decontamination: 1 - point method is the closest analogue; 2 - point by the present method; 3 - line constant dose rate (decontamination factor equal to 1).

6 is a table with data on the number of MFCC extracted from corrosion products on the way - the closest analogue and the claimed method, as well as their specific activity.

Figure 1 presents the source and figure 2 - the final state of the surface the FRC during processing by the method the closest analogue. Light bulbs presents the deposition of Zirconia, and the dark - iron oxide. Different size balls indicates different particle size of the sediments. On figa shows the initial state before the surface decontamination equipment loop of steel. Small strongly coupled particles form on the surface of the steel 1 dense oxide layer 2, consisting mainly of magnetite, but including particles of Zirconia. The average particle size to form a layer of loose sediments 3, the surface of which are the largest particles of the sludge layer 4. In the fluid are present as dissolved corrosion products of iron (Fe3+; Fe2+), and small particles of sediment suspended in the coolant. On figb shows the initial state before the surface decontamination of equipment from zirconium alloy 5, a dense layer 2 which is formed mainly of zirconium dioxide with small inclusions of iron. The composition of the other layers similar figa. Three types of deposits vary in relations between them. A thick layer of sediments linked by chemical forces. Layer of loose sediments coupled through physical adsorption and adhesion. On figa shows the surface condition of the steel and sediment after long-term (20-30 hours) chemical decontamination. Rest the Ren is not only a dense layer of iron oxide, but part of the steel surface. On the surface of the steel is the sludge from the medium and large particles, in which the proportion of zirconium dioxide increased significantly. On the surface of Fe (figb) after decontamination dense film 2 Zirconia remains, as the Zirconia acidic decontamination solution does not dissolve. Sludge depleted in iron oxides and consists mainly of particles of Zirconia. This sludge, mainly consisting of zirconium dioxide, remains on the surfaces. During processing it is partly included in a dense oxide film on the surfaces of equipment made of steel. On the surfaces of zirconium alloys slurry during processing enriched in magnetite and in the further operation gradually forms a loose, and then dense deposits. When conducting decontamination method - the closest analogue nearly all radionuclides excreted from the circuit in conjunction with decontamination solution. Dynamic loosening of the corrosion products layer 3 (see figa and 1B) leads to a weakening of ties between the particles and move them in a state of slurry. Large particles of the slurry are retained on the surface by the forces of adhesion and sedimentation and can be removed by the flow of water at an appropriate speed after chemical loosening. The corrosion products found in the coolant, can be derived from it for the odd filter filter bypass cleanup i.e. the transition of particles in the coolant means removing most of them. Remove the filters corrosion products before the introduction of acid reagents can improve the efficiency of chemical tillage: reduced duration by reducing diffusion losses in the absence of sludge and reducing consumption of acid on the dissolution of the corrosion products of iron contained in the sludge. The dynamic operation of loosening is to periodically raise the temperature of fluid up to 150-200°in the active zone. This is achieved by disconnecting the main circulation pumps (MCP) and the mode of natural circulation of the coolant. Natural circulation occurs due to temperature differences in the active zone and the rest of the circuit. As a result of increasing temperature there is a partial boiling of the coolant in the reactor, the resulting steam bubbles contribute to the loosening deposits 3 on the surface of Fe (figb) and sever the ties between the particles, which are separated from the surface and, due to the low velocity of the brine settle at the bottom of the water communications in the form of sludge. In addition, the temperature is raised to 150-200°C corresponds to the increase in the solubility of magnetite (solubility has a maximum at 150° (C) and additional loosening of otlozhenii the entire path. The fluid with suspended particles is cleared on the mechanical and ion-exchange filters. Flushing circuit is performed after dynamic loosening. The leaching after dynamic loosening with the flow rates of the proposed method allows you to delete some of the loosened deposits. Dynamic loosening and flushing before chemical loosening deposits reduce the proportion of coarse particles in the fluid and reduce the deposition of sludge and secondary sorption in repression of loosening solution. Loosening the sediment surface reduces the contact time with the decontamination solution. The chemical loosening (short-term impacts on surface sediment decontamination solution - 2-10 hours) a thin oxide layer 2 (see figure 1) is stored. Layer of loose sediments 3 goes to the slurry 4, consisting of magnetite, and Zirconia. The sludge remains on the surface and can only be removed by subsequent washing of the circuit. The limitation of chemical loosening over time 2-10 hour after reaching the working concentration of oxalic acid reduces the effects of corrosion decontamination solution and reduce erosion corrosion products and media activity from the active zone of the reactor. Less than 2 hours, the saturation of the solution with radionuclides is not achieved, the more than 10 hours starts to happen dissolution of the base material of the circuit, which leads to the failure of parts made of carbon steel. The part insoluble radioactive sediment in the process of circulation of the solution is deposited in a dead-end zones MFCC. First of all coarse particles of corrosion products accumulate in a cul-stagnant zones handout group of collectors (RGC), in which the horizontal velocity of the coolant is reduced from 5 m/s at the entrance to the RGC to 0 (at the dead end RGC). This leads to increasing doses during repair works on repair of valves and valve (SAV) and the inspection of welded seams RGC. To remove a particle size of from 10 to 100 μm is sufficient flow velocity of 0.12 m/s At a lower flow rate of decrease of the dose rate from the bottoms of RGC is not observed. When the particle size of 10-100 microns, the particles in the flow, down to 1 cm in 0.3-30 s, i.e. during the passage of long vehicles (RGC, BS) and low speed flow, they have time to settle. Therefore, when leaching additionally rinse stub zones RGC. To effectively remove most of the sludge flow rate should be between 0.12 to 0.36 m/s At a lower rate of washout of sludge does not occur. The chemical loosening the complete dissolution of iron oxide deposits are not observed, it is therefore necessary to re carrying out dynamic loosening and washing circuit. When Dina is systematic loosening of the path after the chemical from the surface of the fuel rods are removed particles of sludge 4, composed mainly of Zirconia containing mainly radionuclides95Zr and95Nb. The concentration of corrosion products of stainless steel and zirconium in the fluid also increases. As a result of washing a large portion of the slurry 4 is removed from the surface of a fuel rod, and the inactive part of the circuit. A dense oxide film 2, preventing corrosion, persist in most parts of the circuit, which allows for minimal corrosion. Figure 3 shows the equipment surface MFCC after conducting a sequential dynamic, chemical and then again dynamic loosening with the appropriate leaching. Refer to figure 3 are the same as in figure 1. From the comparison of figure 2 and 3 shows that the deactivation of the proposed method in comparison with the closest analogue has a more complete removal of the particles of corrosion products with radionuclides without damaging the surfaces of the equipment. The combination of dynamic, chemical loosening and washing allows you to get a total effect greater than the sum of the individual operations, due to the fact that each preceding phase is preparatory for the next step.

An example of a specific use of the proposed invention: comparison of the deactivation of the same RBMK-1000 reactor developed by JV the soba and method according to the closest analogue.

Processing circuit according to the method of the closest analogue produced in the following mode: raspolojennom unit when running main circulation pumps introduced a concentrated solution KPO3to achieve its concentration in the FRC 2.1 g/l, was then introduced concentrated solution of N2With2O4to the concentration of 9.8 g/l, during input of potassium nitrate and oxalic acid decontamination solution was heated to 90°With; the circulation of the solution running MCP for 31 h; then the displacement and dilution of decontamination solution to a concentration of 125 mg/l for N2With2O4the introduction of a solution of potassium nitrite to a concentration of 53 mg/l, treatment of internal surfaces lasted 56 hours.

The processing of the proposed method was carried out according to the mode: raspolojennom unit dynamic cultivation was carried out by transferring the active zone in the boiling mode when the temperature of 150-200°With torn circulation, after dynamic loosening followed by washing. The standard scheme of the extraction of suspended particles from the MFCC is a purging system stub zones (PT). During the implementation of dynamic loosening and washing the deposits of corrosion products from the active zone were derived along the lines of the system PT filters BO. Pumps cooling (HP) provided a linear flow rate in which ineach PT not less than 0.12-0.36 m/s In parallel conducted leaching drainage main circulation pumps (MCP), multigenerational (MHZ) and pipeline discharge with emergency feed pumps (NPA). Then when running MCP entered concentrated solution of KNO2to the concentration in the water circuit 2.2 g/l, the input of oxalic acid to a concentration of 10.1 g/l, the solution temperature was 93°S; the duration of the procedure chemical loosening took 5.2 hours; after dilution decontamination solution to 130 mg/l, N2With2O4and finishing concentration KNO3up to 49 mg/l treatment MFCC lasted 12 hours. Then repeat dynamic loosening and washing. The results of both decontamination presents figs.4-6. The histogram shows that the proposed method, the share of the output of the circuit is insoluble in acidic solutions95Zr and95Nb (24.7 per cent) more than in the method prototype (~19.8 per cent). The activity is removed as the chemical and dynamic richline that is because when this happens erosion corrosion products from the surface of the active zone. Points that lie above the line of equal dose (3, 5), consistent with the increasing dose after decontamination on individual sections of the circuit. The deactivation of the way - the closest analogue (item 1) the number of points is located above the dotted line (3), which is found to increase the dose after decontamination in these areas, i.e. the ratio of deactivation (KD) is less than 1, which corresponds to the pollution of the circuit. When using the proposed method, there is a decrease in the dose rate at all sites (point 2). From table 6 it is seen that the specific activity inferred corrosion products in more than 2 times higher than in the method prototype. And almost the same amount smaller than the amount withdrawn corrosion products, i.e. the quantities derived activity by both methods are close to each other. This suggests that the claimed method does not dissolve the metal equipment with greater efficacy of decontamination.

The proposed method when used in a certain sequence of dynamic and chemical loosening and leaching with arranging the movement of water when the linear velocity is not less than 0.12-0.36 m/s increases the efficiency of decontamination of primary circuit without damage to the protective oxide films and dissolution of the base metal equipment. The result is improved reliability of operation of the main equipment of nuclear power plants, which serves as a barrier to the spread of radioactivity. In addition, the more complete removal of radionuclides from the circuit conditions improve inspections and repairs, reduced individual and collective doses of the receipt of the staff.

1. The method of decontamination loop multiple forced circulation of the energy of a nuclear reactor by the influence of the coolant circuit with acid solutions and rinse, characterized in that the impact of acid solutions is carried out in the mode of chemical loosening within 2-10 hours, and before and after chemical loosening perform dynamic loosening by periodically increasing the temperature of the heat carrier in the active zone 150-200°C.

2. The method according to claim 1, characterized in that the washing operation is carried out at a linear flow rate of not less than 0.12-0.36 m/s



 

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