Method of Corrosion Rate Control of Nuclear Power Plant Process Circuit Equipment

Abstract

A method of corrosion rate control of nuclear power plants process circuits equipment. The electrochemical potential of the structural material of heat exchanging tubes and the specific electrical conductivity of blowdown water in steam generators are measured, the polarization resistance of the structural material of the pipelines of the condensate-feeding path and the specific electrical conductivity of feed water in steam generators are measured, and these parameters are automatically averaged and compared with the rated values, which determine various degrees of corrosion activity in relation to the material of pipelines of the feed water circuit in steam generators. Depending on the data comparison, no actions are taken, coolant parameters are adjusted, or the power unit is shut down.

Claims

1. A method of corrosion rate control of nuclear power plant process circuit equipment by using the measurement results related to the values of the electrochemical potential and the specific electrical conductivity of the aqueous medium, automatically averaging of these parameters and comparing them with the rated values, displaying of the values of the electrochemical potential and the specific electrical conductivity on the mnemonic diagram on the monitor screen in the form of points on a two-parameter nomogram with the coordinates of “electrochemical potential and specific electrical conductivity”, quality assessment of the water chemistry and performance of actions aimed at the corrosion rate control, wherein electrochemical potential and specific electrical conductivity of blowdown water from steam generators, as well as polarization resistance and specific electrical conductivity of feed water from steam generators are measured; the electrochemical potential and the specific electrical conductivity of blowdown water from steam generators are displayed in the form of points on a two-parameter nomogram with the coordinates of “electrochemical potential and specific electrical conductivity of the H-cationated sample” of blowdown water divided into areas A, B, D, F which determine various degrees of corrosion activity of blowdown water from steam generators in accordance with the operating conditions; the polarization resistance and the specific electrical conductivity of feed water from steam generators are displayed in the form of points on a two-parameter nomogram with the coordinates of “polarization resistance and specific electric conductivity of the H-cationated sample” of feed water divided into areas G, Y, X, Z which determine various degrees of corrosion activity of feed water from steam generators in accordance with the operating conditions; when the points are located in areas A and G, no action is taken; when the points are located in areas A and Y, the parameters of blowdown water in steam generators are adjusted within the specified period of time; when the points are located in areas B and Y, the parameters of feed water in steam generators are adjusted within the specified period of time; when the points are located in areas D or F and X or Z, the causes for deviations in such parameters shall be determined and eliminated, and, if impossible, the power unit shall be shut down for compensatory measures.

2. The method as defined in claim 1, wherein when the points are located in areas A and Y, the parameters of blowdown water of steam generators are adjusted by increasing its flow rate by (0.5-1.0) wt. % of steam capacity, and by determining and eliminating the causes for an increase in the specific electrical conductivity æ.sub.Hb.

3. The method as defined in claim 1, wherein when the points are located in areas B and Y, the parameters of feed water in steam generators are adjusted by increasing the flow through the filters of the turbine condensate purification system, while changing the flow rate of corrosion inhibitors and that of blowdown water.

4. The method as defined in claim 3, wherein hydrazine solution and/or ammonia solution and/or a solution of organic amines are used as corrosion inhibitors.

5. The method as defined in claim 1, wherein when the points are located in areas D or F and X or Z during power unit shutdown, compensatory measures include technical examination, repair or replacement of the equipment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] This method is illustrated by the drawings, where:

[0025] FIG. 1 shows a two-parameter nomogram with the coordinates of

[0026] “E.sub.h-æ.sub.Hb” for blowdown water in steam generators;

[0027] FIG. 2 shows a two-parameter nomogram with the coordinates of “R.sub.p-æ.sub.Hf” for feed water in steam generators;

[0028] FIG. 3 shows a two-parameter nomogram with the coordinates of “R.sub.p-æ.sub.Hf” for feed water in steam generators for the first corrosion rate control option;

[0029] FIG. 4 shows a two-parameter nomogram with the coordinates of

[0030] “E.sub.h-æ.sub.Hb” for blowdown water in steam generators for the first corrosion rate control option;

[0031] FIG. 5 shows a two-parameter nomogram with the coordinates of “R.sub.p-æ.sub.Hf” for feed water in steam generators for the second corrosion rate control option;

[0032] FIG. 6 shows a two-parameter nomogram with the coordinates of

[0033] “E.sub.h-æ.sub.Hb” for blowdown water in steam generators for the second corrosion rate control option.

THE BEST EMBODIMENT OF THE INVENTION

[0034] Within the scope of this invention, the method for corrosion rate control of the equipment of process circuits of nuclear power plants is implemented as follows: the working medium is supplied automatically from the standard sampling points (upstream of the filters of the steam generator blowdown water purification system and downstream of the group of high pressure feed water heaters) into the pulse tubes to the cells of the sensors measuring En of blowdown water and R.sub.p of feed water. At the same time, the working medium enters the cooler and, at room temperature, passes through the cells of the sensors of the primary circuit automatic chemical monitoring system (ACMS): feed water passes downstream of the group of high pressure heaters through the cells of the sensors measuring the specific electrical conductivity of H-cationated sample, pH.sub.f and hydrazine concentration (N.sub.2H.sub.4); blowdown water passes from the steam generators through the cells of the sensors measuring the specific electrical conductivity of H-cationated sample, pH.sub.b and concentration of sodium ([Na]), chloride ([Cl]), and sulfate ([SO.sub.4]) ions. The sensor signals are sent to the secondary converters, and then, in the adjusted form, to the computer to calculate the values of the above parameters. In this case, the following is carried out: averaging of the measured values of electrochemical potentials, polarization resistance and other automatic chemical monitoring parameters (miff, pH.sub.f, [N.sub.2H.sub.4], æ.sub.Hb, pH.sub.b, [Na], [Cl], [SO.sub.4]); recalculation of the measured values of potentials of the main electrodes into units of the standard hydrogen electrode (SHE); development of still frames of the following nomograms: “Rp and the specific electrical conductivity of the H-cationated sample” for feed water and “electrochemical potential and specific electrical conductivity of the H-cationated sample” for blowdown water. Effective information support of the operator during the equipment corrosion condition assessment and when making a decision with regard to the corrosion rate control shall be based on a combination of direct measurements in the most important circuit flows (feed and blowdown water) with the results of calculations for the physical and chemical models of circuit distribution of the concentrations of working medium impurities and issue of recommendations for possible options depending on the location of the points with “R.sub.p-æ.sub.Hf” for feed water and “E.sub.h-æ.sub.Hb” for blowdown water on the nomograms. When the points are located in areas A and G, no action is taken. When the points are located in areas A and Y, the parameters of blowdown water in steam generators shall be adjusted within the specified period of time. When the points are located in areas B and Y, the parameters of feed water in steam generators shall be adjusted within the specified period of time. When the points are located in areas D or F and X or Z, the causes for deviations in such parameters shall be determined and eliminated, and, if impossible, the power unit shall be shut down for compensatory measures.

[0035] The rate of corrosion of structural materials with the points referring to the corrosion activity of the working medium located beyond areas A and G is controlled as follows: changing of the concentrations of inhibitors by adjusting the flow rate of the latter into the feed water; changing of the working medium flow rate through the filters of the turbine condensate purification system in case of a leakage in the condenser group; changing og the blowdown rate of steam generators; changing, as necessary, of the planned dates for washing, decontamination and technical examination of steam generators.

[0036] In particular, when the points are located in areas A and Y, the parameters of blowdown water in steam generators are adjusted by increasing its flow rate by (0.5-1.0) wt. % of steam capacity, and by determining and eliminating the causes for an increase in the specific electrical conductivity æ.sub.Hb.

[0037] When the points are located in areas B and Y, the parameters of feed water in steam generators are adjusted by increasing the flow through the filters of the turbine condensate purification system, while changing the flow rate of corrosion inhibitors and that of blowdown water. Hydrazine solution and/or ammonia solution and/or a solution of organic amines are injected as corrosion inhibitors.

[0038] When the points are located in areas D or F and X or Z during power unit shutdown, compensatory measures include technical examination, repair or replacement of the equipment.

[0039] This method of corrosion rate control of the equipment in process circuits of nuclear power plants enables the operator to visually assess the location of points corresponding to the current state with respect to the boundaries of the areas with various degrees of corrosion activity of feed and blowdown water. Taking into account the nonlinear nature of the boundaries (FIG. 1, FIG. 2), such a visual assessment contributes, in case of deviations and violations, to determination of the shortest distance from the point to the boundary of the area with low corrosion activity (required impact vector). Using vector projections on the axis of both nomograms (primarily, æ.sub.Hf and æ.sub.Hb), it is possible to obtain the values of the minimum required changes in the controlled parameters to reduce the corrosion activity of the working medium. Maintenance organization, sampling, installation of sensors, converters, fittings, electrical grids and signal cables of the automatic chemical monitoring system are carried out at the power units of nuclear power installations according to the relevant technical documentation. The data shall be recorded on a computer, calculated, archived and communicated to operators using custom designed software.

[0040] Parameters of feed water, to a greater extent, refer to the iron concentration values. At higher values of æ.sub.Hf and lower values of R.sub.p, iron removal and, thus, its concentration are higher. This may occur due to the lack of an inhibitor (hydrazine, ammonia, organic amine), an increased content of strong acid anions and carbonates or due to an unfavorable ratio of the listed components (more often, in case of a change in the power unit capacity). Parameters of blowdown water mainly refer to the concentration of salts in the immediate vicinity of the heat exchanging tubes. The more positive the E.sub.h value and the higher the æ.sub.Hb value, the more salt impurities are retained in the iron oxide deposits on the heat exchanging tubes, the higher the activity of the working medium in steam generators and the higher the possibility of pitting and subsequent stress corrosion cracking of the heat exchanging tubes.

[0041] Certain examples demonstrating the efficiency of this method when operating a VVER-1200 NPP while implementing the procedure for water chemistry quality evaluation according to this method are given below.

INDUSTRIAL APPLICABILITY

[0042] Example 1. FIG. 3 and FIG. 4 show the nomograms for feed and blowdown water during power operation of the power unit with VVER-1200. Operating conditions are as follows: the condensate flow through the filters of the turbine condensate purification system is approximately 10 wt. % of the steam flow through condensers, i.e. 340 t/h; the design cooling water leak through poorly sealed parts of the condenser group is 10.sup.−5 wt. % of the steam flow through condensers, i.e. 0.32 kg/h; the concentration of chlorides in cooling water is 5259 mg/dm.sup.3, the concentration of sulfates is 530 mg/dm.sup.3 and, accordingly, the intake of salt impurities in the form of anions of strong acids into the secondary circuit is 1687 mg/h for chlorides and 170 mg/h for sulfates; the total blowdown of steam generators is at the minimum level and amounts to approximately 0.5 wt. % of the steam capacity, i.e. 30 t/h. The injection of corrosion inhibitors is compliant with the requirements of STO 1.1.1.03.004.0979-2014 “Water Chemistry of the Secondary Circuit during of the Nuclear Power Plant Unit Commissioning under AES-2006 Project. Quality Standards of the Working Medium and Supporting Tools” issued by OJSC Rosenergoatom Concern (http://www.snti.ru/snips_rd3.htm). The inhibitors are ammonia (25% aqueous solution), hydrazine and ethanolamine. FIG. 3 shows a two-parameter nomogram with the coordinates of “polarization resistance and specific electrical conductivity æ.sub.Hf of the H-cationated sample” for feed water. The point (1-0) with coordinates (R.sub.p, æ.sub.Hf) is located in area G and represents a high quality of feed water, in particular, it demonstrates that the concentration of anions of strong acids is very small: 1.11 μg/kg for chlorides, 0.14 μg/kg for sulfates, the specific electrical conductivity of the H-cationated sample, æ.sub.Hf=0.069 μS/cm (the value of the specific electrical conductivity of theoretically pure water is 0.055 μS/cm). A two-parameter nomogram with the coordinates of “electrochemical potential and specific electrical conductivity of the H-cationated sample” for blowdown water of steam generators for the case under consideration is shown in FIG. 4. The point (2-0) with coordinates (E.sub.h, æHb) is located in area B due to the fact that as a result of evaporation that took place in salt compartments of steam generators, the concentration of chlorides reached approximately 128 μg/kg and that of sulphates reached 20 μg/kg. The value of æ.sub.Hb amounts to 1.78 μS/cm. Considering that the volume of cooling water coming from the environment through the poorly sealed parts of the equipment (suction devices) and, accordingly, the concentration of salt impurities in the turbine condensate are insignificant, the increase in the flow of the working medium through the filters of the purification system is undesirable. The working medium, including the condensate, contains reagents, i.e. corrosion inhibitors, ammonia and ethanolamine, concentrations of which exceed the content of salt impurities by more than a thousand times. The filters of the purification system absorb all the impurities and, primarily, the corrosion inhibitors. As a result: the working capacity of the filters is reduced, and more ammonia and ethanolamine must be added to the working medium to maintain the required quality of the water chemistry. Under these circumstances, it is more expedient to increase the blowdown rate of steam generators from 0.5 wt. % of the total steam capacity of steam generators to approximately 1 wt. %, i.e. from 30 t/h to 64 t/h. After increasing the blowdown water flow rate up to 64 t/h, the location of points on the nomograms changed. The point with (R.sub.p, æ.sub.Hf) coordinates moved to the position (1-1) on the nomogram shown on FIG. 3. At the same time, the concentration of chlorides changed to 0.51 μg/kg and that of sulfates changed to 0.08 μg/kg, æ.sub.Hf became 0.062 μS/cm. The point with (E.sub.h, æ.sub.Hb) coordinates moved to the position (2-1) in the green area on the nomogram shown on FIG. 4. At the same time, the concentration of chlorides changed to 32.7 μg/kg and that of sulfates changed to 5.18 μg/kg, æ.sub.Hf became 0.49 μS/cm. Taking into account the high concentration of chlorides in the “salt compartments” (30 μg/kg), the following compensatory action is recommended. When the power during the transient modes is reduced, the blowdown flow rate to the filters of the blowdown water purification system shall be increased to the design maximum value (140 t/h) to remove the salt impurities passing under these conditions from the deposits to the steam generators, and to minimize the possibility of pitting corrosion defects on heat exchanging tubes.

[0043] Example 2. FIG. 5 and FIG. 6 show the nomograms for feed and blowdown water during power operation of the power unit with VVER-1200. Operating conditions are as follows: the condensate flow through the filters of the turbine condensate purification system is approximately 10 wt. % of the steam flow through condensers, i.e. 340 t/h; the design cooling water leak through poorly sealed parts of the condenser group is 10.sup.−5 wt. % of the steam flow through condensers, i.e. 0.32 kg/h; the concentration of chlorides in cooling water is 3143 mg/dm.sup.3, the concentration of sulfates is 363 mg/dm.sup.3 and, accordingly, the intake of salt impurities in the form of anions of strong acids into the secondary circuit is1008 mg/h for chlorides and 117 mg/h for sulfates; the total blowdown of steam generators is at the minimum level and amounts to approximately 1 wt. % of the steam capacity, i.e. 64 t/h. The injection of corrosion inhibitors is compliant with the requirements of STO 1.1.1.03.004.0979-2014 “Water Chemistry of the Secondary Circuit during of the Nuclear Power Plant Unit Commissioning under AES-2006 Project. Quality Standards of the Working Medium and Supporting Tools” issued by OJSC Rosenergoatom Concern (http://www.snti.ru/snips_rd3.htm). The inhibitors are ammonia (25% aqueous solution), hydrazine and ethanolamine. FIG. 5 shows a two-parameter nomogram with the coordinates of “polarization resistance and specific electrical conductivity æ.sub.Hf of the H-cationated sample” for feed water. Point 5 with (R.sub.p, æ.sub.Hf) coordinates characterizing the corrosion activity of feed water is located in the area between G and Y in the immediate vicinity of area Y. The concentrations of strong acid anions are very small: 0.31m/kg of chlorides and 0.06m/kg of sulfates, the specific electrical conductivity value æ.sub.Hf of the H-cationated sample is 0.22 μS/cm. A two-parameter nomogram with the coordinates of “electrochemical potential and specific electrical conductivity æ.sub.Hb of the H-cationated sample” for blowdown water of steam generators for the case under consideration is shown on the nomogram in FIG. 6. Point 6with (E.sub.h, æ.sub.Hb) coordinates is partially located in area A adjacent to area B. The controlled parameters of blowdown water of “salt compartments” of steam generators are as follows: the concentration of chlorides is 19.7m/kg, the concentration of sulfates is 4.2 μg/kg. The value of æ.sub.Hb amounts to 1.15 μS/cm. Theoretical calculation of the specific electrical conductivity values of the H-cationated sample with due account for controlled anions, chlorides and sulfates only, provides the following values: 0.059 μS/cm for æ.sub.Hf and 0.28 μS/cm for æ.sub.Hb. It is quite clear that the process circuit working medium contains uncontrolled salt impurities. It is most likely that these impurities contain fluorides or carbonates of various origins. Carbon dioxide can enter the circuit: from the air in the suction devices of the vacuum part of the turbine; during the thermolysis of both organic amines and neutral organic impurities that are not retained by the filters of the water treatment systems. Carbon dioxide is neutralized with alkaline corrosion inhibitors, so it is not removed from the deaerators. Recommended actions: increase of the flow rate through purification systems up to the maximum available values; improvement of the pressurization of the turbine equipment; improvement of water treatment systems; isolation of makeup water storage tanks, as well as planning of the extended technical examination of the circuit equipment and pipelines in order to determine the areas with corrosion wear exceeding the rated values.

[0044] Thus, the use of this method in practice will allow to actually increase the efficiency of evaluation of the corrosion activity of the coolant in the process circuit of the power unit of a nuclear power plant and, accordingly, to increase the reliability of operation of the equipment of process circuits of nuclear power plants and, mainly, steam generators, by means of timely compensatory measures. In addition, the reliability of determination of the period between inspections increases, which allows to justify its increase and/or the scope of technical examination of various sections of the condensate-feeding path and steam generators while ensuring the level of reliability and safety during operation of the nuclear power plant established by the project.