RU2120143C1 - Water chemistry organizing process - Google Patents

Abstract

FIELD: nuclear power engineering; nuclear power plants using water as coolant. SUBSTANCE: water chemistry organizing process includes introduction of reactive aluminum as correcting additive into coolant circulating in loop; in the process, its molar concentration P_Al in coolant relative to molar concentration of bivalent iron P_Fe in coolant is maintained in proportion meeting condition of P_Al/P_Fe higher than 2. EFFECT: reduced rate of fouling heating surfaces and improved radiation conditions during repairs. 4 cl, 1 dwg, 1 tbl

Description

 The invention relates to the technology of nuclear power plants (AEU) using water as a heat carrier, and in particular to the technology of their water-chemical regime.

 The main tasks of the water-chemical regime of nuclear power plants are: maintaining normalized heat carrier parameters in terms of pH and electrical conductivity, acceptable impurity content, reducing the general and local types of corrosion of structural materials, reducing the rate of formation of deposits on heat-transferring surfaces and improving the radiation situation.

 A known method of organizing a redox water-chemical regime for plants of the WWR type is to introduce hydrazine additives into the coolant (V.I. Pashevich, Report at the All-Union Conference on Water-Chemical Modes and Chemistry of Nuclear Carrier Fluids, L. 1978). In this case, with an increase in the total concentration of radioactive corrosion products in the coolant, a decrease in the level of radioactive contamination of the inner surfaces of the circuits is observed.

 The disadvantage of this regime is the strict requirement to maintain the concentrations of thermally unstable hydrazine in the coolant in a fairly narrow range at a sufficiently low level. Going beyond this interval either leads to increased radiation contamination of the internal surfaces, or causes increased corrosion of the welded joints of the zirconium alloy.

A known method of organizing a water-chemical regime, characterized by the introduction of corrective additives of zinc in the coolant (Water Chemistry of Nuclear Reactor Systems 7. BNES, 1996). The addition of zinc ions, reducing the rate of formation of spinel structures containing cobalt on heating surfaces, contributes to a decrease in the rate of formation of Co ⁶⁰ on these surfaces, to a decrease in its release to the coolant and to a decrease in the level of radioactive contamination of the contour surfaces. For example, the addition of zinc in an amount of up to 15 μg / L approximately halves the activity of Co ⁶⁰ surface deposits compared with the regimen without the introduction of additives. However, this corrective additive does not affect the rate of formation of deposits on heating surfaces, does not cause changes in the direction of transfer of radionuclides, does not change the proportion of corrosion products and radionuclides removed bypass cleaning.

 The objective of the invention is to reduce the rate of formation of deposits on heating surfaces and improve the radiation environment during repair.

 In addition, the developed method increases the role of bypass cleaning when removing corrosion products from the circuit and helps to reduce the output of the highly active sludge component during transient operation of nuclear power plants.

The problem is solved as follows. The method of organizing the water-chemical regime of AEU consists in introducing reactive aluminum as a correcting additive in the coolant circulating in the circuit, while maintaining its molar concentration P _Al in the coolant relative to the molar concentration of ferrous iron P _Fe in the coolant in a ratio satisfying the condition P _Al / P _Fe more than 2.

Aluminum is dosed so that the number of introduced moles of Al ⁺³ exceeds the number of incoming moles of Fe ⁺² not less than 2 times.

 The need to establish a certain ratio between the introduced aluminum and ferrous ions is especially important when a significant part of the iron enters the circuit in the form of its trivalent forms, which cannot form compounds with aluminum.

 For single-circuit plants, reactive aluminum is introduced into the coolant as part of the feed water or condensate by passing the feed water or condensate through a system containing aluminum.

 Dosing of aluminum is carried out in the circuit or bypass cleaning line by passing the coolant through a system containing aluminum, for example, an aluminum alloy.

 The novelty of the invention is characterized by a new set of interrelated operations and their conditions.

 The introduction of reactive aluminum promotes the formation in the coolant of iron aluminates, the solubility of which is significantly higher than the solubility of iron oxide compounds and iron ferrites.

 In the presence of reactive aluminum and ferrous iron in the coolant, the predominant formation of aluminates is observed instead of magnetite and hematite. At the same time, instead of ferrites, including ions of divalent metals (cobalt, zinc, nickel, etc.), mainly aluminates are formed, including these ions. At an aluminum concentration lower than the concentration of divalent metals, the pH of the coolant does not change. Only when the concentration of aluminum exceeds the concentration of divalent metals does pH change. Moreover, for aluminum concentrations up to 0.5 mg / l, pH changes insignificantly.

 Thus, some optimal ratio of the concentration of ionic forms of iron and aluminum in the coolant is necessary, which, on the one hand, provides a reduction in the deposition of iron oxide corrosion products (including Co-containing) on the heating surface, and on the other hand, inhibits the formation of aluminate deposits on the surfaces of pipelines and channel equipment.

 The redistribution of iron, divalent metal ions and radionuclides between aluminum and iron oxide forms helps to reduce the formation of deposits on heating surfaces, to increase the role of bypass cleaning when removing both corrosion products and radionuclides from the circuit.

 Dosing of aluminum allows you to control the process of transfer of radionuclides, to regulate the level of accumulation of radioactivity on the surfaces of the circuit and the level of accumulation of highly active sludge. The latter circumstance allows us to consider the dosing of aluminum, not only as a way to control the transfer and accumulation of radionuclides during stationary operation of the installation, but also as a method of decontamination before a long shutdown.

 Thus, the invention meets the criterion of "significant differences".

 Example 1. In laboratory conditions, the decrease in the influence of aluminum on the amount of deposits on heating surfaces was studied.

 To do this, we used a laboratory setup that included the following elements: a generator of corrosion products with replaceable electrodes, which allows it to be used for electrochemical dosing into a solution of aluminum ions, an electrophoretic cell with flowing electrode spaces, and an indicator of the rate of formation of deposits of corrosion products on fuel surfaces.

The model solution had indicators:
Conductivity, μOhm / cm - 100 + 10
pH - 6.2 + 0.2
The total concentration of iron, mg / l - 0.5 + 0.2
The total concentration of aluminum, mg / l - 0.5 + 0.2
The rate of deposition was determined at a flow rate of 100 l / h, a heat flux of 2.8 MW / m ² . The accumulation time of practically determined amounts was no more than 2 hours.

 It was found that in a closed system, the rate of formation of deposits on the heating surface decreases with time.

 In the table. 1 shows data on the effect of dosing aluminum alloys in tap water of the above composition.

 As can be seen from the table. 1, the dosage of the products of anodic dissolution of aluminum leads to a sharp decrease in the value of deposits (about 3 times). Exposure of the working medium for 5 hours after dosing of aluminum gives a 10-fold reduction in deposits compared with the experiment without dosing of aluminum.

 An additional experiment conducted on water of the same chemical composition, but without aluminum, showed that aluminum in tap water does not affect the rate of deposition.

 The decrease in the rate of formation of deposits is associated with the dosing of reactive aluminum in ionic form.

 Example 2. Bench studies were carried out to determine the effect of dosing aluminum in an offsetless neutral water-chemical regime.

 The aim of the research was to study the effect of aluminum additives on the corrosion processes of structural materials and the formation of deposits of corrosion products on the heating surface in the circuit for an apparatus such as RBMK.

Test Parameters:
The temperature of the coolant is 270 ^o With
Circuit pressure - 7.0 MPa
The pH at 25 ^o With - 6.8
Thermal conductivity - 0.6-0.8 μOhm / cm
The oxygen content in the circuit (ref.) Is 8.0 mg / l.

 Dosing of aluminum in the coolant was carried out by placing samples of aluminum alloy B1T in an autoclave located on the bypass line of the stand. Depending on the temperature of the incoming coolant and its speed, it was possible to vary the amount of aluminum entering the circuit. The maximum concentration did not exceed 0.05-0.1 mg / L. The coolant parameters were maintained at the level of the parameters of the main circuit of the RBMK type apparatus.

 The results of experimental studies are presented in the drawing. The axes show time (in hours) and the concentration of iron (Fe) and aluminum (Al) in units of mg / l.

The research results showed:
- the pH during the first day increased from 6 to 7.6, and then stabilized at this level;
- the concentration of aluminum increased during the first day and stabilized at a value of 0.5 mg / l;
- the concentration of iron oxide corrosion products in the circuit continuously increased and at the time of shutdown was 1.1 mg / l;
- the value of iron oxide deposits of corrosion products on the heater averaged 120-150 μg; (in the absence of dosing of aluminum 1.8 mg);
- aluminum in the composition of the deposits was less than the limit of sensitivity of the measurement method, of the order of 3 μg.

Aluminum deposits appear on the surface of the contour, while these deposits are gray. After a sufficiently long exposure in air, their color changes to orange, characteristic of hematite Fe ₂ O ₃ . This indicates the presence of divalent iron in the composition of the deposits, which, after exposure to air, turns into ferric iron.

 When the stand was operated without dosing aluminum on the heating surfaces, the formation of black deposits characteristic of magnetite was observed.

The results allow us to conclude that the introduction of additional surfaces made of aluminum alloy B1T leads to a decrease in the rate of formation of iron oxide deposits by more than 10 times. The deposition rate of aluminum corrosion products is extremely low (less than 3 μg / (m ² • h).

 Thus, based on the data presented, it follows that a positive effect on the effect of aluminum dosing is observed in the range of both normal temperatures and temperatures characteristic of nuclear facilities.

 Example 3. Bench tests were carried out simulating the operating conditions of AST.

 The stand worked in bypass cleaning mode.

 The aim of the research was to determine the effect of introducing additional anode surfaces made of aluminum alloys on the rate of formation of deposits of iron corrosion products on fuel surfaces and the kinetics of changes in the concentration of corrosion products in the test bed coolant.

 The reactor loop had a closed circular loop. The heat carrier using the main circular pump is fed to the input of the loop channel in which the fuel assemblies are installed. The steam-water mixture formed in the channel is sent to a separator, where the vapor-gas mixture and water are separated. The gas-vapor mixture enters the loop condenser, and the separated water is partially cooled in heat exchangers designed to control the temperature of the coolant, and enters the inlet of the main circular pump. Part of the coolant circulating in the loop with a flow rate of 100 l / h is sent to the bypass cleaning system. Before cleaning, the coolant was cooled in a heat exchanger. The loop used gas compensation for volume changes using helium as a compensating gas.

 The loop has a coolant sampling system with sampling points: at the entrance to the loop channel, at the outlet of the loop channel, steam at the outlet of the separator, separated water, gas-vapor mixture from the condenser, condensate at the outlet of the condenser, water at the entrance and exit of the system cleaning up.

 The technological parameters of the loop during the experiment were as follows.

Pressure at the inlet to the channel, MPa - 2.3-2.5
The pressure at the outlet of the channel, MPa - 1.6
The temperature at the entrance to the channel, ^o C - 150-160
The temperature at the outlet of the channel, ^o С - 200
The flow rate through the channel, m ³ / h - 10.2
The linear velocity of the coolant in the channel, m / s - 1.1-1.7
Thermal power of the channel, kW - 600
The average heat flux from the surface of a fuel rod, kcal / m ² • h - 0.36 • 10 ⁶
The average specific load of the core volume in the channel, kW / l - 50
Loop loop volume, m ³ - 0.8
Samples made of BB1T alloy were installed in the technological channel of the working section of the stand. Heat carrier samples for determining the content of corrosion products of iron and aluminum were taken at a frequency of 1 time per day. The duration of the experiment was 84 hours.

During the experiment, the concentration of ferrous iron in the coolant was determined, which amounted to 60 μg / kg and the total concentration of reactive aluminum, which amounted to 57 μg / kg. When converted to molar concentrations, the ratio of the molar concentration of aluminum to the molar concentration of iron P _Fe in the coolant is 2.1 under the condition of a corrective mode. In the case of the hydrogen-helium regime, the ratio of the molar concentration of aluminum to the molar concentration of iron will be 2.8.

 These ratios corresponded to the rate of aluminum intake with respect to ferrous iron of more than two.

 To determine the concentration of iron in the coolant, the accumulative method was used, based on passing a certain volume of water through nuclear membranes with an alluvial layer.

When the stand was operating in the mode without aluminum, there was no iron in the filtrate. When the stand was operated with aluminum both in neutral and in hydrogen-helium mode, almost all of the iron and aluminum were in the filtrate, which indicates the formation of soluble compounds of the type FeAl ₂ O ₄ .

 When examining the internal surfaces and heating elements, no gray deposits were found, all the surfaces of the stand were black, characteristic of magnetite.

Example 4. To determine the effect of reactive aluminum on reducing the dose rate of gamma radiation from the equipment of the reactor plants, comparative tests were conducted on two reactor loops, the contours of which were made of stainless steel OX18H1OT. The coolant of the loop installations is demineralized water, the radiolysis of the coolant was suppressed by a helium-hydrogen mixture, the temperature of the coolant in the area of the fuel elements (FA) was 120-150 ^o C. The temperature of the coolant supplied to the ion exchange cleaning did not exceed 60 ^o C. A loop installation in which the aluminum concentration was maintained at the level of 10 μg / kg, had 30 fuel assemblies, the loop installation, into which aluminum was not dosed, had 2 fuel assemblies. The concentration of cobalt 60 in the coolant of both loops was at the level of 10 ^-7 ki / l. The research results are presented in table. 2.

 The dose rate of gamma radiation from equipment in a loop with 30 assemblies with the introduction of aluminum was 15–40 times lower than in a loop with 2 assemblies without additives.

 From the above examples, it can be concluded that the dosage of reactive aluminum leads to a decrease in the rate of formation of deposits on heating surfaces and an improvement in the radiation situation during repairs, a decrease in the value of deposits, a decrease in the yield of highly active sludge component during transient conditions, the transition of the bulk of corrosion products to the dissolved state contributes to enhancing the role of bypass cleaning.

 Maintaining the concentration of aluminates below the solubility limit helps to reduce levels of radioactive contamination of the inner surfaces of the circuit and increase the role of bypass cleaning when removing the bulk of corrosion products and radionuclides from the circuit.

Claims (4)

1. The method of organizing the water-chemical regime of nuclear power plants, including the introduction of corrective additives in the coolant circulating in the circuit, characterized in that reactive aluminum is introduced into the coolant and its molar concentration of P _Al in the coolant is maintained relative to the molar concentration of ferrous iron P _Fe in the coolant in a ratio satisfying the condition P _Al / P _Fe more than 2. 2. The method according to p. 1, characterized in that the aluminum is dosed so that the number of introduced moles of Al ⁺³ exceeds the number of incoming moles of Fe ⁺² not less than 2 times.  3. The method according to p. 1, characterized in that for single-circuit plants, reactive aluminum is introduced into the coolant as part of the feed water or condensate by passing the feed water or condensate through a system containing aluminum.  4. The method according to p. 1, characterized in that the dosing of aluminum is carried out in the bypass cleaning line by passing the coolant through a system containing aluminum, for example an aluminum alloy.

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