Patent Application
20080137796
This invention relates generally to nuclear reactors and more particularly, to a method for reducing the corrosion of carbon steel and low alloy steel components in nuclear reactors.
High temperature, high pressure water can be found in a variety of known apparatus, such as water deaerators, nuclear reactors and steam-driven central station power generation systems. For example, a nuclear reactor pressure vessel contains a reactor coolant, e.g., water, which removes heat from the nuclear reactor core. Piping systems and/or circuits carry the heated water or steam to the steam generators or turbines, and circulate feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 megapascals (MPa) and 288° C. for a boiling water nuclear reactor (BWR), and about 15 MPa and 320° C. for a pressurized water nuclear reactor (PWR). The components and materials used in BWRs and PWRs must withstand various loading, environmental and radiation conditions.
Carbon steel components are used extensively in low pressure and high pressure turbine sections, and feedwater heaters and bottom drain lines in nuclear power plants including many ancillary components of the steam-water circuit, such as moisture separators and reheaters. In these components, high flow velocities under single-phase water or two-phase wet steam conditions prevail. Despite careful selection and treatment of these components, corrosion occurs when the components are exposed to the high temperature, high pressure reactor water. Such corrosion contributes to a host of problems including stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves and buildup of gamma radiation-emitting isotopes, such as a Co-60.
In nuclear reactors, the feedwater and bottom drain line piping system components are subjected to flow assisted corrosion. As used herein, references to the term “flow assisted corrosion” are to be understood to refer to an effect of fluid flow that accelerates general corrosion by increasing the rate of mass transport of reactive species to and from the metal surface, and acceleration or increase in the rate of corrosion caused by the relative movement between a corrosive fluid and the metal surface. Corrosion products in reactors cause problems relating to radiation level, radioactive waste and heat transfer. Thus, controlling corrosion within nuclear reactor components is important to maintain the nuclear reactor functioning safely and properly.
Damage associated with flow assisted corrosion of such power plant components generally occurs at locations where there is severe fluid turbulence adjacent to the metal surface, either from high fluid velocities or due to the presence of features such as bends or orifices that generate high local turbulence levels. A thin layer of oxide having a thickness of about one micron or less is normally present on the corroding surface, but the rate of penetration in these localized areas deprived of a characteristic double layer oxide film can reach values as high as 0.1 to 10 millimeters per year. Such rates of metal removal are unacceptable in power plants which have a design lifetime of 30 to 40 years, but even significantly lower rates of penetration may generate undesirable high concentrations of corrosion products in the water circuits.
It is known that oxygen can be added to low-oxygen, hydrazine or hydrogen water, e.g. the feedwater of nuclear reactors, to decrease flow assisted corrosion of carbon steel components. However, it is well documented that stress corrosion cracking of stainless steel, low alloy steel, and nickel based alloys occurs at higher rates when oxygen is present at concentrations of about 5 parts per billion or greater in the high temperature water of a nuclear reactor. Therefore, it is desirable to minimize oxygen concentration in reactor water to reduce stress corrosion cracking of stainless steel, low alloy, and nickel based alloy components.
Although no comprehensive model of flow assisted corrosion which can fully describe the effect of the variables mentioned above has yet been developed, it is believed that the corrosion rates observed at high flow velocities are due to enhanced dissolution of magnetite leading to accelerated metal loss as iron oxidizes to replace the dissolved film.
In one aspect, the present invention provides a method for reducing corrosion of carbon steel components and/or low alloy steel components in a nuclear reactor having an oxide film layer formed on a surface thereof. The method includes injecting a solution of a compound containing zinc into a supply of feedwater introduced into the nuclear reactor. The compound is decomposed under operating reactor thermal conditions to release ions of zinc and/or atoms of zinc. The ions of zinc and/or the atoms of zinc are introduced into the oxide film layer to increase a corrosion resistance of carbon steel when incorporated into the oxide film layer.
In another aspect, the present invention provides a method for improving an erosion corrosion resistance of a carbon steel component including a surface having an oxide film thereon. The method includes immersing the carbon steel surface in high temperature water in which a compound containing zinc is dissolved. The compound is decomposed in the high temperature water to release ions of zinc and/or atoms of zinc. The ions of zinc and/or the atoms of zinc are incorporated into the oxide film such that the ions of zinc and/or the atoms of zinc increase the erosion corrosion resistance of the carbon steel.
In another aspect, the present invention provides a method for reducing corrosion of a carbon steel component and/or a low alloy steel component for a nuclear reactor containing low oxygen feedwater. The method includes adding one of a zinc metal and a zinc oxide as an alloying element to the carbon steel component and/or the low alloy steel component in a concentration not greater than about 20 weight %.
The present invention provides a method for reducing the corrosion of carbon steel and/or low alloy steel components in high temperature, high pressure water. The present invention is described below in reference to its application in connection with and operation of a boiling water nuclear reactor (BWR). However, it will be obvious to those skilled in the art and guided by the teachings herein provided that the present invention is not limited to its application in connection with a BWR. Rather, the present invention may be utilized in other structures in which structural components are exposed to high temperature, high pressure water environments. Such structures include, but are not limited to, pressurized water nuclear reactors (PWRs), such as Russian VVER reactors, pressurized heavy water reactors (PHWRs) such as the Canada Deuterium Uranium (CANDU) reactor, steam-driven turbines, water deaerators and the like. Thus, referring to the drawings in general and to
As used herein, references to “high temperature water” are to be understood to refer to water, steam and/or the condensate thereof, generally having a temperature between about 50° C. and about 350° C., and about 100° C. to about 330° C. in a particular embodiment.
Referring to
A fluid flow through BWR 10 is generally described with reference to
Heat is generated within reactor core 22, which includes fuel assemblies 40 of fissionable material. Water circulated up through reactor core 22 is at least partially converted to steam. In one embodiment, the water enters fuel assemblies 40 disposed within reactor core 22, wherein a boiling boundary layer (not shown) is established. A mixture of water and steam enters core upper plenum 48 under shroud head 26. Core upper plenum 48 provides standoff between the steam-water mixture exiting reactor core 22 and entering vertical standpipes 50, which are disposed atop shroud head 26 and in fluid communication with core upper plenum 48.
The steam-water mixture flows through standpipes 50 and enters steam separators 52. Steam separators 52 separate steam from water, which is recirculated. Residual water is removed from the steam by steam dryers 54. The steam exits RPV 11 through a steam outlet 56 near top head 14. The separated liquid water then mixes with the feedwater in mixing plenum 58, and the mixture then returns to reactor core 22 through annulus 28.
In one embodiment, BWR 10 also includes a coolant recirculation system 59 that provides a forced convection flow through the core necessary to attain the required power density. A pump deck 60, which has a ring shape, extends between shroud support 24 and side wall 16. Pump deck 60 includes a plurality of circular openings 62. Each opening 62 houses a jet pump assembly 64. Jet pump assemblies 64 are circumferentially distributed around shroud 20. A portion of the water is sucked from the lower end of annulus 28 via recirculation water outlet 66 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 64 (only one of which is shown) through recirculation water inlets 68. BWR 10 has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies 64. The pressurized driving water is supplied to each jet pump nozzle 70 through an inlet riser 72, an elbow 74 and an inlet mixer 76. In one embodiment, BWR 10 includes 16 to 24 inlet mixers 76.
Erosion-corrosion is frequently observed in regions of highly disturbed cooling water flow during the operation of BWR 10 under hydrogen water chemistry conditions, such as within the coolant recirculation system and/or the piping systems and conduits carrying heated water or steam, as well as the piping systems and conduits circulating the feedback water through RPV 11. Carbon steel is used extensively in components of BWR 10 and erosion-corrosion of carbon steel in water and wet steam is known to be a major failure problem.
In one embodiment, a method for reducing corrosion of carbon steel components in a water-cooled nuclear reactor and/or associated components having an oxide film layer formed on a surface thereof is provided. In this embodiment, the method includes injecting a solution of a compound containing zinc into the water of the water-cooled nuclear reactor, such as into a supply of feedwater introduced into the nuclear reactor. In a particular embodiment, the temperature of the feedwater is maintained at about 100° C. to about 330° C. Further, the feedwater is maintained in a reducing condition, e.g., low oxygen content, such as low-oxygen, hydrazine or hydrogen water chemistry having an oxygen concentration of less than about 15 ppb. The compound decomposes and/or dissolves under operating reactor thermal conditions to release ions and/or atoms of zinc which incorporate into the oxide film layer. Because the zinc additions become ionic (Zn++ and/or ZnOH+) in high temperature water, the various forms of zinc ions and/or zinc atoms are incorporated into the oxide film layer.
Upon injection of a solution of a compound containing zinc into the feedwater, the compound undergoes decomposition and/or dissolves under operating reactor thermal conditions to release ions and/or atoms of zinc at a rate such that the concentration of zinc in the feedwater is sufficient, once incorporated into the oxide film layer, to enhance a corrosion resistance of the carbon steel component. Such corrosion includes, for example, a flow assisted corrosion and/or an erosion corrosion. Further, the incorporation of zinc into the oxide film layer increases a resistance of the oxide film layer to rupture, as well as increases a strain rate of the oxide film layer.
In one embodiment, the zinc-containing compound includes depleted ZnO, commercial ZnO, Zn compounds like ZnCl2, Zn(NO3)2, Zn acetate, ZnBr2, ZnSO4, fumed Zn compounds, nanoparticles of pure Zn and/or nanoparticles of Zn compounds. The compound is injected into a supply of the feedwater in an amount sufficient to produce a zinc concentration of about 0.1 ppt to about 200 ppb, and about 50 ppb to about 150 ppb in a particular embodiment. In an alternative embodiment, the compound contains a plurality of zinc and/or zinc oxide nanoparticles that are injected into the feedwater. The zinc and/or zinc oxide nanoparticles have a mean diameter not greater than about 100 nanometers. In other alternative embodiments, at least a portion of the zinc and/or zinc oxide nanoparticles have a mean diameter greater than about 100 nanometers depending upon the solubility of the zinc-containing compound injected into the feedwater. The nanoparticles are distributed and/or redistributed on the oxide film layer in response to an interaction with electrostatic forces of the feedwater.
The zinc-containing compound or the zinc-containing nanoparticles can be introduced into the high temperature, high pressure water during various stages of operation of BWR 10. Further, the zinc-containing compound or the zinc-containing nanoparticles are provided to the high temperature, high pressure water during full power operation, cool down, outage, heat-up, hot standby and/or low power operation of BWR 10.
The zinc-containing compound or the zinc-containing nanoparticles are introduced into the high temperature, high pressure water at any location within BWR 10 where thorough mixing of the zinc-containing compound or the zinc-containing nanoparticles occurs. The locations at which the zinc-containing compound or the zinc-containing nanoparticles are introduced include, without limitation, a residual heat removal piping, coolant recirculation system 59, jet pump assemblies 64, feedwater lines such as at feedwater inlet 30 and/or feed water sparger 32, core delta P lines, control rod drive cooling water lines, water level control points and/or reactor water clean-up systems.
In one embodiment, the erosion corrosion resistance of a carbon steel surface having an oxide film thereon is increased by immersing the carbon steel surface in high temperature water in which a compound containing zinc is dissolved. The compound includes depleted ZnO, commercial ZnO, Zn compounds like ZnCl2, Zn(NO3)2, Zn acetate, ZnBr2, ZnSO4, fumed Zn compounds, nanoparticles of pure Zn and/or nanoparticles of Zn compounds. As the compound decomposes and/or dissolves in the high temperature water, atoms of zinc are released from the compound, which incorporate into the oxide film. Zinc increases the erosion corrosion resistance of the carbon steel when incorporated into the oxide film. In this embodiment, the compound dissolved within the high temperature water produces a zinc concentration not greater than about 150 ppb, and about 100 ppb to about 150 ppb in a particular embodiment. In alternative embodiments, the dissolved compound produces a zinc concentration greater than about 150 ppb.
By introducing compounds and/or nanoparticles containing Zn or ZnO into high temperature water, the Zn-containing oxide, ZnFe2O4, enhances a strain rate of the carbon steel oxide and, thus, increases the resistance of the oxide film layer to rupture. The fine particulates or nanoparticles are responsive to electrostatic forces or zeta potentials in the high temperature water and can be distributed on the oxide film layer surface. Thus, the presence of Zn or ZnO materials on the oxide film layer surface can reduce the corrosion rate of carbon steel in high temperature water containing H2.
In an alternative embodiment, a suitable coating of zinc metal or zinc oxides is at least sufficient to increase the corrosion resistance of the carbon steel. For example, a suitable coating can be formed by electroless plating for about 30 seconds or more, having a thickness of about 0.3 microns or greater. The zinc metal or zinc oxide coating can be deposited by methods well known in the art for depositing continuous or substantially continuous coatings on metal substrates, such as plasma spraying, flame spraying, chemical vapor deposition, physical vapor deposition processes such as sputtering, welding such as metal inert gas welding, electroless plating and electrolytic plating. It is apparent to those skilled in the art and guided by the teachings herein provided that in alternative embodiments, any suitable method is used to deposit the zinc metal or zinc oxide coating on the carbon steel components. In a particular embodiment, a zinc metal or zinc oxide is added as an alloying element to the carbon steel components and/or the low alloy steel components. The zinc metal or zinc oxide is added in a suitable concentration, e.g., up to about 20 weight %.
In one embodiment, a coating of zinc metal or zinc oxide on carbon steel components increases the corrosion resistance of the carbon steel component in high temperature, high pressure water. As a result, the thin oxide layers that are soluble in flowing high temperature, high pressure water and lead to unacceptable levels of flow assisted corrosion are not formed on the components. Rather, flow assisted corrosion is substantially reduced on the coated components when the water is provided with a compound containing a concentration of zinc of about 0.1 ppt to about 200 ppb, and in a particular embodiment about 50 ppb to about 150 ppb.
Additional features and advantages of the method of this invention are further shown by the following example.
Several carbon steel test specimens having an ASTM designation UNS G10180 comprised of about 0.17 weight percent carbon and 0.8 weight percent manganese, were used to test the effectiveness of a zinc-containing compound in reducing the corrosion of carbon steel components within a high temperature water environment. A control group of five carbon steel test specimens were exposed to high temperature water containing 150 ppb hydrogen (H2) without the addition of zinc. Additionally, a test group of five carbon steel test specimens were exposed to high temperature water containing 150 ppb H2 and including zinc (Zn). The zinc was introduced into the high temperature water in the form of Zn(NO3)2.
Each of the carbon steel control specimens and each of the carbon steel test specimens were immersed in water having a temperature of 180° C. for a testing time period of two weeks. The control test group specimens were immersed in high temperature water containing 150 ppb H2. The test group specimens were immersed in high temperature water containing 150 ppb H2 with Zn(NO3)2 introduced into the high temperature water to provide a zinc concentration within the high temperature water of 20 ppb Zn.
After the two week corrosion testing period, the thickness of the oxide layer on each specimen was measured with a x-ray proton spectroscope (XPS).
The above-described method for reducing the corrosion of carbon steel components in high temperature, high pressure water facilitates extending a service life of carbon steel components of a nuclear reactor. More specifically, the method facilitates a decrease in oxide layer thickness by enriching the oxide layer with zinc. As a result, the corrosion of carbon steel components in high temperature, high pressure water is reduced, thereby increasing the service life of these carbon steel components.
Exemplary embodiments of a method for reducing the corrosion of carbon steel components in high temperature, high pressure water are described above in detail. The method is not limited to the specific embodiments described herein, but rather, steps of the method may be utilized independently and separately from other steps described herein. Further, the described method steps can also be defined in, or used in combination with, other methods, and are not limited to practice with only the method as described herein.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
