A device and a method for preventing shadow corrosion

Abstract: A device comprising a first component which comprises a zirconium-based alloy, and a second component of a type that can form a counter-electrode in relation to the first component, said first and second components being arranged at a short distance from each other, with an electrically conducting medium between them and being subjected to radioactive radiation through a radiation field. The device comprises a means arranged to electrically insulate at least a part of the second component from the first component in order to avoid shadow-corrosion on the first component. A method for preventing shadow-corrosion on the first component is characterised in that at least a part of the second component is electrically insulated from the first component.

Description: THE BACKGROUND OF THE INVENTION AND PRIOR ART The present relates to a device, comprising a first component, which comprises a zirconium-based alloy, and a second component of such a type that it can form a counter-electrode in relation to the first component, said first and second components being arranged at a short distance from each other, with an electrically conducting medium between them, and being subjected to radioactive radiation through a radiation field. The invention also relates to a method for preventing shadow-corrosion on a first component, said component comprising a zirconium-based alloy and being intended to be arranged in a field of radioactive radiation at a short distance from a second component, said second component being of a type that can form a counter-electrode in relation to the first component, and said first and second components being subjected to the radiation of the field. Today, devices of the above type are often used in nuclear power plants. In recent time, it has been noticed that zirconium-based alloys that are arranged at a short distance from a component made of, for example, stainless steel or a nickel-base alloy present an elevated tendency of corroding in the environment in which they are arranged inside a nuclear power plant in comparison to the case in which they are arranged at a larger distance from said component in the same environment. In this environment, the zirconium-based alloy is subjected to radiation as well as corrosive media. Until now, there has been a general opinion that the reason for this corrosion, so-called shadow-corrosion, is an elevated radiolysis due to an extensively elevated β-field due to a nuclear reaction with the alloying element manganese. Accordingly, prior art within this field is primarily focused on solving the problem of the shadow-corrosion by avoiding said nuclear reaction with the alloying element manganese in some way. However, this has not resulted in any commercially practical solutions, and therefore, the shadow-corrosion of zirconium-based alloys arranged at a short distance from components made of stainless steel or nickel based alloys has, so far, been accepted as unavoidable under the conditions given in nuclear power plants. EP-A2-0 798747 describes a device according to the preamble of claim 1. The English abstract of JP-A-08-313683 describes that galvanic corrosion in a Zircaloy channel box can be suppressed by coating the surface of a handle of a control rod and guide roller with an insulation ceramic, such as alumina or spinel. The English abstract of JP-A-07-248390 describes that local corrosion of a contact part (upper end plug and lower end plug of a clad pipe) can be reduced by forming an oxide film on one side or both sides of a constitutive member of the contact part. SUMMARY OF THE INVENTION One object of the present invention is to provide a device in which a first component that comprises a zirconium-based alloy arranged at a close distance from a second component made of a nickel based material presents a substantially reduced tendency to shadow-corrosion in relation to devices according to prior art. This objective is achieved by means of a device according to claim 1. The shadow-corrosion problem is thus attacked in a way substantially different from the ones proposed through prior and research. The solution is based upon the new insights about the reasons to the shadow-corrosion phenomena which the inventors have come to. Thereby, one has come to the conclusion that it is a combination of radiation (mainly γ-radiation) and a not very well passivated (from an electrochemical, not chemical point of view) component at a short distance from the zirconium material which gives rise to the shadow-corrosion. The γ-radiation results in occasional conducting channels for charges, which results in the corrosion being substantially increased locally, and also integrally over the whole oxide film which is present on the surface of the zirconium-based alloy in these environments. In order to avoid shadow-corrosion, the first component should thus be electrically insulated from the second component, at least in the region where the two components are arranged at a distance from each other which is below a certain shortest distance. According to the invention, the device comprises an insulating coating disposed on the second component. Thereby, an advantageous shielding of the very reason for or source of the shadow-corrosion on the first component is obtained. According to the invention, the coating comprises zirconium dioxide. With today's technique, this kind of coating can be deposited in a suitable way onto the component in question. It have qualities to cope with the corrosive environment which it will be subjected to in a nuclear power plant, while simultaneously being subjected to radiation, for example neutron radiation and γ-radiation. When the coating comprises a plurality of layers, as, for example, in connection with deposition by means of PVD or CVD, one or more of the layers may comprise a generally pure metallic material, e.g. from the group comprising Pt, Ni, Pd, Rh, Ti, Cu, and Zr. According to a preferred embodiment, the coating has a thickness in the range of 0.01-1000 µm. The choice of thickness depends on a plurality of factors, such as the type of second component, the position of the component, and the environment in which it is arranged. For spacers and spacer springs which normally comprise nickel based material, the coating preferably has a thickness in the range of 0.01-50 µm. The electrical resistance of the layer should be in the range of 1·10 3 - 1·10 10 Ω·cm 2 , preferably 1·10 6 - 1·10 10 Ω·cm 2 to obtain a well insulating effect. There is a further object of the invention to propose a method for preventing shadow-corrosion on a first component which comprises a zirconium-based alloy and is provided to be arranged in a field with an elevated radioactive radiation on a short distance from a second component, the second component being such that it can form a counter-electrode in relation to the first component, and the first and second component being subjected to the radiation of the field. The second component is made of a nickel based material. Preferably, it is arranged in a nuclear power plant and can be any one of a spacer and a spacer spring. This object is obtained by means of a method according to claim 5. The method has, as is the case with the device discussed above, been developed in the light of the new insight about γ-radiation giving rise to occasional conducting channels between the first and second components. Partly due to the surrounding environment, the components will thereby define two electrodes between which charges are interchanged. Normally, the second component will thereby form an electrolytic counter-electrode in relation to the first component when the components are at a short distance from each other, normally less than or equal to 10 mm. The shadow-corrosion of the first component can be substantially reduced by the second component being insulated electrically from the first component, as proposed by the invention. Because the shadow-corrosion only is a problem when said first and second components are arranged at a very short distance from each other, below approximately 10 mm, the second component should be electrically insulated from the first component, preferably in the region where the components are at a distance from each other which is below said shortest distance. Hereby, it should be realised that said shortest distance may vary depending on different external parameters, such as the type of medium in which the components are arranged. According to the invention, the second component is insulated from the first component by means of an electrically insulating coating which is deposited onto the second component. Thereby, it becomes possible to very precisely control which region of the second component should be insulated by means of said coating. Also a plurality of other properties of the coating, such as the thickness and the electrical resistance, may easily be controlled during the very depositing process. It is also an advantage that any one of the large number of the advanced deposition techniques which are accessible today can be used in order to accomplish such an insulation. According to a preferred embodiment of the method, the deposition is done by means of at least one of the methods PVD, CVD, heat treatment in gases or gas mixtures, deposition where a fine powder provided in a gel is deposited onto the component and is permitted to sinter as a coating on the surface of the component under the affection of heat, deposition by means of flame-spraying, and spray deposition. All these techniques have been extensively practised and have the advantage of permitting a very precise control of the coating thickness and which material or materials that are to be included in the coating. They also make it possible to arrange a plurality of different layers which are of different compositions or have different material structures and which together define said coating. A particularly advantageous technique is the one by which a fine powder, preferably a ceramic powder, which is mixed together with a liquid and inorganic or organic compounds to a so-called solgel, is deposited onto the component in question and then is permitted to sinter to a coating on the surface of the component under the affection of heat. Further features and advantages of the present invention will be defined in the following description of different embodiments of the invention. DETAILED DESCRIPTION OF EMBODIMENTS In a nuclear power plant, which preferably is a boiling-water reactor (BWR) but also could be a pressure-water reactor (PWR) or other commercial or non-commercial fission or fusion reactors, or any other environment where there are substantial radiation fields of neutrons, γ-radiation or β-radiation, a first component and a second component are arranged at a short distance from each other, less than approximatively 10 mm, and these components are arranged in contact with an electrically conducting medium. Normally, this medium is water. The first component comprises a zirconium-based alloy and may be any one of the components in a nuclear power plant that are comprised by such a zirconium-based alloy, e.g. a spacer frame of a spacer in a light-water reactor. The second component is made of a nickel based alloy with the capability of acting as an electrolytic counter-electrode in relation to the first component, with or without any externally applied electrical field, and which is also located in a radiation field. The second component may be a spacer made of a nickel base material for BWR and PWR or spacer springs made of nickel base material for BWR and PWR. The second component is provided with a coating which comprises zirconium dioxide. The coating has a thickness of 0.01-1000 µm, preferably 0.01-50 µm for spacers and spacer springs. Its electrical resistance should be 1·10 3 - 1·10 10 Ω·cm 2 , preferably 1·10 6 - 1·10 10 Ω·cm 2 . Of course, a plurality of alternative embodiments will be obvious for a man skilled in the art without thereby leaving the scope of the invention, such as defined in the appended claims.

Claims: A device comprising a first component which comprises a zirconium-based alloy and a second component made of a nickel based material, wherein the second component is of such a type that it can form a counter-electrode in relation to the first component, wherein the first and second components are arranged at short distance from each other with an electrically conducting medium between them and being subjected to radioactive radiation through a radiation field, characterised in that the second component is a spacer or spacer spring for a nuclear boiling-water reactor or a nuclear pressure-water reactor, wherein the first and second components are located less than or equal to 10 mm from each other, wherein the device comprises an electrically insulating coating comprising zirconium dioxide, deposited on the second component so as to electrically insulate at least a part of the second component from the first component in order to avoid shadow-corrosion on the first component. A device according to claim 1, characterised in that the coating has a thickness in the range of 0.01-1000 µm. A device according to any one of the preceding claims, characterised in that the coating has a thickness in the range of 0.01-50 µm. A device according to any one of preceding claims, characterised in that the coating has an electrical resistance in the range of 1·10 3 - 1·10 10 Ω·cm 2 , preferably 1·10 6 - 1.10 10 Ω·cm 2 . A method for preventing shadow-corrosion on a first component, said first component comprising a zirconium-based alloy and being arranged to be positioned in a field of radioactive radiation at a short distance from a second component, said second component being made of a nickel based material and being of such a type that it can form a counter-electrode in relation to the first component, and the first and second components being subjected to the radiation of the field, characterised in that the second component is a spacer or spacer spring for a nuclear boiling-water reactor or a nuclear pressure-water reactor, wherein the first and second components are located less than or equal to 10 mm from each other, wherein at least a part of the second component is electrically insulated from the first component by providing the second component with an electrically insulating coating comprising zirconium dioxide in order to avoid shadow-corrosion on the first component. A method according to claims 5, characterised in that the coating is applied by at least one of the methods comprised by PVD, CVD, heat treatment in gases or gas mixtures, the deposition of a mixture of a fine powder in a gel onto the component where it is permitted to sinter as a coating on the surface of the component under the affection of heat, deposition of a coating by means of flame-spraying, and spray deposition. A method according to any one of claims 5-6, characterised in that the coating is given a thickness in the range of 0.01-1000 µm. A method according to any one of claims 5-7, characterised in that the coating is given a thickness in the range of 0.01-50 µm. A method according to any one of claims 5-8, characterised in that the coating has an electrical resistance of 1·10 3 - 1·10 10 Ω·cm 2 , preferably 1·10 6 - 1·10 10 Ω·cm 2 .

Inventors: WIKMARK GUNNAR, WIKMARK, GUNNAR
Applicants: WESTINGHOUSE ATOM AB
Application year: 1999
Main classification: G21C3
IPC codes: G21C7, G21C3, G21D1, C23C4