Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Encased with nonfuel component
Reexamination Certificate
1998-11-10
2003-01-28
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Encased with nonfuel component
C376S410000, C376S414000, C376S416000, C376S305000
Reexamination Certificate
active
06512806
ABSTRACT:
THE FIELD OF THE INVENTION
The present invention relates to a component designed for use in a light water reactor and at least partly comprised of a metal and/or a metal alloy with at least one surface that presents a coating. The present invention also relates to a method of creating a coating resistant to hydration on at least one surface of a component, which is designed for use in a light water reactor and which is partly comprised of a metal and/or metal alloy by subjecting the component to a treatment with a gas mixture while heating.
BACKGROUND OF THE INVENTION
Components in nuclear plants are often subjected to attacks caused by hydration, oxidation and/or wear, and it is often necessary to deposit a coating onto the surface of the components in order to protect the latter. Cladding tubes for nuclear fuel form an example of such components. In the worst scenario, attacks on a cladding tube for nuclear fuel result in a damage extending through the total thickness of the cladding tube in such a way that the radioactive nuclear fuel inside the cladding tube leaks out to the surroundings. This may be caused by primary as well as secondary damages on the cladding tube.
Primary damages are created by attacks on the outer surface of the cladding tube, the said attacks being caused by oxidation, due to the contact between the cladding tube and the cooling water, or due to wear. A primary damage extending through the total thickness of the cladding tube implies that water, water steam or a combination thereof flows through the damage, so that a space between the fuel and the inner surface of the cladding tube is filled by the water, water steam or the combination thereof. The presence of the water, water steam or the combination thereof in this space implies that the cladding tube runs the risk of being damaged by attacks from inside the tube. This attack often takes place through hydration. Such a damage is called a secondary damage and can only occur when a primary damage already has occurred. Secondary damages extending through the total thickness of the cladding tube result in a leakage of the nuclear fuel inside the cladding tube, and thereby of radioactivity, to the surroundings. Secondary damages can occur at relatively long distances from the primary damage and, therefore, often have the shape of long cracks, which make them a serious type of damage.
A lot of work has been done to develop a coating on such a cladding tube in order to make the coating more resistant to hydration, oxidation and/or wear and thereby able to prevent damages to the cladding tube. Particularly, it has been difficult to produce coatings at the inside of the cladding tube, where the coatings resulting in a good protection against secondary damages. At such a location, a coating which is particularly resistant to hydration is required.
In order to test the resistance to hydration and oxidation of a coating on a cladding tube, a method is normally used where the cladding tube is autoclaved by conditions similar to the ones that the cladding tube is subjected to during use thereof in a nuclear plant, whereafter the presence of hydration and oxidation, respectively by the coating and the cladding tube is examined. The autoclaving of the cladding tube by this method is not to be confused with an autoclaving that may be used for creating protective coatings on cladding tubes. The latter type of autoclaving will be described more in detail later in this text.
Until the seventies, contents of hydrogen in the shape of water in uranium dioxide in fuel pellets resulted in the cladding tube being subjected to hydration at its inside. These damages are called blisters and differ from secondary damages, even though both of them are caused by hydration at the inside of cladding tubes. These uranium is substantially free from hydrogen, and, thereby, the problem with blisters has disappeared.
During the sixties and seventies, a coating was produced in a cladding tube for nuclear fuel by autoclaving the cladding tube in substantially saturated water steam at a temperature of approximately 425° C. and at a pressure of from 0.1 to 0.5 MPa for 24 hours. The result thereby was a coating comprised by zirconium dioxide (ZrO
2
), which normally had at thickness of from 0.5 to 1 &mgr;m. This coating had a relatively low resistance to hydration and, therefore, it had no substantially protective effect with regard to secondary damages on the cladding tube.
German patent document DE-A-2 429 447 discloses a cladding tube made of zirconium or niobium alloy with a coating comprised by an oxide layer arranged at an inner surface as well as an outer surface of the cladding tube.
During the eighties, a liner layer preferably made of zirconium was applied to the inside of the cladding tube for protection against stress corrosion caused by iodine formed in the uranium dioxide during the fission.
In Japanese patent document JP-A-63 179 286, a liner layer of zirconium on an inner surface of a cladding tube made of zirconium alloy is combined with a subsequent autoclaving which produces a coating of zirconium dioxide on the outer surface of the cladding tube and on the surface of the liner layer. The autoclaving was performed at a pressure of 5 MPa and at a temperature of at least 400° C. The difference between the autoclaving according to this Japanese patent document and the autoclaving according to the method used in the sixties and the seventies described herein is that the autoclaving according to the Japanese patent document takes place in the presence of at most 10% water steam, while the autoclaving according to the technique of the sixties and seventies was performed in the presence of generally 100% water steam. However, in the Japanese patent document, there is only one example of an embodiment with autoclaving in a dry atmosphere. A coating on the liner layer on the inner surface of the cladding tube had a thickness of least 0.2 &mgr;m and preferably 0.5 to 1.0 &mgr;m.
The inventors have experienced that the coating according to Japanese patent document JP-A-63 179 286, in comparison with a coating produced by means of autoclaving in the presence of substantially 100% water steam according to the above technique from the sixties and seventies, results in an improved protection against H
2
-absorption, but that it does not provide a sufficient barrier against H
2
-permeation to prevent secondary damages on the cladding tube.
By methods of producing a coating on a surface, where the methods are performed by autoclaving the component under pressure, the reaction speed between active constituents in the gas and the material in the surface of the component is regulated by, amongst other factors, varying the pressure. This is a very bad way of regulating the reaction speed. Relatively small pressure variations during the course of the treatment may cause large variations with regard to the reaction speed, which in turn might lead to the appearance of defects in the coating. Autoclaving according to prior art is therefore performed at a constant pressure, that is, at static conditions.
By a production of a cladding tube designed for use in a light water reactor, a series of annealings of the uncoated cladding tube is performed in order to provide good mechanical properties to the tube. By employing methods according to prior art the cladding tube is then moved to a special final annealing plant which permits treatment under pressure action, for execution of a final annealing and, thereby, a production of a coating. For economical reasons it is not acceptable to execute the annealings of the cladding tube for obtaining good mechanical properties in a plant which makes pressure treatment possible, as these annealings do not require the use of pressure. Therefore, the final annealing, and, thereby, the production of the coating, constitutes one step in the total treatment of the cladding tube, the step being separated from previous treatment steps.
Accordingly, prior art does not offer any coating at the inside of a cladding tube f
Hallstadius Lars
Rudling Peter
Vesterlund Gunnar
Carone Michael J.
Keith Jack
Swidler Berlin Shereff & Friedman, LLP
Westinghouse Atom AB
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