Patent Number: 
Section: description

In FIG. 1 shows a part of a cladding tube 1. The tube is arranged in a light water reactor and nuclear fuel is provided therein as fuel pellets 2. On its outer surface 3, the cladding tube 1 presents a coating 4 according to the invention. The cladding tube 1 also presents a liner layer 6 on its inner surface 5, on which layer a coating 7 according to the invention is provided. The coating 7 may be deposited by means of CVD-technique. By use in a light water reactor the coating 4 on the outer surface 3 of the cladding tube 1 is in contact with a primary cooling circuit that comprises water, water steam or a combination thereof. The coating 4 on the outer surface 3 of the cladding tube 1 has as its task to protect the outer surface 3 of the cladding tube against attacks, preferably caused by oxidation, due to the presence of the water, water steam or the combination thereof, or wear due to the contact with other components in the light water reactor. The coating 4 thus presents a good resistance against oxidation and wear. If, despite this, the coating 4 will get a damage extending through the total thickness of the coating 4, an area of the outer surface 3 of the cladding tube 1 will be exposed to the water, water steam or the combination thereof, whereby this area will oxidate until a damage extending through the total thickness of the cladding tube 1 finally will be created. If the oxidation continues, a damage extending through the total thickness of the liner layer 6 will finally be created. Thereby, a damage extending through the total thickness of the coating 4, the cladding tube 1, the liner layer 6 and the coating 7, a so called primary damage, is formed. In such cases the water, water steam or the combination thereof will penetrate through the primary damage to an inner space 8 between the coating 7 and the fuel pellets 2. Thereby, the water, water steam or the combination thereof will fill the inner space 8 and attack the coating 7. These attacks may occur at long distances from the primary damage and be caused by hydration. Thanks to the coating 7 of the invention having a high resistance against hydration mostly no damages extending through the total thickness of the coating 7 are formed. The coating 7 and the combination of the coating 7 and the liner layer 6 thereby significantly reduce the risk of secondary damages being formed on the cladding tube 1 in comparison with uncoated cladding tubes. It also possible to exclude the liner layer 6 and still obtain a good protection against hydration at the inner surface 5 of the cladding tube 1. In FIG. 2 shows a part of a cladding tube 1 according to prior art. The tube being arranged in a light water reactor and nuclear fuel is arranged therein as fuel pellets. By use in a light water reactor, an outer surface 3 of the cladding tube 1 is in contact with a primary cooling circuit comprising water, water steam or a combination thereof. Water, water steam or a combination thereof has an oxidating effect on the outer surface 3 of the cladding tube 1. The outer surface 3 of the cladding tube 1 is also subjected to wear from other components present in the light water reactor. The material of the cladding tube 1 has not a sufficient resistance against wear and oxidation to prevent the creation of damages by these attacks. When such a damage is well initiated on the outer surface 3 of the cladding tube 1, through the action of oxidation or wear, the oxidation progresses at the location of this damage. Finally, the result thereof is a damage extending through the total thickness of the cladding tube 1. By such a primary damage the nuclear fuel in the fuel pellets 2 may leak through the damage to the primary cooling circuit and thus spread radioactivity to the circuit. The damage extending through the total thickness of the cladding tube 1 also implies that the water, water steam or the combination thereof from the primary cooling circuit penetrates through the damage into the cladding tube to an inner space 8 located between the fuel pellets 2 and an inner surface 5 of the cladding tube 1. The water, water steam or the combination thereof is spread in the inner space 8 and has a hydrating effect on the inner surface 5 of the cladding tube 1. The material of the cladding tube 1 has not a sufficient resistance against this hydration, and damages will therefore be created on the inner surface 5 of the cladding tube 1. These damages may occur at long distances from the primary damage due to the fact that the water, water steam or the combination thereof causing the damage is spread over so large areas in the inner space 8. The damage created on the inner surface 5 of the cladding tube 1 then grows until, finally, a damage extending through the total thickness of the cladding tube is formed. The nuclear fuel from the fuel pellets 2 may leak out through such secondary damages and further spread radioactivity to the primary cooling circuit. An uncoated cladding tube for nuclear fuel with a liner layer provided on an inner surface of the cladding tube was subjected to a final annealing in order to produce a coating according to the invention on an inner surface onto the liner layer as well as an outer surface of the cladding tube. This final annealing was performed at atmospheric pressure by treating the cladding tube for 90 minutes at a temperature of 565xc2x0 C. under the action of a gas mixture comprising oxygen, argon and water steam. This treatment resulted in a coating of zirconium dioxide (ZrO2) on an inner surface on the liner layer as well as on an outer surface of the cladding tube. This coating presented a good resistance against hydration, oxidation and wear. The final annealing was executed in the same way as in example 1 with the only exception that the gas mixture contained nitrogen instead of oxygen. The result thereof was a coating comprised by zirconium nitride (ZrN) on an inner surface, upon the liner layer, and on an outer surface of the cladding tube. This coating had a good resistance against hydration, oxidation and wear. The thickness of the coating according to the invention may vary from at least 1 xcexcm or at least 3 xcexcm to at most 10 xcexcm or at most 25 xcexcm in order to obtain a good resistance against hydration, oxidation and wear. Generally, the method comprises the provision of a coating of zirconium oxide or zirconium nitride on the inside of a cladding tube by subjecting said tube to an environment that comprises a controlled gas mixture that comprises metal organic compounds and also one or more other gases, such as oxygen gas, carbon dioxide, methane and/or nitrogen gas. By controlling the temperature, reaction amounts, pressure and gas content of said environment, an even coating which is very dense and resistant to hydration may be provided. The thickness thereof is preferably between 1 and 10 xcexcm.