Patent Number: 044118619
Section: summary

The invention of the instant application relates to a method for protecting the casing or cladding tubes of nuclear reactor fuel rods, which are formed of zirconium alloy, against the attack of radioactive fission products, such as iodine, especially. The fuel rods, especially of light-water reactors, are formed of a cladding or casing tube to which end caps have been welded at both ends thereof, and a charge of generally pellet-shaped nuclear fuel, such as uranium dioxide or a mixture of uranium dioxide and plutonium dioxide, for example. The wall thickness of the cladding tubes is about 0.3 to 0.7 mm. The operating temperature in the nuclear reactor is in the order of 400.degree. C., but this temperature can be exceeded for short periods of time in the event of accidents. Operating experience with such nuclear reactor fuel rods over many intervening years has shown that presently employed fuel rod constructions have a high degree of reliability. Occurrence of damage to nuclear reactor fuel rods has been rare and has required in most cases no immediate interruption of the operation of the respective nuclear reactor. This experience prevailed especially for such nuclear reactors which were operated with uniform load. It was found, however, that more fuel rod defects occurred when nuclear reactors had to be operated with varying load. This varying load resulted in varying temperatures of the nuclear reactor fuel rods, which were consequently also stressed cyclically, in addition, by thermal expansion. It was accordingly found that stress-crack corrosion set in at the inner wall surface of the cladding tubes, radioactive fission products, especially iodine, being found to be the corroding media. The problem therefore arose of finding ways and means to prevent such stress-crack corrosion to the greatest extent possible and, thereby, to make the fuel rods as reliable under varying-load operation as under constant load. It is accordingly an object of the invention to provide a method of protecting the casing or cladding tubes of nuclear reactor fuel rods from such stress-crack corrosion and for, thereby, rendering the fuel rods equally reliable under varying-load as well as constant-load operation. With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of protecting a zirconium-alloy cladding tube of a nuclear-reactor fuel rod against attack by radioactive fission products, such as iodine especially, which comprises applying an internal pressure to the cladding tube at a temperature of 300.degree. to 500.degree. C. so as to deform the cladding tube, depending upon the geometric dimensioning thereof, in the elastic range and up to nearly the yield point thereof and, while this condition exists, reacting a medium previously introduced into the interior of the cladding tube with the inner surface of the cladding tube to form a protective layer. At the end of the empirically determinable time for the formation of the protective layer, the temperature and the pressure are lowered again so that the cladding tube resumes virtually the original dimensions thereof. The protective layer produced during this treatment is thereby compressed and is under great internal compressive stress. This stress does not also disappear during operation of the reactor because in the course thereof, a similar cladding-tube deformation cannot occur due to the externally applied high pressure of the coolant. It has now been found that cladding tubes of nuclear fuel rods treated in this manner have become virtually insensitive to stress-crack corrosion. This also corresponds to experience obtained in other fields of engineering with work pieces which were provided with an internal compressive stress zone as protection against stress-crack corrosion. In this connection, reference may be had to heat exchanger tubes which are provided on the outside thereof with a compressive stress layer by rolling or blasting with glass beads. In accordance with another measure of the method of the invention, the cladding tube is closed at both ends thereof, and the introduced medium serves simultaneously for applying the internal pressure thereto. In accordance with a further measure of the method invention, a given quantity of water serves as the introduced medium, the vapor thereof forming a protective layer of zirconium dioxide on the inner surface of the cladding tube. In accordance with an added measure of the method invention, a given quantity of H.sub.2 O.sub.2 serves as the introduced medium, the vapor thereof and liberated atomic oxygen forming a protective layer of zirconium dioxide on the inner surface of the cladding tube. In accordance with an additional measure of the invention, the method includes welding a plug beforehand to one open end of an open-ended tube so as to seal the respective open end thereof, thereafter introducing the given quantity of medium into the tube thus sealed at the one end thereof, temporarily closing off the other open end of the tube, then heating the tube to vaporize the medium that has been introduced therein and so as to cause it to react with the inner surface of the cladding tube, opening the temporarily closed other end of the tube after an empirically determined time period during which the protective layer is formed, and cooling the cladding tube. In accordance with yet another measure of the method invention, the medium is formed at least partly of a respective substance for increasing the internal pressure in the cladding tube and for forming the protective layer. In accordance with yet a further measure of the method invention, the substances are gaseous. In accordance with an alternate measure of the method invention, the substances are gas-forming. In accordance with a concomitant feature of the invention, the method includes filling the cladding tube sealed at the respective one end thereof with nuclear-fuel pellets, and sealing the other end of the cladding tube to form the fuel rod. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a method for protecting the casing tubes of nuclear reactor fuel rods, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.