Patent Number: 044118619
Section: description

Referring now to the drawing and first, particularly, to FIG. 1 thereof, there is shown a fuel rod cladding tube 2 in an end view before the method according to the invention is performed. In the interior of the tube 2, a pressure P.sub.o prevails, which also corresponds to external pressure. FIG. 2 shows that through application of a high internal pressure P.sub.1 as well as of a temperature of 300.degree. to 500.degree. C., an enlargement or widening of the cladding tubes has occurred. In this expanded cladding tube 2, the deformation of which has been effected, while yet in the elastic range or region, nearly to the yield point, a protective layer 3 is formed by an introduced medium. This condition is shown in FIG. 3. After this protective layer 3 has been formed, and pressure and temperature have been reduced, the fuel rod cladding tube 2 returns to its original shape according to FIG. 4, the protective layer 3' formed in the interior thereof being compressed i.e. being stressed in compression. The diameter increase shown in the figure is, obviously, not to scale, but rather, has been greatly exaggerated in order to illustrate the principle of the invention. FIG. 5 illustrates two possibilities for performing the method practically. The fuel rod cladding tubes 1 are tightly welded at the one end thereof to a conventional end plug 4. The tubes 1, at the other end thereof, are welded to a provisional or temporary end plug 5 which is connected to a pressurized-gas source 52 through a line 51. To heat such a fuel rod uniformly, it is placed in a furnace 8, indicated only diagrammatically in FIG. 5, for carrying out the method. Another heat source, such as inductive heating, could obviously also be used for this purpose. To apply the required high internal pressure for producing the elastic expansion which, for conventional dimensions of a fuel rod cladding tube, is more than 100 bar, pressure gas, such as nitrogen, for example, is delivered through the line 51. This gas additionally contains a given amount of the substance forming the protective layer, such as oxygen, for example. With this entrained oxygen, the inside wall surface of the cladding tube is oxidized and ZrO.sub.2 is formed. The same effect is also obtained through the application of high-pressure steam. The mechanism for forming the protective layer corresponds to that known heretofore from the technology of the autoclaving of zirconium cladding tubes. In the latter process, however, the cladding tube per se is not stressed mechanically, so that the protective layers formed thereon show no internal residual stresses. Another elegant procedure for performing the method of the invention is shown in FIG. 5 in regard to the lower cladding tube 1 illustrated therein. The latter tube 1 is disposed in the same furnace 8 and is closed by a provisional or temporary end plug 6. The end plug 6 is absolutely tight, no connection being provided to any source of pressurized gas. Before the temporary plug 6 was inserted, however, a given amount of water (note the drop 7) was introduced into the lower cladding tube 1 shown in FIG. 5. The amount of water in the drop 7 was determined as the amount which will produce the internal pressure required for the elastic deformation at the temperature provided by the furnace. Instead of the drop of water 7, a corresponding amount of hydrogen peroxide could also be used. When the latter evaporates, atomic oxygen is produced which causes the inner wall surface of the cladding tube to oxidize more rapidly than has been possible by any of the methods mentioned hereinbefore. Instead of liquids, other gas-yielding or generating substances can, of course, also be introduced, it being important that these substances be able to be reliably metered. The excellent protective action against stress-crack corrosion achieved by this method invention is explainable not only by the compressive-stress layer but also by the procedure due to which the protective layer is applied to the widened or expanded cladding tube. This protective layer is formed also, for example, in gaps between the end plugs and the open cladding tubes, as well as at the boundary of the welded seam connecting them one to the other. The same thing applied as well to surface defects which can stem from the process of manufacturing the open-ended cladding tubes per se. As compared to the normal autoclave technology, a complete coating or layer is thus produced which extends into the microscopic range and has a thickness of up to 5 .mu.m advantageiously. A further advantage connected with this method invention ought not to be omitted. This is that while this method is being performed, a possibility is simultaneously provided for checking the tightness of the welding seam between the end plug 4 and the previously open-ended cladding tube 2. This can be accomplished if the interior of the furnace 8 is tightly closed off, and the pressure therein is monitored. It should also be noted that this method according to the invention extends not only to the formation of oxide layers, but that, carbide and silicide layers and the like could also be utilized to achieve stress-crack corrosion resistance. The choice of the most suitable protective layers depends upon the selection of the cladding tube material, which may well always be a zirconium alloy, as well as upon the specification of the nuclear fuel. It should be further noted that it is also possible to apply this method invention to previously completed nuclear fuel rods i.e. fuel rods already containing their nuclear fuel charge. In that case, the oxidant, for example, must be introduced before the open-ended cladding tube is finally closed off. This oxidant can be of such composition then as to provide simultaneously for the so-called initial internal pressure of the nuclear reactor fuel rod during the operation of the fuel rod.