METHOD FOR MANUFACTURE OF TUBES OF A ZIRCONIUM-BASED ALLOY

Abstract: Method for manufacturing cladding tubes for fuel and tubes for construction of fuel rod skeletons made of zirconium-based alloy for nuclear reactors and use of the method. The tubes will be ultrasmooth grounded after conservative manufacturing regarding all steps of manufacturing from melting of blocks to the final vaccuum annealing, essentially in the final dimension, to a surface roughness of </=0.2 mu mRa at which the tubes outer surface will be grounded by a belt grinding machine with a belt device consisting of conventional SiC-based belts, so called "dressed" SiC-based belts with cotton back and conventional SiC-cork belts. The tubes may possibly be polished after ultrasmooth grinding with wheel to a surface roughness <0.25 mu mRa.

Description: Method for manufacture of tubes of a zirconium-based alloy The present invention provides a method of ultrasmooth grinding of tubes of a zirconium-based alloy for cladding of fuel provided to be used in thermal rectors, including conventional manufacturing of tubes from the stadium of ingot to the final annealed tubes, ultrasmooth grinding and after that non-destructive and destructive testing according to conventional techniques. The invention has a particular significant application - but not exclusively - for the manufacturing of cladding tubes for fuel rods for boiling water reactors (BWR) and compressed water reactors (PWR). The invention has even a significant application in the area of construction tubes of type guide thimble tubes and instrumentation tubes for fuel rods for PWR and construction tubes for fuel rods for BWR. Among the alloys, which are most customary used, especially two alloys, called "Zircaloy" occur, of which Zircaloy-2 and Zircaloy-4 have the following composition in accordance with ASTMB 811 according to the following table. Besides the above requirement, it is requested that the content of oxygen for these alloys should be between 900 and 1600 ppm for cladding tubes and construction tubes for fuel element skeletons. Furthermore, Zircaloy-2 and Zircaloy-4 are commercial used with insignificant, but well-defined additives of Si and/or C, preferably in contents of 50-120 ppm Si and 80-270 ppm C. These zirconium-based alloys are chosen because of their nuclear properties, foremost the low ability to absorb neutrons, mechanical properties and corrosion resistance in water and steam at high temperatures and under radiation. The development goes to meet longer service times for fuel rods, this means to higher extent of rates of burn-up. It has been proved thereby to be of importance to be able to improve the corrosion resistance to not exceed the design related maximum oxide thickness of the zirconium-base alloyed fuel rods. Furthermore, a very important factor is to minimize the activity build-up on the fuel, i.e. on the outer surface of the fuel rods. Since the service time is extended in relation there are made higher demands on the outer surface of the fuel rods referring to their condition and surface roughness. Experience has shown that a pickled and preautoclaved surface with fine surface roughness minor contributes to deposition of activity-contributing elements on the surface of the fuel rod than in case of fuel rods with conventional surface finishes with coarser surface roughness. It has been a widely used technique to manufacture cladding and construction tubes in zirconium alloys by a manufacturing process, which includes hot working of an ingot to a bar, quenching from the beta area, machining to a hollowed work piece, extrusion of the hollowed workpiece in a α-area to a tube hollow, reducing in several operations by cold rolling to substantially final dimension with a recrystallisation annealing in the α- area for each cold rolling operation. The cold rolled tube, substantially in the final dimension, will be vacuum annealed in the α-area at a temperature in the interval of 450-650°C, surface-conditioned by some of the methods blasting or pickling, alternatively a combination of these methods and also surface-conditioned on the outside by mechanical treatment in form of one or a combination of the methods belt grinding, belt polishing, contact grinding and polishing. The object of the presented invention is to obtain an application for the manufacturing of tubes of zirconium alloys for cladding of fuel and for construction of fuel rods for skeletons, which are better than earlier known manufacturing methods, including mechanical treatment of the outer surface, after the final vacuum annealing, meet the requirements for fine surface roughness in order to minimize the activity build-up. Certainly, a fine surface roughness and by that a low activity build-up as above mentioned could be performed by that the outer surface of the tubes after the final vacuum annealing will be pickled and preautoclaved in high-pressure steam. However, several disadvantages are found with this method. At first the pickling must be done in a mixture of nitric acid and hydrofluoric acid, a mixture, which causes big disadvantages regarding to the working environment and also the taking care of the consumed acid. The handling contributes to a very unwanted increasing of the environmental pollution. At second the autoclaving contributes to corrosion-produced hydrogen, which will be absorbed by the zirconium alloys. This makes that the cladding and construction tubes get undesirable shortening of the total operating time, since the autoclave produced hydrogen adds to the hydrogen that will be absorbed during operating. According to the present invention it has reveled to be possible to manufacture cladding tubes and construction tubes of zirconium alloys for nuclear reactors according to a method that contains ultrasmooth grinding, which gives a considerable finer surface roughness the outer surface than for conventional manufactured tubes and closer determined, which gives a surface roughness, which is considerably finer than that represented by pickled and preautoclaved tubes. Further it is experienced that tubes with ultrasmooth ground surface have a corrosion rate, which is approximately 20 % lower than for conventional manufactured tubes under autoclave testing in steam at 415°C. This subsequent result is entirely unexpected. The invention provides to a method of manufacturing cladding tubes and construction tubes of zirconium-based alloy for nuclear rectors, at which the zirconium-based alloy is manufactured according to conventional and known methods referring to melting, hot working and beta quenching before extrusion, extrusion, cold rolling with intermediate recrystallisation annealing, final vacuum annealing, inner surface conditioning and at which the zirconium-based alloy will be ultrasmooth ground instead of going through a conventional outer surface conditioning. The ultrasmooth grinding will be done in a belt grinding machine, consisting of a feeder, a grinding machine and a feeding out device. The grinding machine consists of a number of belt heads. Each belt head consists of two belt units, operated of one and the same engine. The belt unit consists of a driving wheel and a steering wheel, which runs and stretches the grinding belt and also a pressure wheel, which presses down the belt towards the tube while grinding. The material removal from the tube-surface will be adjusted by vertically adjusting of each belt head. The belt grinding machine can be equipped with up to 5 belt heads, i.e. 10 pieces of belt units. The ultrasmooth grinding will be performed with a sequence of belts consisting of on one hand conventional abrasive SiC-belts, conventional polishing SiC-cork belts and also so called "dressed" SiC-based grinding belts with cotton back. FigJ shows a belt-grinding machine with 3 belt heads. The zirconium-based alloy consits preferably of a zirconium-tin alloy, for example the alloys known under the trade marks Zircaloy-2 and Zircaloy-4, that have a content of alloying elements in the range between 1.2-1.1% for tin, 0.07-0.24% for iron, 0.05- 0.15% for chromium, 0-0.08% for nickel and 0.09-0.16% for oxygen, small additions of silicon and carbide, the balance zirconium and optionally occurring inevitable impurities, where the mentioned contents as well as the for the rest of the mentioned contents in the application refer to percentage by weight and also the alloys of type Zircaloy-3 with 0.5-1.0 % Sn, 0.3-0.6% Fe and 0-0.2% Cr and supplements and occurring impurities corresponding to Zircaloy-2 and -4. Zircaloy-2 contains 1,2-1,7% Sn, 0,07-0,20 % Fe, 0,05-0,15 % Cr, 0,03-0,08 % Ni and 0,09-0,16 % O. Zircaloy-4 contains 1.2-1.7% Sn, 0.18-0.24% Fe, 0.07-0.13% Cr, 0.09-0.16% O, no Ni. The invention will be explained in detail by describing one of the embodiments. Cladding tubes in the alloy Zircaloy-2 were manufactured according to the conventional methods regarding to all process steps from melting of the ingot to the final annealing at 575°C. After the conventional surface conditioning of the inner surface the product is after that exposed to utrasmooth grinding at which the product will be ground in a grinding machine, consisting of 6 belt units. In order to obtain the optimal surface roughness the tubes are exposed to two grinding passages in the machine with the following sequence of belts: Pass 1 : 2 x 320 grit, conventional SiC-belts 2 x 400 grit, conventional SiC-belts 2 x 600 grit, "dressed" SiC-based belts with cotton back Pass 2: 2 x 800 grit, "dressed" SiC-based belts with cotton back 2 x 1000 grit, conventional SiC-cork belts 2 x 1200 grit, conventional SiC-cork belts After the ultrasmooth grinding the tubes were sp lifted into two lots. One of the lots was after the ultrasmooth grinding exposed to another surface treatment in form of polishing with 800-grit wheel. Under measuring the surface roughness of the tubes according to the present invention it was observed that the surface roughness after ultrasmooth grinding is <0J μmRa and after ultrasmooth grinding and polishing with 800-grit wheel <0J5 μmRa. The surface roughness for the conventional manufactured tubes is in the interval 0.3-0.5 μmRa and for conventional manufactured tubes, which were exposed to an outside pickling as well as autoclave treatment, the surface roughness is <0J5 μmRa. Under corrosion testing in steam at 415°C for 90 days - a test that is experienced simulating the conditions regarding to the resistance against general corrosion during reactor operating - tubes, which were manufactured according to the present invention exhibit a better resistance against general corrosion than conventional manufactured tubes with proved good resistance against general corrosion, which were included in the trial for comparison. Measured out between the exposing time 60 and 90 days the corrosion rate, expressed in mg/dm /day, is as follows: Ultrasmooth ground tubes: 0.53 Ultrasmooth ground plus polished tubes: 0.60 Conventional manufactured tubes: 0.67 The ultrasmooth grinding leads to a decreasing of the corrosion rate with 20 % and ultrasmooth grinding plus polishing a decreasing with 10 % compared with conventional manufactured tubes.

Claims: Claims 1. Method of manufacturing cladding tubes for fuel and tubes for construction of fuel rod skeletons for nuclear reactors made of zirconium-based alloy, characterized in, that the tubes after conventional manufacturing regarding all steps of manufacturing from melting of ingots to the final vacuum annealing, essentially in the final dimension, being ultrasmooth ground to a surface roughness of < 0J μmRa at which the tubes outer surface will be ground by a belt-grinding machine with a sequence of belts consisting of conventional SiC-based belts, so called "dressed" SiC-based belts with cotton back and conventional SiC-cork belts. 2. Method according to claim 1, characterized in, that the ultrasmooth grinding will be performed in a grinding machine with 6 to 10 belt units. 3. Method according to some of the prior claim, characterized in, that the ultrasmooth grinding will be performed with a sequence of belts consisting of conventional SiC-belts with grit size 240-600, "dressed" SiC-belts with cotton back and grit size 600-1000 and also conventional SiC-cork belts with grit size 800-1200. 4. Method according to some of the prior claim, characterized in, that the tubes after the ultrasmooth grinding will be polished by a wheel to an surface roughness of < 0J5 μmRa. 5. Method according to claim 4, characterized in, that the polishing would be performed with a wheel with grit size 600-1200. 6. Method according to some of the prior claim, characterized in, that the ultrasmooth grinding would be performed before or after a conventional surface conditioning of the inner surface. 7. Application of the method according to the claims 1 -6 under the manufacturing of cladding tubes for fuel and tubes for construction of skeletons for fuel elements in zirconium alloy for fuel elements for nuclear reactors. 8. Application according to claim 7, characterized in, that the zirconium alloy contents 1.2-1.7% Sn, 0.07-0.24% Fe, 0.05-0.15% Cr, 0-0.08% Ni, 900-1600 ppm O, 50-120 ppm Si, 80-270 ppm C, and the remainder zirconium and optionally occuring inevitable impurities. 9. Application according to claim 8, characterized in, that the zirconium alloy contents 1.2-1.7% Sn, 0.07-0.20% Fe, 0.05-0.15% Cr, 0.03-0.08% Ni, 900-1600 ppm O, 50-120 ppm Si, 80-270 ppm C and and the remainder zirconium and optionally occuring inevitable impurities. 10. Application according to claim 8, characterized in, that the zirconium alloy contents 1.2-1.7% Sn, 0.18-0.24% Fe, 0.07-0.13% Cr, 900-1600 ppm O.50-120 ppm Si, 80-270 ppm C and the remainder zirconium and optionally occurring inevitable impurities. 11. Application according to claim 7, characterized in, that the zirconium alloy contents 0.5-1.0% Sn, 0.3-0.6 Fe, 0-0.2% Cr, 900-1600 ppm O, 50-120 ppm Si, 80-270 ppm C and remainder zirconium and optionally occurring inevitable impurities.

Inventors: ANDERSSON THOMAS, SOEDERQVIST LEIF, ANDERSSON, THOMAS, SOEDERQVIST, LEIF
Applicants: ANDERSSON THOMAS, SANDVIK AB, SOEDERQVIST LEIF, ANDERSSON, THOMAS, SANDVIK AB (PUBL), SOEDERQVIST, LEIF
Application year: 1999
Main classification: B24B1
IPC codes: B24B1, B24B7, C22F1, G21C3