Patent Number: 058870458
Section: summary

The present invention concerns zirconium-based alloy tubes for use in nuclear reactor fuel assemblies. Tubes of that type are usable in particular for constituting fuel rod cladding, for forming the external portion of such cladding, or for forming guide tubes which receive the rods of control clusters. Cladding of that type is frequently constituted by tubes made from an alloy known as "Zircaloy 4" which contains, in addition to zirconium, 1.2% to 1.7% by weight of tin, 0.18% to 0.24% by weight of iron, 0.07% to 0.13% by weight of chromium and 0.10% to 0.16% by weight of oxygen. A number of alloys which are derived from those previous alloys have also been proposed, in particular alloys in which the chromium is either completely or partially replaced by vanadium and/or in which the oxygen content exceeds that given above, with a corresponding reduction in the contents of some of the other addition elements. Particular qualities which are required in a tube for use as cladding are good resistance to corrosion by water at high pressure and at high temperature, limited long term creep, long term retention of mechanical properties, limited expansion on irradiation and reduced sensitivity to lithium; in addition, these properties must be reproducibly obtainable, and the alloy must have metallurgical properties at the various production stages (in particular rollability) which keeps the rejection rate down to an acceptable value. The behavior of Zircaloys on irradiation constitutes a factor which is inhibiting advances in operating conditions for nuclear reactors as regards increasing cycle time. This is mainly due to uniform corrosion. A particular aim of the invention is to provide a tube with improved characteristics which can be in the recrystallized state when good creep behavior is required above all, or which can be in a metallurgically stress-relieved state, which is more easily manufactured econom-cally to within strict dimensional tolerances (in particular as regards circularity errors) , and which is better as regards generalized corrosion. For that purpose, there is provided a zirconium-based alloy tube containing, by weight, 1% to 1.7% of tin, 0.55% to 0.8% of iron, 0.20% to 0.60% in total of at least one element selected from chromium and vanadium, and 0.10% to 0.18% of oxygen, the carbon and silicon contents being controlled and being respectively in the range 50 ppm to 200 ppm and in the range of 50 ppm to 120 ppm, the alloy further containing only zirconium and unavoidable impurities. The tube, in its final state, is either stress-relieved or recrystallized depending on the required properties. Vanadium is essentially present in fine precipitates in the form Zr(Fe,V).sub.2 ; this is also the case for chromium, which is present in precipitates in the form Zr(Fe,Cr).sub.2. A high Fe/(V+Cr) ratio, which may exceed 3/1, can further improve resistance to corrosion in a lithium-containing medium. As a general rule, this ratio will be close to 2/1. It is generally preferable to use either chromium alone, or vanadium alone rather than a combination of the two. The precise composition selected from the above range will depend on the properties which are to be prioritized. Usually, an alloy containing 1.3% Sn, 0.60% Fe, 0.25% V or Cr, 0.14% O.sub.2, 140 ppm C and 90 ppm Si will be a good compromise. The presence of vanadium reduces the fraction of absorbed hydrogen and improves resistance to corrosion in an aqueous medium at high temperature and high pressure, even in the event of localized boiling. If one requirement is to reduce creep as much as possible during the initial stage of reactor use, it may be advantageous to have a high tin, carbon, and/or oxygen content. A carbon content of more than 100 ppm is favorable as regards creep; but above 200 ppm, expansion on irradiation becomes large. The silicon content is "controlled" to take advantage of its regulatory effect on structures and its favorable influence on corrosion resistance. A high value for the sum of the beta-producing elements (Fe+V+Cr) contributes to reducing the grain size of the metallurgical structure, which is a factor for good resistance to stress corrosion, ductility after irradiation, mechanical properties, and shaping. This sum is frequently at least 0.70%. The invention also provides a process for the production of an alloy tube of the type defined above, comprising successively: casting an ingot and forging to a solid bar; water quenching the bar after heating, generally by induction, in the .beta. phase; optional annealing in the range 640.degree. C. to 760.degree. C. (advantageously about 730.degree. C.) to form the .alpha. phase; drawing a pierced billet to a tubular blank; optional annealing in .alpha. phase in the range 600.degree. C. to 750.degree. C. (advantageously about 650.degree. C.); successive cold rolling steps to form tubes of decreasing thicknesses, with intermediate annealing steps in an inert atmosphere or in a vacuum at a temperature in the range 640.degree. C. to 760.degree. C., advantageously about 730.degree. C. for the first two steps and 700.degree. C. for subsequent steps; and a final annealing step in an inert atmosphere or in a vacuum at a temperature in the range 450.degree. C. to 500.degree. C. (advantageously about 485.degree. C.) if a stress-relieved structure is required, or in the range 565.degree. C. to 630.degree. C. (advantageously about 580.degree. C.) if a recrystallized structure is required. The set of heat treatments is advantageously such that the heat treatment parameter .SIGMA.A is in the range 10.sup.-18 to 10.sup.-16, .SIGMA.A g equal to the product of time t in hours multiplied by exp (-40000/T), T being expressed in Kelvins. The first annealing step, after quenching, is advantageously carried out at 730.degree. C.; the second, after extruding, is advantageously carried out at about 650.degree. C. The tube produced does not undergo any further heat treatment which would modify its metallurgical structure until the time it is used as a cladding tube or a guide tube. However, it does undergo more surface treatment and is then examined. The surface treatment may in particular comprise blast cleaning and film removal followed by rinsing. The surface treatment can be completed by polishing using a wheel. It is checked conventionally, either visually, and/or using ultrasound and/or using eddy currents.