Patent Number: 059011939
Section: description

DESCRIPTION OF THE PRESENT INVENTION The invention is based on the realization that a fuel element with a two-layer cladding, in order to obtain a small amount of precipitated hydrides in the boundary zone between the layers and a tangential hydride distribution in both the inner and outer parts, shall be composed of zirconium alloys whose Sn content does not differ more than 0.7%. Since the cladding is built up in such a way that the difference in Sn content between the inner and outer parts does not exceed 0.7%, both parts have a relatively similar recrystallization temperature. This means that a finishing stress-relieve anneal of the cladding results in the outer layer being stress-relieve-annealed or only partially recrystallized. Sn and O increase the recrystallization temperature at a given degree of processing, whereas other alloying elements such as Fe, Cr, Ni and Nb reduce the recrystallization annealing temperature. By a two-layer cladding, good corrosion properties may be utilized in an alloy for the outer layer together with good mechanical and creep properties of a conventional construction material for a fuel cladding such as Zircaloy-2 and Zircaloy-4, which constitute the inner supporting part. Zircaloy-2 contains 1.2-1.7% Sn, 0.07-0.2% Fe, 0.05-0.15% Cr, 0.03-0.08% Ni, 0.07-0.15% O and the total amount of Fe, Cr and Ni is within the interval 0.18-0.38%. zircaloy-4 contains 1.2-1.7% Sn, 0.18-0.24% Fe, 0.07-0.13% Cr and 0.10-0.16% O, and the total amount of Fe and Cr is within the interval 0.28-0.37%. According to the invention, a good corrosion resistance and a small hydrogen pick-up are obtained for a fuel element with a cladding tube whose outer part consists of a zirconium alloy with addition of Sn within the content interval 0.65 to 0.95% and Fe within the interval 0.35 to 0.5%. The inner part of the cladding may consist of a conventional zirconium alloy such as zircaloy-4 and zircaloy-2 with a normal content of Sn (within the interval 1.2 to 1.7%), but may also consist of another zirconium alloy with sufficiently good mechanical and creep resistance properties to be able to constitute a supporting part of a fuel cladding. Of importance, however, is that during stress-relieve annealing of the inner part at 450 to 510.degree. C., the cladding is given an only partially recrystallized outer part. When estimating the recrystallization temperature, all the alloying elements in the inner layer may be taken into consideration, which inner layer may comprise, in addition to Sn, Fe, Cr, Ni and Nb. The Sn content in the outer part and the inner part shall be relatively equal and not differ more than at most 0.7% in order for a final heat treatment of the cladding at 450 to 510.degree. C. to result in the supporting inner part being stress-relieve-annealed and the outer layer being only partially recrystallized. In this way, it may be ensured that hydrogen which is absorbed by the fuel cladding during operation will be precipitated evenly in the cross section of the cladding and preferably not in the boundary zone between the layers and, in addition, a largely tangential precipitation of the hydrides is obtained. Since the Fe content in the outer layer is relatively high and in partially recrystallization-annealed state, a harder outer layer is obtained, which facilitates the mounting of fuel rods in the fuel bundle. During manufacture of a cladding tube for fuel rods included in a fuel element according to the invention, an inner tube of Zircaloy-4 is joined together with an outer part of zirconium with 0.8% Sn and 0.4% Fe. The inner part of zircaloy-4 is chosen so that the Sn content in the material does not exceed the Sn content in the outer layer by more than at most 0.7%. In both Zircaloy-4 and the outer layer, the other substances in the materials are limited to the maximally allowed values for reactor grade zirconium. The parts are joined together by means of extrusion so as to become metallurgically bonded, whereupon tube manufacture in conventional manner using cold-rolling operations and intermediate heat treatments is performed. The final heat treatment is performed at 450-510.degree. C. for 2-5 hours. Measurement of the hydride orientation shows that both the inner and outer parts are given an f.sub.n -value &lt;0.05. By f.sub.n -value is meant the percentage of hydrides oriented within 45.degree. from a radial direction in relation to the total number of hydrides. During a normal hydride test, comprising intentional hydrogenation of the tube so that this contains at least 100 ppm hydrogen, few hydrides visible in a microscope are present in the outer layer or in the bonding zone between the layers. The outer layer constitutes approximately 10-25% of the wall thickness of the cladding tube. During manufacture of the cladding tube, it is advantageous if the so-called annealing parameter A is high. An aim is that log A should be greater than -13. A is a measure of the sum of all the heat treatments during the tube manufacture and is defined as A=.SIGMA..sub.i t.sub.i .multidot.exp(-Q/RT.sub.i), where t.sub.i =annealing time in hours, T.sub.i =annealing temperature in .degree.K, Q=the activation energy=15000 J/mole, R is the general gas constant.