Patent Number: 044915403
Section: description

Referring first to FIG. 1, a number of spent nuclear fuel rods 11 from a nuclear reactor are arranged in a copper container 10. The fuel rods, which consist of zircaloy cladding tubes containing pellets of uranium dioxide, remaining attached to spacers 12 which retained the fuel rods in bundles in the nuclear reactor. These spacers 12 can be of stainless steel. In FIGS. 1 and 2, four fuel rod bundles 13, 14, 15 and 16 are shown. The fuel rod bundles may possibly rest on supports (not shown) spacing them from the bottom of the container 10 or they can be placed on a bed of copper powder. The container 10 is then filled in its entirety, while being vibrated, with a mixture 17 consisting of 70 parts by weight of a copper powder with spherical particles having diameters in the range 0.5-1.5 mm and of 30 parts by weight of a copper powder with spherical particles having diameters in the range 0.1-0.2 mm. A lid 18 of copper is then placed on the container 10. The container, the lid and the powder are all of the previously mentioned copper quality containing 99.95% Cu (including small amounts of Ag). The circumferential part 19 of the lid 18, which makes contact with the container 10, has a stepped shape to provide a central lower portion 20 of the lid which projects into the container. The confronting surfaces 10a and 18a of the container 10 and the lid 18, respectively, are roughened or otherwise textured, as is indicated in FIG. 3. The surfaces 10a and 18a are well cleaned and freed from oxide by acids before fitting the lid 18 onto the container 10. The container 10, its contents 11, 12, 17 and the lid 18 are arranged in a capsule 21 of copper sheet or of steel sheet, the lid 22 of which, made of copper sheet or steel sheet is welded to the capsule by forming a gas-tight joint 23. The lid 22 is provided with a tube 24 of copper or steel, respectively, which can be connected to a vacuum pump for evacuation of the capsule with its contents. After evacuation, the capsule is sealed by closing the tube 24 above the upper surface of the lid (e.g. by cold or hot welding). The sealed capsule 21, 22 with its contents is then subjected to hot isostatic pressing in two stages employing a gas, for example argon, as the pressure medium in a high pressure furnace of the kind disclosed in U.S. Pat. No. 4,172,807. In the first stage, the capsule is subjected to a pressure of 80 MPa and to a temperature of 450.degree.-500.degree. C. for a period of 2-10 hours. During the first stage, the copper in the container 10, the lid 18 and the powder 17 undergo a creep deformation, which results in the copper filling powder 17 providing an efficient all-round support for the fuel rods 11, which prevents creep rupture in the zircaloy cladding tubes as a result of an increase in pressure of the gas, present in these tubes, during continued heating. However, this first stage does not result in the powder grains, the container and the lid forming a coherent unit with a fully developed bonding. Such a result is achieved during the second stage in which the temperature in the furnace is increased to about 700.degree. C., while the pressure is increased, without additional supply of gas, to about 100 MPa, and by maintaining these conditions for 1-4 hours. When the capsule with its contents has been subjected to the second stage of the isostatic pressing, the capsule with its contained material is allowed to cool, whereafter the pressure is reduced to atmospheric pressure and the capsule is removed from the furnace. Normally, the capsule is allowed to remain around the compressed product 10, 11, 12, 17, 18 when it is to be deposited for long-term storage. In an alternative example, the mixture 17 consists of 55 parts by weight of a copper powder with spherical particles having diameters in the range 0.8-1.0 mm and 45 parts by weight of a copper powder with spherical particles having diameters in the range 0.2 mm and below. A fill density of 81% of the theoretical density can then be obtained by vibrational filling. After evacuation of the capsule 21 with its contents, the capsule is heated to 350.degree. C., whereupon it is filled with hydrogen gas with a pressure of 0.1 MPa. When this temperature has been maintained for 1/2 hour, the capsule is re-evacuated and is then refilled with hydrogen gas. This treatment with hydrogen gas at 350.degree. C. is repeated a plurality of times, for example 7 times, suitably with a successively longer treatment time after each refilling. The final treatment time could be 10 hours. The cyclic treatments with hydrogen gas result in a reduction of possibly existing oxides of copper. After completion of the cyclic treatments with hydrogen gas, the capsule 21, 22 is evacuated and sealed as in the previously described case. During the isostatic pressing, a temperature of 400.degree.-450.degree. C. is used in the first stage and a temperature of 525.degree. C. is used in the second stage. This described alternative example is otherwise carried out under the same conditions as the previously mentioned case. In the embodiment illustrated in FIG. 2, the surrounding capsule 21, 22 is dispensed with. Instead, the container 10 and the lid 18 are provided with flanges 25 and 26, respectively. After placing the fuel rods 11 in the container and filling this with copper powder 17, the flanges 25 and 26 are joined together by welding or cold pressing to form a gas-tight joint 27. The lid 18 is provided with a tube 28 of copper which is sealed after evacuation of the container and its gas-tight lid. After sealing of the tube 28, the closed container is subjected to isostatic pressing in two stages in either of the manners described for the sealed capsule in accordance with FIG. 1.