Patent Number: 062352237
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention and its advantageous will be explained in further detail below in terms of a reference sintered nuclear fuel body or compact, which has been produced approximately in accordance with German Published, Non-Prosecuted Patent Application DE 38 02 048 A1, and for sintered nuclear fuel bodies according to the invention including (U, Pu)O.sub.2 mixed crystal: In order to produce the reference sintered nuclear fuel body, 70 g of UO.sub.2 powder and 30 g of PuO.sub.2 powder are mixed with 1 g of zinc stearate powder and ground for 16 hours in a ball mill. The ground product is then granulated in a granulating vessel and mixed in a conical mixer with a further 400 g of UO.sub.2 powder. Some of this mixed powder is then compressed into a body with a density of from 5.4 g/cm.sup.3 to 6.5 g/cm.sup.3 . This compressed body or compact is sintered in the sintering atmosphere, which includes 4% hydrogen and 96% nitrogen. The natural oxygen impurities in the nitrogen bring about an oxygen partial pressure of 10.sup.-20 atmospheres. After a holding time of 3 hours for the sintering, the sintered nuclear fuel body is cooled down in the sintering atmosphere. The final result is a reference sintered nuclear fuel body with a sintering density that is 95.7% of the theoretical density and an open porosity of 1.9% of the total volume of the reference sintered nuclear fuel body. The (U, Pu)O.sub.2 mixed crystal of the reference sintered nuclear fuel body has a mean particle size of 5.1 .mu.m. In order to produce a first sintered nuclear fuel body according to the invention, the same starting quantities of powdered UO.sub.2 and powdered PuO.sub.2 as in the production of the reference sintered nuclear fuel body are mixed, but together with one gram of powdered aluminum distearate, and compressed and sintered in the same way as in the production of the reference sintered nuclear fuel body. The result is a first sintered nuclear fuel body according to the invention, having a density of 95.5% of the theoretical density, an open porosity of 1.3% of the total volume of the sintered nuclear fuel body, and a mean particle size of the (U, Pu)O.sub.2 mixed crystal of 8.2 .mu.m. In order to produce a second sintered nuclear fuel body according to the invention, 470 g of powdered UO.sub.2, 30 g of powdered PuO.sub.2 and 250 ppm of TiO.sub.2 are mixed together. The mixture is then ground for 45 minutes in an attritor mill with steel balls. The ground product is then granulated for 20 minutes, and some of this granulated ground product is finally compressed into a body having a density of 5.4 g/cm.sup.3 to 6.5 g/cm.sup.3. This body is sintered in the same sintering atmosphere and in the same way as the body for producing the reference sintered body. The result is a second sintered nuclear fuel body according to the invention, having a density of 96.3% of the theoretical density, an open porosity of 0.4% of the total volume of the sintered nuclear fuel body, and a mean particle size of the (U, Pu)O.sub.2 mixed crystal of 28 .mu.m. An identical body to that used for making the reference sintered nuclear fuel body is heated in the same sintering atmosphere as in the production of the reference sintered nuclear fuel body to 1750.degree. C. and held at this temperature for one hour. CO.sub.2 gas is then added to the sintering atmosphere in increasing amounts until an oxygen partial pressure in the sintering atmosphere of 10.sup.-8 atmospheres is brought about. At the same time, the sintering temperature of 1750.degree. C. is maintained for a further two hours. The feeding of CO.sub.2 is then terminated, and the sintered body is cooled down in the hydrogen-containing sintering atmosphere. The result is a third sintered nuclear fuel body according to the invention, having a density of 95.3% of the theoretical density, an open porosity of 0.08% of the total volume of the sintered nuclear fuel body, and a mean particle size of the (U, Pu)O.sub.2 mixed crystal of 15 .mu.m. Finally, an identical body as in the production of the first sintered nuclear fuel body according to the invention is sintered and cooled down in the same sintering atmosphere as in the production of the third sintered nuclear fuel body according to the invention and in the same manner as in the production of this third sintered nuclear fuel body. The result is a fourth sintered nuclear fuel body according to the invention, having a density of 95.2% of the theoretical density, an open porosity of 0.03% of the total volume of the sintered nuclear fuel body, and a mean particle size of the (U, Pu)O.sub.2 mixed crystal of 29 .mu.m. Similarly advantageous values for the density, open porosity and mean particle size of the mixed crystal and therefore a correspondingly good retention capability of the sintered nuclear fuel bodies for gaseous fission products can be attained by the method according to the invention even if the UO.sub.2 powder that is used is produced, not by the ammonium uranyl carbonate (AUC) method as is the UO.sub.2 powder for the reference sintered nuclear fuel body and the four sintered nuclear fuel bodies according to the invention, but rather by the ammonium diuranate (ADU) method or by dry conversion (see the book entitled "Gmelin Handbuch der Anorganischen Chemie" [Gmelin Manual of Inorganic Chemistry], Springer-Verlag Berlin, Heidelberg, N.Y., 1981; Uran, Erganzungsband A3 [Uranium, Supplemental Volume A3], pp. 99-115). Using the four sintered nuclear fuel bodies according to the invention, including (U, Pu)O.sub.2 mixed crystal in a high-power nuclear reactor during four usage cycles showed that only about half as much fission gas is released during the four usage cycles, as compared with the reference sintered nuclear fuel body including (U, Pu)O.sub.2 mixed crystal.