Patent Number: 043483399
Section: description

DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, sintering additives with grain-growth-promoting action are added to the nuclear fuel powder with arbitrary oxygen-to-metal ratio to adjust the micro structure, and the powder is pressed into blanks. The blanks are subsequently sintered in a continuous sintering furnace in an oxidizing atmosphere and are then treated in a reducing atmosphere. The oxidizing gas consists of technically pure carbon dioxide and is conducted through the furnace space in the same direction as the blanks, i.e. concurrent flow. The reducing gas, on the other hand, which may be mixed with a neutral protective gas i.e. a gas which is inert under the conditions of operation, such as nitrogen, is conducted through the furnace against the direction of motion of the blanks, i.e. counter-flow. The reducing gas may be provided with an appropriate degree of moisture for adjusting the residual fluorine content in the nuclear fuel. This method can, therefore, be called a two-stage process having an oxidative-sintering stage in a carbon-dioxide atmosphere and a reducing stage in a hydrogen-containing atmosphere. Both steps are carried out in a single sintering furnace. The furnace has two zones, i.e., the sintering zone and the reduction zone with the zones separated by a gas lock which also serves as a means for discharging the two gases, i.e. the oxidizing gas from the oxidative-sintering zone and the reducing gas from the reduction zone. The micro structure of the sintered bodies produced in this manner at low temperatures of 1000.degree. to 1400.degree. C. consists of finer grains and coarser grains, which are distributed homogeneously. This is achieved by adding sintering additives, to be discussed. The starting powder to be pressed into blanks to be sintered for manufacture of nuclear fuel does not consistently have the same oxygen-to-metal ratio but generally varies, sometimes quite frequently and to a considerable extent, i.e. from a ratio of 2.0 to 2.2 and higher. Such variable starting powders have been termed nuclear fuel powder with "arbitrary" oxygen-to-metal ratio. The method of the present invention overcomes the difficulty of starting powders with arbitrary oxygen-to-metal ratios and reliably produces uniform end products even with starting powders with arbitrary oxygen-to-metal ratios. The present method will now be described in detail with the aid of FIGS. 1 and 2, and compared with the state of the art. FIG. 1 shows schematically the construction of a high-temperature sintering furnace such as is in use at the present time. This furnace, designated by numeral 1, contains a reduction zone R, in which a temperature of 500.degree. to 600.degree. C. prevails. Adjoining the latter is a sintering zone S with a temperature of 1700.degree. C. On the output side of the furnace is a cooling zone A. The material to be sintered is moved, i.e. pushed or pulled into this furnace in the direction of the arrow 3. The reducing atmosphere 4 which contains hydrogen flows through the furnace against the travel direction of the material to be sintered. FIG. 2, on the other hand, shows, likewise schematically, a low-temperature sintering furnace for carrying out the method according to the invention. This furnace is designated by numeral 2. The arrow 3 indicates the entering direction of the material to be sintered. The latter first gets into the sintering zone S with a temperature of 1100.degree. C. and the CO.sub.2 -gas flows over it in the same direction. This gas is discharged through the adjacent lock 7, which in addition is flushed with a stream of nitrogen. Gas-locks are known in the art. The material to be sintered then travels through the lock into the reduction zone R, in which the same temperature of 1100.degree. C. prevails, and subsequently, into the cooling zone A. A reducing gas, which consists, for instance, of 94% nitrogen and 6% hydrogen, flows against the travel direction of the material to be sintered through zones A and R and is likewise discharged through the lock 7. An over-stoichiometry required for the sintering process at temperatures from 100.degree. C. on is adjusted through the use of carbon dioxide of technical purity. This stoichiometry occurs when the blanks pressed from oxide powders are heated up, remains constant during the sintering and is completely independent of the ratio of starting oxygen to metal of the metal powder used. In this process, the same sintering density is achieved as with the sintering process in the high-temperature furnace as per FIG. 1. This density is .gtoreq.94% of theoretical density and is reached after very short sintering times. The following table shows the relationship between the sintering time and the sintering temperature according to this method: ______________________________________ Sintering Time: Sintering Temperature: ______________________________________ 30 min. 1000.degree. C. 20 1100 10 1200 10 1300 5 1400 ______________________________________ Longer sintering times or higher temperatures increase the sintering density only inappreciably and can therefore be used to adjust the microstructure variables. In the reduction zone R, hydrogen, hydrogen/inert gas or hydrogen/nitrogen mixtures are used dry or moistened. As shown in the figure, gas mixtures with only 6% by volume of hydrogen are sufficient to obtain an oxygen-to-metal ratio of 2.0.+-.0.02 after 30 minutes at the temperatures give. Humidification of the reduction gas leads to lower fluorine contents in the sintered bodies. The fluorine contents are safely below 10 ppm. Also with this method, microstructure variables are obtained such as have been optimized with the high-temperature method. The desired pores in the range of 1 to 10 .mu.m are generated by the addition of U.sub.3 O.sub.8. The mean grain size is 4 to 10 .mu.m, depending on the sintering temperature and time. Controlled adjustment of this microstructure via the addition of U.sub.3 O.sub.8 yields a matrix grain size of about 2 .mu.m, in which a grain fraction of 20 to 50 .mu.m is embedded. This bimodal grain structure exhibits better plasticity because the fine-grain regions form the skeleton of the sintered bodies and take up the mechanical load. In addition, the fission gas liberation during the operation of the reactor is lowered by the coarse and growth-stable bodies. This grain structure therefore is an optimum compromise with respect to the fuel properties, plasticity and fission gas retaining capacity. The mentioned addition of up to 25% U.sub.3 O.sub.8 allows the use of dry-processed revert material from the pellet production, which is converted into U.sub.3 O.sub.8 by annealing. In this manner, it is possible to recycle nuclear fuel scrap into the production and to adjust at the same time the microstructure, as already mentioned. These U.sub.3 O.sub.8 additions remain stable during the sintering and are converted to UO.sub.2 only in the reduction stage. Thereby, this additive acts as a pore former and thus lowers the density. This lowering of the density is directly proportional to the amount of U.sub.3 O.sub.8 additive (obtained from scrap). From this discussion it is evident that in spite of substantially lower temperatures than with the previous high-temperature sintering method, products of equal quality are obtained. The lower temperatures, however, make possible substantially lower furnace heating power and in addition, the wear of the materials is substantially lower, which is reflected particularly advantageously in the operating costs. These are also affected advantageoulsy by the protective gases used in comparison with pure reduction gases. The results obtained with this method will be demonstrated with the aid of the following embodiment examples. The pressed bodies were prepared by directly pressing UO.sub.2 powder or powder mixtures of UO.sub.2 with gadolinium oxide or with PuO.sub.2. The UO.sub.2 -powder had the following powder data: Specific surface: 5 to 6 m.sup.2 /g PA1 Bulk density: 2 g/cm.sup.2 PA1 Average particle size: 6 .mu.m No lubricants, binders or pore formers of any kind were added to the powders or powder mixtures. In part of the samples, the only addition was recycled sintered material annealed to form U.sub.3 O.sub.8. The density of the blanks was always 5.6 g/cm.sup.3. The powder composition, sintering conditions and pellet properties can be seen from the examples 1 to 7 of the following Table. TABLE __________________________________________________________________________ Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 __________________________________________________________________________ Powder compo- 100% UO.sub.2 100% UO.sub.2 100% UO.sub.2 100% UO.sub.2 100% UO.sub.2 100% UO.sub.2 100% UO.sub.2 sition Powder O/U 2.11 2.11 2.08 2.15 2.10 2.10 2.08 U.sub.3 O.sub.8 addition -- 10% -- -- -- -- 8% Sintering- CO.sub.2 CO.sub.2 CO.sub.2 CO.sub.2 CO.sub.2 CO.sub.2 CO.sub.2 gas Sintering- 1100.degree. C. 1100.degree. C. 1100.degree. C. 1100.degree. C. 1000.degree. C. 1400.degree. C. 1100.degree. C. temperatures Sintering- 1 h 1 h 1 h 1 h 5 min 1 h 2 h time Reducing gas H.sub.2 H.sub.2 H.sub.2 H.sub.2 H.sub.2 H.sub.2 94% N.sub.2 / 6% H.sub.2 Reduction- 1100.degree. C. 1100.degree. C. 1100.degree. C. 1100.degree. C. 1000.degree. C. 1400.degree. C. 1100.degree. C. temperature Reduction- 15 min 15 min 15 min 15 min 15 min 15 min 2 h time Pellet pro- perties: Sintering density (g/cm.sup.3) 10.51 10.32 10.51 10.52 10.27 10.64 10.44 % of theoret. 95.9 94.2 95.9 96 93.7 97 95 density Fluorine &lt;5 ppm &lt;5 ppm &lt;5 ppm &lt;5 ppm 3 ppm O/U 2.00 2.00 2.00 2.00 2.00 2.00 2.00 __________________________________________________________________________