Patent Number: 041586816
Section: description

DETAILED DESCRIPTION OF THE INVENTION The nuclear fuel pellets first travel through a reduction furnace with an adjustable velocity or dwelling time for adjusting the stoichiometry of the nuclear fuel oxides. The pellets are then taken in cooled-down condition to a checking station and pass subsequently through a sintering furnace with independently adjustable dwelling time or throughput velocity. The reduction of such pressed blanks initially in stoichiometric excess of oxygen is accomplished, for example, in an externally heated muffle furnace, the temperature profile of which can be fitted by appropriately controlling individual heater circuits within wide ranges (100.degree. to 1000.degree. C.) to the temperature curve which is optimum for conducting the reaction as a function of various conditions (stoichiometric excess of oxygen, quantity of pressed blanks per unit time, shape of the pressed blanks, reactivity of the powders). In order to keep the water concentration low in accordance with the course of the reduction, additional gas injection stations are provided at different points. After leaving the muffle, the blanks can quickly be checked in the control station to determine whether the reduction has taken place completely. The check can be made, for example, optically; thus, the color hue of the pressed blanks gives information regarding the stoichiometric condition. This has the advantage that it can be ascertained at an early stage of the operation whether the pressed blanks are sufficiently reduced. For example, in case of insufficient reduction of the blanks, defects would be produced in the sintering which would limit their usability. Such inadequately reduced pressed blanks might then even have to be scrapped as rejects. A nitrogen/hydrogen mixture can be used as the reduction gas, which has considerable cost advantages over the use of a mixture of rare gas and hydrogen. A mixture of nitrogen and carbon monoxide may also be used as the reduction gas. Larger amounts of carbon monoxide as compared to hydrogen may be safely tolerated in the gas mixture. The hydrogen content in the gas mixture is usually 4 to 8 % while the carbon monoxide content is 4 to 12 %. The push-through velocity and the dimensions of the muffle can be matched optimally to the given reduction and material conditions. In particular, the push-through velocity is independent of the push-through velocity of the sintering furnace. The extent of reduction of the pellets depends on a number of factors, including throughput or quantity of pellets fed to the furnace per unit time, velocity of the pellets through the furnace, length of the furnace, furnace temperature and gas composition. The effect of these factors on the pellets in the furnace could be referred to as exposure of the pellets to conditions in the furnace, which shall be designated as "residence time". Normally the dimensions of a furnace are fixed by previous design of the furnace. Also to some extent temperature variations are limited by previous design of the furnace. While gas compositions may be varied, in general, since the hydrogen content is not to materially exceed 8 %, changes in the hydrogen content would be small. Usually in an operation, the temperature and gas composition are set and any variation or adjustment or regulation to obtain the desired reduction and sintering of the pellet is readily and conveniently accomplished by varying the velocity of the pellets through the furnace or by varying the throughput, i.e., increasing or decreasing the quantity of pellet passing through the furnace, or a combination of a change of velocity and a change of throughput. Through the use of a closed, externally heated muffle, further advantages accrue due to the fact that the replacement of defective heater circuits is possible without the aggravating conditions of working with radioactively contaminated work pieces; that the gas flow conditions and the mass balance can be controlled better than in furnaces equipped with gas-permeable linings; and finally, that the decomposition products of the lubricant or/and the binder and lubricating agents are no longer condensed uncontrollably at colder points of the furnace jacket but are removed from the muffle with the hot gas. Through using nitrogen/hydrogen mixtures, the employment of electrostatic dust separators becomes furthermore possible, which remove, in addition to dust, also the decomposition products of the lubricating oil and/or of the binder and lubricating agents. Electrostatic separators cannot be used in the presence of argon (the cheapest rare gas), as argon is ionized already at the relatively high voltage and the separator then breaks down. A high degree of separation is necessary, however, as the furnace gas must be discharged to the outside air only via absolute filters. The sintering of the reduced pressing blanks is performed, after they pass through an intermediate or checking station, in a resistance-heated furnace lined with highly refractory blocks. As all or substantially all the reduction had previously been effected in the reduction furnace and no further reduction need take place, the reduction potential in the furnace can be adjusted to any required order of magnitude without effect on the preceding reduction. Overall, only a small quantity of a rare gas/hydrogen mixture is necessary for this purpose, as the material to be sintered is already reduced and no additional water is therefore generated. This is accompanied, in addition, by considerable cost advantages over the process technique customary heretofore. By decoupling the reduction from the sintering, the length of the sintering furnace and the push-through velocity can be optimally matched to the operational requirements such as space required and maximum loading on the one hand and the requirements as to the sintered oxide such as, for example, a minimum dwelling time in the high-temperature zone. The operation is best conducted with temperatures lower in the reduction furnace than in the sintering furnace. In general, the reduction furnace operates from about 700.degree. C. to about 1000.degree. C., preferably about 1000.degree. C., and the sintering furnace operates at from about 1000.degree. C. to about 1760.degree. C., preferably about 1600.degree. C. to 1700.degree. C. The reduced pellets in the intermediate station wherein the pellets are held for checking or temporary storage or both are at a low temperature, preferably below 100.degree. C., desirably about ambient temperature. The equipment for carrying out the method is shown schematically in the drawing. The reduction furnace 3 is watercooled by passing cooling water through cooling coil 32 on the outside. The heater winding 31 is situated outside the furnace chamber proper, which is connected via the lines 33a, b and c to a source, not shown, of an N.sub.2 /H.sub.2 gas mixture. The gas mixture leaves the furnace chamber via the line 34 and is then purified in a cleaning device 35. There, binder agents which may have been driven off are condensed and dust is separated electrostatically. The material to be sintered is loaded on transport boats, not shown, of highly heat-resistant material such as, for example, molybdenum and is placed at the inlet 1 into the transport canal 19 which goes through the whole installation. After the inlet 1, an input lock 2 is provided, which shuts the interior of the reduction furnace 3 against the outside atmosphere. After this furnace is traversed, an outlet rail 4 of similar design is provided again, which serves the same purpose. Ahead of it, this canal 19 is further provided with water cooling 12, which continues to cool the material to be sintered to room temperature after it has already cooled down in the furnace 3. After passing through the outlet lock 4, the transport boats arrive at a control station 5 which may also be designated as an intermediate storage station. There, it is ascertained, for example, that the reduction process performed in the furnace 3 has taken place properly. The intermediate storage station 5 makes possible, furthermore, different throughputs in the reduction furnace and in the following pushthrough furnace 7. The latter is again equipped with external water cooling means 72. The electric heater winding in sintering furnace 7, which makes possible sintering temperatures to maximally 1760.degree. C., is located inside the furnace chamber proper. A mixture of argon and hydrogen with controllable water vapor content is fed-in and discharged via the lines 73 and 74. The locks 6 and 8 ahead of and behind the sintering furnace 7 ensure that no harmful atmosphere gets into the interior of the transport canal 19. The water cooling 13 of the transport canal 19 takes care of cooling the finished pressed bodies which leave the furnace in sintered condition. At the outlet 9 of the transport canal 19, the transport boats can then be taken from the furnace installation and the nuclear fuel pellets can be passed on for further processing, e.g., grinding. The following examples illustrate the present invention: EXAMPLE 1 Uranium oxide/Plutonium oxide powder mixtures with 2.2 stoichiometry (oxygen-to-metal ratio) are pressed without binder to form pressed bodies in the density range of 5.5 grams per cm.sup.3. These pressed bodies are loaded into transport boats of molybdenum, each transport boat taking a pressed body weight of about 4 kg. These transport boats are then run into the reduction furnace 3 via the lock 2 as illustrated in the drawing. The furnace has a temperature profile such that the temperature increases from room temperature in the first quarter of the furnace to 1000.degree. C. This temperature is maintained over one-half the length of the furnace and then drops again to room temperature in the last quarter of the length. A total gas quantity of 35 m.sup.3 per hour of nitrogen with 8% hydrogen flows-in through the furnace via the lines 33a, b, and c. The humidity content in the entering gas is less than 10 ppm. The total gas quantity is fed into the furnace 3 in such a manner that 15 m.sup.3 per hour flow in via the line 33a at the furnace exit and 10 m.sup.3 per hour each are introduced into the hot zone by two further gas supply lines 33b and 33c. The push-through or travel velocity of the transport boats is chosen so that about 12 kg UO.sub.2 pressed bodies, i.e., 3 transport boats, get into or leave the furnace per hour. The humidity of the sinter gas leaving the furnace 3 in a collecting pipe 34 is measured continuously. If the former exceeds a value of 8000 vpm H.sub.2 O, an alarm is given and either the push-through velocity is reduced or a smaller amount of pressed bodies is loaded into the individual transport boats. After being cooled down to room temperature, the transport boats are removed from the furnace 3 and taken to the checking station or intermediate storage station 5. There, the stoichiometry is checked by sampling. If it is smaller than UO.sub.2.05, the transport boats are placed in the sintering furnace proper 7 with a temperature higher than 1600.degree. C. The push-through velocity through this furnace is controlled uniformly for all pressed bodies in such a manner that the residence times remain the same in the zone of the highest temperature and corresponds to the requirements desired for the nuclear fuel. Through this sintering furnace flows a gas mixture of argon and 8% hydrogen as well as an adjustable water content. This water content is adjusted so that the oxygen potential (hydrogen:water ratio) of the gas at the sintering temperatures is equal to the oxygen potential in the nuclear fuel pellets of the desired stoichiometry at the same temperature. The quantity of gas to be passed through is limited here to maximally 10 m.sup.3 per hour. EXAMPLE 2 Uranium oxide/Plutonium oxide powders with 2.2 stoichiometry are pressed after the addition of binder and/or lubricating agents to form pressed bodies in the density range of about 5.6 grams per cm.sup.3 and after being pressed are loaded into the transport boats. Here, too, a pressed body weight of about 4 kg is loaded per boat, and the latter are then run into the reduction furnace 3. The temperature profile of this furnace as well as the gas supply for the reduction process are the same as in Example 1. However, the push-through velocity is to be chosen in such a manner that the driving-out of the binder or lubricating agent does not lead to permanent damage at the pressed body. The upper velocity is determined simply by examining the pressed bodies at the intermediate station. The gas leaving the furnace 3 in the collecting pipe 34 is conducted through the device 35, where the binder and lubricating agent, which have been driven out and carried from the furnace by the hot gas stream, are precipitated. Likewise, dust separation of the gas stream by electrostatic means takes place there. The further treatment of the pressed bodies is the same as in Example 1. The travel-through velocities will vary, as these depend on the composition of the nuclear fuel pellets as well as on their geometrical dimensions and can be readily determined. This procedure of separating reduction processes and the sintering proper allows one to adjust and maintain optimum operating conditions for both zones of heating, so that an end product of the highest possible quality is obtained.