Abstract:
Manufacture of very dense oxidic fuel bodies of UO 2  with rare earth oxides in which pressed blanks are subjected to sintering in an oxidizing atmosphere at relatively low temperature and are sintered in a reducing atmosphere at a higher temperature. This avoids sintering-inhibiting phases and permits very dense bodies with greater content of rare earth oxides to be produced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for the manufacture of very dense oxidic nuclear fuel bodies of UO 2  with additions of rare earth oxides such as gadolinium oxide. Such nuclear fuel bodies are required particularly for the fuel assemblies of nuclear light-water reactors; the rare-earth oxide, especially Gd 2  O 3 , plays the role of a burnable poison. 
     2. Description of the Prior Art 
     In sintering directly mixed uranium-gadolinium powders pressed without additions, it has been found that the sintering process deviates from that with pure UO 2  in important ways. In particular, the densities required in the range around 6% by weight can barely be achieved or maintained in spite of very long sintering times. Thorough investigations have shown that in the course of the sintering process, diffusion-stable and therefore sintering-inhibiting phases can form. This manifests itself particularly by the fact that the sintering densification takes place with delay in the temperature range above 1200° C. This effect is pronounced particularly with gadolinium contents around 6% by weight, but extends over the entire range of about 4 to 8% by weight. 
     Up to now, mainly Gd 2  O 3  concentrations of 2 to 4% have been used in light-water reactors. Since, however, an increase of the concentration is desirable, especially in view of the use in fuel assemblies with high target burnup, the problem arose to find a sintering method with which it is possible to also obtain the higher weight contents of gadolinium oxide without the difficulties mentioned, especially with respect to the sintering density with controlled structure. 
     SUMMARY OF THE INVENTION 
     With the foregoing and other objects in view, there is provided in accordance with the invention a method which is characterized by the following steps: 
     (a) Mixing the nuclear fuel powder with up to 10% by weight powder of rare earth oxide, 
     (b) pressing into blanks, 
     (c) sintering in an oxidizing atmosphere at 800° to 1400° C. for a period of 15 minutes to 2 hours, and 
     (d) sintering in a reducing atmosphere at temperatures above 1650° C. for a period of 30 minutes to 4 hours. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method for the manufacture of very dense oxidic nuclear fuel bodies, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawing, in which is diagrammatically illustrated the flow of blanks through a sintering zone cocurrent with an oxidizing medium, then through a gas lock flushed with an inert gas, and then through another sintering zone at a higher temperature counter current to a reducing gas. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     By sintering in two temperature steps, the main densification processes can take place below the starting temperature for the mixed-crystal phases which form the solid solutions which inhibit the sintering. The overstoichiometric state adjusted by an oxidizing atmosphere largely prevents the formation of the sintering-inhibiting phases which can be formed in a reducing atmosphere and causes heavy densification at low temperatures via the activation of transport processes. 
     The sintering in the second step at temperatures of above 800° to 1400° C. to more than 1650° C. in a reducing atmosphere serves for setting the stoichiometry in the fuel. A fine adjustment of the density can be made via the holding time at temperatures above 1650° C. 
     For further explanation, the individual process steps will now be given once more together with the changes in the nuclear fuel caused thereby: 
     First, the starting powders of flowable UO 2 , which was prepared, for instance, by the AUC process, and of rare earth oxides, specifically gadolinium oxide with concentrations of up to 10% by weight are mixed directly. This mixture is subsequently pressed into blanks without the need to add lubricants, binders or pore formers. It is possible, however, to admix dry-processed production runback directly up to 20% by weight, which has been annealed to form U 3  O 8 . This addition of scrap has a pore-forming effect in the finished sintered nuclear fuel and thereby permits a targeted control of the density as well as the setting of a suitable pore and grain structure of the sintered bodies. 
     These blanks are then placed in a two-zone sintering furnace, as is shown schematically in the drawing together with the temperature profile. 
     The sintering furnace is loaded on the low-temperature side which has temperatures of 800° to 1400° C., for instance, 1100° C. In this zone, an oxidizing atmosphere prevails, as for example CO 2  /CO mixtures or technically pure CO 2  (without repurification), which is fed into the furnace space, for instance, in the travel direction of the blanks. Formation of a solid solution which would have the effect of inhibiting the sintering, does not yet take place in this temperature range. This means that the overstoichiometric state set by the sintering conditions mainly causes the UO 2  particles to be sintered, but the added rare earth oxides remain largely untouched thereby. 
     Subsequently, the material to be sintered is pushed through a lock flushed with nitrogen into the second part of the furnace which is heated to above 1700° C., generally not higher than about 1900° C. There, a reducing atmosphere prevails, for example technically pure hydrogen or mixtures of hydrogen with dry or moistened inert gases, for instance, 8% H 2  /92% N 2 , as well as ammonia. These gases flow through the high-temperature zone, preferably against the travel direction of the bodies to be sintered. Their residence time in the high-temperature zone is varied between 30 minutes and 4 hours. The formation of solid solution of UO 2  and rare earth oxides takes place during this isothermal sintering process. The microstructure can be set via the sintering time. 
     The mentioned humidification of the reducing gas atmosphere is known and serves for better fluorine depletion in the nuclear fuel pellets. 
     The attached Table shows the sintering results obtained with this method as a function of the weight content of gadolinium oxide as well as of the holding times and temperatures in the two sintering stages. 
     Accordingly, the fuel bodies produced by this method attain high densities, in particular, more than or equal to 94% of theoretical density. 
     The densities obtained are largely independent of the concentration of the rear earth oxide, particularly the gadolinium oxide, in the concentration range 0 to 10% by weight. 
     The high sintering densities permit the addition of up to 20% U 3  O 8  as a pore former and for controlling the density for a stable behavior during the burnup in the nuclear reactor. 
     From the above, it will be seen that the previous difficulties mentioned at the outset are practically eliminated by this relatively simple method. 
     
                                           TABLE__________________________________________________________________________(a)    (b)    (c)    (d)    (e)    (f)  (g)UO.sub.2 /Gd.sub.2 O.sub.3  Temperature         Holding time                Temperature                       Holding time                              Sintering                                   Sinteringfuel body  of the during the                of the zone                       during the                              den- densitywith x oxidizing         oxidizing                of reducing                       reducing                              sity in % ofweight %  sintering         sintering                sintering                       sintering                              in   theoreticalGd.sub.2 O.sub.3  in °C.         in minutes                in °C.                       in minutes                              g/cm.sup.3                                   density__________________________________________________________________________X = 4  1100   30     1750   120    10.52                                   97.4    4  1250    0     1750   120    10.41                                   96.4    5  1000   30     1750   240    10.24                                   95.2    5  1100   10     1750   240    10.25                                   95.2    6   950   60     1750   180    10.35                                   96.5    6.5   900   60     1750   240    10.33                                   96.5    6.5  1000   45     1750   180    10.33                                   96.5    6.5  1100   60     1750   240    10.36                                   96.8    7   920   60     1750   180    10.07                                   94.3    7   970   60     1750   180    10.12                                   94.7__________________________________________________________________________