Industrial presses used to form complex parts by powder metallurgy includes several tools (punches, cores and die) which can move separately in a specific sequence that prevent powder transfers from a part of high density to low density one during pressing.
Even for a simple shape like a fuel pellet, the displacement of upper punch that control the volume of powder is accompanied by a downward movement of the die which influences friction forces and density gradient as a consequence. Three main cases can be considered according to the ratio between the speed of the upper punch, Vp, and those of the die Vm (cf.
figure 8). Simple effect modes lead to a monotonic decrease in density, the higher density
H/D = 0.25 H/D = 0.5 H/D = 1
H/D = 1.5
zone being in contact with the moving punch (relative to the die). Double effect mode (Vm/ Vp
= 0,5) produces a symmetrical density distribution, which gives a final “hourglass-like” shape after sintering.
Vp≠ 0 and Vm = 0 Simple mode (upper punch)
Vm/Vp = 0.5 Double effect
Vm/Vp = 1 Simple mode (lower punch) FIG. 8. powder density distribution according to die-pressing conditions for a cylindrical part.
However, the density gradients may differ from these ideal cases when the speeds ratio change during the pressing stage. We have considered the actual sequence where firstly both the upper punch and die move in a quasi double effect mode (Vm/ Vp ~ 0,5) and then, at a given moment, the die stops and the upper punch moves downward till the end of pressing.
Simulation shows that the time when the die stops is a key parameter. An optimized value of this time has been numerically determined and then experimentally tested. Comparisons of two pellet diametrical profiles obtained after sintering are reported on the figure 9, one concerns a reference pressing cycle, the second is related to an optimized cycle. In the first case, the volume of material removed by grinding to obtain a perfect cylinder reaches 0.92%
of the pellet volume (0.75% calculated) while it is lowered to 0.29% (0.23 calculated) after optimization. In these calculations, the diameter of the cylinder is defined as the minimal value obtained on the sintered pellet and it has to be noticed on figure 9 that this value also depends on the cycle.
Moreover two limiting cases have been reported on figure 9: an experimental profile obtained when the die stops early in the cycle (quasi simple mode) and a profile obtained when the die moves till the end of the cycle and goes on after the upper punch has stopped. In the latest case, a complete inversion of the profile is observed as a result of a stresses redistribution (at constant volume) due to friction between powder and the moving die. It is thus shown that the variation of pellet shapes may be sharply correlated with the variation of a simple press parameter.
High density
low density
4,030
FIG. 9. Pellet diametrical profiles obtained after different pressing cycles : reference cycle (gray) and optimized cycle (black), symbols denote measured values, continuous lines represent the calculated profiles. Open marks combined with doted lines correspond to 2 cases: a case where die stops early (squares) and a case where die stops after the punch has stopped (circles)
6. CONCLUSION
FEM simulations and experimental results clearly show that the diametrical profile of fuel pellet depends on the die-pressing conditions as a consequence of frictional forces between die and powder. Since friction depends on relative movements, the pressing cycle parameters (speeds ratio and changes) may be adapted to minimize the density gradient and to lower the geometrical defects of the sintered pellet. In this study, the volume of material that has to be removed to obtain a perfect cylinder has been reduced by a factor 3, just by fitting a single press parameter.
FEM simulation of shaping process constitutes an interesting tool to perform one or even multi-parameter optimizations (a specific algorithm has been implemented in the PreCAD® software). This code may be also used for design purposes (tools, cycles…) or to perform sensitivity analysis (powder properties, friction coefficient…). Experimental characteri-zation of powders remains necessary to supply model parameters. On-going programs aimed at extending the database to actual MOX powders are performed in the CEA facilities (Cadarache). Further calculations will be also carried out to determine optimized cycle conditions adapted to this material.
REFERENCES
[1] BACCINO R., MORET F. Numerical modeling of powder metallurgy processes, Materials and Design, 21, 359–364 (2000).
[2] DELLIS C. et al.: PRECAD, a Computer Assisted Design and Modelling Tool for Powder Precision Moulding, HIP’96 Proceeding, Proceedings of the International Conference on Hot Isostatic Pressing, 20–22 May 96, Andover, Massachusetts. Pages 75–78.
[3] PAVIER E., PhD thesis, Institut National Polytechnique de Grenoble, France. (1998).
[4] PAVIER E et al., Analysis of die compaction of tungsten carbide and cobalt powder mixtures, Powder Metallurgy, vol. 42, n°4, 345–352 (1999).
[5] FOURCADE, J. Thesis, Université de Montpellier 2 (2002).
MIXED OXIDES PELLETS OBTENTION BY THE
“REVERSE STRIKE” CO-PRECIPITATION METHOD J.E. MENGHINI, D.E. MARCHI
V.G. TRIMARCO, E.H. OROSCO Comisión Nacional de Energía Atómica, Centro Atómico Constituyentes,
Buenos Aires, Argentina Abstract
In order to obtain mixed oxides of uranium/plutonium and uranium/gadolinium, the “reverse strike”
co-precipitation method was studied. The objective was to verify that it is possible to obtain sintered pellets with the required physicochemical characteristics, furthermore a good micro homogeneity and, in the case of plutonium, an easy scrap recycling by dissolution with nitric acid without the fluorhidric acid adding. This method consists in the co-precipitation of ammonium diuranate (ADU) and plutonium hydroxide, Pu(OH)4, or ADU and gadolinium hidroxide, Gd(OH)3, from its nitric mixed solutions using gaseous ammonia and keeping the pH and temperature controlled. The tests with uranium and plutonium were carried out inside glove boxes using a mixed solution of this element where the Pu/(U+Pu) ratio is 20% (w/w). The tests with U and Gd were carried out using solutions with different Gd/(U+Gd) ratios between 0 % and 8% (w/w).The different steps of these processes in order to obtain the fuel pellets and the characterization of intermediate products are shown. The results show that this method assures a mixed-homogeneous precipitate obtaining. Regarding the ADU-Gd(OH)3, the existence of a mixed phase was verified. In sintered pellets a single phase of solid solution was observed. In the case of plutonium, its solubility in nitric acid without FH (fluorhidric acid) was rapidly reached. Sintered pellets showed high density and an inhomogeneous pore distribution, probably, due to problems during the pressing because of the low powder density. The low content of actinides in the filtrate was verified; therefore a previous treatment of it before its discarding is unnecessary. The method is appropriate for the obtention of mixed-oxide pellets having high densities and a good micro- homogeneity. It also assures the formation of a solid solution in the ceramic structure.
1. INTRODUCTION
Several methods have been studied in the world in order to obtain mixed oxides containing uranium for the fabrication of fuel pellets used in power reactors. We are especially interested in those fuels that use plutonium because of the recycling of this element proceeding from the burned fuels and from the scrap of fabrication, and also those containing gadolinium as a burnable poison.
The aim is to obtain a final product within the required specifications, having a good micro homogeneity and features that allow to improving its behavior inside the reactor. In the case of the uranium-plutonium, furthermore, the simple operability of the method and the easy treatment of generated wastes, are considered. For this, it is very important also, a fast and simple dissolution of the fabrication scraps.
One of the evaluated methods in our laboratory, alongside the direct denitration using microwaves, is the “reverse strike” co precipitation method .In this method, contrary to the direct precipitation one ( where the chemical species are precipitated from the acid solution by means of the addition of ammonia gas or ammonium hydroxide with a gradual increase of pH of the medium ), the reactive and the solution containing the actinides are added simultaneously inside the reactor using a controlled temperature, agitation, and pH previously
fixed which remains constant during all the process. Thus, the two species, uranium-plutonium or uranium-gadolinium, precipitate at the same time and in a quantitative way.
By means of the “reverse strike” co precipitation method, it is possible to obtain a precipitate, and consequently, a mixed oxide powder for the pellets fabrication with a high homogeneity grade regarding the dispersion of one element to the other, all this due to the simultaneous formation of precipitated species, as well as a good sinterability behavior.
In the present work, the method and the experiments carried out with U-Pu and U-Gd solutions are described, and the obtained results are shown.