Patent Number: 
Section: description

Other characteristics and advantages of the invention will be seen more clearly upon reading the following examples, which are naturally given as an illustration and are not restrictive. The following examples illustrate the preparation of phosphosilicate apatites corresponding to four embodiments of the invention. In all these examples, the method according to the invention is used to prepare said phosphosilicate apatites from the following reagents: CaF2, SiO2, Ca2P2O7, CaCO3 and PuO2 with Na2CO3, GdF3 and/or Gd2O3, if required. Na0.45Ca9.1Pu0.45(PO4)5.55(SiO4)0.45F2xe2x80x83xe2x80x83III. To obtain 10 g of britholite complying with the above formula, a first mixture is first of all prepared in acetone using the following quantities of reagents: CaCO3: 2.3405 g PuO2: 1.1307 g SiO2: 0.2479 g Ca2P2O7: 6.4662 g, and Na2CO3: 0.2187 g. All the reagents, except CaF2, are mixed in acetone and dried in an oven at 100xc2x0 C. for 1 hour. They are then ground to obtain a particle size of 50 xcexcm and calcined for 1 to 2 hours at 900xc2x0 C. to break down the carbonates. After cooling, 0.7160 g of CaF2 is added, and the two powders are again mixed in acetone. After the acetone has evaporated completely in an oven at 100xc2x0 C. (around thirty minutes), grinding is performed with 50% by weight of distilled water in jars in ZrO2 to obtain a powder with a particle size of 10 xcexcm. The reagent powder is then compacted to 400 MPa (4000 bar) with the application of slow and progressive pressure (20 MPa/min; 200 bar/min). This makes it possible to increase the thermal conductivity of the powder and the calcination will be affected. The reaction time for a given temperature and pressure will be decreased. The pellet obtained is then calcined at 1500xc2x0 C. for 6 hours in a nitrogen atmosphere. Under these conditions, there is no loss of fluorine due to volatility during the reagent sintering. Ca9.46 Gd0.08 Pu0.46 (PO4)5 SiO4 F2xe2x80x83xe2x80x83V. In this case, the same procedure as for example 1 is followed, but the reagents used to prepare the first mixture are present in the following proportions: CaCO3: 2.7945 g PuO2: 1.1425 g SiO2: 0.5446 g Ca2P2O7: 5.7585 g, and Na2CO3: 0.4804 g. After heat treatment to break down the calcium carbonate, 0.7078 g of CaF2 is added and the final mixture of the powders is performed, as in example 1. A homogeneous dense ceramic containing plutonium and gadolinium is thus obtained. Ca9.04Gd0.48Pu0.48(PO4)4.56(SiO4)1.44F2xe2x80x83xe2x80x83VII To obtain 10 g of britholite corresponding to the above formula, the following reagents in the following proportions are used to prepare the first mixture: CaCO3: 3.0089 g PuO2: 1.1361 g SiO2: 0.7474 g Ca2P2O7: 5.0047 g, and Gd2O3: 0.7515 g. All these reagents are mixed in acetone and dried in an oven as in example 1. They are then ground to obtain a particle size of approximately 50 xcexcm and the mixture is heated to 900xc2x0 C. to break down the carbonates. After cooling, 0.6745 g of CaF2 is added and the final mixture and grinding are performed as in example 1. The powder obtained is shaped in a carbon mould using a piston and the mould is then heated by induction. A pressure of 25 MPa is then applied at the end of the 15 minute stage at 700xc2x0 C. and calcination is continued at a temperature of the order of 1100xc2x0 C. for one hour, applying a pressure of 25 MPa. If the homogeneity of the matrix is not satisfactory, the pellet undergoes fine grinding, followed by annealing at a very high temperature (1600xc2x0 C.) in a neutral atmosphere without any risk of loss of fluorine. Ca9.55PU0.45(PO4)5.1(SiO4)0.9F2xe2x80x83xe2x80x83IX. In this example, the same procedure as in example 3 is followed using the following quantities of reagents: CaCO3: 3.1481 g PuO2: 1.1241 g SiO2: 0.4930 g Ca2P2O7: 5.9073 g, and CaF2: 0.7118 g. The product obtained following the pressurised sintering is a dense, homogeneous ceramic wherein all the fluorine is incorporated. It is to be noted that the improved incorporation of fluorine in the method according to the invention is due to the use of a neutral or reducing atmosphere which does not favour fluorine-oxygen exchanges, and due to the fact that the synthesis and densification are carried out in only one step at a high temperature. The dense ceramics obtained in this way are more homogeneous since the calcination and/or annealing temperatures are higher. These two improvements made to the method described in document [1] improve the conditioning of plutonium considerably. In addition to the intrinsic properties offered by the present invention, additional confinement properties are offered in that the products obtained come in the form of dense monoliths, which reduces the exchange surface for leaching. In addition, according to the invention, it is possible to add a neutrophage product such as Hf or a neutron poison such as Gd in the form of microinclusions or as a substitution to reduce criticality risks. [1]: WO-A-95/02886.