#Suvorova V.A. and Kotelnikov A.R. Synthesis of ceramic phosphate-containing matrices for immobilization of radionuclides of rare-earth elements (La, Ce).
key words [radionuclides of Ce and La immobilization ceramic matrix mountain rocks burial]
The purpose of the work was to study the possibility of binding radionuclides of rare-earth elements, lanthanum and cerium contained in waste of nuclear fuel, to ceramic materials by their synthesis from phosphates of imitators of the corresponding elements and easily accessible starting raw materials, natural mountain rocks.
The synthesis makes it possible to obtain a cheap final product corresponding to the principles of (1) multi-barrier character of protective compositions and (2) phase and chemical correspondence in the matrix-host rock system [1]. The multi-barrier protective compositions obtained consist of monazites and alumosilicates, each of which is a barrier for radionuclide loss, because it binds them chemically or mechanically; host rocks with which the ceramics synthesized is in the phase and chemical correspondence (equilibrium) is the third barrier.
For mass burial of highly radioactive waste (HRW), the problem of preparation of the material without high material and energy expenditures is still urgent. Ceramics obtained by caking of grains of minerals of finely grind rocks and calcinated radioactive waste can serve as this material. It was shown in [2] that rocks are appropriate for binding Na, Ca, Cs, and Sr from HRW. Based on the requirement of easy incorporation of lanthanides and REE in the structure of minerals, we chose for experiments the rocks containing alkali pyroxenes and silicates of REE, granites and tuffs.
Starting material: natural granites (Le2 and Le7), tuff (211A), and cerium and lanthanum orthophosphates synthesized by the special procedure [3].
Experimental. Silicate components and phosphates were grind in an agate mortar for 1.5 h until the uniform composition was achieved. Pellets with a mass of 1.5 g, height 5-6 mm, and diameter 8 mm were prepared from the mixtures at room temperature by the cool molding method at a pressure of about 100 kg/cm2. The pellets obtained were caked in platinum crucibles for 3 days at 1180oC in a KO-14 electroheating furnace (runs Fm1-Fm5), and runs Fm7-Fm8 were performed at 1040oC.
Analytical procedures. The complex thermal analysis of samples of the starting silicates was performed on a Q-1500D derivatograph, which decreased the ceramization temperature to 1040oC (the temperature of the phase transition).
All samples (both of the starting materials and ceramics obtained) were studied by X-ray phase analysis. The ceramic samples had the compositions corresponding to natural granite and tuff with additions of REE phosphates.
Microprobe analysis (on a Camebax instrument) was used to determine the compositions of the starting materials and products of experiments. The results are presented in Tables 1 and 2.
Table 1. Chemical composition of the starting rocks
Rock |
Na2O | MgO | Al2O3 | SiO2 | K2O | CaO | TiO2 | FeO |
| Granite Le2 | 2.89 | - | 14.69 | 74.58 | 5.67 | 0.23 | 0.59 | 1.43 |
| Granite Le7 | 3.37 | - | 13.70 | 77.00 | 4.77 | 0.17 | 0.61 | 1.12 |
| Tuff 211A | 2.59 | 7.71 | 22.53 | 51.41 | 0.02 | 15.27 | 0.03 | 6.38 |
Table 2. Chemical composition of ceramics obtained
N |
Composition | Na2O | MgO | Al2O3 | SiO2 | K2O | CaO | FeO | Ce2O3 | P2O5 | La2O3 | SrO |
| Fm1 | LaPO4+Le2 | 4.48 | - | 12.38 | 66.78 | 4.03 | - | 1.14 | - | 5.21 | 5.47 | 0.43 |
| Fm2 | LaPO4+Le7 | 5.43 | - | 10.20 | 61.06 | 4.04 | - | 0.95 | - | 7.78 | 10.08 | 0.41 |
| Fm3 | CePO4+Le2 | 1.11 | - | 6.10 | 34.87 | 2.36 | 0.22 | 0.56 | 36.07 | 18.17 | - | 0.39 |
| Fm4 | CePO4+Le7 | 0.87 | - | 5.96 | 26.60 | 1.58 | 3.00 | 0.39 | 39.17 | 22.10 | - | 0.19 |
| Fm5 | CePO4+.Tuff | 1.36 | 1.93 | 12.35 | 29.45 | 0.44 | 5.84 | 2.03 | 30.57 | 15.62 | - | - |
| Fm7 | CePO4+Le7 | 2.64 | - | 8.58 | 31.96 | 3.46 | 0.42 | 1.05 | 25.41 | 13.26 | - | - |
| Fm8 | CePO4+Tuff | 2.28 | 3.18 | 11.26 | 29.68 | 0.50 | 5.71 | 5.65 | 27.27 | 13.81 | - | - |
Table 3. Leaching rates V (g/m2·days) of elements from ceramic samples
| Sample no | Density, g/cm2; | Leaching element | Wt.% in starting sample | 0-1 days | 1-7 days | 7-14 days | 7-28 days |
| Fm1+LaPO4 + + granite Le2 |
1.91 | Al P La Na |
5.56 2.94 4.65 3.32 |
0.890 3.300 0.079 7.802 |
0.15 1.260 0.025 2.030 |
0.050 0.400 0.078 0.580 |
0.022 0.230 0.008 0.386 |
| Fm2+LaPO4 + + granite Le7 |
1.95 | Al P La Na |
5.41 4.39 8.54 4.02 |
1.020 1.540 0.079 - |
0.240 0.650 0.010 0.991 |
0.056 0.180 0.009 0.352 |
0.020 1.103 0.009 0.191 |
| Fm3+CePO4 + + granite Le2 |
1.90 | Al P Ce Na |
3.23 10.24 30.66 0.82 |
0.280 0.015 0.029 - |
0.170 - 0.005 0.140 |
- 0.012 0.005 0.050 |
0.001 0.048 0.003 0.022 |
| Fm4+CePO4 + + granite Le7 |
2.24 | Al P Ce Na |
3.16 12.16 33.29 0.64 |
0.304 0.009 0.183 - |
0.110 0.148 0.004 - |
0.004 - 0.002 - |
0.002 0.006 0.002 - |
| Fm5+CePO4 + + tuff |
2.05 | Al P Ce Na |
6.55 8.31 24.58 1.00 |
0.220 0.220 0.013 - |
0.103 0.075 0.004 0.186 |
0.005 0.006 0.003 0.067 |
0.004 - 0.003 0.029 |
Fig.1. Kinetics of leaching of La and Ce from ceramic samples.
Fig. 2. Kinetics of leaching of phosphorus from ceramic samples.
Determination of leaching rate. The quality of ceramics was estimated from the leaching rates of elements from the samples in distilled water at 90oC (MAGATE MSS-1 test [4]). The results of the experiments on leaching of elements from the ceramic samples are presented in table 3 and Figs. 1 and 2.
Results and discussion. As can be seen in tables and plots presenting the results of experiments on leaching, the samples studied bind elements by ceramization at 1040-1180oC of the mixtures of their precipitated concentrates with natural mountain rocks.
The data on leaching of phosphate-silicate ceramic matrices obtained in the first series of experiments demonstrated their sufficiently high stability despite the low density. The analysis of results indicates that the leaching rate in the first 8 days of testing has the maximum value for all samples. During this period, only 0.002-0.015% lanthanum and 0.001-0.008% cerium contained in the sample are transferred to the leaching medium. The denser samples of the second series (2.74 and 2.89%) allow one to expect lower leaching rates.
After 50-day exposure under MSS-1 test conditions, for the granite-based samples, the mean leaching rate of cerium was 2.78x10-3 g/m2 day, and that of lanthanum was 8.24x10-3 g/m2 day, which is comparable with the leaching rates of elements from synroc samples [5]. The predomination of the leaching rate of phosphorus over that of REE allows one to consider that REE are bound in structures of newly formed minerals.
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