B. E. Burakov, E. E. Strykanova
V. G. Khlopin Radium Institute
28, 2^nd-Murinskiy Ave., St. Petersburg, 194021, Russia
Tel: +7+812 346-1186, Fax: +7+812 346-1129, E-mail: burakov@riand.spb.su

Weapons plutonium pits contain a significant admixture of Ga (up to 3 wt.%) in the form of a metallic alloy with Pu. The different chemical behavior of Ga in comparison with Pu can impact the process for treating weapons grade plutonium to produce a durable waste form. As was shown in previous work, yttrium-aluminum garnet (YAG) Y₃Al₅O₁₂ is a very durable crystalline host-phase with a lattice of high capacity for incorporating actinides. Crystalline ceramics based on YAG have been proposed for actinide transmutation and as components of Pu-containing nuclear fuel. This paper considers a solid solution of YAG with other garnets: Gd₃Ga₅O₁₂ (GGG) and Y₃Ga₅O₁₂ (YGG) as a prospective host-phase for immobilization of weapons grade Pu in the presence of Ga. The advantages are:

1. GGG and YGG form unlimited solid solution with YAG that are also durable phases;
2. a direct conversion of plutonium pits to the stable ceramic can be achieved avoiding chemical separation of Ga;
3. the possibility of incorporating in the lattice of this host phase not only Ga but also Gd as an effective neutron absorbent, all rare-earth elements in different valence states, and Fe, Ca, Cr, Zr, etc.; and
4. the possibility of using the YAG­GGG­YGG system for stabilization of Pu­containing wastes with complex chemical compositions.

The physico-chemical features of a YAG­GGG­YGG solid solution are described. As a conclusion the optimal methods of YAG­GGG­YGG synthesis using existing technologies are discussed.

Weapons plutonium pits contain a significant admixture of Ga (up to 3 wt.%) in the form of a metallic alloy with Pu. Also, some Pu-containing wastes have Ga admixture up to 8 wt.%. The different chemical behavior of Ga in comparison with Pu can impact the process for treating weapons grade plutonium to produce a durable waste form. The crystalline host-phases usually considered for Pu immobilization: zircon, monazite, zirconolite, etc. do not contain Ga in their lattices. There is also no information concerning Ga incorporation by these minerals in the form of solid solution. We suggest the system of crystalline garnet: Y₃Al₅O₁₂ (YAG) –Gd₃Ga₅O₁₂ (GGG)–Y₃Ga₅O₁₂ (YGG) as a prospective host-phase for immobilization of weapons grade Pu in the presence of Ga.

Synthetic garnet with a general formula A₃B₂C₃O₁₂ where A, B and C are three types of structural cation positions, is a well-known durable compound widely used in industry such Nd-doped YAG^1,2 as in the field of laser materials.³ Garnet is also desirable in the microelectronics industry as substrate for magnetic films with cylinder magnetic domains⁴ and as an artificial gem stone with a hardness higher than quartz and without cleavage problems.^5,6 YAG was proposed previously as a host-phase for actinide immobilization.^7-10

Garnet provides a high lattice capacity for incorporation of various chemical elements because of its flexible structure as described in the following:

1. is a structural position for large cations: Na⁺, Ca²⁺, Sr²⁺, Ba²⁺, Cd²⁺, Fe²⁺, Mg²⁺, Y³⁺, Ln³⁺, Zr⁴⁺, Hf⁴⁺ in cubic (8) coordination; Pu is proposed to occupy this position;
2. is characterized by octahedral (6) coordination and a high isomorphous capacity for the following elements: Fe²⁺, Mn²⁺, Mg²⁺, Al³⁺, Ga³⁺, Y³⁺, Sc³⁺, Fe³⁺, Cr³⁺, Ln³⁺, Si⁴⁺, Ge⁴⁺, Sn⁴⁺, Zr⁴⁺, Hf⁴⁺;
3. is a position with tetrahedral (4) coordination common for tri-and tetra-valent cations: Al³⁺, Fe³⁺, Ga³⁺, Si⁴⁺, Ge⁴⁺, Ti⁴⁺, Sn⁴⁺.

The same elements can occupy different positions; for example, Al – "B" and "C", Gd – "A" and "B", Ga – "B" and "C". However, this does not decrease the stability of garnet crystalline structure.

Usually synthetic garnet is used in a form of high quality monocrystals, which are produced by melting methods^11-13 at temperature from 1770^oC (melting point of YGG) to 1950^oC (melting point of YAG). However, garnet can be formed at significantly lower temperature (900^oC for YAG) by solid-phase synthesis.¹⁴

In our experiments of synthesis of garnet solid solution YAG­GGG­YGG, we consider the possible application of cold-crucible melting techniques, which are widely considered as an attractive method for high level radioactive waste (HLW) treatment.¹⁵

A combined nitrate solution of Ce and Ga (Ce/Ga ratio was 99/1) simulating weapons Pu pits dissolved in nitric acid was mixed with nitrate solutions of Y, Al, Gd. The concentrations of all elements in mixed solution was controlled to provide the stoichiometry of the final garnet YAG­GGG­YGG with the following formula (Y, Ce, Gd)₃ (Al, Ga)₅ O₁₂ containing about 8 wt.% Ce and 0.6-0.7 wt.% Ga. The hydroxides of Y, Al, Ce, Gd, Ga were co-precipitated by adding excess of NH₄OH. Dried hydroxide powder was sintered at a temperature 800^oC for 30 minutes. The resulting product was milled and cold pressed to form pellets 8 millimeters in diameter and 20 mm in height. Pellets were heated in air for 20-30 seconds under a temperature gradient ranging from 1500°C in the bottom part to 2000°C in the top part of the pellet. Heating was stopped when melting of the top part of the pellet was visible. Samples of the sintered ceramic were examined by X-ray diffraction, and the melted part of the samples was also examined by SEM.

XRD-analysis of synthesized samples show that the yield of garnet (Y,Ce,Gd)₃(Al,Ga)₅O₁₂ varied from 80 wt.% in the unmelted part to 87 wt.% in the melted part. The content of the other Ce-doped phase (Y,Gd)AlO₃ with a perovskite structure is correlated inversely with the content of separated cerium phases: Ce₂Y₂O₇ and CeO₂. The unmelted part of the samples contains up to 13 wt.% of those separated Ce-phases, while the melted part contains significantly less – about 1-2 wt.%. At the same time a content of perovskite phase does not exceed 4 wt.% in the unmelted part but reaches 10-12 wt.% in the melted part. Cell parameters of the perovskite phase are significantly higher in the melted part compared with the unmelted part. Therefore, the formation of Ce-doped perovskite during melting is correlated to Ce assimilation from the initial separated Ce phases. This is also confirmed by the high Ce content of 11 to 26 wt.% in the perovskite from melted samples.

Microprobe analysis shows that the distribution of Ce and Gd in the garnet matrix is not uniform (Table I). Generally, the highest concentration of Ce is correlated with the highest concentration of Gd. The melted part of the samples is characterized by two main crystalline phases responsible for Ce-incorporation in their lattices: garnet (Y,Gd,Ce)₃(Al,Ga)₅O₁₂ and perovskite (Y,Ce,Gd)AlO₃ (Figure 1).

It is important to note that Pu valence behavior is not equivalent to Ce; and, in Pu experiments, we can expect other Pu distributions between garnet and perovskite phases, even the formation of a garnet mono-phase. In case of actinide incorporation in a tetravalent state, the necessity of using bivalent elements for charge compensation is proposed: (Y,Gd,Pu,Np,Me)₃(Al,Ga)₅O₁₂, where Me is a bivalent element.

The results allow us to make the following preliminary conclusions:

1. The garnet solid solution YAG­GGG­YGG is an attractive crystalline host-phase for Pu immobilization in the presence of significant admixtures of a few wt.% Ga and other elements in a different valence state.
2. The formation of Ce-doped garnet during melting under non-equilibrium conditions is accompanied with the formation of Ce-doped perovskite YAlO₃.
3. Since Pu valence behavior is expected to be different from Ce, additional experiments with other simulants such as U, Th or Pu are needed to determine Pu distribution and yield between these two phases and the isomorphous capacity for Pu incorporation.
4. Garnet synthesis by sintering is assumed to result in the formation of some Pu separated phases. By using a melting method, such as a cold-crucible, a smaller amount of separated Pu phases is expected. However, approval of the use of high temperature (near 2000^oC) for Pu treatment would be required.

The proposed schedule of Pu conversion into garnet-based ceramic is shown on Figure 2.

The authors are very grateful to Mrs. M. Zamoryanskaya and Mr. V. Garbuzov for their help. The authors thank Dr. D. Knecht for the comments and corrections.

1. Handbook of Lasers with Selected Data on Optical Technology, Chemical Rubber Co., Cleveland (1971).
2. KAMINSKY, "Physics and Spectroscopy of Laser Crystals," Moscow, Science (1986) (in Russian).
3. M. J. WEBER, Handbook of Laser Science and Technology, Boca Raton: CRC Press, Vol. 1 (1982).
4. J. P. M. DAMEN, J. A. PISTORIOUS, J. M. ROBERTSON, "Calcium Gallium Germanium Garnet as a Substrate for Magnetic Bubble Applications," Mater. Res. Bull., Vol. 12, No. 1, p. 73 (1977).
5. W. ANDERSON, Gem Testing, Butterworths & Co, London (1980).
6. ELWELL, Man-Made Gemstones, Elwell/Ellis Horwood Ltd., Publishers (1979).
7. ANDERSON, B. E. BURAKOV, V. G. VASILIEV, "A Creation of Crystalline Matrix for Actinide Waste in Khlopin Radium Institute," Proceedings of the International Conference SAFE WASTE-93, Avignon, France, Vol. 2, pp. 29-33 (June 13-18, 1993).
8. PRUNIER, M. SALVATORES, Y. GUERIN, A. ZAETTA, "First Results and Future Trends for the Transmutation of Long-Lived Radioactive Wastes," Proceedings of the International Conference SAFE WASTE-93, Avignon, France, Vol. 2, pp. 298-311 (June 13-18, 1993).
9. B. ANDERSON, B. E. BURAKOV, V. A. STARCHENKO, V. G. VASILIEV, N. SHULYAK, R. G. G. HOLMES, W. WEAVER, D. GODDARD, R. CLEGG, S. RICHARDSON, "UK-Russian Collaboration High Level Waste Immobilization Studies," Proceedings of the International Conference GLOBAL-95, Versailles, France, Vol. 1, pp. 216-223 (September 11-14, 1995).
10. K-I. KURAMOTO, Y. MAKINO, T. YANAGI, S. MURAOKA, Y. ITO, "Development of Zirconia- and Alumina-Based Ceramic Waste Forms for High Concentrated TRU Elements," Proceedings of the International Conference GLOBAL-95, Versailles, France, Vol. 2, pp. 1838-1845 (September 11-14, 1995).
11. B. RUBINSTEIN, R. L. BARNS, "Crystallographic Data for Rare-Earth Aluminum Garnets: Part II," American Mineralogist, Vol. 50, p. 782 (1965).
12. B. J. COCKAYNE, "The Melt Growth of Oxide and Related Single Crystals," Journal of Crystal Growth, Vol. 42, p. 413 (1977).
13. G. PETROSYAN, Kh. S. BAGDASAROV, T. J. BUTAEVA, et. al., "Growth and Properties of Lu₃Al₅O₁₂-RE³⁺ Single Crystals," Kristall und Technik, Bd 10, p. 467 (1975).
14. BONDAR, L. N. KOROLEVA, E. T. BEZRUK, "Physico-chemical properties of yttrium aluminates and gallates," Inorg. Mater. J., Issue of USSR Acad. of Sciences, Vol. 20, No. 2, pp. 257-261 (1984) (in Russian).
15. T. ADVOCAT, G. LETURCQ, J. LACOMBE, G. BERGER, R. A. DAY, K. HART, E. VERNAZ, A. BONNETIER, "Alteration of Cold Crucible Melter Titanate-Based Ceramics: Comparison with Hot-Pressed Titanate-Based Ceramic," Mat. Res. Soc. Symp. Proc., Vol. 465, pp. 355-362 (1997).

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