Fabrication of Zircon Leads to a Pu Stabilization Alternative

An article by R. Ewing (UNM) appeared in the Fall 1995 issue of the Actinide Research Quarterly. The article in this current issue describes the work initiated at Los Alamos.

Background and Research Objectives

Zircon, because of its well-known, long-term durability (~109 years) in the geologic environment, has been proposed as a host medium for storage of Pu and other actinides recovered from dismantled nuclear weapons. Much is known about the structure of this single-phase mineral and its homologs (e.g., Hf, Th, Pa, U, Np, Pu). It is clear that zircon-type orthosilicate structures can accommodate metal cations having radii both smaller and larger than that of the Zr ion in zircon. Natural zircon has been found to contain approximately 5000 ppm of U and Th. Some of the cations found in natural zircon (e.g., Gd, Hf) are strong absorbers for thermal neutrons. Laboratory-scale samples of zircon doped with up to 10 wt % of Pu have been prepared for conducting accelerated tests on the effect of radiation damage on the structure, property changes, and phase stability in simulated geological conditions (by W. J. Weber at the Pacific Northwest National Laboratory). Furthermore, small samples of pure PuSiO4 have been prepared (by C. Keller in Karlsruhe, Germany), which suggests that it is possible to substitute Zr with more than 10 wt % of Pu. From the point of view of nuclear waste minimization, it is desirable that the upper limit of Pu solubility in zircon be determined. While exploring the potential for increasing the Pu loading in zircon, we need to investigate the issues of criticality safety so that the optimum waste form can be determined.

Figure 2. The tantalum container after the first HIP run. This figure shows that the welds are intact, and the body of the container remains in good condition after 2 hours at 1450C and 4000 psi.

Various methods for synthesizing laboratory quantities of zircon have been investigated. However, simultaneous application of high temperature and high pressure to enhance the solid-state reaction has not been attempted, especially for zircon doped with large quantities of Pu. With the existing equipment, such as hot presses and hot isostatic presses in the Plutonium Facility, LANL is in a good position to develop the technology for large-scale fabrication of Pu-bearing zircon and, therefore, to provide the nation with an alternative method for disposing of nuclear materials. The success of this work will allow us to address the technical problems related to short-term and long-term storage of nuclear materials recovered from weapons. The results of this effort will enhance TA-55's capabilities in solving plutonium disposition problems. In this work we set out to obtain the following information, essential to large-scale fabrication of Pu-bearing zircon: 1) the process parameters for large-scale fabrication of zircon with desirable Pu-loading and 2) the solubility limit of Pu in zircon. What we learn will be important from the standpoint of minimizing waste volumes. This article presents the results of the first study.

Scientific Approach and Accomplishments

Since the main objective of this work was to investigate the feasibility of large-scale fabrication of Pu-bearing zircon, it was of utmost importance to be able to achieve this goal without releasing contamination to the work environment. Therefore, the approach taken in this work was to use the synthesis of zircon to explore potential problems in the containment of materials during processing. Furthermore, since PuO2 is thermodynamically less stable than ZrO2, the processing parameters developed for the synthesis of zircon would be applicable to those for Pu-zircon.

The process used was hot isostatic pressing (HIP). The equipment provides high temperature in the reaction chamber through resistively heated graphite heating elements. It provides high pressure by compressing inert argon gas supplied from gas cylinders to the reaction chamber. The hot press is capable of attaining temperatures up to 2000°C and pressures up to 30,000 psi. The material to be "hipped" is contained inside compressible containers, typically fabricated from thin metals, and is compressed with equal force from all directions.

For hipping zircon, a container made of refractory metal that melts at high temperature is desirable. However, most metals are thermodynamically unstable, (and more so at higher temperatures), with respect to the formation of their oxides. Since the starting materials for zircon are oxides, the oxidation of the container materials is inevitable unless some sort of barrier (inner container) is provided to separate the reaction mixture from the metal container. But this inner container must also be compressible as well as chemically inert to the reaction mixture.

In anticipation of fabricating Pu-zircon in PF-4, the LANL Plutonium Facility, some schemes need to be developed for packaging the reaction-mixture/container assembly. First, the powder mixture has to be loaded into the inner container inside a glove box. Then, for safety reasons, the container has to be assembled and sealed without the use of excessive heat. Thereafter, this inner container will be transferred from a glove box into the "cold" outer metal container in an introductory hood while the outer container is maintained free of contamination. The metal container will then be welded in a separate "cold" tungsten/inert gas (TIG) welding hood. Finally, the outer container has to maintain its integrity after the processing; otherwise the process equipment contained inside the hood will be contaminated.

Based on these criteria, tantalum was chosen for the outer container, and quartz the inner one. Tantalum was chosen based on its high melting point of 3017°C, its availability, and its relative ease of welding. The choice of quartz was based on the known phase behavior of the ZrO2/ SiO2 system. The phase diagram of this system indicates that zircon does not form any compounds with SiO2 in the temperature range of interest. Furthermore, quartz softens at temperatures above 1200°C and, therefore, can be compressed. Nevertheless, the potential for reactions among Ta, ZrO2, and SiO2 still exists. Based on this containment system, the range of temperature, pressure, and time were explored to provide a reasonable processing scheme.

Results and Accomplishments The tantalum container was compressed as expected, the welds were intact in all cases, and the quartz joint held together in one piece. The product was monolithic. The results from x-ray powder diffraction showed that 30 lines were observed in the starting material, and that almost all lines can be assigned to either monoclinic-ZrO2 or SiO2 (quartz). In the first HIP run, 39 lines were observed in the product. Out of the 39 lines, 25 are assignable to zircon, 11 are due to monoclinic ZrO2, and only 3 are due to SiO2 (cristobalite). However, the 3 strongest lines are due to ZrO2 and SiO2. This indicates that zircon begins to form in approximately 2 hours of reaction time, but the reaction is nowhere near completion. The other samples from all products showed almost identical patterns and relative intensities. This indicates that the extent of reaction did not differ significantly for reaction times ranging from 2 to 8 hours. A more quantitative analysis is necessary using x-ray diffraction on samples that are mixed with standard reference material. Further product material characterization by chemical, analytical, and metallographic techniques are underway.

Figure 3. Cut-up view of a tantalum container after processing. The figure shows that the quartz joint holds together. The quartz container has broken into pieces and can be separated from the product.

Under the high-temperature and high-pressure conditions employed in this work, hundred-gram batches of ZrO2/SiO2 mixture began to react and form zircon in approximately 2 hours. Considering the thermo-dynamic stability of PuO2 relative to ZrO2 (i.e., PuO2 has less negative standard Gibb's free energy of formation than ZrO2), it is expected that the fabrication of Pu-bearing zircon will be accomplished more easily than the fabrication of zircon alone. The result of this preliminary work indicates that it is feasible to fabricate large quantities of Pu-zircon; however, further developmental work is necessary. This work revealed potential container problems that could be encountered at the developmental stage and suggested practicable solutions. The quartz inner container, the glass sealant, and the tantalum outer container all showed expected behavior. The process developed for joining quartz parts with the use of a special glass sealant, as an alternative to high-temperature fusion, inside glove boxes is an innovative approach that is applicable to other operations in the Plutonium Facility. The containment system allowed the reaction mixture to be compressed isostatically. The separation of final products from the container materials was not difficult. The reactions between tantalum, SiO2, and ZrO2 could be prevented by providing a sacrificial barrier between the inner and the outer containers. Clearly, further developmental work is needed to establish a safe process for large-scale fabrication of Pu-bearing zircon.

Principal investigators are Kyu C. Kim, NMT-DO, and John Y. Huang, NMT-6. Others contributing to the project are Patricia L Serrano and Mark A. Williamson, NMT-6; Mary Ann Reimus, Gary H. Rinehart, Christina M. Lynch, Paul Contreras, and Paul Moniz, NMT-9; Stanley Pierce, MST-6; Rudolph Fernandez, NMT-4; McIlwaine Archer and Richard G. Logsdon, MST-7; and Luis A. Morales, NMT-5.

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