Zirconia ceramics for excess weapons plutonium waste

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Abstract

We synthesized a zirconia (ZrO2)-based single-phase ceramic containing simulated excess weapons plutonium waste. ZrO2 has large solubility for other metallic oxides. More than 20 binary systems AxOy–ZrO2 have been reported in the literature, including PuO2, rare-earth oxides, and oxides of metals contained in weapons plutonium wastes. We show that significant amounts of gadolinium (neutron absorber) and yttrium (additional stabilizer of the cubic modification) can be dissolved in ZrO2, together with plutonium (simulated by Ce4+, U4+ or Th4+) and impurities (e.g., Ca, Mg, Fe, Si). Sol–gel and powder methods were applied to make homogeneous, single-phase zirconia solid solutions. Pu waste impurities were completely dissolved in the solid solutions. In contrast to other phases, e.g., zirconolite and pyrochlore, zirconia is extremely radiation resistant and does not undergo amorphization. Baddeleyite (ZrO2) is suggested as the natural analogue to study long-term radiation resistance and chemical durability of zirconia-based waste forms.

Introduction

The directive resulting from the 1996 Moscow Summit between President Clinton and President Yelstin calls for surplus plutonium to be converted into forms that are resistant to reuse in nuclear weapons. The question is how to safely dispose of 100 tons of weapons grade plutonium declared surplus at the end of the Cold War. Additionally, there are impure chemical forms of plutonium that are also considered surplus.

It has been suggested to convert some forms of plutonium (Pu–Ga alloys and PuO2) into mixed-oxide (MOX) fuel and to irradiate this fuel in commercial nuclear power reactors. In this way electrical energy would be generated and the remaining plutonium would be contained in discharged (spent) reactor fuel. In particular, it has been proposed that burning plutonium in a non-fertile fuel based on a zirconia matrix that may constitute a viable, final waste form [1], [2]. Plutonium is believed to be sufficiently proliferation resistant because the spent non-fertile fuel is shielded by strong radiation. The deterrent provided by spent fuel is called the spent fuel standard [3]. The validity of the spent fuel standard as a barrier against fast and efficient recovery of plutonium can be questioned [4], since shortcuts have been published [5] and new ones are occasionally discovered [6].

Chemical forms of plutonium not appropriate for conversion into MOX require other methods of treatment and conversion into proliferation-resistant waste forms, suitable for storage and disposal. In the US, this inventory may be as high as 50 tons of plutonium. The waste forms currently under development are ceramics, though glasses have been studied as well [7]. If properly selected and manufactured, ceramic and vitreous host phases can accommodate the chemically impure forms of plutonium currently stored in weapons plutonium processing facilities. Generally, ceramic forms are chemically far more durable than vitreous waste forms, particularly at higher temperatures or in flowing water [8]. There are chemically durable and radiation-resistant minerals, e.g., baddeleyite (ZrO2) that form solid solutions with plutonium oxide and are candidate waste forms for interim storage and disposal of pure plutonium [9], should the option of re-use as MOX fuel not be pursued. Radiation resistance is important, because the waste form is expected to confine fissile material, 239Pu (2.43 × 105 yr Half-life) and its daughter 235U (t1/2=7 × 108 yr) for a very long time.

Glass and ceramic waste forms can provide proliferation resistance comparable to that of spent fuel. There are approaches to development of such waste forms, as the following examples show:

  • 1.

    A can-in-can concept, where the Pu waste form (ceramic or glass) is embedded in vitrified high-level radioactive waste.

  • 2.

    If ceramic or glass waste forms are not embedded in vitrified high-level radioactive waste, an equivalent radiation field can be provided by a admixture of 137Cs. There is plenty of 137Cs in storage in the US. Between 10 and 300 yr after discharge of the fuel from the reactor, 137Cs determines the dose rate of spent fuel and of vitrified high-level waste.

  • 3.

    Plutonium could be burnt in non-fertile reactor fuel, where UO2 is replaced by another ceramic such as ZrO2 or MgAl2O4. Calculations have shown that over 93% of the fissile plutonium can be burnt [10]. The spent non-fertile fuel would meet the spent fuel standard.

  • 4.

    A chemical method to discourage extraction of plutonium would be to make solid solutions of plutonium and thorium oxides. Dissolution of such systems is extremely slow [11].

  • 5.

    Plutonium could be disposed of in deep boreholes without a radiation shield [3].

The most promising mineral host phases for Pu include apatite, pyrochlore, zirconolite, monazite, zircon and zirconia. The physical and chemical properties of these minerals, except zirconia, were reviewed recently by Ewing et al. [12].

A pyrochlore/zirconolite ceramic, containing other phases such as brannerite, actinide oxides and rutile, has been selected for plutonium wastes in the US [13] and is currently under development at Lawrence Livermore National Laboratory. Pyrochlore is a derivative of the fluorite structure type. In the general formula A2B2X6Y larger cations are in the A-site such as Na, Ca, U, Th, Y, and rare-earth elements (REE), and smaller, high-valent cations preferably in the B-site (Nb, Ta, Ti, Zr, Fe3+). Zirconolite (CaZrTi2O7) is one of the three main phases in Synroc, the most extensively studied ceramic waste form for high-level radioactive waste [8]. Monoclinic zirconolite is a fluorite-derivative structure closely related to pyrochlore. Zirconolite is the primary actinide host in Synroc with the actinides accommodated in the A-site. Pu incorporation into zirconolite has been studied [14], [15]. The pyrochlore phase Gd2Ti2O7 has been extensively studied in terms of radiation damage [16], [17] as was shown to become fully amorphous at a dose of 3.1 × 1018 α-decay events/g. The crystalline-to-amorphous transition is accompanied by a factor of 50 decrease in chemical durability. Amorphization induced by alpha-decay events in zirconolite has been observed for and 244Cm-doped [16] and 238Pu-substituted [16], [18], [19]. Pyrochlores occur naturally with up to 30 wt% of uranium in the A-site. Natural occurrences of zirconolite are rare, but samples have been studied extensively [17], [20], [21], [22]. Both minerals are often found to be metamict with chemical alterations in nature [23], [24], [25], [26]. Zirconolite is less susceptible to radiation-induced amorphization than is pyrochlore [27].

Usually, multi-phase ceramics are easier to make than are single-phase ceramics, if a wide range of waste constituents must be accommodated in solid solutions. Even in multi-phase ceramics, small amounts of waste constituents may concentrate in minor additional phases, sometimes as a silicate glass [7], [13]. In the multi-phase pyrochlore/zirconolite ceramic waste form for plutonium waste, plutonium and neutron absorbers (e.g., Hf and Gd), as well as waste impurities (e.g., U, Ca, Mg, Fe) enter different crystalline hosts according to their individual crystallochemical behaviors. The Gd is partitioned preferentially into the pyrochlore phase and Hf into the zirconolite phase. U and Pu may also separate into different phases, e.g., actinide oxide phases. Heterogeneous partitioning and phase separation may cause a decrease in chemical durability.

The objective of this investigation is to study the ternary system ZrO2–MO2–REE2O3 and to find a field of compositions for single-phased ceramics of cubic structure with excess weapons plutonium wastes. M stands for Pu, U and Th. We simulate plutonium by cerium. Uranium is the decay product of plutonium and may also be a waste component initially. Thorium may be included in the waste form and will complicate extraction of plutonium. REE2O3 includes Gd2O3 as a neutron absorber to prevent criticality and Y2O3 as a stabilizer of the cubic modification of ZrO2.

Section snippets

Zirconia

Zirconia, ZrO2, occurs in nature in its monoclinic modification as the mineral baddeleyite. Structural transformation from monoclinic to tetragonal takes place at 1170°C and from tetragonal to cubic at 2370°C. This modification is stable between 2370°C and the melting point at 2680°C [28]. In the cubic modification, the cations are arranged in a face-centered lattice (fluorite-type structure). Cubic ZrO2 can be stabilized by tetravalent cations in ZrO2–MO2 systems, where M=Ce4+, Th4+, U4+ [29],

Experimental

Zirconia solid solutions were prepared by reaction sintering of cold-pressed powders at temperatures between 1400°C and 1600°C. Powders were obtained either by grinding and mixing the respective metal oxides or by a sol–gel process. The solid solution compositions are listed in Table 3.

Powder method: The starting materials were powders of oxides: ZrO2, CeO2, Gd2O3, Y2O3, MgO, CaO, Fe2O3 and SiO2. All chemicals were analytical grade. Oxides in desired molar ratios (Table 3, samples #1–4) were

Results

In Table 3 we list our zirconia solid solution compositions, methods and temperatures of preparation, the number of phases, and structural information. Cerium doped samples (#1–4 in Table 3) were prepared by reaction sintering of oxide powder mixtures at 1600°C. All other samples were prepared by the sol–gel method. The temperatures for reaction sintering of powders derived from the sol–gel process were 1400°C and/or 1500°C. The second last column of Table 3 shows that all solid solutions are

Discussion

Concentrations of 1 to 10 wt% in excess weapons plutonium waste forms are under consideration [13], [71]. In these studies, We took 15 mol% as an upper limit of MO2 and 26 mol% as an upper limit of REE2O3.

Between 1400°C and 1600°C, our temperature range for ceramic syntheses, pure ZrO2 is stable in its tetragonal modification. Depending on their concentrations, many compounds, including rare-earth oxides (Table 1), form single-phase solid solutions with ZrO2 at 1400°C, stabilizing zirconia’s

Conclusions

Ceramics containing simulated excess weapons plutonium waste in solid solution with zirconia can be synthesized using sol–gel and powder methods. Zirconia ceramics with the fluorite-type cubic structure exhibit significant compositional flexibility to incorporate high concentrations of plutonium, neutron absorbers, and impurities contained in plutonium wastes. Ce4+ was found to be the best substitute for Pu4+ in this system. Synthesis of single-phase zirconia ceramics for Pu wastes is simple.

Acknowledgments

The TEM work was completed in the Microbeam Analysis Facility of the Department of Earth and Planetary Sciences at the University of New Mexico. We thank Donggao Zhao at University of Michigan for XRD analysis of some samples and Nidal Jadalla, Sergey Ushakov, and Abdelessam Abdelouas at University of New Mexico for help with preparation of some sol–gel samples.

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