Towards nuclear waste confinement: solid solutions and phase stability in (Th/Ce)-Y-zirconia systems

INTRODUCTION

Safe disposal of spent nuclear fuel (SNF) requires matrix materials with strong resistance against corrosion and dissolution over a period of 106 years. Derivatives of zirconium-based ceramics, in particular zirconia, ZrO2, are promising materials for these applications since these phases are known to remain stable in geological cycles of up to 109 years. Here scientific and technological goals are to obtain zirconium-based ceramic materials containing maximum possible tetravalent actinides (An) without Zr/An phase separation. In addition, structural stability of these phases under various external parameters, e.g. temperature (T), pressure (P), irradiation and leaching resistance is essential in order to exclude possible discharge of the incorporated radioactive elements over a long time scale.
Five different structural modifications of zirconia are known to exist. At ambient pressure undoped ZrO2 phase exhibits three different polymorphs as a function of temperature - low-temperature (LT) monoclinic (7-fold coordination of Zr atoms, P21/c ) and parent HT tetragonal (T > 1440 K), and cubic (T > 2640 K) forms (8-fold coordination, P42/nmc and Fm-3m, respectively) [1]. Upon application of high pressure (HP) monoclinic modification of zirconia transforms into orthorhombic-I phase (Pbca, 7-fold coordination of Zr atoms similar to that observed for the parent monoclinic P21/c phase) at pressure of ~ 4 GPa [2]. Upon further compression at P > 25 GPa orthorhombic-I modification transforms into orthorhombic-II phase (Pnma, 9-fold coordination) [3] and this phase was found to be stable up to at least 100 GPa [4].
Various properties of ZrO2 can be efficiently controlled by doping. In particular, introduction of Y3+ ions is known to significantly influence phase stability range of zirconia and to stabilize desired HT modifications. Specifically, tetragonal Y-stabilized zirconia (YSZ) phases, denoted as t′, can be obtained at ambient temperature for Y content of ~ 3 - 15 at.% and the cubic YSZ phase can be stabilized for Y content > ~15 at.% [5]. The YSZ phases are, however, much more poorly studied as a function of temperature and pressure compared to the parent ZrO2 compound and the reported results are sometimes contradictory. In this work we present synchrotron radiation diffraction studies on incorporation of Th and Ce atoms in various YSZ phases as a function of composition, temperature and pressure.

DESCRIPTION OF THE WORK

Five series of samples have been synthesized for the current study: ZrxY0.11ThyO2-z (y = 1 – 7%, ~ 1% step), ZrxY0.14ThyO2-z (y = 4, 7, 10, 12%), ZrxY0.21ThyO2-z (y = 0 – 11%, ~ 3% step), ZrxY0.10CeyO2-z (y = 0 – 8%, ~ 1.5% step) and ZrxY0.16CeyO2-z (y = 0 – 8%, ~ 1.5% step). All samples have been obtained via precipitation of the corresponding metal salts by increasing the pH (pH equal 8 for Th- and 11 for Ce-containing samples, correspondingly). Obtained suspensions were subsequently centrifuged and the residues were dried at 350 K. Final oxide phases were formed by annealing at 1673 K for two hours with further quenching.
Ambient, T- and P-dependent in situ synchrotron radiation diffraction experiments were performed at the ROBL BM20 beamline [6] at ESRF, Grenoble. HT was obtained with hot gas blower and HP was generated using diamond anvil cells (DAC). Diffraction data were collected on high resolution XRD1 (Pilatus 100k) and multipurpose XRD2 (Pilatus3 X 2M, HT and HP experiments) diffractometers of ROBL [6].

RESULTS AND DISCUSSION

For the tetragonal ZrxY0.11ThyO2-z and ZrxY0.14ThyO2-z series maximum Th intake was found to be ~ 10 at.%, as concluded from the corresponding expansion of the unit cell volume as a function of % Th (Fig. 1, left). In addition, appearance of ThO2 in the sample with 12 at. % Th also confirms this solubility limit. Introduction of Th atoms into the YSZ system induces flattening of the ZrO8 polyhedra (Fig. 2). This behaviour is explained by the larger ionic radius of Th4+ compared to Ce4+ (1.19 vs. 0.98 Å, respectively, in 8-fold coordination [7]). Thus, insertion of Th atoms introduces additional volume in the unit cell allowing for the coordinating oxygen atoms to arrange in a more symmetrical way with more equilibrated Zr-O distances. Accordingly, the higher Th at. % content may be expected to be favoured by higher (cubic) symmetry. Indeed, cubic ZrxY0.21ThyO2-z system featured intake of Th up to at least 11 at.%, as concluded from the corresponding expansion of the unit cell volume (Fig. 1, right). Investigations in the cubic YSZ system for higher Th content are in progress.
Structural stability of An-containing compounds is one of key requirements for introduction of these materials in the underground nuclear waste repositories (NWR) for long-term storage. This includes resistance against corrosion and internal irradiation. While the underground T-P conditions at the NWR level (typically 500 m below the ground) are rather mild (T ~ 310 K, P ~ 100 bars (or 0.01 GPa)), partial subduction of NWR over a period of million years can not be excluded. This would expose An-containing phases to more extreme temperatures and pressures. In addition, elevated T can be produced in case of a fire outbreak. Therefore, studies of structural stabilities of the corresponding matrix materials under extreme T-P conditions allow to simulate and accelerate processes which can possibly occur during the storage period.
For the corresponding studies we have synthesized Ce-containing YSZ series. Ce is widely used as surrogate atom to simulate tetravalent radioactive elements like Th, U or Pu. In situ T-dependent diffraction studies on tetragonal ZrxY0.10CeyO2-z and cubic ZrxY0.16CeyO2-z series in a RT-1150 K range revealed excellent structural stability for all the studied compounds. In particular, occupancy of Ce4+ atoms as a function of temperature does not decrease in these systems (Fig. 3, Ce0.05Y0.10Zr0.85O1.95 phase is shown as an example) indicating that the mobility of these ions does not increase with temperature. Within the error range unit cell volume increases linearly for all the phases. The corresponding coefficients of thermal expansion, defined as α = 1/V0*((V-V0)/(T-T0)), with V0(V) and T0(T) being the initial (final) unit cell volume and the sample temperature, are listed in Table 1. Cubic Ce-YSZ samples are slightly stiffer than the corresponding tetragonal phases.
Application of external pressure on the Ce0.05Y0.10Zr0.85O1.95 phase induced a structural transformation to a higher cubic symmetry around the P ~ 8.5 GPa. Interesting, occupancy of Ce4+ remains stable throughout the transition. This together with T-dependent data indicates excellent affinity of Ce atoms with the host YSZ matrices. The parent YSZ phases are, therefore, promising candidates as host matrices for radiotoxic tetravalent elements like U, Th or Pu.

ACKNOWLEDGEMENTS

We acknowledge the Federal Ministry of Education and Research (Germany) for the support of this project (BMBF grant 02NUK060).

REFERENCES

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