An investigation of the thermal stability of NdxYyZr1−x−yO2−δ inert matrix fuel materialsby John R. Hayes, Andrew P. Grosvenor, Mouna Saoudi

Journal of Alloys and Compounds


Mechanical Engineering / Mechanics of Materials / Materials Chemistry / Metals and Alloys


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of ud

Received 1 December 2014

Received in revised form 28 January 2015

Accepted 2 February 2015

Available online 21 February 2015


X-ray absorption spectroscopy

Nuclear fuel

An important step in achieving a closed uranium fuel cycle is to develop new inert matrix fuel (IMF) materials for use in the burn-up of transuranic species (TRU; i.e., Pu, Np, Am, Cm). Cubic fluorite zirconia burn-up efficiency of IMF materials means that a smaller IMF loading is required in the reactor core, and makes IMFs economically comparable to MOX fuels [5]. In addition, IMFs may be designed to act as a geological storage matrix, eliminating the need for expensive post-service reprocessing [1–3,5–7]. chemical stability ditions. As such, e in IMF a nly stable a peratures above 2370 C, and adopts a tetragonal struct temperatures between 1170 C and 2370 C and a mon structure at temperatures below 1170 C [19,20]. These stru are highly related, and the tetragonal and monoclinic structures may be considered as distortions of the parent cubic structure [21]. It is well known that the cubic structure can be stabilized by doping the material with aliovalent cations such as Ca2+ or

Y3+. The size difference between Zr and the doped cation, in addition to O-vacancies generated due to charge-balancing requirements, act to stabilize the cubic structure across a range of ⇑ Corresponding author. Tel.: +1 (306) 966 4660; fax: +1 (306) 966 4730.

E-mail address: (A.P. Grosvenor). 1 Formerly, Atomic Energy of Canada Limited (AECL).

Journal of Alloys and Compounds 635 (2015) 245–255

Contents lists availab

Journal of Alloys a .e lTRU species in nuclear waste [4]. IMFs consist of actinides embedded in a neutron transparent (inert) matrix, and, unlike the mixed-oxide (MOX) (Pu,U)O2 fuels currently used, allow TRU burn-up without breeding Pu in the process [5]. The higher that has been shown to retain its mechanical and under extreme temperature and irradiation con this material has been studied extensively for us tions [3,8–18]. However, pure cubic zirconia is o 0925-8388/ 2015 Elsevier B.V. All rights reserved.pplicat temure at oclinic ctures1. Introduction

The use of inert matrix fuels (IMF) has been proposed to ‘‘burn-up’’ (transmute) transuranic elements (TRU; i.e., Pu, Np,

Am, Cm) using currently available commercial pressurized-water reactors (PWR) [1–3] to reduce the heat load generated by these

In order for a material to be considered for use in IMF applications, the material should exhibit: favorable neutronic properties, compatibility with the reactor coolant, ability to withstand high radiation doses, high thermal conductivity, and the ability to incorporate burnable poisons such as Gd and Er [5]. Cubic zirconia (ZrO2), adopting the fluorite-type structure (Fig. 1), is a materialYttria-stabilized zirconia

Structural characterization

Inert matrix

Fuel cycle(ZrO2) has ideal properties for use in IMF applications, but it is not stable at room temperature and must be stabilized through the addition of small amounts of dopants such as Y. While Y-substituted zirconia (YSZ) has been extensively studied, relatively little work has been done to investigate how the addition of an actinide to the YSZ system affects the properties of these materials. To this end, the long-range and local structures of a series of NdxYyZr1xyO2d compounds (Nd was used as a surrogate for Am) were studied using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption spectroscopy (XAS) at the Zr K-, Zr L3-, Y K-, and Nd L3-edges. The thermal stability of Nd–YSZ materials was also investigated by annealing the materials at temperatures ranging between 600 and 1400 C.

These studies showed that the thermal stability of the NdxYyZr1xyO2d system was improved by the addition of small amounts of Y (i.e. P5 at.%) to the system. Additionally, the XAS results showed that the local structure around Zr remained relatively constant; only changes in the second coordination shell were observed when the materials were annealed. These results strongly suggest that the addition of Y can significantly improve the thermal stability of zirconia-based IMFs. This study has also confirmed the importance and value of using advanced characterization techniques that are sensitive to the local structures of a material (i.e., XAS).  2015 Elsevier B.V. All rights reserved.Article history:An investigation of the thermal stability matrix fuel materials

John R. Hayes a, Andrew P. Grosvenor a,⇑, Mouna Sao aDepartment of Chemistry, University of Saskatchewan, Saskatoon, SK S7N 5C9, Canada bCanadian Nuclear Laboratories Limited, Chalk River, ON K0J 1J0, Canada1 a r t i c l e i n f o a b s t r a c t journal homepage: wwwNdxYyZr1xyO2d inert i b le at ScienceDirect nd Compounds sevier .com/locate / ja lcom were set to be consistent with the ideal stoichiometry of each material. As an

Supra-55 WDS-VP SEM coupled with an Energy Dispersive X-ray Spectrometer andtemperatures [22,23]. Yttria-stabilized zirconia (YSZ) has been extensively studied for use in IMF applications, likely due to the extensive knowledge base generated on YSZ from its use and study as a structural ceramic in the nuclear industry [24,25].

Additionally, YSZ has been widely studied as a solid-oxide fuel cell material and a thermal barrier coating [26–28]. However, relatively little research has been performed to understand how the addition of an actinide element to the YSZ system will affect the thermal stability of these materials.

The extremely complex reactions and changes in composition that occur within a nuclear fuel during service make it necessary to understand how the structure of actinide-doped YSZ materials will change when they are subjected to a wide range of temperatures over a range of actinide doping levels. A phase change from a cubic structure to a lower symmetry structure will result in a decrease in the thermal conductivity of the fuel material, which could lead to an unsafe increase in the reactor core temperature [29–31]. Further, the phase transition from the cubic to monoclinic structure is accompanied by a 3–5% increase in volume