An investigation of the thermal stability of Nd Y Zr1−−O2− inert matrix fuel materials

Abstract

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 (ZrO 2) 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 Nd x Y y Zr 1-x-y O 2-δ 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 L 3-, Y K-, and Nd L 3-edges. The thermal stability of Nd-YSZ materials was also investigated by annealing the materials at temperatures ranging between 600-1400 °C. These studies showed that the thermal stability of the Nd x Y y Zr 1-x-y O 2-δ system was improved by the addition of small amounts of Y (i.e. ≥ 5 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).

Investigation of NdxY0.25–xZr0.75O1.88 inert matrix fuel materials made by a co-precipitation synthetic route

“…These results were compared to similar results from Nd-YSZ materials made via a ceramic method reported previously. 39 This study has shown that the local bonding structures around the metal centres in the materials made by the co-precipitation synthesis method or the ceramic synthesis method are similar. However, the intermediate and long-range structures of the materials made by the coprecipitation method were more disordered compared to the structures of the materials made by the ceramic method.…”

“…48 These parameters are described in detail in previous publications. 39,48 The Zr K-edge EXAFS spectra were analyzed by calculating F j (k), j (k), and for each set of scattering pairs in a given cluster using the FEFF6 code. 49 The Zr-Zr and Zr-Y paths were considered as equivalent, due to the similar atomic number of Zr and Y.…”

“…39 (Nd was used as a surrogate for Am.) However, Nd was not evenly distributed in these materials, and it is therefore of interest to investigate if the Nd distribution can be improved using alternate synthetic techniques.…”

“…However, Nd was not evenly distributed in these materials, and it is therefore of interest to investigate if the Nd distribution can be improved using alternate synthetic techniques. 39 Further, ceramic synthesis is generally costly, and the presence of powdered radioactive material would make large-scale syntheses envisioned for safe nuclear fuel production difficult. 40 A low-temperature, liquid-based approach could be more easily automated as well, reducing the costs of safely producing nuclear fuels bearing TRU species.…”

“…Aliquots of the as-synthesized materials were then annealed at 1400 and 1500°C as free powders for 24 h. The Nd x Y 0.25-x Zr 0.75 O 1.88 materials prepared by the ceramic method using binary metal oxides as reactants were synthesized for a previous study, and the full experimental details of the synthesis of these materials have been previously described. 39 The phase identification of the as-synthesized and annealed materials was performed using powder XRD. The powder XRD patterns were collected using a PANalytical Empyrean diffractometer using either Cu K ␣1,2 ( = 1.5406 Å) or Co K ␣1,2 ( = 1.7890 Å) radiation.…”