Event Timeslots (1)

Day 2 – June 21
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Disposal mixed oxide (d-MOX) fuels are a candidate wasteform for the immobilisation of Pu. d-MOX fuels are fabricated by diluting PuO2 within a UO2 matrix and adding a neutron poison, such as Gd. The resulting wasteform will ideally adopt the cubic fluorite structure (Fm-3m) of the matrix, allowing it to retain high corrosion resistance of UO2, whilst Gd mitigates criticality in the disposal environment.

In the present work, a series of four simulant d-MOX compositions were prepared using both solid-state and wet co-precipitation fabrication routes. Th, acting as a Pu surrogate, and Gd were doped into UO2 to produce sintered pellets of simulant d-MOX. Each composition was characterised using XRD, SEM-EDS, Raman, XANES and ICP-OES, revealing the effects of composition and fabrication route on the phase formation, lattice parameter, U oxidation state and grain size.

Both solid-state and co-precipitation synthesis routes yielded dense pellets of simulant disposal-MOX with grain sizes comparable to PuO2-UO2 MOX fuel. Gd3+ and Th4+ were incorporated within the fluorite (Fm-3m) UO2 matrix, substituting directly for U4+ on the FCC cation sub-lattice. Trivalent Gd3+ was charge compensated through both the generation of oxygen vacancy defects, as observed by Raman spectroscopy analysis, and oxidation of U4+ to U5+, as determined by high energy resolution fluorescence detection x-ray absorption near edge spectroscopy (HERFD-XANES). These data demonstrate that, in principle, disposal-MOX materials are a viable candidate wasteform for Pu immobilisation.