Co-immobilization of a PuO2 surrogate and contaminated stainless steel within a zirconolite matrix

The aqueous reprocessing of spent nuclear fuel generates a considerable amount of plutonium alongside contaminated stainless steel, necessitating meticulous handling for safe decommission and long-term management. The present work investigated the co-immobilization of CeO 2 (as an inactive surrogate for PuO 2 ) with metallic Fe and Cr (representing a simplified stainless steel) within a zirconolite ceramic wasteform, nominally targeting Ca 1− x Ce x ZrTi 2−2 x Fe x Cr x O 7 . After sintering at 1400°C under an air atmosphere, the zirconolite phase constituted 90.8–95.1 wt.% of the product across the solid solution range of 0.05 ≤ x ≤ 0.25, alongside perovskite and baddeleyite secondary phases in varying proportion; no evidence of the unincorporated CeO 2 or metallic and oxidized Fe or Cr were identified. Above x = 0.30 CeO 2 was detected inferring, the solubility limit was reached. A polytype transformation from zirconolite 2M to 3T was confirmed by X -ray diffraction and selected area electron diffraction results, with the relative fraction of the 3T phase gradually increasing to 49.5 wt.% at x = 0.30. Deconvolution of X-ray photoelectron spectroscopy data revealed the partial reduction of Ce 4+ –Ce 3+ , whereas Fe and Cr species maintained trivalent, in agreement with the targeted substitution scheme. Benefitting from the excellent chemical flexibility of zirconolite structured compounds, the co-immobilization approach may be an effective disposal pathway for Pu-containing wastes and contaminated stainless steel residues.